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1 PETE 411 Well Drilling Lesson 21 Prediction of Abnormal Pore Pressure Slide 2 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 Slide 3 3 HW #11 Slip Velocity Due 10-28-02 Read: Applied Drilling Engineering, Ch. 6 Slide 4 4 Shale Resistivity vs. Depth 1. Establishtrend line in normally pressured shale 2. Look for deviations from this trend line (semi-log) Slide 5 5 EXAMPLE Shale Resistivity vs. Depth 1. Establish normal trend line 2. Look for deviations (semi-log) Slide 6 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) Slide 7 7 Shale Density, g/cc Depth, ft Slide 8 8 Mud Temperature in flowline, deg F Depth, ft Slide 9 9 Example 8.2 X Why? Slide 10 10 Example 8.8 X Thermal conductivity, heat capacity, pore pressure... Slide 11 11 Drilling Rate, ft/min P HYD - P PORE, psi Slide 12 12 P = (P 2 - P 1 )1,000 Effect of Differential Pressure Slide 13 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 Slide 14 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 Slide 15 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. Slide 16 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 Slide 17 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. Slide 18 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. Slide 19 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. Slide 20 20 Determination of Abnormal Pore Pressure Using the d c - exponent From Ben Eaton: Slide 21 21 Where Slide 22 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 Slide 23 23 Example From Ben Eaton: Slide 24 24 Example Slide 25 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?) Slide 26 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. Slide 27 27 Interval Travel Time, sec per ft Depth, ft Slide 28 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 Slide 29 29 Ben Eaton From Ben Eaton, ( t 1/v ) Slide 30 30 Ben Eaton From Ben Eaton Note: Exponent is 3.0 this time, NOT 1.2! = (0.6904 / 0.052) = 13.28 lb/gal p = 0.6904 * 13,500 = 9,320 psig Slide 31 31 Equations for Pore Pressure Determination Slide 32 32 Pore Pressure Determination Slide 33 33 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. Slide 34 34 Pressure, psi Depth, ft Casing Seat 10,000 Mud Wt. Grad = 0.52 psi/ft FractureGradient = 0.73 psi/ft 0.73 0.52 = 0.21 (psi/ft) 5,200 2,500 Slide 35 35 Example 3 - Solution The danger here is fracturing the formation near the casing seat at 2,500 ft. The fracture gradient at this depth is 0.73 psi/ft, and the mud weight gradient is 0.52 psi/ft. So, the additional permissible pressure gradient is 0.73 0.52 = 0.21 psi/ft, at the casing seat. This corresponds to an additional pressure of P = 0.21 psi/ft * 2,500 ft = 525 psi Slide 36 36 Example 3 Solution contd This additional pressure, at 10,000 ft, is also 525 psi, and would amount to an additional pressure gradient of: 525 psi / 10,000 ft = 0.0525 psi/ft This represents an equivalent mud weight of 0.0525 / 0.052 = 1.01 lb/gal This is the kick tolerance for a small kick! Slide 37 37 Problem #3 - Alternate Solution When a well kicks, the well is shut in and the wellbore pressure increases until the new BHP equals the new formation pressure. At that point influx of formation fluids into the wellbore ceases. Since the mud gradient in the wellbore has not changed, the pressure increases uniformly everywhere. Slide 38 38 Wellbore Pressure, psi Depth, ft PP Casing Seat at 2,500 ft Kick at 10,000 ft Before Kick After Kick and Stabilization 525 Slide 39 39 At 2,500 ft Initial mud pressure = 0.52 psi/ft * 2,500 ft = 1,300 psi Fracture pressure = 0.73 psi/ft * 2,500 ft = 1,825 psi Maximum allowable increase in pressure = 525 psi At 10,000 ft Maximum allowable increase in pressure = 525 psi (since the pressure increases uniformly everywhere). This corresponds to an increase in mud weight of 525 / (0.052 * 10,000) = 1.01 lb/gal = maximum increase in EMW = kick tolarance for a small kick size. Slide 40 40 Wellbore Pressure, psi Depth, ft PP Casing Seat at 2,500 ft Kick at 10,000 ft 1,300 psi 1,825 psi 5,725 psi 5,200 psi Slide 41 41 Wellbore Pressure, psi Depth, ft PP Casing Seat at 2,500 ft Kick at 10,000 ft Before Kick After Small Kick and Stabilization After Large Kick and Stabilization