oa Deep Pore Pressure Prediction in Challenging Areas, Malay Basin, SE Asia

Accurate pore pressure prediction is not trivial anywhere, but becomes especially challenging in HPHT environments, where many geological processes commence and make rock properties inherently less predictable. In such an environment the traditional methods to estimate pore pressures ahead of the bit must be modified, and in some cases replaced. Heavy reliance on seismic-derived pore pressures is HP/HT environments is likely to lead to unacceptable uncertainties, and should be replaced by models based on geological processes. The traditional approach to pore pressure predictions, which works well in young, rapidly deposited and low temperature sediments, such as occur in Tertiary Deltas (e.g. Gulf of Mexico, Nile Delta), is based on principles which govern compaction of compressible sediments such as shales. During burial, as compaction proceeds, porosity is reduced in the sediment, driven by stress. If sediments are not sufficiently permeable to allow complete dewatering within the time frame that a stress is imposed (for example during and after addition of load during sedimentation) the increment of additional stress is distributed only partially on the grains and the remainder on the fluids. Incomplete dewatering leads to the overpressure mechanism termed compaction disequilibrium where the magnitude of overpressure is controlled by the weight of the added load (vertical stress), as well as rock properties such as compressibility and permeability. Typically, pore pressure profiles evolve with depth to be overburdenparallel. Current pore pressure prediction capability is well optimised for these where compaction disequilibrium is the primary source of overpressure. However, as industry drills deep targets where temperatures typically exceed 100-120oC (~250oF) and often much higher, the ability for conventional porosity-based pore pressure prediction methods to deliver satisfactory results diminishes. Above this threshold temperature pore pressures are likely to be underestimated, as techniques using interval velocities, wireline or Logging While Drilling data such as sonic and resistivity become increasingly unreliable. In these higher temperature conditions, additional pore pressure can also be generated by fluid expansion mechanisms (aquathermal pressuring, hydrocarbon maturation, inter-granular water released during clay diagenesis) and framework weakening/load transfer (the modification of the load-bearing part of the sediment such that the rock becomes weaker/more compressible, for example when smectite re-crystallises as illite or when kerogen transforms to oil/gas and residual kerogen). Fluid expansion and framework weakening causes the pore pressure to increase at a rate faster than rate of increase of overburden stress. Overpressures generated by compaction disequilibrium and fluid expansion methods have been quantified by Swarbrick et al. (2002). Load transfer/framework weakening has been quantified by in Lahann et al. (2001) using data from the Gulf of Mexico. Our recent work in the High Pressure/High Temperature region of Mid-Norway estimates a contribution to pore pressures of approximately 17 MPa (2500 psi) overpressure at depths of 4500m (15,000 feet) through the mechanism of framework weakening. In HP/HT environments, therefore, very significant contributions of secondary overpressure (in addition to that from compaction disequilibrium) can be expected as temperatures well in excess of 100-120oC are encountered. If not anticipated prior to drilling, this additional overpressure leads to major drilling surprises with implications for health and safety (as well as geological implications such as hydraulic failure of top-seals in reservoirs and re-migration of hydrocarbons). Steps towards an improved understanding of these processes and their<br>contribution to overall sediment overpressure would provide a significant contribution to pore pressure prediction modelling in deep and hot environments. Therefore, this paper is designed to bring together some new research results from studies of overpressure in the Malay Basin as well as Gulf of Mexico, SE Asia and Northern Europe to develop a workflow and methodology to characterize and quantify pore pressure in deep targets and inform the next generation of pore pressure prediction capability in HP/HT environments.