Deriving the Unknown Sublayer's Pressure in Producing Gas Wells Using the Selective Inflow Performance Workflow

The accurate measurement of sub-zone pressures is crucial for monitoring the performance of reservoirs in gas fields. However, this can be challenging due to the commingled flow of multiple reservoirs and sub-zones within a single well. In this study, we evaluated the use of Production Logging (PLT) and Selective Inflow Performance (SIP) workflows for computing sub-layer pressure in carbonate reservoirs in Pakistan. The study was conducted on carbonate reservoirs located in Pakistan. The usual wells are vertical and have numerous perforations, resulting in combined production across multiple reservoirs and sub-zones. To determine the pressure of a specific sublayer, Production Logging (PLT) was carried out to collect data at various choke sizes. The data was then analyzed through Selective Inflow Performance (SIP) workflow. The Rawlins and Schell-Hardt equation was used to plot the relationship between flow rate and pressure, enabling analysts to match the trendline with the acquired data points. The proposed analysis revealed that at least three different chokes should be tested during PLT logging to ensure accurate results. In addition, for accurate analysis it is preferred that a single-phase fluid is flowing at high flow rates, to be able to detect contrasts between inflows from different zones. In Case-1 SIP workflows were applied on two independent reservoirs with multiple sub-zone entries and found that Reservoir-1 had a current pressure of around 1480psi, whereas Reservoir-2, which was more depleted, contributing more than 80% to production had a current pressure of around 1050psi. Shutting down the wells showed an estimated crossflow of 0.43 MMSCF/D from one reservoir to another. The results demonstrated a good relationship between PLT estimated flow rates and pressures across all producing sub-zones. In Case-2, it was observed that the highest contributing layer on every choke size became a thief zone in shut-in conditions. Such kind of a reservoir that contain mulitple isolated stringers that are commingled together in the well make it challenging for reservoir management. The successful application of SIP workflow allows to perform pressure time-lapse monitoring and adjust the dynamic model accordingly for future reservoir development. This study highlights the importance of periodically measuring sub-zone pressures for effective reservoir performance monitoring in gas fields.