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3. METHODS USED IN
OVERPRESSURE ANALYSIS
03. ABNORMAL PRESSURES
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There are several methods that can be applied to predict, evaluate and
calculate the pressures existing in a well (or below, while drilling is still in
progress). They could be divided into three main categories:
1. (not always reliable) Methods used BEFORE DRILLING A WELL,
and in some cases even instead of drilling a well Seismic Data Analysis and Interpretation
Data of Reference Wells, if available and (again) reliable
2. Methods used WHILE DRILLING A WELL (qualitative vs quantitative)
Drilling Data Processing and Interpretation
MWD, LWD, and also LWF Processing and Interpretation
3. Methods usually used AFTER DRILLING A BOREHOLE SECTION Wireline Log Analysis (Sonic Log, Resistivity Log, etc.)
3.1. INTRODUCTION
03. ABNORMAL PRESSURES
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METHOD WHEN WHAT HOW
SURFACE SEISMICS BEFORE DRILLING Interval velocities From reflection times
to transit times
DRILLING
PARAMETERS
WHILE DRILLING ROP, WOB, W, MW,
HS, RPM
Drilling exponent and
Sigmalog
CUTTINGS
ANALYSIS
WHILE DRILLING CUTTINGS Features and size of
drilled cuttings
TEMPERATURE WHILE DRILLING MUD
TEMPERATURE
Measurement of mud
temperature inside and
outside the well
WELL BEHAVIOUR WHILE DRILLING Overpulls,
reaming, drilling
break, gas
detection, torque,
water influx
Anomalies in parameter
values, salinity control
MWD- LWDLOGGING
WHILE DRILLING Resistivity andSonic Log
Resistivity and transittime
WIRE LINE LOGS AFTER DRILLING A
HOLE SECTION
Resistivity and
Sonic Log
Resistivity and transittime
3.1. INTRODUCTION
03. ABNORMAL PRESSURES
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3.2. PRESSURES PREDICTION ANDEVALUATION
FROM SEISMIC DATA ANALYSIS
(not always reliable),BEFORE DRILLING A WELL
03. ABNORMAL PRESSURES
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A. AVERAGE VELOCITY vs
REFLECTION TIME
B. VELOCITY ANALYSIS
(example)
3.2.1. Seismic Data Presentation Formats ENI example
03. ABNORMAL PRESSURES
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Average
Velocity
Two-Way
Time
Cumulative
Depth
Interval
Velocity
Interval
Velocity
Correct
Interval
Depth
Correct
Cumulative
Depth
Correct
Average
Velocity
Correct
Interval
Travel
Time
CORRECTIONS
Vav,i ti Hi Vint, i Vint,c, i Hint,c, i Hi, c Vav,c, i tint,i
m/sec sec m m/sec m/sec m m m/sec sec/ft
(1) (2) (3) (4) (5) (6) (7) (8) (9)
1,600 0.000 0.00
1,815 0.200 181.5 1,815 181.5 181.5 168
1,876 0.300 281.4 1,998 1,992 99.6 281.1 1,874 153
1,936 0.400 387.2 2,116 2,106 105.3 386.4 1,932 145
1,991 0.500 497.7 2,211 2,197 109.9 496.3 1,985 139
2,042 0.600 612.6 2,297 2,280 114.0 610.3 2,034 134
2,095 0.700 733.2 2,413 2,383 119.4 729.7 2,085 128
2,151 0.800 860.4 2,543 2,508 125.4 855.3 2,138 122
2,214 0.900 996.3 2,718 2,665 133.2 988.3 2,198 114
2,291 1.000 1,145.5 2,964 2,893 144.7 1,133.0 2,266 105
3.2.1. Seismic Data Presentation Formats ENI example
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1. The cumulative depth (Column 3) can be easily determined with the following relationship:
Hi = (Vav, i ti)/2
2. The interval velocity (Column 4), that is the velocity with which the sound waves travel within
each single depth interval can be obtained by applying the following relation:
Vint, i = [(Vav, i ti) (Vav,i-1 t i-1)]/(ti t i-1)3. The interval velocity, Vint, c,i , (Column 5) can be corrected by using the following equation:
Vint,c,i = [(V2 av, i ti) (V2 av,i-1 ti-1)]/(ti ti-1)4. Once the interval velocity for each time interval is determined, the thickness of the interval
characterised by given value of Vint,c,i, (Column 6) can be obtained from the relation:
Hint,c, i = [Vint,c,i (ti t i-1)]/25. By summing up the thickness of each single interval, the corrected cumulative depth, Hi,c, is
calculated (Column 7):
H c,i= H int,c, I6. Knowing the interval velocities, corrected according to Dix (Point 3), the corrected average
velocities can be recalculated (Column 8) by using this equation:
Vav,c, i= [Vint,c,i (ti t i-1)]/ti7. At this point the corrected interval velocities are transformed into their reciprocal, that is into
interval travel times, t int,i in sec/ft, (Column 9):t int, I = (0.3048 106)/Vint,c, i
3.2.1. Seismic Data Presentation Formats ENI know how
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3.2.2. Geopressures Evaluation from Seismics
The seismic data (transformed in terms of depths, interval
velocities and interval travel times), at this point are ready
to be used for pressure gradient calculation, that is for the
determination of:
overburden pressure and gradient;
pore pressure and gradient;
fracture pressure and gradient (when possible).
Accord ing to ENI standards, heresec means seconds ,
to be prop er ly ind icated bys
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As a start point, the following relationship between t and can be written:
t = tmax (1) + tfl
where:
- t = transit time of sound through the formation (sec/ft)- tmax = transit time of sound through the solid frame (sec/ft)- tfl = transit time of sound in the pore fluid (sec/ft)
- = porosity, fraction
The previous equation can be also expressed in terms of porosity:
= (t - tmax)/( tn - tmax)
where (in the opinion of ENI):- tfl = assumed equal to 200 sec/ft (a conservative value for pore pressure
gradient calculations)
- tmax = values which depend on lithology
3.2.2. Geopressures Evaluation from Seismics
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BUT: Eni E&P has found from laboratory tests that the previous equationcan better represent the behaviour of non-cemented or undercompacted
formations (i.e. sand, gravel, shales), if written in the following form:
=1.228 [ ( t - tmax) / ( t + 200) ] experimental eq.
By combining this equation with the following equation, which gives the bulk
density of a rock, b:
b= fl + (1) max
the relations expressed in next slide can be derived, which allow thecalculation of bulk densities from travel (transit) times or interval
velocities.
3.2.2. Geopressures Evaluation from Seismics
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3.2.2.1. Overburden Pressures and Gradients Calculation
BULK DENSITIES CALCULATION
Bulk densities can be computed with the following equations, proposed byEni E&P (but NOT shared worldwide):
In terms of interval travel-times:
In terms of interval velocities:
min
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BULK DENSITIES CALCULATION
where:
-b
= bulk density of the formation, g/cm3
-max = matrix density, g/cm3 (an average value of 2.75 g/cm3 is usually assumed)
-tint = interval transit-time obtained from analysis of the seismic data, sec/ft
-tmax = interval transit-time of the rock matrix, sec/ft (assumed between 43.5 47.0
sec/ft)
-tfl = interval transit time of the fluid present in the rock, sec/ft (equal to 200 sec/ft)
-Vint = interval velocity obtained from analysis of the seismic data, m/sec
-Vmax
= velocity of sound in the rock matrix, m/sec (assumed to be 6,485 - 7,000 m/sec)
- Vmin = minimum velocity of sound corresponding to the first superficial layer, m/sec
(generally around 1,500 m/sec).
