USING SIMPLE LOADING MODELS TO PREDICT CRESTAL PORE PRESSURES IN MIOCENE CARBONATE EXPLORATION TARGETS, LUCONIA, SARAWAK

摘要

Miocene carbonate reservoirs form frequent exploration targets in basins such as North Lombok and Central Luconia, Sarawak. One of the key risks associated with these traps is that of mechanical seal failure, whereby pore pressure at the crest of the structure exceeds the fracture strength of the seal. Therefore, quantifiying this risk pre-drill can help prevent the drilling of dry holes in Luconia, Sarawak. Predicting pore pressure in these basins is hampered by the frontier nature of much of the drilling i.e. little well calibration and poor/limited seismic and/or log data. Moreover, carbonates in general do not have a porosity/effective stress relationship which enables traditional pressure prediction techniques to be used. Therefore, in order to provide accurate pore pressure for use in estimation of breach risk, we propose a two step method that relies on simple loading and required only the use of chrono-stratigraphic markers derived from seismic. Step 1 uses rates of sedimentation and stratigraphic ages to determine theoretical shale pressure profiles. In thick shale sequences, where disequilibrium compaction is the mechanism for generating overpressure, pore pressure builds parallel to the loading or evolving overburden. If loading rates are high, then these profiles start at depths less than 1.0km. Therefore, using this approach allows us to produce a model for shale pressure in these basins. Thick sand-rich sequences are likely to have lower reservoir (and shale) pressures. As these pinnacle reef targets are steeply inclined by up to 1km of relief, the differential overpressure in the shales between the crest of any structure and the down-dip spill point can be significant. This difference in overpressure is transferred to the carbonate reservoir such that crestal pressure at the reservoir/seal interface can be elevated in excess of the surrounding shale pressure. This process is termed lateral transfer. Thus, step 2 is to determine this additional presssure at the crest and add this to the shale pressure as calculated in step 1. Using this simple workflow, a kick taken in Central Luconia well A drilled by SapuraKencana Energy Inc. is matched in the post-drill analysis. Moreover, the risk of mechanical seal failure can be more accurately addressed. This will help to prevent the risk of drilling dry holes in the future. Where sufficient seal capacity is demonstrated, hydrocarbon column lengths can be calculated.