Abstract
The pore pressure profile of an exploration well in high pressure and high temperature (HPHT) A-Field of Nam Con Son Basin was initially predicted along the wellpath in tandem with the analogy to an offset well to design a drilling program of penetrating the unreachable sedimentary formation. The scheme had driven mud weight to encounter the pressure ramp but resulted in underbalance and influx into the wellbore that incurred downtime for well control. Subsequently, formation pressure measurement conceded the substantial disparity over the offset well so the post-drill study was conducted towards with 3D geological modeling and fault seal analysis to gain insight into overpressure generation mechanisms of the field.
Seismic interval velocity, density, resistivity data are applied for generating the validated pore pressure profile of exploration well on both Eaton and Bowers methods with calibration on formation pressure measurement for Middle and Lower Miocene Sequences. Besides, the cutting edge 3D modeling is approached to construct a robust structural and fault framework as well as to condition and upscale ultimate shale volume, pressure gradient, and overburden stress for facies and pressure distribution. The sealing capacity of a fault is quantified in terms of the pressure acting on the fault surface that is required to be exceeded for the fault to become unstable and slip including simulations of formation juxtaposition mapping, fault clay content prediction, fault flow indicators, and transmissibility.
Upon completion of the 3D model, the fault surfaces which are mapped by shale volume could provide a detailed geometry and lithology juxtaposition analysis for the fault planes. The results of the high Shale Gouge Ratio (SGR), very low fault permeability, and relatively high fault rock thickness imply that the studied faults act as a baffle to fluid flow. However, from juxtaposition observation, the displacement broadens with depth in some parts of faults and the lateral stress increases through the sync-rift stage of Middle Miocene could be a cause of overpressure in this studied area.
The 3D pore pressure and stress regime integrated with fault seal analysis in the model are generally obtained to provide both vertical and spatial overpressure characterization and advantages for well drilling plan and reservoir production. From the drilling aspect, a fault stability study can optimize the maximum allowable mud weight to not exceed while drilling so that fault reactivation does not take place. From a depletion perspective, understanding of stress variations due to lowering reservoir pressure with time can be incorporated with fault seal analysis.