3.2.2.1. Overburden Pressures and Gradients Calculation
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OVERBURDEN PRESSURE CALCULATION
The following equation is applied:
which expressed in terms of interval transit-time becomes:
3.2.2.1. Overburden Pressures and Gradients Calculation
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OVERBURDEN GRADIENT CALCULATION
The Overburden Gradient can be calculated with the following equation:
where:- Gov = overburden gradient, kgf/cm
2 / 10 m
- Pov = overburden pressure, kgf/cm2
- H = depth, m
3.2.2.1. Overburden Pressures and Gradients Calculation
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OVERBURDEN GRADIENT CALCULATION EXAMPLE
Results of calculations are as here below shown.
Depth Interval Transit Time Bulk Density Overburden
Pressure
Overburden
GradientH, m tint,c,i , sec/ft g/cm
3 kgf/cm2 kgf/cm2/10 m
181.5 168 2.056 37.32 2.056
281.1 153 2.116 58.40 2.078
386.4 145 2.151 81.05 2.098
496.3 139 2.177 104.98 2.116
610.3 134 2.200 130.06 2.131
729.7 128 2.229 156.67 2.147
855.1 122 2.259 185.00 2.163
988.3 114 2.300 215.64 2.182
1133.0 105 2.349 249.63 2.203
1285.1 100 2.377 285.78 2.224
1450.5 92 2.425 325.89 2.247
1629.7 85 2.469 370.13 2.271
1802.7 88 2.450 412.52 2.288
1928.3 85 2.469 456.68 2.305
3.2.2.1. Overburden Pressures and Gradients Calculation
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BULK DENSITY
(kg/dm3)
OVERBURDEN
GRADIENT
(kgf/cm2/10m)
2.0 2.5zero 0
1000
2000
3000
4000
5000
6000
1.5
Profonditm
BULK DENSITY
INTEGRATED OVERBURDEN
DENSITY
3.2.2.1. Overburden Pressures and Gradients Calculation
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3.2.2.2. Pore Pressures and Gradients Calculation
Two methods of analysis of the data can be applied for Pore
Pressure calculation (the final goal, of course). They are:
1) (Interval Velocity versus Depth) or, more commonly,Interval Transit Times versus Depth Plots, finally leading
to a quantitative assessment performed either by theEQUIVALENT DEPTH method or by EATON method
2) (empirical method) Interval Velocity / Theoretical VelocityRatio, R = V1/V2, forclastic formations.
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The calculation procedure can be summed up in the following points:
determination of the interval velocity, Vint;
determination of the depth H relative to each interval velocity;
transformation of the interval velocities into interval travel times, tint; construction of the graph tint versus H on semilog paper (see next slide).Generally, for plotting the data a two cycle semilog paper is used, with depth on
the ordinates and tint on the abscissae (logarithmic scale). It is also advisable toplot the depth in such a way as to have 1 cm corresponding to 100 - 200 m;
once interval transit times as a function of depth have been plotted, the
interpretation of the curve results easier if the following points are taken into
consideration:
in normal compaction conditions, tint decreases with depth; Gp isnormal and equal to 1.031 kgf/cm2/10 m. Through these points, thereference trend line can be drawn;
when tint values start to increase, this means abnormal compactionconditions and an abnormal pore pressure gradient, which has to be
calculated.
3.2.2.2.1. Interval Transit Times versus Depth
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0
1000
2000
3000
4000
5000
10 100 1000
Depth(m)
The tint decreases with
depth down to 2900 m,where the data start to
increase.
3.2.2.2.1. Interval Transit Times versus Depth
3 2 2 2 1 I t l T it Ti D th
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0
1000
2000
3000
4000
5000
10 100 1000
Depth(m)
Overpressures
Top
The tint decreases with
depth down to 2900 m,where the overpressure
top is located.
Below 2900 m,
overpressures areencountered.
3.2.2.2.1. Interval Transit Times versus Depth
3 2 2 2 1 I t l T it Ti D th
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The methods used to quantify pore pressure gradients
using the travel time vs depth plots usually are:
A.theEQUIVALENT DEPTHmethod (most used)
B.EATONmethod
3.2.2.2.1. Interval Transit Times versus Depth
3 2 2 2 1 I t l T it Ti D th
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A. EQUIVALENT DEPTH METHOD PRINCIPLE
1. As already pointed out, the overburden pressure acting at a certain depth, H, is the sum of the
effective pressure, Peffor, and the pore pressure, Pp, as given by the equation:
Pov = Peff+ Pp
2. If at the considered depth H1, the rock has been able to dissipate the pressure generated within
its pores as a consequence of its burial at major depth, its compaction is considered normal and
also normal is its pore pressure, assuming a value equal to the hydrostatic trend.
3. If, for instance, the rock laying at the depth H2 was impeded, for any reason, to expel the excess
of water located within its pores during the burial process as a consequence of the increasing
weight of the sediments above it, it will show a porosity higher than normal, and will be therefore
undercompacted and will have a pore pressure above the hydrostatic one.
4. If the two rocks are made both by the same lithology (i.e. both are shales) and have the same
transit time, though being at different depths, this means that they have the same porosity andthat have been subjected to the same compaction pressure.
3.2.2.2.1. Interval Transit Times versus Depth
3 2 2 2 1 Interval Transit Times versus Depth
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5. Therefore, once calculated the overburden pressure at the depth H1 and knowing that here the
pore pressure gradient is normal (equal, for instance to 1.031 kgf/cm2/10 m), by applying the
above mentioned equation the effective compaction pressure can be easily calculated:
Peff,H1 = Pov,H1 Pp,H1 (at depth H1)
6. Moving to the depth H2, the overburden pressure is easily calculated, being the overburden
gradient already available; by subtracting from the overburden pressure the effective
compaction pressure, as obtained at the previous point, the pore pressure is finally determined
by the difference:
Pp,H2 = Pov,H2 Peff,H1=H2(at depth H2)
Operatively you must:
3.2.2.2.1. Interval Transit Times versus Depth
3 2 2 2 1 Interval Transit Times versus Depth
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1.Select the depth at which you want to compute the Pressure Gradient (i.e. point H2 at 3500 m).
2. Intersect vertically the Normal Compaction Trend line (point H1 at 2300 m).
This means that shales at 3500 m are as compacted as those at 2300 m.
