It has been long recognized that using the conventional production logging tool in combination with the electrical probes measurement for water holdup can resolve the source identification of water inside the borehole. The source of water for single layer reservoir can be identified easily and can be complex for borehole involving multilayers and multiphase. The current limitations with the conventional production logging tool is that it covers the quantitative measurements inside borehole and to some extent qualitative measurement behind casing e.g. temperature profile.
The conditions get complex in the absence of cement behind casing and multiple pay zones behind the production casing and thus mixture profile tends to complicate the identification of phases within borehole. An alternative way to identify the hydraulic isolation between layers is to detect fluid movement directly behind the casing. Using a novel approach of Oxygen activation of water, the axial movements of water can be identified and quantified.
A Solid state neutron source in a stationary logging tool is first turned on activating the oxygen in the region near the wellbore, and then turned off. If the oxygen in stationary, decay is exponential and predictable. However, if flow is present, axial fluid movement transports some of the temporarily activated atomic oxygen in the water past three detectors where gamma ray count rate transient responses are recorded. Analysis of transients allows direct detection of water flow in annular space, a velocity measurement, and an estimate of the flow magnitude.
The case study presents oil producing well with two reservoirs started producing excessive amount of water. Where top reservoir was cement squeezed. The water producing zones across bottom reservoir was to be identified and to investigate if top reservoir contributing this water behind the liner. First time in Pakistan Oxygen activation technique with detectors at bottom (inverted mode) was utilized along with conventional production logging tool to resolve the source of water flowing downward behind casing.