Bài giảng Well drilling engineering - Chapter 6: Prediction of Abnormal Pore Pressure (Part 2) - Đỗ Quang Khánh

1 Well Drilling Engineering  Prediction of Abnormal Pore Pressure Dr. DO QUANG KHANH 2 Prediction of Abnormal Pore Pressure Resistivity of Shale Temperature in the Return Mud Drilling Rate Increase d c - Exponent Sonic Travel Time Conductivity of Shale 3 HW #  Read:  Applied Drilling Engineering, Ch. 6 4 Shale Resistivity vs. Depth 1. Establish trend line in normally pressured shale 2. Look for deviations from this trend line (semi-log) 5 EXAMPLE Shale Resistivity vs. Depth 1. Establish normal trend line 2. Look for deviations (semi-log) 6 Shale Resistivity vs. Depth 1. Establish normal trend line 2. Look for deviations 3. Use OVERLAY to quantify pore pressure (use with caution) Pore Pressure (lb/gal equivalent) 16 14 12 10 9 ppg (normal) 7 Shale Density , g/cc Depth, ft 8 Mud Temperature in flowline, deg F Depth, ft 9 Example 8.2 X Why? 10 Example 8.8 X Thermal conductivity, heat capacity, pore pressure... 11 Drilling Rate, ft/min P HYD - P PORE , psi 12 D P = (P 2 - P 1 )1,000 Effect of Differential Pressure 13 Typical Drilling Rate Profiles - Shale The drilling rate in a normally pressured, solid shale section will generally generate a very steady and smooth drilling rate curve. The penetration rate will be steady and not erratic (normally pressured, clean shale). Shale 14 Typical Drilling Rate Profiles - Sand The drilling rate in a sand will probably generate an erratic drilling rate curve. Sands in the Gulf Coast area are generally very unconsolidated. This may cause sloughing, accompanied by erratic torque , and temporarily, erratic drilling rates . Sand 15 Typical Drilling Rate Profiles - Shaley Sands This is generally the most troublesome type drilling rate curve to interpret. Many times this curve will look similar to a solid shale curve that is moving into a transition zone. Shaley Sands Note: This is a prime example why you should not base your decision on only one drilling parameter , even though the drilling rate parameter is one of the better parameters. 16 Typical Drilling Rate Profiles If you are drilling close to balanced, there will probably be a very smooth , (gradual) increase in the drilling rate. This is due to the difference between the hydrostatic head and the pore pressure becoming smaller. Transition Zone Shale 17 Typical Drilling Rate Profiles Transition Zone Shale As the pressure becomes very small, the gas in the pores has a tendency to expand which causes the shale particles to pop from the wall. This is called sloughing shale. The transition zone generally has a higher porosity, making drilling rates higher. In a clean shale the ROP will increase in a smooth manner. 18 Typical Drilling Rate Profiles Note: If you are drilling overbalanced in a transition it will be very difficult to pick up the transition zone initially. This will allow you to move well into the transition zone before detecting the problem. 19 Typical Drilling Rate Profiles This could cause you to move into a permeable zone which would probably result in a kick . The conditions you create with overbalanced hydrostatic head will so disguise the pending danger that you may not notice the small effect of the drilling rate curve change. This will allow you to move well into that transition zone without realizing it. 20 Determination of Abnormal Pore Pressure Using the d c - exponent From Ben Eaton: 21 Where 22 Example Calculate the pore pressure at depth X using the data in this graph. Assume: West Texas location with normal overburden of 1.0 psi/ft. X = 12,000 ft . X 1.2 1.5 d c 23 Example From Ben Eaton: 24 Example 25 E.S. Pennebaker Used seismic field data for the detection of abnormal pressures . Under normally pressured conditions the sonic velocity increases with depth. (i.e. Travel time decreases with depth) (why?) 26 E.S. Pennebaker Any departure from this trend is an indication of possible abnormal pressures. Pennebaker used overlays to estimate abnormal pore pressures from the difference between normal and actual travel times . 27 Interval Travel Time, m sec per ft Depth, ft 28 Ben Eaton also found a way to determine pore pressure from interval travel times. Example: In a Gulf Coast well, the speed of sound is 10,000 ft/sec at a depth of 13,500 ft . The normal speed of sound at this depth, based on extrapolated trends, would be 12,000 ft/sec . What is the pore pressure at this depth? Assume: S/D = 1.0 psi/ft 29 Equations for Pore Pressure Determination 30 Pore Pressure Determination 31 EXAMPLE 3 - An Application... Mud Weight = 10 lb/gal. (0.52 psi/ft) Surface csg. Set at 2,500 ft. Fracture gradient below surf. Csg = 0.73 psi/ft Drilling at 10,000 ft in pressure transition zone * Mud weight may be less than pore pressure! DETERMINE Maximum safe underbalance between mud weight and pore pressure if well kicks from formation at 10,000 ft.