3. From the Integrated Overburden Gradient curve, obtain the Overburden Gradientat 3500 m and 2300 m:
GOV at 3500 m (H2) = 2.42 kgf/cm2/10m GOV at 2300 m (H1) = 2.34 kgf/cm
2/10m
4. Calculate the Compaction Pressure at Depth H1:
(GOVH1 - Gp,n) x H1 (2.34 - 1.03) x 2300Pc = = = 301,3 kgf/cm
210 10
5. Calculate the Overburden Pressure at Depth H2:GOVH2 x H2 2.42 x 3500
POV = = = 847,0 kgf/cm2
10 10
6. Calculate the Pore Pressure at 3500 m: PP = POV - Pc = 847,0 - 301,3 = 545,7 kgf/cm2
7. Calculate the Pore Pressure Gradient at 3500 m:GP = (PP x 10)/H2 = (545,7 x 10)/3500 = 1.56 kgf/cm
2/10 m
3.2.2.2.1. Interval Transit Times versus Depth
3 2 2 2 1 Interval Transit Times versus Depth
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Depth
m
010 100 1000
500
1000
1500
2000
2500
3000
3500
4000
Transit time - sec/ft
H4
H 3
H1
H2
THE EQUIVALENTDEPTH METHOD
3.2.2.2.1. Interval Transit Times versus Depth
3 2 2 2 1 Interval Transit Times versus Depth
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0
500
1000
1500
2000
2500
3000
3500
4000
4500
1.5
Verticaldepthm
2.0 2.5 3.0
Overburden Gradient, kgf/cm2/10 m
THE EQUIVALENT DEPTHMETHOD
H1
H2
Gov,H1 Gov,H2
3.2.2.2.1. Interval Transit Times versus Depth
3 2 2 2 1 Interval Transit Times versus Depth
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B. EATON METHOD
A different and simpler method to calculate the pore pressure gradient is
the method proposed by B. Eaton (1969), which is, however, less carefulwith respect to the equivalent depth method. The Eaton relationship is
based on the ratio, at the examined depth, between the normalt,tNCT,as read on the normal compaction trend line, and the computed value,
tcalc, that is:
n
calc
NCTsedsedp
t
tGGG 03.1
The exponent n depends on the method used to define the normalcompaction trend line and is, usually, taken equal to 3 if the Sonic Log (ora seismic data set) has been performed and evaluated, or to 1.5 if aResistivity Log has been considered.
3.2.2.2.1. Interval Transit Times versus Depth
EATON METHOD
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EATON METHOD
AA
Gov = 2.10 kgf/cm2/10 m
n = 3
tNCT (A) = 50 sec/ft
tcalc (A) = 70 sec/ft
Gp = 1.71 kgf/cm2/10 m
n
calc
NCTsedsedp
t
tGGG 03.1
3 2 2 2 2 Interval Velocity/Theoretical Velocity Ratio R = V1/V2
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It is an empirical method, developed within Eni E&P (but not shared by
other OC) to solve some interpretation problems, sometimes encountered
when using the two previously mentioned techniques in clastic formations.
It is based on the calculation and subsequent graphic representation of theratio, indicated as R, between the interval velocity, Vint, and the soundvelocity, Vs, as follows:
R = Vint/Vswhere:
- Vint= interval velocity as obtained from seismics, sec/ft- Vs = sound velocity, that is the velocity of the sound that should beobserved within normally compacted shales, sec/ft
Depending on the values assumed by the quantity R, the followingconditions will be possible:
if R =1 normally compacted and pressured formations if R < 1 undercompacted and abnormally pressured formations if R > 1 overcompacted formations or carbonates
3.2.2.2.2. Interval Velocity/Theoretical Velocity Ratio, R V1/V2
3.2.2.2.2. Interval Velocity/Theoretical Velocity Ratio, R = V1/V2
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R ratio
0
0,5 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0
500
1,0001.500
2.000
2.500
3.000
3.500
4.000
4.500
5.000
5.500
6.000
6.500
7.000
Very porous oroverpressuredformations (R
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0
0,5 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0
500
1.000
1.500
2.00
02.500
3.000
3.500
4.000
4.500
5.000
5.500
6.000
6.5007.000
Overcompactedformations (R>1)
R ratio
Very porous oroverpressuredformations (R
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Once the overburden and pore pressures gradients have been calculated from
the analysis of seismic data as seen before, the fracture gradients can be
easily derived by applying the following equations:
a) The fracture gradient, showing the rate at which the fracture pressure
changes with depth, is given, for a rock having an elastic behaviour, by the
followng expression:
Gfr= Gp + ( 2 ) (Gov Gp)
1-
b) If the drilling fluid is water or whenever the drilling fluid invades in depth the
formation, we have:
Gfr= Gp+ ( 2 ) (Gov Gp)
c) If the rock has a plastic behaviour, the fracture gradients is given by the
formula:Gfr= Gov
3 3 actu e essu es a d G ad e ts
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3.3. PRESSURE PREDICTION AND
EVALUATION
FROM DRILLING DATA ANALYSIS
03. ABNORMAL PRESSURES
33Enis Way
3 3 1 INTRODUCTION
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The processing and interpretation of drilling parameters represents a very
important group of techniques, which have the advantage to be available
more or less in real time and to be referred to any possible well in progress.
These methods can be:
qualitative, which, if analyzed in their completeness, can anyway provide
significant information about the actual status of the well and alert the drilling
engineering staff in case dangerous and abnormal conditions are in progress;
quantitative, which ensure the quantification of the pressures acting in andaround the well and the related risk levels.
3.3.1. INTRODUCTION
03. ABNORMAL PRESSURES
34Enis Way
3.3.2. QUALITATIVE INDICATORS
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Among the qualitative techniques, based on drilling and geological parameters
recording, processing and interpretation, we can recall the following ones:
rate of penetration
torque
overpulls
cavings and hole tightening
flow rate and pumping pressure
mud level in the pits
cuttings increase at shale shakers
hole fill up
mud resistivity and chlorides concentration
pH
shale resistivity and shale density
gas shows
mud temperaturemontmorillonite concentration in the mud (MBT test)
Q
03. ABNORMAL PRESSURES
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3 3 2 1 QUALITATIVE DRILLING INDICATORS
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DRILLING RATE OR RATE OF PENETRATION ROP
If everything else remains constant, the drilling rate (here also calledpenetration rate orrate of penetration, ROP) gradually declines as depth
increases due to the decreasing porosity caused by the weight of the
overlying sediments and the increase in differential pressure between the
hydrostatic head of drilling mud in hole and the pressure of formation fluids.
But raw penetration rate values are affected by so many influencing factors(characteristics of formations, bit types and wear, WOB, RPM, mud type
and weight, etc), that it is impossible to use them directly within a reliable
and efficient detecting method. Furthermore, penetration rate values
provide only a qualitative indication about formations porosity and do not
allow any quantitative evaluation of existing (or not) abnormal pressures.
03. ABNORMAL PRESSURES
36Enis Way
3.3.2.1. QUALITATIVE DRILLING INDICATORS
3.3.2.1. QUALITATIVE DRILLING INDICATORS
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TORQUE
The amount of torque required during the drilling operations can supply
reliable information about the presence of abnormally pressured formations.
It must be remembered that the amount of required torque depends on the
resistance met by the bit, which is a function of: the weight on bit, the
coefficient of friction of the formation and the amount of restoring torque, thelatter parameter being significantly dependent on the amount of frictional
force developed against the wellbore walls. Any change in the torque value,
therefore, can be due to a change in weight on bit, to a change in the type
of formation or to bit balling, and not only to hole tightening, which could
be indicative of wellbore instability and of presence of abnormal pressure
values.
