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TAMU - Pemex Well Control

2. Logging While Drilling . Sonic Travel Time Resistivity and Conductivity Eaton's Equations (R, C, Dt, dc) Natural Gamma Ray Other

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TAMU - Pemex Well Control

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    1. TAMU - Pemex Well Control Lesson 10 Logging While Drilling (LWD)

    2. 2 Logging While Drilling Sonic Travel Time Resistivity and Conductivity Eaton’s Equations (R, C, Dt, dc) Natural Gamma Ray Other…

    3. 3 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.

    4. 4 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.

    5. 5 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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    9. 9 Resistivity and Conductivity The 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/m

    10. 10 Resistivity and Conductivity Rock 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.

    11. 11 Resistivity and Conductivity Under normal compaction, R increases with depth. Deviation from the normal trend suggests abnormal pressure

    12. 12 Resistivity and Conductivity FR = Ro/Rw FR = formation resistivity factor Ro = resistivity of water- saturated formation Rw = resistivity of pore water

    13. 13 Resistivity of formation water Rw reflects the dissolved salt content of the water, and is dependant upon temperature. Equation shows that Rw decreases with increasing temperature, and consequently, decreases with depth.

    14. 14 Porosity, f

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    19. 19 Example 2.21 Matthews and Kelly

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    22. 22 Eaton’s Equations

    23. 23 Eaton’s Equations These 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 years

    24. 24 Example 2.22 In 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 Eaton’s technique Using Hottman and Johnson’s

    25. 25 Solution Eaton From 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/ft

    26. 26 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 ppg

    27. 27 Discussion Actual 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.

    28. 28 Discussion In older sediments, exponent may be lowered to 1.0 for resistivities. Service companies may have more accurate numbers for exponents.

    29. 29 Natural Gamma Ray Tools measure the natural radioactive emissions of rock, especially from: Potassium Uranium Thorium

    30. 30 Natural Gamma Ray The 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 minerology

    31. 31 Natural Gamma Ray Pore pressure prediction using MWD is now possible (Fig. 2.68). Lower cps (counts per second) may indicate higher porosity and perhaps abnormal pressure.

    32. 32 Natural Gamma Ray

    33. 33 Pore pressure gradient prediction from observed and normal Gamma Ray counts

    34. 34 Example 2.23

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    36. 36 Effective Stress Models Use 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 determined

    37. 37 Dr. Choe’s 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.

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