Rapid sedimentation in the deep water of the young Tertiary basin in southeast (SE) Asia is one of the primary causes for the development of overpressured systems. In this setting, pore water is unable to escape naturally during deposition. This causes increased pore pressure above hydrostatic pressure. Shale porosity has often been used to estimate in situ fluid pressure. A typical approach is to examine the porosity profile in a zone of known fluid pressure (i.e., the hydrostatic zone) and then use this empirical relationship to predict pressure where it is unknown (i.e., the overpressured zone). The resulting shale based predictions are then compared to in situ fluid pressure measured in reservoir rock (e.g., sand). A difference between predicted and measured pressure is then interpreted simply as a deficiency of the model. However, a more recent belief is that, in overpressure zones, a pressure difference in sand and shale is not uncommon in the presence of structural inclination.
This study continues to explore differences between shale predicted pore pressure and in situ pressure in sand and how it affects well design. The authors interpret observed deviations between measurements and predictions as actual differences between sand and shale pressure attributed to lateral transfer of fluids along inclined reservoirs. The elevation where shale and sand pressure are in equilibrium is called a centroid effect.
Three offset wells are used to build a centroid model in Field DW for the purpose of predicting pore pressure in the proposed well located at the crest of the structure. Industry standard practice is followed to build the centroid model to help prevent pitfalls.
Geopressure is the fluid pressure within earth formations, also referred to simply as formation pore pressure. High formation pressure exceeding normal hydrostatic pressure is termed overpressure (Dutta 1987). High formation pressure is typically found in young, rapidly deposited clastic rocks (primarily sandstone, shale, and siltstone) because of incomplete dewatering of fine-grained rock, such as shale, which is known as compaction disequilibrium (Osborne and Swarbrick 1997; Chapman 1994; Mouchet and Mitchell 1989). This process indicates that the more shale existing in the sedimentary succession, the greater the likelihood of high pressure. The amount of overpressure is a function of the permeability of the rock (how fast the water can escape when buried) and the rate of burial. However, there are other reasons for high pressure primarily related to deeper processes occurring in the rock column at elevated temperatures. It follows from the previous description of the mechanisms that three primary components control where overpressure occurs—rate of sediment burial, temperature, and sediment permeability.
