Pore Pressure Prediction using Seismic and Well Log Data by Employing Eaton’s and Bowers’ Methods

Summary: Pore pressure is defined as the pressure of the fluid inside the pore space of the formation that is also known as the formation pressure. When the pore pressure is higher than hydrostatic pressure, it is referred to as overpressure. Knowledge of this pressure is essential for cost-effective drilling, safe well planning, and efficient reservoir modeling. To optimize drilling decisions and well planning in overpressure areas, it is essential to predict pore pressure in the first step. Before drilling, reliable prediction of pore pressure is critically important at different stages of petroleum engineering investigations. Pore pressure prediction has an important application in proper selection of casing and reliable mud weight. Currently pore pressure prediction in carbonate reservoir is still far from satisfaction, and there is no specific method widely accepted. The existing methods and theories in the pore pressure prediction community are almost all based on the shale properties. Although these methods are not the proper way to predict pore pressure in carbonates and may probably lead to dangerous errors, they are still used in the field practice of carbonate reservoirs. Introduction: The generation of abnormal pore pressure in carbonate formation is different from a reservoir to another reservoir because of different depositional and geological conditions. Although most studies currently point out that under-compaction mechanism is the dominant mechanism of abnormal pore pressure generation, the role of this mechanism in carbonate formations needs further study and discussion. Over-pressure can be identified by direct and indirect methods. Direct methods such as measured pressure data obtained from drill stem test (DST), repeat formation tester (RFT), modular dynamic test (MDT) and mud weight, and they are reliable evidences that reflect the over-pressure phenomenon in permeable reservoirs. Methodology and Approaches: Pressure coefficient, which is the ratio of the actual fluid pressure versus corresponding normal hydrostatic pressure at the same depth, has also been proposed and applied to investigate over-pressure. On the contrary, indirect methods, such as well logging, seismic and rock mechanic data, can identify abnormal pressure zones based on various response anomalies compared with the normal pressure system. The main objective of this study is to determine pore pressure using Eaton’s and Bowers’ methods and to compare their accuracy and usage. To obtain this goal in Bowers’ method, researchers try to directly link the formation velocity to the effective stress. One of these models was developed by Bowers (1995). Results and Conclusions: Sonic and density data in the distance between wells have been predicted using a combination of sequential Gaussian simulation and collocated cokriging techniques. Then, effective stress cube and overburden pressure cube have been predicted using velocity to effective stress transform and density to overburden pressure relation. Eventually, formation pore pressure has been predicted using Terzaghi’s method. Another way to estimate pore pressure is used from well logging data by applying Eaton's method (1975) with some modifications. In this way, sonic transient time trend line is separated by lithology changes determined from Zhang’s method. Our results show that the best correlation with the measured pressure data is obtained by the modified Eaton’s method with the Eaton’s exponent of about 0.4. Pore