03. ABNORMAL PRESSURES
37Enis Way
3.3.2.1. QUALITATIVE DRILLING INDICATORS
3.3.2.1. QUALITATIVE DRILLING INDICATORS
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OVERPULLS
When tripping out a string, the amount of hook load is approximatelyproportional to the depth reached. For various causes, not alwaysimputable to the presence of an abnormal pressure, the predicted value
may be exceeded, thus causing an overpullcondition. The main causes of
hook load increase can be represented by the following items:
bit balling, stuck drill string;
unusual swabbing effects;
severe dog-legs.
If these above stated causes can be ruled out, the overpull may be
attributed to underbalance conditions with hole tightening, and then to thepossible presence of overpressures.
03. ABNORMAL PRESSURES
38Enis Way
3.3.2.1. QUALITATIVE DRILLING INDICATORS
3.3.2.1. QUALITATIVE DRILLING INDICATORS
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PUMPING PRESSURE
If a fluid, having a density which is lower than the mud density in the hole, enters the
well, a decrease in pumping pressure can be observed, because the hydrostatic
pressure in the annulus becomes lower than the pressure in the drill pipes
determining as a consequence a decrease also in pumping pressure for the U tube
effect. In these conditions also the friction losses in the annulus decrease.
FLOW RATE-IN AND FLOW RATE-OUT
If the mud flow rate exiting the hole is higher than the mud flow rate pumped into the
drill string, that clearly indicates underbalance conditions, before any mud pit
volume (say surface level position) change can be appreciated.
HOLE FILL-UP
If, during tripping the drill string out of the hole, less mud than expected is required
to fill the hole, this indicates underbalance conditions and then the necessity to
increase the mud density, unless evident signs of swab are recognized.
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MUD SALINITY, RESISTIVITY, pH
If an increase in the salinity (mainly
chlorides) of the mud is observed, this
means that water, present in the rockpores, has entered the hole, because of
insufficient pressure exerted by the
mud column. Being the formation water
usually more salty than the water used
for preparing the mud, an increase in
salinity is therefore experienced.
The inflow of salty formation waters into
the hole determines, not only an
increase in salinity, but at the same
time provokes a decrease in mud
resistivity and in its pH. If these
quantities are continuously monitored
and plotted versus depth, theoccurrence of abnormally pressured
formations can be detected.
RESISTIVITY - The resistivity analysis of the salty water that contaminates the drillingmud (from overpressured layers) tends to show slowly decreasing values.
MUD CHLORIDES An increase in Chlorides concentration is an overpressure index.
Depthm
RESISTIVITY
OVERPRESSURETOP
Depthm
CHLORIDES
OVERPRESSURETOP
MUD RESISTIVITY - CHLORIDES
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MUD DENSITY/GAS RELATIONSHIP
The volume of gas released from a drilled formation will be dependent upon: porosity,
permeability, gas saturation and differential pressure of this formation.
The progressive and continuous increase of gas in mud while drilling (background
gas) and while connecting pipes to the drill string (pipe connection gas) could be
regarded as another way to monitor abnormal pressure conditions, provided that other
causes such as swab or drilling through gas bearing rock could be excluded.
The influx of gas into the wellbore determines a decrease in mud density, which can becalculated by using the relation:
with:
- W1 = gas-cut mud density, lb/gal
- W2 = uncut mud density, lb/gal
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CUTTINGS CHARACTERISTICS
When underbalance conditions
are approached, an increase in
penetration rate occurs; in this
case the amount of the cuttingsproduced and lifted to the surface
increases too. The extra-quantity
of produced cuttings depends on:
length of the interval drilled in
underbalance;
differential pressure between themud and the formation;
bit penetration rate.
When drilling in underbalanced
conditions, also cavings are
produced. Cavings are largepieces of formation detached from
the wellbore walls and not due the
bit activity (as the cuttings are).
A. TYPICAL SHALE CAVINGS
PRODUCED BY
UNDERBALANCED DRILLING
B. TYPICAL ARGILLACEOUS
CAVINGS PRODUCED BY
STRESS RELIEF
A B
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SHALE DENSITY
This has been a very popular technique in the past, when sensors and computers were
not so common at rig sites.
The method is based on the measurements of shale density, assuming that
overpressured shales be undercompacted, therefore more porous and with a lower
density than normally compacted shales. By plotting the density, as obtained from
cuttings collected at the shale shakers, versus depth, a graph is built, which shows an
increase in shale density if normal conditions exist and a decrease when overpressures
are entered.
The magnitude of the bulk density change will vary with the type and magnitude of the
geopressure. Bulk density may also decrease, but it may remain constant (due to
lithology) or continue to increase at a lower rate than the previously established trend
due to the geopressure mechanism.
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OVERPRESSURED shales are UNDERCOMPACTED, have a LOWER DENSITYthan what they should have at the depth where they are located.
Verticaldepthm
Verticaldepthm
DENSITY g/cm3
2.2 2.3 2.4 2.5
DENSITY g/cm3
2.2 2.3 2.4 2.5
OVERPRESSURETOP
SHALE DENSITY
NORMAL
COMPACTIONUNDERCOMPACTION
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SHALE DENSITY
Ideal Clay
DensityResponses in
Geopressured
Zones Caused
by Different
Mechanisms
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TEMPERATURE
The geothermal gradient, or the rate at which subsurface temperatures increase with
depth, can be calculated by using the relation:
GT = 100 [(TF2 TF1)/(D2 D1)]
where:
- GT = geothermal gradient (C / 30 m)
- TF1 = temperature (C at depth D1, m)
-TF2 = temperature (C at depth D2, m)
While the average temperature gradient across normally pressured formations may beconstant, pressured formations exhibit abnormally high geothermal gradients, due to
their higher porosity and higher fluid content, which make them very poor heat
conductors. Therefore, overpressured shales will heat the mud much more than other
normally pressured rocks.
Monitoring and recording mud flowline temperature is a practical method to determinetemperature gradient, provided variable factors such as pump rate, lag time,
environment temperature, lithology and temperature changes at the surface (due to
mud mixing and to chemical treatments) can be accounted for.
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TEMPERATURE
A. Geothermal Gradient Change
Through an Insulating Geopressured
Zone
B. Expected Flowline Temperature WhenDrilling Through a Geopressured Interval
A
B
Prior to reaching a
geopressured zone, a
ttz (say a temperature
transition zone) will
be encountered in
which, due to distortion
of the isothermal lines,
there will be areduction in
geothermal gradient,
followed by an
extremely large
increase, occurring as
the geopressured zoneis penetrated.
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TEMPERATURE
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TEMPERATURE
At the rig-site flowline
temperature is
plotted againstdepth. The transition
zone is characterized
by a decrease in
geothermal gradient,
while the entrance into
the overpressuresshows an increase in
geothermal gradient.
The mud temperature
is affected by many
variables and can giveonly qualitative type
responses.
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NORMALIZED PENETRATION RATE
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NORMALIZED PENETRATION RATE
As seen before, the rate at which a formation can be drilled is determined by a number
of factors, such as:
weight on bit, WOB; rotary speed, RPM;
bit tooth efficiency;
differential pressure;
hydraulics;
rock matrix strength;
formation compaction.
To make penetration rate info more useful, several attempts have been made in the
past to correct them for some of the most important parameters which affect drilling, in
particular WOB, RPM, hole size, differential pressure and mud density and,
consequently, a number ofdrillability ornormalized penetration rate formulationshave been proposed in order to simplify the treatment of the effects of so many drilling
variables.
