Well ControlLesson 10 Logging While Drilling (LWD)

TAMU - Pemex

Logging While Drilling Sonic Travel Time Resistivity and Conductivity Eatons Equations (R, C, t, dc) Natural Gamma Ray Other2

Logging While Drilling (LWD)The parameters obtained with LWD lag penetration by 3 to 60, depending on the location of the tool. Some tools have the ability to see ahead of the bit. These are most commonly used for Geo-steering, but can be used in detection of abnormal pressure.3

Logging While Drilling Any log that infers shale porosity can indicate the compaction state of the rock, and hence any abnormal pressure associated with undercompaction.

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Logging While Drilling Most of the published correlations are based on sonic and electric log data. Density logs can also be used if sufficient data are available.

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Pore Pressure Gradient vs. difference between actual and normal sonic travel time From Hottman and Johnson

gp, psi/ft

LA Upper TX Gulf Coast

to tn, sec/ft

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Normal

Matthews and Kelly

gp, psi/ft

to tn, sec/ft

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Relationships vary from area to area and from age to age But, the trends are the same.

gp, psi/ft

to tn, sec/ft

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Resistivity and ConductivityThe ability of rock to conduct electric current can be used to infer porosity. Resistivity -- ohm-m2/m or ohm-m Conductivity -- 10-3m/ohm-m2 or millimhos/m9

Resistivity and ConductivityRock grains, in general, are very poor conductors. Saline water in the pores conducts electricity and this fact forms the basis for inferring porosity from bulk R or C measurements.

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Resistivity and ConductivityUnder normal compaction, R increases with depth. Deviation from the normal trend suggests abnormal pressure

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Resistivity and Conductivity FR = Ro/Rw FR = formation resistivity factor Ro = resistivity of watersaturated formation Rw = resistivity of pore water12

Resistivity of formation waterRw reflects the dissolved salt content of the water, and is dependant upon temperature.Rw2 T1 + 6.77 = R w1 T + 6.77 2

where T1 and T2 are in oF

Equation shows that Rw decreases with increasing temperature, and consequently, decreases with depth.

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Porosity, Porosity of water-saturated rock, = aF If a = 1, and m = 2, then = F So, = (Ro/Rw)-0.5 Rw in shales cannot be measured directly so Rw in a nearby sand is used instead. Ro would tend to increase with increasing depth under normally pressured conditions. See Fig. 2.63. 14-0 .5 R

1 / m R

Fig. 2.63 Normal Compaction

Depth, ft

Ro , . m

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Example 2.20Rw estimated from nearby well. Estimate the pore pressure at 14,188 ft using Foster and Whalens techinque. So, at 14,188 ft,Ro 0.96 FR = = R w 0.034

FR = 28.24

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Using Eatons Gulf Coast correlations, ob = 0.974 psi/ft or 13,819 psig at 14,188 Eq. Depth = 8,720 obe = 0.937 psi/ft or 8,170 psig at 8,720 pne = 0.465*8,720 Transition at ~11,800 = 4,055 pp = ppe + (o b

-

oe b

)

= 4,055+(13,816-8,171) = 9,703 psig17

= 13.16 ppg

Fig. 2.65 -Hottman & Johnsons upper Gulf Coast Relationship between shale resistivity and pore pressure

Gp, psi/ft

Rn/Ro

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Example 2.21 Matthews and KellyDetermine the transition depth and estimate the pore pressure at 11,500

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Example 2.21 Fig. 2.67Transition is at ~9,600 ft. At 11,500 ft: Co = 1,920, and Cn = 440 Co/Cn = 1,920 / 440 = 4.36 gp = 0.81 psi/ft (Fig 2.66)20

Fig. 2.66 gp = 0.81 psi/ft p = 15.6 ppg pp = 9,315 psig

4.3621

Eatons Equations t n g p = g ob ( g ob g n ) t o Ro g p = g ob ( g ob g n ) R n Cn g p = g ob ( g ob g n ) C o3

Eq. 2.34

1 .2

Eq. 2.35

1 .2

Eq. 2.361 .2

d co g p = g ob ( g ob g n ) d cn

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Eatons EquationsThese equations differ from the earlier correlations in that they take into consideration the effect a variable overburden stress may have on the effective stress and the pore pressure. Probably the most widely used of the log-derived methods Have been used over 20 years23

Example 2.22In an offshore Louisiana well, (Ro/Rn) = 0.264 in a Miocene shale at 11,494. An integrated density log indicates an overburden stress gradient of 0.920 psi/ft. Estimate the pore pressure. Using Eatons technique Using Hottman and Johnsons24

SolutionEatonFrom Eq. 2.35, gp = gob - (gob - gn)(Ro/Rn)1.2 gp = 0.920 - (0.920 - 0.465)(0.264) 1.2 gp = 0.827 psi/ft25

Solution Hottman & Johnson Rn/Ro = 1/(0.264) = 3.79 From Fig 2.65, we then get gp = 0.894 psi/ft Difference = 0.894 0.827 = 0.067 psi/ft Answers differ by 770 psi or 1.3 ppg26

DiscussionActual pressure gradient was determined to be 0.818 psi/ft! In this example the Eaton method came within 104 psi or 0.17 ppg equivalent mud density of measured values This lends some credibility to the Eaton method.27

DiscussionIn older sediments, exponent may be lowered to 1.0 for resistivities. Service companies may have more accurate numbers for exponents.

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Natural Gamma RayTools measure the natural radioactive emissions of rock, especially from: Potassium

Uranium Thorium

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Natural Gamma RayThe K40 isotope tends to concentrate in shale minerals thereby leading to the traditional use of GR to determine the shaliness of a rock stratum. It follows that GR intensity may be used to infer the porosity in shales of consistent minerology30

Natural Gamma RayPore pressure prediction using MWD is now possible (Fig. 2.68). Lower cps (counts per second) may indicate higher porosity and perhaps abnormal pressure.

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Fig. 2.68

Natural Gamma RayIn normally pressured shales the cps increases with depth Any departure from this trend may signal a transition into abnormal pressure32

Pore pressure gradient prediction from observed and normal Gamma Ray counts

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Example 2.23From table 2.17, determine the pore pressure gradient at 11,100 ft using Zoellers correlation. Use the first three data points to establish the normal trend line.

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At 11,100 NGRn / NGRo 57/42

= 1.36

From below, gp = 0.61 psi/ft or 11.7 ppg

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Effective Stress ModelsUse data from MWD/LWD Rely on the effective-stress principle as the basis for empirical or analytical prediction Apply log-derived petrophysical parameters of the rock to a compaction model to quantify effective stress Knowing the overburden pressure, the pore pressure can then be determined36

Dr. Choes Kick Simulator Take a kick Circulate the kick out of the hole Plot casing seat pressure vs. time Plot surface pressure vs. time Plot kick size vs. time etc.37