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

摘要

Miocene carbonate reservoirs form frequentrnexploration targets in basins such as North Lombokrnand Central Luconia, Sarawak. One of the key risksrnassociated with these traps is that of mechanical sealrnfailure, whereby pore pressure at the crest of thernstructure exceeds the fracture strength of the seal.rnTherefore, quantifiying this risk pre-drill can helprnprevent the drilling of dry holes in Luconia,rnSarawak.rnPredicting pore pressure in these basins is hamperedrnby the frontier nature of much of the drilling i.e.rnlittle well calibration and poor/limited seismicrnand/or log data. Moreover, carbonates in general dornnot have a porosity/effective stress relationshiprnwhich enables traditional pressure predictionrntechniques to be used. Therefore, in order to providernaccurate pore pressure for use in estimation ofrnbreach risk, we propose a two step method thatrnrelies on simple loading and required only the usernof chrono-stratigraphic markers derived fromrnseismic.rnStep 1 uses rates of sedimentation and stratigraphicrnages to determine theoretical shale pressure profiles.rnIn thick shale sequences, where disequilibriumrncompaction is the mechanism for generatingrnoverpressure, pore pressure builds parallel to thernloading or evolving overburden. If loading rates arernhigh, then these profiles start at depths less thanrn1.0km. Therefore, using this approach allows us tornproduce a model for shale pressure in these basins.rnThick sand-rich sequences are likely to have lowerrnreservoir (and shale) pressures. As these pinnacle reef targets are steeply inclinedrnby up to 1km of relief, the differential overpressurernin the shales between the crest of any structure andrnthe down-dip spill point can be significant. Thisrndifference in overpressure is transferred to therncarbonate reservoir such that crestal pressure at thernreservoir/seal interface can be elevated in excess ofrnthe surrounding shale pressure. This process isrntermed lateral transfer. Thus, step 2 is to determinernthis additional presssure at the crest and add this tornthe shale pressure as calculated in step 1.rnUsing this simple workflow, a kick taken in CentralrnLuconia well A drilled by SapuraKencana EnergyrnInc. is matched in the post-drill analysis. Moreover,rnthe risk of mechanical seal failure can be morernaccurately addressed. This will help to prevent thernrisk of drilling dry holes in the future. Wherernsufficient seal capacity is demonstrated,rnhydrocarbon column lengths can be calculated.