--> Pore Pressure & Gas While Drilling Analysis For Seal Integrity Assessment In Clastic Reservoir

AAPG Asia Pacific Region GTW, Pore Pressure & Geomechanics: From Exploration to Abandonment

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Pore Pressure & Gas While Drilling Analysis For Seal Integrity Assessment In Clastic Reservoir

Abstract

One the main criteria for CO2 Storage Site is to evaluate the sealing potential of respective potential storage sites candidates. It is important to investigate the seal as it will ensure long term containment for CO2 sequestration program. Four (4) fields from Peninsular Malaysia Basin has been shortlisted to undergo Level #1 Study which focused on integrated subsurface evaluation which include Geophysical & Geological (G&G) containment integrity study, rough estimation on CO2 storage capacity and sensitivity on potential CO2 injection rate as well as well engineering analysis on integrity of the wells. As part of G&G containment study, evaluation of sealing potential were focus on overpressures seal perspectives together with potential of leakage pathway from reservoir to seal formations. The study involved integration of pore pressure analysis, gas while drilling study and understanding of the geology of field area.

The pore pressure analysis were conducted mainly for exploration wells whereby both resistivity and sonic logs were utilized for this analysis. Normal compaction trend were identified based on the log trend to get the estimated pore pressure which further calibrated with available MDT or RFT data. Fracture gradient analysis also has been computed to get the potential fracture limit for both reservoir (sand) and seal (shale) sections. Formation Integrity test (FIT) and Leak off test (LOT) data has been used to calibrate the fracture gradient. At the same time, Gas While Drilling (GWD) method is used to identify any leakage C1, C2 and C3 from the reservoir into the seal, which indicate the caprock seal integrity of the reservoir. From the mud logs data, gas peaks were selected and plotted into the mixture graph crossplot for gas component separation distribution (C1-C4) and the trend were analyzed and compare within the reservoir and target seal formations. The data also used to further calibrate with pore pressure analysis to investigate further on overpressure area. The four (4) fields that has been identified is mainly interbedded reservoir with potential seal thickness at shallow section ranges from 50 to 300m metres. From pore pressure analysis, most of the pressure trend shows it is hydrostatic pressure in seal. However, overpressure seals is possible in these multistack reservoirs as it can be due to lateral transfer or sealing faults. The gas while drilling shows majority of lower gas component at shallow section ~ 0.78% of C1 and only 1 (one) field shows potential of gas of C1 to C3 migrated from lower reservoir through existing micro faults and fractures.

The outcome for this study has been integrated further with seismic and geological structural interpretation to summarize the overall subsurface risk from G&G points of view. Meanwhile, pore pressure and fracture gradient profile were further integrate with reservoir engineering study to provide rough estimation of CO2 capacity and as input data for Level 2 Study in building geomechanics model whereby maximum injection pressure were determine based cap rock stress profiles.