Life of a well
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1 1 Life of a well Life of a well Schlumberger 1999 A
Transcript of Life of a well
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Life of a well
Life of a well
© Schlumberger 1999
A
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Life of a well
Life of a well-1
Drilled well Cased Well Perforated Well
Need to find:
Saturation casing integrity
Porosity cement quality
Zones
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Objective of a wellThe Objective of most wells is to find hydrocarbons. The volume of hydrocarbons in place is given by:
H=Constant x Swh
whereH = initial oil in place = effective porositySw= initial water saturationh = productive interval A = drainage area
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Hydrocarbon in PlaceThis is simple to visualise
A - area of the reservoir h - the thickness of the reservoirtogether the product gives the total volume of rock
- percentage of pore space in that volume of rock. i.e. the volume that contains fluids
Sw = percentage of the pore space containing water so(1-Sw) = percentage of pore space containing hydrocarbons
Hence the equations for the hydrocarbons in place
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Open Hole Measurements
Open Hole Measurements are made by two methods;
1) The traditional wireline logginghere the tools are lowered into the well on the end of an electrical line. Measurement is usually made pulling out of the hole.
2) Logging While Drillingthe tools are built into drill pipe. Logging is made while drilling the hole and data is stored in downhole memory as well as being transmitted up hole.
3) Logging on drill pipehere the standard wireline tools are attached to drill pipe. A cable is still used for data transmission. Logging is made both down and up.
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Use of Open Hole logs
The basic parameters needed are:
Thickness - measured by the tool depths
Porosity - measured by porosity tools
Saturation - computed from a combination of porosity and
lithology
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Life of a well-2
Well Produced Workover activity Recompleted
Need to know:
Production Perforation efficiency Flow rates
fluid mix new zones Zone Production
Pressures Flow rates Pressures
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Perforation
Gun systems use three components:
- detonator - primary high explosive ignited by heat or shock
- primacord - secondary high explosive ignited by the detonator, burns at 8400 m/sec
- shaped charges - create the perforations, detonated by the primacord.
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Shaped charge
The explosion forces the liner to flow inwards and out.It forms into a characteristic shape, the jet.This is moving rapidly and has extremely high pressures at the tip.The dimensions of the perforation, length of the tunnel, and diameter of the entrance hole are linked and depend on the geometry of the shaped charge.
Case
ExplosiveCharge
Liner
PrimacordPrimerCharge
Jet
Tip
Slug
7000 m/s
500 m/sp=100GPa
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Types of Perforation
Three Types of perforated completion
a) Wireline - Carried on an electric line
1) Casing Gun Completion
Well Pressure > Formation Pressure
Overbalanced perforating
Large diameter carrier gun
2) Through Tubing Perforation
Well Pressure < Formation Pressure.
Completion and final surface production equipment, or a temporary completion and testing facilities are in place
Underbalanced perforating, with pressure control equipment
Through tubing gun (small guns)
Gauges can be run with the string
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Tubing Conveyed Perforating
b)Carried on Drill Pipe or Tubing
3) Tubing Conveyed Perforating
Perforation gun is carried on either the drill pipe or on tubing.
Well Pressure < or > Formation Pressure
Large interval of perforation in one run - in - hole
High explosive content, perforation spacing
Gauges can be run at the same time
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Perforation Characteristics
Guns are classified by the number of shots per foot, spf.
The current maximum is 21 spf.
Perforat ionDi rections
90? pha s i ngGuns are also described by their Phasing- the directions of the perforations. This ranges from 0Þ to 30Þ/60Þ
The example shows 90Þ.
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Cement EvaluationCementation of wells is of vital importance for the wells performance as it seals one zone from another,
To properly evaluate any measurement in cased hole the quality of the cement has to be evaluated
Unwanted fluidflow
Bad Cement
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Cement Bond Logging ToolsThere are three types of tool in current use
1) Cement Bond Log (CBL)- Variable Density Log (VDL)
a) CBL measures the amplitude of signal reflected from the casing wall. The higher the amplitude the lower the amount of cement.
b)VDL image of the recorded wavetrain. The only log to see beyond the first casing into the formation.
2) Pulse Echo type tool
measures the acoustic impedance of the casing-cement interface using ultrasonics.
The latter tool is either segmented using individual transducers or rotating covering the entire casing
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Fluid Flow
Fluid flow in the borehole is complex. The fluid may start as oil but then bubbles of gas come out of solution.
There may also be mixed flow of water and oil.
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Flow Regimes
.
1 10 102 103
1
10
102
10-1
BUBBLE FLOW
PLUG FLOWS LUG FLOW
MIS T FLOW
REGION IIIREGION II
REGION I
GAS VELOCITY
TRANSITION
LIQUID VELOCITY
FLOW REGIMES
The actual flow regime depends on a number of factors, such as gas-oil-ratio.