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NORMALIZED PENETRATION RATE
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NORMALIZED PENETRATION RATE
Among the many drillability formulations, proposed by several authors to correctpenetration rates for the variables affecting them, one particularly used in the Industry,
at least as a starting point, was that developed by G.F. Combs (1968):
where:
- R0= penetration rate with a new bit and P=0, ft/h- W = weight on bit per hole size unit, lbs/in- Dh = hole diameter, in
- N = rotary table revolutions per minute, rpm
- Q = flow rate, gal/min
- Dn = bit nozzle diameter, 1/32ths of an inch
- aw = exponent of weight on bit
- an = exponent of rotary table speed- aq = exponent of hydraulics
- P = differential pressure, psi- T = bit teeth wear index
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From this basic drillability equation proposed by Combs, two are the most commonsimplified methods derived to obtain a quantitative evaluation of pressures and relevant
gradients while drilling a well, that is:
the drilling exponent, d-exp, with its more advanced version, that is the corrected
drilling exponent, dc-exp, method;
the Sigmalog method, developed also by Eni E&P (but different from other versions).
1. Both methods are semi-empirical and are based on a relationship between
penetration rates and the selected drilling parameters.
2. If all other conditions remain the same, these drillability indices are proportional to
the depth. In other words, in normal compaction conditions, they increase as the depth
of the well increases, because the rock becomes harder and the differential pressure
between the mud in hole and the formation pressure increases too, while in presence of
overpressures the drillability index decreases, because the porosity of the rockincreases and the differential pressure decreases..
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3.3.3.1. THE DRILLING EXPONENT AND
CORRECTED DRILLING EXPONENT METHOD
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M.G. Bingham (1965) proposed the following relationship between the drilling rate,
weight on bit, rotary speed and bit diameter.
where:
- R = drilling (penetration) rate, ft/h- N = rotary speed, rpm
- D = bit diameter, ft
- W = Weight on bit, lbs force
- a = matrix constant, dimensionless
- d = drillability exponent, dimensionless
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This mathematical relationship was revised and adapted to field requirements by Jorden
and Shirley (1966), solving ford and by introducing constants to take into account the
units of measurement commonly used in the petroleum industry in order to obtain valuesthat vary within an acceptable range. It assumed the following form which is known as
drilling exponent or simply d-exp:
where:
- d-exp = drilling exponent. dimensionless
- R = drilling or penetration rate. ft/h
- N = rotary speed, rpm
- W = weight on bit, Ibs force
- D = bit diameter, in
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3.3.3.1. THE DRILLING EXPONENT AND
CORRECTED DRILLING EXPONENT METHOD
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Using metric units and changing from natural to common logarithms, the previous
equation becomes:
where:
- d-exp = drilling exponent. dimensionless
- R = drilling or penetration rate, m/h
- W = weight on bit, ton force- D = bit diameter, in
- N = rotary speed, rpm
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CORRECTED DRILLING EXPONENT METHOD
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Since differential pressure depends upon the mud density and formation pore pressure,
whenever there is any change in the mud density this will promote an unwanted change
in the d-exp; for this reason, the corrected drilling exponentordc-exp was proposed(Rehm and McClendon, 1971), whose expression, in metric units, is the following:
- Gpn = normal pore pressure (equal to 1.031), kgff/cm2/10 m
- ECD = Equivalent Circulating Density (mud density plus friction losses), kg/litre
or more simply:
dc-exp = d-exp/MW
with:- d-exp = uncorrected drilling exponent
- MW = mud weight (referring to the density of the mud in use, given in kg / litre)
log
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3.3.3.1. THE DRILLING EXPONENT AND
CORRECTED DRILLING EXPONENT METHOD
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The calculation sequence generally adopted in the field is the following:
the drilling parameters (ROP, WOB, RPM, HS) are recorded every 1 meter drilled
and processed by using the equations previously seen;
the resulting d-exp and dc-exp values are plotted versus depth on a semilogarithmic
paper;
in normal compaction conditions, the d-exp, and also the dc-exp, will increase with the
depth and all points, taken in correspondence of shales, will lay on what is called the
normal compaction trend line;
as soon as the d-exp and dc-exp values, always taken in correspondence of shalelevels, start to decrease, this means that abnormal formations are entered and the
more these values depart from the reference trend line the higher is their overpressure.
Of course, the true overpressure values have to be calculated on the dc-exp curve,
because it has been corrected for the mud weight present in the well. The following slide
shows that the d-exp curve, with respect to the dc-exp curve, is masked as the mudweight seems to result increased, thus indicating (apparently) a lower pressure regime.
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d Exponentd Exponent dc
Exponentdc Exponent
Depth
Normally
Compacted Zone
Mud Masking
Effect
Overpressured
Zone
Overpressure
Top
d-exp and dc-exp
trends versus depth
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3.3.3.1. THE DRILLING EXPONENT AND
CORRECTED DRILLING EXPONENT METHOD
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The quantitative calculation of pore pressures and gradients can be performed with
one of the three following methods, that is:
A.the equivalent depth principle, as seen when dealing with the interval travel timesin seismics;
B. (slightly modified) the so-called Eaton method;
C. the r ratio method (abacus construction) which is based on the expression:
Po = PN [(dc-exp)N/(dc-exp)p]
where:
- Po = the actual pore pressure at the depth, H, of interest, kgf/cm2
- PN = normal pore pressure, obtained from (H x 1,031)/10, kgf/cm2
- (dc-exp)N = actual dc-exp value at the depth of interest;
- (dc-exp)p = value at the depth of interest as read on the normal compaction trend line
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3.3.3.1. THE DRILLING EXPONENT AND
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ddcc--exponentexponent
Vertical
Depth
Vertical
Depth
A2
A1H1
H2
dc-EXPONENT: A) EQUIVALENT DEPTH METHOD
Pc,H1 = Pov,H1 Pp,H1
Pov,H1 = (G ov,H1 H1)/10
Pp,H1 = (1.03 H1)/10
Pc,H1 = Pc,H2
Pov,H2 = (G ov,H2 H2)/10
Pp,H2 = Pov,H2 Pc,H1=H2
Gp,H2 = (Pp,H2 10)/H2
Gov,H1 = 2.13 kg/cm2/10 m
Gov,H2 = 2.28 kg/cm2/10 m
H1 =2300 m
H2 = 3400 m
Pov,H1 = 490 kg/cm2
Pp,H1 = 237 kg/cm2
Pc,H1 = Pc,H2 = 253 kg/cm2
Pov,H2 = 775 kg/cm2Pp,H2 = 522 kg/cm
2
Gp,H2 = 1.54 kg/cm2/10 m
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3.3.3.1. THE DRILLING EXPONENT AND
CORRECTED DRILLING EXPONENT METHOD
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2.1
03.1
norm
compovbdovbdp
dcdcGGG
dc-EXPONENT: B) EATON METHOD
A A
Gov = 2.10 kg/cm2/10 m
dccomp (A) = 0.4
dcnorm (A) = 0.6
Gp = 1.44 kg/cm2/10 m
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CORRECTED DRILLING EXPONENT METHOD
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dc-exp
DEPTH
dc-EXPONENT: C) r RATIO METHOD (ABACUS)
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211
131
131
131
111
Bitrundepthm
Sand
Line
Gp Lines
1.7 1.5 1.3 1.1
SHALES
Line
bit
Wear
Bit Wear
Top of Overpressure
Porosity effect
The interpretation of the
dc-exp curve is facilitatedby taking into
consideration:
lithology;
bit runs,
casing setting points;
hole difficulties;
knowledge of the area.
