Improved Log Analysis in Shaly-Sandstones--Based on S_w and Hydrocarbon Pore Volume Routine Measurements of Preserved Cores Cut in Oil-Based

摘要

Routine measurements of properly preserved cores cut in oil-based mud (OEM) give accurate porosity (PHI) and water saturation (S_w) values in hydrocarbon reservoirs. Above mobile water zones the Hydrocarbon Pore Volume (HPV) of these OBM cores is usually accurate in both clean and shaly sand reservoirs. HPV evaluations from the 'core' and the 'total' and 'effective' porosity systems must all be the same: HPV = PHI_(core) (1 - S_(wcore)) = PHI_t (1 - S_(wt)) = PHI_e (1 - S_(we)), where the subscripts denote the specific system being addressed. In shaly reservoirs, core porosity (PHI_(core)) and total porosity (PHI_t) may be larger than effective porosity (PHI_e), with the extra porosity being generated from the drying of the clay minerals, especially smectites. The water volumes measured by Dean-Stark extraction of OBM core plugs includes this clay-bound water, so OBM PHI_(core) and S_(wcore) used together as a pair give an accurate evaluation of HPV. Example core and log data from a shaly-sand formation are evaluated using several well-known log analysis models. As tested, the Dual-Water, Cyberlook and Indonesia models all provide quite similar S_w results and agree fairly well with the OBM S_(wcore) corrected to reservoir conditions. Because the standard Cyberlook and Indonesia S_w results are close to S_(wt), they are paired with total porosity for HPV calculations. They have previously been thought to give S_(we), not S_(wt), and have been paired with 'effective' porosity giving pessimistic HPV results. The Archie total porosity model also gives useful, but pessimistic, results. The Archie effective porosity model gives very pessimistic S_w and HPV results in shaly sands. The most accurate evaluation method, the OBM core analysis, can be used to calibrate current resistivity-S_w models for shaly sands. This process leads to improved estimates of oil- and gas-in-place and, potentially, to improved shaly-sand models for application in worldwide log analysis.