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Production Tools
Current tools consist of a number of sensors. The major ones are
Flowmeter flow measurement
Gradiomanometer fluid density/ hold up, fluid mix
Pressure reservoir, borehole pressures, fluid density
Temperature production temperature, flow measurement, cement evaluation, channel identification
Caliper borehole size, Flowmeter correction
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Production Log InterpretationStep 1
The first thing that is done is to look at the logs.
Questions asked:
• do the spinners change at the perforation depths
• is the fluid density "sensible"
• does the shut in pass show anything e.g. crossflow
• does the temperature have any anomalies
• are there any stable zones for calibration
• what's the problem
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Example 1
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example-1 solution
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Flowmeter Interpretationmonophasic flow
0 10 20
2.5 rps
3 .0 rp s
1.5 rps
7.0 rps
cps
35.7% oftotalf low
42.9% oftotalf low
21.4% oftotalf low
A
B
C
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Multiphase Flow
Monophasic flow
simple to analyse
The flowmeter gives the flowrate , rest is known.
Diphasic flow ( the well is producing two fluids)
also relatively simple to analyse. the flowmeter gives the total flowrate
the gradio gives the fluid mix theoretical correlations are used to compute the relative flowrates of each phase
assumption is that the flow regime is simple
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Triphasic flow
Triphasic flow ( the well is producing water, oil and gas)
analysis very difficult
the flowmeter still gives the total flow rate
the gradio gives the fluid mix
no correlations to separate the fluids, the flow regimes are too complex and variable
Only method is to assume one phase constant and solve for the other
two
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Flowmeter response
The spinner is reacting to the flow of the general fluid plus the slugs passing by.
The gradio response will be similar as the different fluids go past.
The result is a log impossible to interpret.
spinner RPS
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Other Measurement
Hydrocarbon
Water entry
1st Oil entry
2nd Oil entry
3rd Oil entry
Stagnant water
Friction effects
Jetting, Venturieffects
Mud
Gradiomanometer
water hold up
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Diphasic flow
0 10 20
2.5 rps
3.0 r ps
1.5 rps
7.0 rps
cps
35.7% oftotalflow
42.9% oftotalflow
21.4% oftotalflow
A
B
C
sludge at bottomof the well
producing onlyoil
producing waterwith some oil
producing mostly oil
percentage oilproduction
100% 0%
WATER
OIL
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Saturation and Time
Matrix
oi l
Sw- original
water
water
ΔSw
Over time the saturation undergoes another change with the oil being displaced by the invading water
The water could be of a different salinity to that originally in place
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Saturation Monitoring
The objective is to monitor the depletion of the producing zone, i.e. the difference between the original and the current oil saturation.
It is also to detect problem zones such as water fingering or coning.
Remaininghydrocarbon
displaced hydrocarbon
original waterin placeP
erforations
Porosi ty %50 0
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Monitoring Tools
Two measurement methods exist:
Pulsed neutron capture logging
Carbon Oxygen logging
Both use an electronic source and pairs of detectors measuring gamma rays
PNC measures the capture cross section of the formation. This is related to the amount of chlorine and hence the water
C/O measures the relative amounts of carbon and oxygen. This is related to the amounts of hydrocarbon and water.
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PNC Method and Analysis
The log reading is a linear mixture of the matrix and the fluid:
The fluid term can be expanded to:
Hence if Sw, Sma, Sh and the porosity, are known the saturation Sw can be obtained.
S log = Σf φ+ 1 − φ( )Σma
S f = ΣwSw + 1 − Sw( )Σh
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Pulsed Neutron Capture
Interpretation
LS = maS −φ + S φ
S φ = hS φ − WS + WS φ WS
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Time Lapse Example
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Carbon Oxygen Analysis
The tool measures carbon (in the hydrocarbons) and oxygen (in the water)
Combining these gives the saturation
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C/O Saturation
The plot is of the Far C/O ratio against the Near
The combination gives both the formation water percentage Sw and the borehole percentage Yo
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Far C/ O ratio
Near C/ O Ratio
Sw=0, Yo=100
Sw=0, Yo=0
Sw=100, Yo=0
Sw=100, Yo=100
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Vertical Wells
Wells can be split into three categories1) Vertical
• drilled to a specific target• measured depth = true depth
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Deviated well
2) Deviated• usually from a platform or• from land to near offshore• measured depth has to be converted to true vertical depth
possible well tracks
Target formation
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Horizontal well
3) Horizontal• drilled to maximise production or minimise problems such as coning• well is precisely guided along a predetermined track
Vertical section
Curvature
Ramp
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Measurements in Open Hole
The measurements made in the borehole are affected by the environment.
The major effects are:
Borehole size and shape
Borehole Fluids
Borehole Temperature
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Measurements in Cased Hole
The cased hole environment is more complex than open hole because of
casing and tubing
• steel affects measurements
cement
• to be measured but unknown in most cases
unknown fluids
• to be measured, affects tools
limited tools
• slim hole restrictions plus casing etc limits tools which can be run
lot of unknowns
• have to take into account reservoir behaviour, fluid dynamics as well as rock mechanics and formation properties
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Measuring in the borehole
The formation to be measured is masked by the borehole.