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The Sigmalog method has been developed during the 1970s also (but not
only) by Eni E&P to overcome the limits of the dc-exp technique experienced
while drilling deep wells in the Po Valley Basin, in particular its deficiency in
identifying overpressures within carbonatic reservoirs.
The Sigmalog takes into account the mud density effect on penetration rates
and is based on the drillability concept, as all other methods which process
drilling data. Again the parameters involved are ROP (m/h), RPM (rpm), WOB
(ton force) and bit or hole size HS (in).
The Sigmalog does not allow only the calculation of the pore pressure
gradients, but also of the overburden and fracture gradients through steps
very long and tedious indeed; for this reason, the overburden gradients can be
more easily determined from seismic interpretation or from Sonic Log analysis.
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The basic equation of Sigmalog is as follows:
t= WOB0,5 x RPM0,25
Dh x ROP0,25
where:
- WOB : weight on bit, ton force
- RPM: revolutions per minute of the rotary table
- Dh: hole size (also indicated as HS), in
- ROP: rate of penetration, m/h
To compensate for values excursion at shallow depth, a correction factor,
depending from depth, has been introduced:
t = t + 0,028 (7 H/1000)
where:- H: depth, m
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In order to take into account the effect of the differential pressure, P, on the
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penetration rate, the following relationship is introduced:
P = (MWGp) x H/10
where:
- MW: mud weight, kg / litre
- Gp: pore pressure gradient at the depth H, kgf/cm2/10m (taken equal to 1.031)
Because ROP does not change linearly with P, a correction factor takes into
account this occurrence:
F* = 1 + 1 - 1 + n2P2
n Pwhere:
- n = 3,2/(640 t) if t 1
- n = 1 x (4 0,75 ) ift > 1640 t
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The value of Sigmalog which is plotted versus depth is given by the following
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The value of Sigmalog, which is plotted versus depth, is given by the following
equation:
o = F t
The interpretation criterion is the same as per the dc-exponent method., that is:
- if the o values increase with depth in homogeneous formations (shales), it meansthat normal conditions exist and the pore pressure is normal;
- if the o values tend to decrease with depth, always in homogeneous formations, it
means that abnormal conditions are encountered and that overpressures arepresent.
A reference trend line r, indicating normal compaction, can be drawn throughthe o values in the section of the hole where they constantly increase with depth.
The departure of the ovalues from the r trend line is proportional to theamount of overpressure.
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Th l ti t d li h t t i li ti
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The normal compaction trend line has a constant inclination:(angular coefficient a = 0,088)
Trend line equation:
where: H = depth, mb = intersection between the trend line and the x-axis
sr = aH
1000+ b
Pore Pressure Gradient Calculation
3. Calculate the PORE PRESSURE GRADIENT
2. Recalculate the mud/poredifferential pressure
F*t
r=
'
s
sP = 2 (1-F)
1 (1-F)
1
n2
sO s't1. Once youknow
GP =d - p 10H
Mud
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TRUE SIGMALOG
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NORMAL COMPACTION
TREND LINE
ABNORMALLY
COMPACTED AND
OVERPRESSURED ZONE
TRUE SIGMALOG
DEPTH
OVERPRESSURE TOP
True Sigmalog
Versus Depth
Plot Example
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Sigmalog:
Determination of
the ReferenceTrend Line
DETERMINATION OF THE
NORMAL COMPACTION TREND
LINE:
Vertic
aldepthm
Normal
compaction
trend
sO
sr
THE SIGMALOG METHOD: INTERPRETATION
b
SLOPE: 0.088
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THE SIGMALOG METHOD: INTERPRETATION
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Correction of the
Reference Trend
Line by Shiftsin the Normally
Compacted
Zones
sO
Vertic
alDepthm
Vertic
alDepthm
SHIFTS OF THESIGMALOG CURVE
ALLOW THE
CORRECTION OF
THE REFERENCE
TREND LINE
b1 b2b3b4
SLOPE: 0.088
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THE SIGMALOG METHOD: INTERPRETATION
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Correction of the
Reference Trend
Line by Shiftsin the
Abnormally
Compacted
Zones
VerticalD
epth
m
0,2 0,4 0,6b1
2
0s
0s
1
0s
b2
THE SIGMALOG METHOD: INTERPRETATION
CORRECTION OF THEREFERENCE TREND LINE BYSHIFTS IN THEOVERPRESSUREDINTERVALS
OVERPRESSURE
TOP
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2. THE SIGMALOG METHOD
CALCULATION OF PORE PRESSURES AND GRADIENTS
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CALCULATION OF PORE PRESSURES AND GRADIENTS
Once the term b is defined for each shift recognizable on the Sigmalogcurve, the pore pressure gradient at any particular depth is calculated usingthe following procedure:
calculate the values assumed by the reference trend line as seen above;
calculate the term F from the equation:
define the differential pressure P as:P = [2 (1 F)]/[1 (1 F)2] (1/n)
calculate the pore pressure gradient as:
Gp = df- [(P 10)/H]
where:
- df= mud density, kg / litre
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THE SIGMALOG METHOD: PRESENTATION EXAMPLE Example of Pore
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1.1 1.3
1.50.
8
1.71.
2
1.81.
6
2.22.
00.1 0.3 0.5 0.7 0.9
124
134
128
136 231
126
124
527
134
124
124
131
131
131
131
111
111
111
500
1000
1500
2000
2500
3000
3500
A
C
B
Kick
A B C
A: Sigmalog before
interpretation with
representation of the
reference trend lines
B: Sigmalog after
interpretation with
representation of
only one reference
trend line
C: Pore pressuregradient trend with
representation of the
mud densities used
while drilling the well
At the left and right
of the curves, bit
runs, hole sizes andlithology are shown
Pressure
Gradient
Development
from
an Interpreted
Sigmalog Curve
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3.4. PRESSURES PREDICTION
AND EVALUATION
FROM WIRELINE LOGS
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3.4.1. INTRODUCTION
The analysis of wireline logs allows the calculation of:
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- Overburden Gradients
- Pore Pressure Gradients
- Fracture Gradients
The wireline logs generally used for pressure prediction and evaluation are:
Sonic Log (SL)
Induction Log (IES)
Formation Compensated Density Log (FDC)
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The Sonic Log is interpreted in the same way as the seismic data; in fact
3.4.2. SONIC LOG
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both are based on the use of travel times, t, plotted versus depth.
The Sonic Log allows the calculation of:
overburden pressures and gradients
pore pressures and gradients
fracture pressures and gradients
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As already seen, the following relationship between t and can be written:
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t = tmax (1) + tfl
where:
- t = transit time of sound through the formation (sec/ft)- tmax = transit time of sound through the solid frame (sec/ft)-tfl = transit time of sound in the pore fluid (sec/ft)
The previous equation can be also expressed in terms of porosity:
= (t - tmax)/( tn - - tmax)
where:
- tfl = assumed equal to 200 sec/ft (a conservative value for pore pressure
gradient calculations)- tmax = values depend on lithology
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SOLID Density tMATTER g/cm3 microseconds/ft
Dolomite 2.87 43.5
Limestone 2.71 43.5 - 47.5
Anhydrite 2.96 50
Shale 2.70 47
Transit Times and Density for Some Rock Matrices
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Laboratory tests proved the following relations correct.
