Formation Tester Optimization for the XiHu Basin: Observations and Case Studies from the East China Sea

Abstract

In recent years, low-porosity and -permeability gas reservoirs have become an important exploration target for the industry in general, and the China National Oil Company (CNOOC), Shanghai branch, in particular. Formation evaluation of these resources is inherently challenging, and the application of formation testers has become an integral part of the evaluation workflow to address these challenges. The XiHu basin, located offshore China in the East China Sea, is a frontier exploration area for CNOOC Shanghai branch. Drilling safety in the XiHu basin is a concern owing to the intermediate high-pressure sands and overlying coals. Drilling with high overbalance and long openhole sections to mitigate these challenges has in turn caused problems for the formation testing program that are manifested as elevated pressures and long sampling times. The optimization of formation testing began with the statistical analysis of historical data to identify trends in pressure measurement and sampling. This analysis clearly indicated that continuous filtrate invasion and water slumping processes contributed significantly to the long sampling times and apparent elevated pressures observed. Therefore, these mechanisms were studied by modeling the invasion process quantitatively using a finite-difference simulator. After matching the simulated pressures with measured pressures in a candidate well, the effect of these invasion mechanisms on the formation testing program and ways to mitigate them was investigated. Our study shows that the invasion profile in these reservoirs is different from the classic invasion profile. The classic model shows minimal invasion at the top of the sand, but we observed the longest cleanup times were generally at the top of the reservoir. Using realistic physical and drilling parameters, we demonstrate that invasion effects are often more complicated than the classic model. Optimized tool selection and testing processes have been proposed to mitigate the effects of the invasion processes. The applicability of new technology in these challenging environments was also studied and demonstrated through case studies in which an overall efficiency improvement in the formation testing program was observed.