Analysis of wireline formation tester data for determination of fluid type in low permeabilities reservoirs

Abstract

Formation tests in low-permeability reservoir have been impeded by several problems, such as excessively long buildup time, supercharging, and loss of packer seals. A long pressure buildup time is not desirable because the possibility of the tools getting stuck would increase and waste of rig time. This thesis studies the pressure response data from wireline formation tester in low permeability reservoirs, focusing on pressure behavior and method to identify fluid type in reservoir. The study is conducted by simulating the pressure response in the low permeability reservoir condition with reservoir permeability as low as 0.1 mD, and with various fluid types in the reservoir. The concept of restricted zone is introduced to take into account shale in a reservoir that may cause difficulty in fluid flow into a probe when the probe is inserted into the shale region in the reservoir. According to this study, the restricted zone acts as a choke to reduce flow from the reservoir into the probe and chamber, hence slowing down the pressure response. This assumption allows us to simulate the phenomena when the probe is inserted into a shale region in a low to moderate permeability reservoir. When there is no restricted zone existing in low permeability reservoir (0.1 mD), the 5-minute buildup time is enough to identify the reservoir pressure and also identify pressure gradient or fluid type. For the case with restricted zone around the probe, the buildup time can be as long as 35 days. The pressure gradient or fluid type determination using pressure from too short period of time can lead to wrong conclusion due to unstable conditions caused by wellbore (chamber) storage effect and/or transition regime.