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y p g
Relat ion between transi t t ime and poro si ty
for conso l idated and cemented rocks
for uncon sol idated sands
for shales
Relat ion between densi ty and trans i t t ime
for cemented and compacted formations
for non-cemented formations
f = (1)t- t153
ma
ft +
(2)= 1.228 t- tma
200
f (3)= 1.568 t - tt +
ma 200
d = 3.28 - t89
(4)
d =2.75-2.11 tt+
- tma
200
(5)
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Comparisons with density values detected by the FDClog (formation
density compensated log) attested and verified that the following relation
is valid for all formations:
b +2 75 2 11
47
200. .
t
t
where:
47 = t max (transit time through the rocky matrix assumed to be equal to 47 sec/ft
200 = t in water, sec/ft
03. ABNORMAL PRESSURES
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To determine the exact transit time in an interval ofH thickness, the following procedurecan be applied:
th b f illi d l d f th d t th f ti i t l
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the number of milliseconds elapsed for the sound wave to cross the formation interval
having thickness H is read (this is done by counting the ITT pips on the SL to the left)
on the log; this value is multiplied by 1,000 to change milliseconds into microseconds, and is
divided by the value ofH; in order to obtain the values oft, measured in sec/ft, hereH must be changed from meters into ft by dividing by 3.28; the final expression represents the average transit time in the H interval and takes thesimple form:
t = (K 1000)/(3.28 H)
where:
- K = milliseconds required by the sound wave to pass through a section of height H:- H = depth interval, m
The t values can be also more rapidly read on the log by taking into account theintervals characterized by more or less the same values.
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SONIC LOG INTERPRETATION
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1 82
SONIC LOG INTERPRETATION
ITT = INTEGRATED TRANSIT TIME
H = 15.5 m
K = 8.2
t = 161 sec/ft
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Once determined the bulk density versus depth with the equations seen above,
the procedure for the calculation of the overburden pressure and relevant
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83
the procedure for the calculation of the overburden pressure and relevant
overburden gradient using the transit times read on a Sonic Log is the same as
discussed when dealing with seismic data interpretation, to which the reader
has to refer. Therefore, the basic equations to consider are, as already known,
the following:
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Overburden Gradient,kgf/cm2/10 m
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BULK DENSITY
0
1000
2000
3000
4000
5000
1,6 1.8 2.0kg/dm3
2.2 2.4
0
500
1000
1500
2000
2500
3000
3500
4000
4500
1.5
V
erticaldepthm
2.0 2.5 3.0kgf/cm2/10 m
Enis Way
20 60 100 200The plot of Transit Times t, asobtained from Sonic Log readings,
t (s/ft)
3.4.2.2. SONIC LOG: PORE PRESSURE GRADIENT CALCULATION
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85
500
1000
1500
2000
2500
obtained from Sonic Log readings,
versus depth is the most common
method for overpressure detectionand calculation, being a quick and
reliable tool. As in the case of
seismics, the transit times in shales
decrease regularly with depth in
normally compacted and normally
pressured formations. This is due tothe fact that the density of the rocks
increases with depth in normal
conditions and increases also the
velocity with which the sound waves
propagate through them. The transit
times will lay on a straight line, whichis the normal compaction trend line.
Enis Way
10080 200
Considering always valid the assumption
that density, porosity and relevantt (s/ft)
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Ve
rticalDepthm
0
500
1000
1500
2000
2500
60 10080 200y, p y
pressures (effective and pore pressure)
are correlated, it derives very clearly that
the t will decrease regularly with thedepth when normal conditions exist and
that an increase in its values, on the
contrary, will determine its departure from
the reference trend line and will be
indicative of abnormal conditions
(undercompaction and overpressures). OVERPRESSURES TOP
OVERPRESSURED shales are
UNDERCOMPACTED and have aHIGHER PORE WATER CONTENT.
Therefore they have a HIGHER t,compared to the depth at which theylie.
01008060 200t (s/ft)
In the t vs depth plots, it is possibleti t b th hift d
3.4.2.2. SONIC LOG: PORE PRESSURE GRADIENT CALCULATION
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Verticalde
pthm
500
1000
1500
2000
sometimes to see both shifts and
slope changes in the reference trend
line. Very often these anomalies,clearly indicated by an abrupt
dislocation of the points rightwards or
leftwards, are not related to
overpressures but to particular
geological conditions (overcompacted
formations, for instance in the upper
part of the well) and, therefore, have to
be corrected during interpretation.
TRUE OVERPRESSURES TOP
ERRONEOUS OVERPRESSURES TOP
OVERCOMPACTED
SECTION
Once the curve has been interpreted and adequately corrected, the pore
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pressure gradients are calculated by the usual equivalent depth method orby the Eaton method, as already seen.
microseconds/ft
0
01.5 2.0 2.5 3.0
kgf/cm2/10 m
2 36 2 50
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Verticaldepth(m)
10 20 30 50 100 200 300 500 1.000
500
1.000
1.500
2.000
2.500
3.000
3.500
4.000
4.500
5.000
500
1000
1500
2000
2500
3000
3500
4000
4500
Verticaldepthm
POINT A:- Pov = 1125 kgf/cm
2
- Pc = 332 kgf/cm2
- Pp = 1125 - 332 =
793 kgf/cm2
- Gp = 1.76 kgf/cm2/10m
POINT B:- Pov = 590 kgf/cm2
- Pp = 258 kgf/cm2
- Pc = 590 - 258 =
332 kgf/cm2
POINT A
POINT B
2.36 2.50
2500 m
4500 m
4500 m
2500 m
EQUIVALENT DEPTH PRINCIPLE
Once the overburden and pore pressures gradients have been calculated, the
fracture gradients can be easily derived by applying the following usual
ti
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equations:
a) The fracture gradient, showing the rate at which the fracture pressurechanges with depth, is given, for a rock having an elastic behaviour, by the
followng expression:
Gfr= Gp + ( 2 ) (Gov Gp)1-
b) If the drilling fluid is water or invades in depth the formation, the relationships
becomes:
Gfr= Gp + ( 2 ) (Gov Gp)
c) If the rock has a plastic behaviour, the fracture gradients is given by the
formula:
Gfr= Gov
The resistivity of a rock depends on its porosity and on the amount of fluids
present in the pores; therefore low porosity rocks are usually high resistivity
3.4.3. SHALE RESISTIVITY: PORE PRESSURE GRADIENTS CALCULATION
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present in the pores; therefore low porosity rocks are usually high resistivity
formations (for instance, compacted limestone, volcanic rocks, etc).
If all other conditions are the same, the resistivity of a rock depends on:
saline concentration of the fluids within the pores;
rock composition;
formation temperature.
As depth increases, shales are more and more compacted and less porous,
therefore their resistivity tends to progressively increase.
03. ABNORMAL PRESSURES
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There are two methods of overpressure analysis based on
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Shale Resistivity:
Method 1 - Find shale resistivity on the recorded electric log,
and plot it directly on a semi-logarithmic scale,
without any elaboration.
