--> Role Of Thermal Parameters In Modelling Fracture Gradient And Overpressure Mechanism In Southern Part Of West Baram Delta, Sarawak, Malaysia

AAPG Asia Pacific Region GTW, Pore Pressure & Geomechanics: From Exploration to Abandonment

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Role Of Thermal Parameters In Modelling Fracture Gradient And Overpressure Mechanism In Southern Part Of West Baram Delta, Sarawak, Malaysia

Abstract

In oil and gas exploration and production overpressure mechanism is one of a crucial parameter in estimating or predicting overpressures before drilling. The correct geo-pressure parameters in deep reservoirs are required to improve drilling safety and preventing the drilling hazard. Furthermore, efforts in modelling fracture gradients based on natural fracture is necessary to gauge the formation integrity so as to plan mud density in drilling proposal. Thorough, investigations are recommended for fracture gradients and overpressure mechanisms analysis to overcome the shortcomings in overpressure prediction encountered. Correct estimation of fracture gradient is essential in planning for drilling a safer and an economic well. It is extremely important especially in the areas of abnormally pressured formations. Moreover, understanding the nature of fracture is essential in order to gain a better outlook on the fracture gradient. Analysis on fracture attributes, pore pressure and the rock properties especially thermal conductivities for several selected wells in southern sector of West Baram Delta, Offshore Sarawak were carried out, to determine overpressure mechanisms, fracture gradients and pore pressure and temperature gradients. The West Baram Delta that was influenced by both gravity-driven deformation and regional stress, exhibits variation of growth faults, fold and anticline. The study was conducted by integration of rock physics and pore pressure trends analysis within the shale and sand intervals. The pore pressure, electric log and thermal trends are used to determine predominant overpressure mechanism at each well. Meanwhile, theoretical method to calculate fracture gradient from well logs using Ben Eaton methods was carried out. Comparison with the actual leak-off test fracture pressure was done to validate the accuracy of the fracture gradient model. The relative changes of fracture gradient are inversely proportional to Pressure-Temperature gradient (P-T gradient) where higher fracture gradient occupies areas of low P-T gradients. Towards the northern sector, the P-T gradients increase probably due to occurrence of shale diapir with fluid expansion mechanism of overpressure. This is also an area of lower fracture gradient. The fracture gradient for average depth of 2000m to 3000m is 13 psi/ft to 16 psi/ft. In the southern sector, near to the coastline, the P-T gradient is lower ranging from 0.4 to 1.2 Mpa/°C while the fracture gradient is higher at 16 to 18 psi/ft. for the depth interval. Whereas, expansion mechanism dominates the North Eastern sector, where the P_T gradient is high. Compaction overpressure mechanism prevails in the North East and Southern coastal sector. Major factors controlling the fracture development are lithology and mineral composition, as well as abnormal high pressure. The variability in P-T gradients over the area influence the fracture gradients’ distributions. Integration of thermal and logs data provide the significant analytical means in investigating the overpressure mechanisms and fracture gradient modelling. While using seismic velocity is a common method for overpressure prediction, subsequent contribution of thermal parameters in understanding the mechanisms of overpressure would enhance in pore pressure prediction methodology. Thus, integrated thermal and facies analysis technique shall compliment the velocity analysis, to investigate the overpressure mechanism as well as fracture gradient modelling. This will lead to enhancement the methodology in pore pressure estimation for future oil and gas exploitation.