Overpressures in the Northern Malay Basin: Part 2 - Implications for Pore Pressure Prediction

Abstract

Abstract Examination of plots of sonic velocity against effective stress from the Northern Malay Basin indicate that fluid expansion or another unloading mechanism contribute to the total overpressure in addition to the more commonly recognised disequilibrium compaction mechanism. The aim of this study was to build a pressure prediction model from spatially extensive seismic velocity data. The pressure prediction model was calibrated by sonic velocity data acquired at well locations where extensive pressure datasets as a function of depth were measured using wireline equipment. Analysis of pressure and velocity data from wells in the Northern Malay Basin shows that pore pressure can be accurately predicted from sonic velocity using a modified Eaton approach and the seismic velocity is closely fit by filtered sonic velocity data. This modified Eaton technique requires the derivation of a robust normal compaction velocity trend and the selection of suitable empirical coefficients. The spatial variation of the fitting coefficients of an empirical normal compaction velocity and the empirical Eaton overpressure prediction relationships were assessed for 31 wells. Results show that in the Northern Malay Basin the compaction coefficient varies spatially with the geological structure and the spatial variation of the Eaton coefficient was found to be positively correlated with the magnitude of pore pressure. Accurate pressure prediction from seismic at locations away from calibration wells requires that seismic velocity analysis outputs sufficiently accurate interval velocities at a suitable resolution. The assessment of the accuracy of seismic velocities was performed on pre-stack data for a number of inlines over the study area. Seismic velocities were carefully re-picked and the resulting interval velocity at the well locations found to closely match the filtered sonic velocities. As a result of the remote nature of the seismic measurement there remains some error in the interval velocities predicted from the seismic due to the presence of shallow gas accumulations which affect the transmission of seismic energy and residual multiples in the processed reflection data. Introduction Drilling operations for hydrocarbon exploration and development in overpressured formations are potentially hazardous from an HSE perspective and can often lead to difficulties such as kicks, blow-outs, borehole instability issues, stuck pipe and lost circulation (Sayers, 2006). Overpressure occurs when the pore pressure at any given depth exceeds the hydrostatic pressure of a column of water (or formation brine). If the pore fluid pressure magnitude exceeds a threshold dictated by the strength of the rock, the rock will fail and no longer seal the reservoir causing the release and migration of hydrocarbons and the breaching of the trap (Dutta and Khazanehdari, 2006).