An Innovative Approach to Predict Viable Pore Pressure in Carbonate Rocks in the Middle East

Abstract Abnormal pore pressures have been frequently observed in various regions globally, particularly in exploration areas. Accurate Pore Pressure Prediction and designing safe mud weights during drilling execution are crucial for ensuring safety and efficient operations. This paper presents an innovative approach to predict pore pressure prediction in carbonate reservoir, analyzing the elastic properties changes after prolong production/injection and incorporate an adjustment factor to predict the pore pressure that matches with actual measurement The primary goal of pore pressure prediction is to provide pressure data before drilling operations. Historical studies indicate that pore pressure prediction equations based on sonic, resistivity, and porosity are not effective in carbonates (O’ Connor et al, 2010). However, the current study confirms that the pore pressure correlates greatly with interval velocities, therefore pore pressure prediction in carbonate rocks can be quite accurate if the parameters are optimized and adjustment factor is applied. This paper presents a workflow that discusses data gathering & quality control for fifty wells, logs correction, sensitivity scenarios, overburden and hydrostatic gradient estimation to predict pore pressure within carbonate reservoirs. The paper also covers modelling part to predict the expected change in elastic properties after prolong production/injection to calculate the adjustment factor for accurately estimating pore pressure that matches with actual measurements. Choosing the appropriate parameters in Eaton’s equation resulted in a fairly accurate transformation from velocity to pore pressure. The sonic logs for fifty wells have been corrected for drift and invasion. Based on the actual samples taken from three formations, salinity appears to change drastically from shallow to deeper formations and therefore the hydrostatic pressure gradients. For pore pressure prediction, an average value of 0.485psi/ft has been used. The predicted pore pressure in the overburden shows 98% match with measured pressures without incorporating adjustment factor. This is because the pore pressure and the logs were acquired on the same date. In the reservoir section, the predicted pore pressure shows 150psi to 200psi difference with initial and last measured pressures respectively. Therefore, an adjustment factor is needed which depends on the cumulative injection/production in vicinity and it is estimated through fluid substitution modelling. The modelling results shows that Vp (Primary velocity) increases by 6.5% when seawater displaces 90% oil in the reservoir section. Based on this, an adjustment factor has been designed and incorporated in pore pressure prediction equation to estimate the current reservoir pressure that can be used for Mud Weight (MW) designing. The resultant pore pressure values show fair to good correlation with the history matched model and actual measurements from the well. To address pore pressure challenges in the Middle East carbonate reservoirs, the optimized parameters have been chosen along with an adjustment factor that work effectively, thereby minimizing drilling risks in overburden and aiding in drilling long lateral horizontal wells within the reservoir section.