An offshore gas-producing well recently encountered an increase in water production. The well featured multiple completion zones, creating ambiguity and uncertainty in the water source and production zone. Due to the significant deviation of the well, coiled tubing (CT) provided the means of conveyance for an advanced array of well diagnostic tools selected to evaluate the multiple downhole scenarios that may lead to the increase in water production, including completion failures, saturation depletion, and flow from unexpected sources.
This paper discusses the integrated analysis and methodology behind the successful determination of the water source in an offshore gas-producing well. The deployed tools include a multi-detector pulsed-neutron tool (MDPNT), an advanced acoustic array leak detection tool (ALDT), and a suite of production logging tools. The logging suite was tailored to address the wealth of possible scenarios for the increase in water production. Logging was performed in shut-in and flowing environments to understand the behavior of the well in dynamic conditions. Following the acquisition, the data sets were integrated and analyzed, leading to the successful determination of the problem zone.
The well featured two perforated reservoirs, each separated by packers and sliding sleeves. The deeper zone was determined to be water-saturated and should have been isolated by a closed sliding sleeve beneath a packer. MDPNT oxygen activation eliminated the possibility of channeling between the water-producing zone and the gas-producing zone. The packers in the well proved to be intact based on the interpretation of the ALDT acoustic noise data and the count rate reading of the MDPNT. The investigation then focused on the completion string jewelry and involved computing a two-dimensional (2D) radial flow map from beamforming of the ALDT-recorded acoustic noise activity around the wellbore, which revealed a closed sliding sleeve within the water-producing zone was likely leaking and, therefore, ineffective. The saturation results using the MDPNT in sigma mode indicated that the perforated reservoirs contained a low remaining gas saturation. Following the diagnosis, the operator ensured the sliding sleeve was closed reducing water production by more than 50%, resulting in an improved well performance.
Compatibility with CT enabled the advanced suite of pulsed neutron, acoustic noise array, and PL wireline sensors to be deployed in a well with significant deviation. As the water production source was unknown, many possible scenarios had to be tested. Careful job planning and interpretation of the acquired log data eliminated scenarios and led to the conclusion that the completion integrity was compromised at a sliding sleeve, therefore enabling flow from a water-saturated zone.