Open Hole Formation Testing and Sampling in Shallow and Low Pressure Gas Reservoirs: Best Practices and Lessons Learned

Abstract

Cendor Graben Shallow sands are 35 Km2 of gas accumulation at depth varying from 350-450mss over the Jambu-Liang anticline, defined by high amplitude in seismic, containing hydrocarbon gas (C1-C3gas indicated by mudlogs) with the possibility of high CO2; possibly charged from deeper sources though fault seeps. Shallow Gas reservoirs also due to the unconsolidated formation present unique measurement and operations challenges to the wireline formation tester (WFT) in pretesting and sampling due to its nature of high compressibility especially at low pressure. Often the high gas compressibility results in a larger pretest drawdown volume needed to decompress the flowline and to reduce the flowline pressure below the formation pressure for a valid formation pressure measurement. The good mobility formation further complicates the flowline decompression due to a very small pressure drawdown. In addition, at the start of pretest build-up period, a pressure build-down behavior was observed and has greatly affected the formation pressure stabilization and mobility calculation. On the other hand, gas sampling in such low pressure and unconsolidated formation introduces great difficulties in balancing the samples bottle filling and sand production initiation which may potentially plugged the downhole pump-out module. The station time limitation commonly imposed by the drillers making the sampling process even more challenging. In the past, companies often considered these sands as drilling hazards and not targets for exploitation. This testing validated the importance of formation testing in shallow unconsolidated gas reservoirs by preventing formation failure that could lead to plugging. The small sand particle sizes and unconsolidated nature of the formation makes it very difficult to control sand production. This work presents an exploration work done in a very shallow and low pressure gas bearing formation, which exposes the WFT to a combination of unconsolidated formation and high gas compressibility challenges. Best practices are established and discussed in this paper to address the aforementioned challenges to enable a successful WFT evaluation in the shallow, low pressure gas reservoirs.