Method 2 - Analyze the F shale factor (shale formation
factor).
03. ABNORMAL PRESSURES
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3.4.3.1. SHALE RESISTIVITY PLOT
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RESISTIVITY LOG
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METHOD 1The resistivity of the cleanest shales is plotted Vs depth, on a semi-logarithmic scale. The
inversely proportional relationship existing between resistivity and porosity (meant as fluid content)
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y p p p g y p y ( )
will produce a diagram, whose values will increase with the depth in case of normal compaction
conditions.
Depth
Shale resistivityTherefore, in case of normally
compacted and normally pressured
formations, the resistivity values
will increase with the depth and will
lie on the normal compaction trend
line, as shown in Figure aside.
Formation with a NORMAL
pressure gradient.
As depth increases, so does
compaction, while porosity
decreases
03. ABNORMAL PRESSURES
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METHOD 1In case of overpressured formations, the resistivity values, as obtained from the log
readings, will tend to depart from the normal compaction trend line, assuming lower values
3.4.3.1. SHALE RESISTIVITY PLOT
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than expected for that depth of burial.
Overpressure Top
Shale resistivity
Depth
This depends on the fact that more
fluid with more salt is present in the
pores of the rock and this makes the
resistivity to decrease. The more the
points depart from the reference
trend line, the higher the pore
pressure gradient will be.
OVERPRESSURED formations
Undercompacted shales with
high porosity, compared to
the depth where they lie.
03. ABNORMAL PRESSURES
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METHOD 2In this case, it is not made reference only to the resistivity values as read on the log,
but it is
necessary to determine the Shale Formation Factor, or simply the Fshale factor, which is the
3.4.3.2. SHALE RESISTIVITY: SHALE FORMATION FACTOR, FSH
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F shale- Normal-Gradient Formations
y p y
ratio between the shale resistivity and the formation water resistivity, as shown below:
Vertical
depth
m
00.1000.0800.060 0.200
500
1000
1500
2000
2500
wshalew
shaleshale
RCR
RF
1
where:
- Rshale = shale resistivity
- Cshale = shale conductivity
- Rw = formation water resistivity
03. ABNORMAL PRESSURES
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0 1000 0800 060 0 200
METHOD 2Also in this case, if overpressured
formations are encountered, a decrease in
3.4.3.2. SHALE RESISTIVITY: SHALE FORMATION FACTOR, FSH
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,
00.1000.0800.060 0.200
500
1000
1500
2000
2500
F shale-
OverpressuredFormations
Verticaldepth
m
OVERPRESSURE
In presence of undercompacted
shales, hencein OVERPRESSURE,
Fshale values decrease whencompared to the normalcompaction trend.
,
Fsh values can be observed; again, higher
is the departure of the points from thereference trend line, higher will be the pore
pressure gradient values.
OVERPRESSURES
TOP
03. ABNORMAL PRESSURES
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CALCULATION SEQUENCE
The operational sequence required to compute pore pressure gradients by means of the
3.4.3.2. SHALE RESISTIVITY: SHALE FORMATION FACTOR, FSH
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Fshale technique is as follows:
1. Calculate RW, that is the formation water RESISTIVITY along the whole well profile.
2. PlotRWvalues on a semi-logarithmic scale.
3. Read the Condu ct iv i tyvalue in the login correspondence of clean shales, along the whole
well profile.
4. Calculate the F shalevalue.
5. Plot the F shale on semi-logarithmic paper.
6. Draw the F shalenormal compaction trend.
7. Interpret the F shale variation and observe the possible presence of overpressures.
03. ABNORMAL PRESSURES
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1. FORMATION WATER RESISTIVITY (RW) calculation along the entire well profile
CALCULATION SEQUENCE
3.4.3.2. SHALE RESISTIVITY: SHALE FORMATION FACTOR, FSH
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1. Create a chart, copying at different depths the well temperatures recorded on the electric log.
2. Read the Spontaneous Potential (SP) variation value in correspondence of the cleanestsands.
3. Find the Rmf(resistivity of mud filtrate in well) value with the corresponding temperature.on the log heading
4. Enter the mud and temperature Rmfvalues read on the log heading in theSCHLUMBERGER Gen 9 A-6 diagram , and read the Rmfvalues at depth andtemperature of SP values.
5. Calculate the (Rmf)evalue: if Rmf at 75 F is > than 0.1 ohm-m(Rmf)e=Rmfx 0.85. If Rmf at 75 F is < than 0.1 ohm-m, (Rmf)e is to be foundin the Schlumberger SP-2 A12 diagram.
Please note: This is true for water-based muds with the exclusion of lime and
gypsum-based muds.
6. Enter the pair ofSP and temperaturevalues in the SCHLUMBERGER SP-1 A-10diagram, and read the (Rmf)e/ (RW)e ratio values.
03. ABNORMAL PRESSURES
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CALCULATION SEQUENCE (continued)
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8. Obtain RW for the entire well. Entering the (RW)e values and correspondingtemperatures in the Schlumberger SP-2 A12 diagram, obtain RW values along thewhole well profile.
( )RW e
( )Rm f e
( )Rmf e
( )RW e
=
7. Once the (Rmf)e terms and the (Rmf)e/ (RW)e ratio are known,
calculate (RW)e using this equation:
03. ABNORMAL PRESSURES
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010 100 1000
The Rw values, as
obtained with the
d d t il d
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1000
2000
3000
4000
5000
Verticaldepth
(m)
MEAN RW VALUES(RAVENNA SEA ZONE) OHMm
procedure detailed
above, are then plotted
versus depth, obtaining a
curve, similar to that
shown in Figure.
Each well or area is
characterized by a
specific Rw curve.
03. ABNORMAL PRESSURES
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CALCULATION SEQUENCE (continued)
3.4.3.2. SHALE RESISTIVITY: SHALE FORMATION FACTOR, FSH
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9. In the Resistivity/Conductivity log read the Conductivity value in correspondence ofclean shales .
10. Compute the F shale value using the following relation:
11. Plot the F shale values on semi-logarithmic paper.
12. Draw the shale normal compaction trend and interpret F shale variation to detect thepossible presence of overpressures.
F C Rshale shale W
=1
x Cshale= Shale condu ct iv i ty
03. ABNORMAL PRESSURES
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" Fshale
0,01 0,10 1,00
The Fshale values
plotted versus
d th i di t
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Verticaldepth
m
0
1000
2000
3000
4000
5000
depth indicate
the presence of
overpressures
when they
depart
(decreasing)
from the normal
compaction
trend line.
Overpressures Top
Normally Pressured
Zone
Overpressured Zone
03. ABNORMAL PRESSURES
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The main limits of the methods based on Resistivity Measurements
can be summarized as here follows:
3.4.3.2. SHALE RESISTIVITY: SHALE FORMATION FACTOR, FSH
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They can never be applied reliably to CARBONATES.
They can only be applied in presence of frequent interbeddings
of shales and sands.
The SP (Spontaneous Potential) value between shales andsands must be clearly detectable.
Shalesmust be clean.
Fluids contained in shales (gas or oil) modify the Conductivity value.
Borehole must not be caved in (say its geometry must be regular).
03. ABNORMAL PRESSURES
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