In a deepwater environment, testing of wells can be an expensive process coupled with safety concerns. Properly characterizing a well has a significant impact on asset management. Wells are routinely stimulated to remove formation damage, but subsequent testing to determine the skin after stimulation is rarely performed. This paper discusses a case study where multirate multizone (MRMZ) pressure-transient testing and production logging was performed in a deepwater well in the Gulf of Mexico (GOM) to determine the effectiveness of the stimulation treatment.
MRMZ production logging was performed across the targeted pay intervals. The process was to flow the well at three different flow rates followed by a shut-in period. During these different rates, production logging was performed by three up and three down passes at each choke setting, followed by a stationary measurement at a predetermined depth for 10 to 15 minutes. Following the last production logging run, the tool was parked between the two pay zones, and the well was shut in for a pressure buildup until radial flow was fully established. After that, the tool performed the final routine production logging passes while the well was still shut in.
Although the oil production almost doubled after the initial stimulation treatment with xylene, the high skin observed in real time from the MRMZ pressure-transient analysis prompted the operator to change the formation damage removal program and stimulate with hydrochloric acid (HCl) by coiled tubing rather than only xylene. This resulted in an immediate fourfold improvement of productivity index with a potential of further doubling.
MRMZ testing is a relatively low-risk, low-cost method for acquiring valuable formation data. This paper additionallydiscusses best practices that should be performed while attempting MRMZ and how deviating from these practices can lead to increased uncertainty in the acquired data.