8Some Causes of Abnormal Pressure 1. Incomplete compaction of sedimentsFluids in sediments have not escaped and help support the overburden.2. Tectonic movementsUpliftFaulting
9Some Causes of Abnormal Pressure 3. Aquifers in Mountainous RegionsAquifer recharge is at higher elevation than drilling rig location.4. Charged shallow reservoirs due to nearby underground blowout.5. Large structures...
10HIGH PRESSURE NORMAL PRESSURE Thick, impermeable layers of shale (or salt) restrict the movement of water. Below such layers abnormal pressure may be found.
11HIGH PRESSURENORMAL PRESSUREHydrostatic pressure gradient is lower in gas or oil than in water.
12When crossing faults it is possible to go from normal pressure to abnormally high pressure in a short interval.
34Procedure for Determining Pore Pressure From dc - Exponent Calculate dc over ft intervalsPlot dc vs depth (use only date from Clean shale sections)Determine the normal line for the dc vs. depth plot.Establish where dc deviates from the normal line to determine abnormal pressure zone
35Procedure for Determining Pore Pressure From dc - Exponent Normal TrendNormalDepthAbnormaldc - Exponent
36Procedure for Determining Pore Pressure From dc - Exponent If possible, quantify the magnitude of the abnormal pore pressure using overlays, or Ben Eaton’s MethodPore Pressure Grad.Normal Pore Pressure Grad.Overburden Stress Grad.
37In normally pressured shales, shale compaction increases with depth
38Shale Density - Mud Cup Method 1. Fill mud cup with shale until the weight is 8.33.2. Fill to top with water, and record the reading Wtot.Note: Dry sample carefully with towel. Do not apply heat.
39Alternate Method: Use variable density column. See p. 270 in text
40Pore Pressure from Resistivity DepthShale resistivity plots may be developed from (i) logs or (ii) cuttings10,000’What is the pore pressure at the point indicated on the plot?[Assume Gulf Coast]. Depth=10,000 ft