The borehole contains fluids and is of an irregular shape.
The sensor has to be able to measure the formation property accurately and send the information to surface.
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Borehole -Size and Shape
Perfect shape no problems except if very large.
Ovalised hole; will give problems for some tools. Best to run two calipers.
Irregular borehole, gives problems for most tools.
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Tool Positioning - 1
Some tools are run centralised in the borehole in order to measure properly.
These include laterolog and sonic devices.
Special centralisers are put on the tool.
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Tool Positioning - 2
Some tools are run eccentred, pushed, against the borehole wall.
In some cases this is done with an eccentraliser.
In other cases a caliper arm does this job.
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Tool Positioning - 3
Some tools are run with “stand-offs” to position them at a fixed distance from the wall.
The induction family are usually run in this manner.
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Borehole - Fluids
The borehole fluid can be
- water based mud
fresh or
salt saturated
- oil based mud
varying quantities of water
- air
- foam
In addition there are a number of additives to increase weight, viscosity and so on.
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Borehole fluids 2
Oil based mud will not allow current to pass so electrical logs will not work.
Foam and air muds will not transmit sonics signals. Neutron tools are also affected.
Mud salinity affects electrical and induction tools in different manners.
Additives such as barite affect density, gamma ray and photoelectric effect measurements.
The mud type, salinity and additives must be known so that the appropriate corrections can be made.
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Borehole - Temperature
Increasing temperature affects the measurements in some tools. The most affected is the thermal neutron devices.
High temperature also affect the performance of the electronics in the tools.
Temperature affects the mud resistivity (it decreases with increasing temperature).
Temperature is measured during each logging run.
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Volume of Investigation
The tool shown here measures all around the borehole. It is omni-directional.
An example of this type of tool is the Gamma Ray.
Some of the “signal” is in the borehole. Most comes from the invaded zone.
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Volume of Investigation 2
This type of investigation is also omni-directional but it reads mainly in the virgin zone.
This pattern is that of the deep resistivity tools.
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Volume of Investigation 3
This type of measurement has the sensor facing in one direction only.
Examples of this are the neutron porosity and bulk density measurements.
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Vertical Wells
In vertical wells, with homogeneous layers all types of tool are reading in the same formation.
In horizontal (or highly deviated) wells the deep reading resistivity tools may read a different layer to the shallow reading tools.
In addition the omni-directional tools (e.g. GR) may read different layers from the single direction devices.
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Drilling Objective
A well is drilled to a pre-determined objective:
An exploration well targets a suspected reservoir.
An appraisal well evaluates a discovery.
A development well is used for production.
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Pre-Drilling Knowledge Exploration
Structural information obtained from surface seismic data.Rough geological information can be provided by nearby wells or outcrops.Approximate depths estimated from surface seismic data.
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Pre-Drilling Knowledge Appraisal
Detailed structure from logs taken in previous wells.
Time-to-depth conversion for surface seismic from logs taken in previous wells.
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Appraisal Wells - Continued
Geological data from cuttings, cores and logs from previous wells.
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Pre-Drilling Knowledge Development
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Tool History1927 - First electrical log recorded.
1930s - SP, Short Normal, Long Normal and Long Lateral combined, Core Sample Taker.
1940s - Gamma Ray and Neutron, 3-arm Dipmeter using SP, then electrical measurements, Induction tool.
1950s - Microlog tool, Laterolog tool, Sonic tool, Formation Tester.
1960s - Formation Density tool.
1970s - Dual Spacing Neutrons, Advanced Dipmeters, Computerised Surface Systems, Repeat Tester tools, Electromagnetic Propagation tool.
1980s - Resistivity Imaging tool, Advanced Sonic tools
1990s - Advanced testing tools, Induction imaging tools, Azimuthal Laterolog tools, Ultrasonic imaging tools, Epithermal porosity tools, Magnetic resonance tools
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Early Interpretation
Early resistivity logs were used to find possible producing zones.
high resistivity = hydrocarbon
SP was used to define permeable beds, compute Rw and determine shaliness.
Resistivity was also used to determine "porosity".
Archie developed the relationship between resistivity, porosity and saturation.
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Interpretation Procedure 2
Resistivi ty Porosi tyGamma Ray
Water
Water
Shale
Hydrocarbon
The simplest evaluation technique consists of recognising the hydrocarbon zone using the porosity and resistivity curves
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Interpretation Procedure
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Zoning
Zoning is the first step in any interpretation procedure. During zoning, the logs are split into intervals of:
1) Porous and non-porous rock.2) Permeable and non-permeable rock.3) Shaly and clean rock.4) Good hole conditions and bad hole
conditions.5) Good logs and bad logs.
Zoning Tools:SP.GR.Caliper.Neutron Density-Pef.Resistivity.
