Detection of Over Normal Pore Pressure Intervals by Using Well Logs

Abstract

Pore pressure is very important parameter that impacting on drilling, production planning and operations. Drilling and production processes cannot be beginning if pore pressure is not estimated. There is a limit of difference between hydrostatic pressure of mud column and pore pressure during drilling to ensure that the layers are preserved from fracturing, as well as that kicking does not occur inside the well. Difference between pore pressure and bottom hole flowing pressure is a key for production process. Over (Abnormal) pressure intervals are causing many problems during drilling. In present study, pore pressure is estimated firstly as a hydrostatic pressure, and secondly, after determination of shale flag, two methods of Eaton slowness and Bowers original are used for detecting of over (Abnormal) pressure shale intervals. Compressional and density logs of three wells (X3, X4, and XD) located at Y oil field and producing from Asmari formation are used to perform the present study. Density log is linearly extrapolated to estimate bulk density from zero depth to last depth point at reservoir. Vertical stress is predicted for these three wells. The vertical stress gradients were 1.03, 0.99, and 0.93 psi/ft for XD, X3, and X4 wells respectively. Results are reveals that Bowers original better than Eaton slowness in detection of over (abnormal) pressure intervals where the last did not cut all shale intervals by compressional slowness shale base line that equal to 80 us/ft so, Eaton slowness method provided either very high over pressure in some shale intervals or subnormal pressure to other shale intervals and that inaccurate while Bowers original method approximately provided all shale intervals as over pressure in reasonable values. Modular dynamic tester measurements for pore pressure of these three wells are used for calibration. Maximum percent error between predicted and measured pore pressure of wells (X3, X4, and XD) are 1.2%, 0.89% and 3% respectively where these percent are very acceptable. Maximum over pressure values in Asmari formation zones at wells are as follows: 6328 psi at depth 3225 m in well X3, 7538 psi at depth 3080 m in well X4, and 6731 psi at true vertical depth 3067 m in well XD.