Time-lapse Monitoring of Pressure Depletion for High-Temperature Gas-Condensate Reservoirs to Optimize In-Fill Perforation Using Pulsed-Neutron Well Logging

Abstract When infill perforation intervention opportunities are evaluated, it is essential to confirm reservoir pressure. Without appraisal of reservoir pressure, perforation of depleted reservoirs can negatively impact the overall well performance and even introduce early water breakthrough. We successfully evaluated the reservoir pressures of unperforated high-pressure, high-temperature (HPHT) gas-condensate-bearing reservoir sands using pulsed-neutron well logging technology. This paper discusses multiyear time-lapse pulsed-neutron well logging in an HPHT gas-filled borehole using previous- and next-generation sleeved-multidetector instruments for through-casing reservoir pressure depletion monitoring. We employed a previous-generation sleeved-pulsed-neutron tool to acquire three-detector-based pulsed-neutron capture (PNC) data through a gas-filled wellbore to analyze reservoir pressure depletion in 2022. The challenge in the first logging run was a high-temperature environment close to the tool operating limit. Surveillance revealed that the upper target sand was not depleted, whereas the lower sands were depleted. The operator planned to perform time-lapse monitoring of the target sands in 2024 before the final perforation decision. The next-generation sleeved-pulsed-neutron tool was deployed in the 2024 run. PNC data were recorded twice as fast as the first run using the legacy tool. Crucial technological advancements in the next-generation pulsed-neutron tool enable time-efficient PNC data acquisition. First, the pulsed-neutron generator emits twice as many high-energy neutrons as the legacy tool. Second, the adoption of denser scintillation material for gamma-ray detectors allows for recording higher gamma-ray count rates with higher resolution. Finally, a high-speed digital electronics system processes gamma rays faster. The logging speed of the multidetector PNC mode with the new tool was twice that of the legacy tool at 20 ft/min. Benefiting from a faster logging speed, thus reducing the time the tool is in the hole, the heat in the borehole, approximately 350 degrees Fahrenheit, was not a significant concern. The data obtained using the next-generation pulsed-neutron tool demonstrated reliable coherence with the previous data acquired via the legacy tool, confirming a smooth transition from the old tool to the new tool and ensuring seamless time-lapse data analysis. Additionally, extensive Monte Carlo N-Particle (MCNP) models under various reservoir pressure conditions for legacy and new tools were integrated into the reservoir depletion analysis. Time-lapse analysis revealed that the upper sand remained under the virgin reservoir condition but that the lower sands unconnected to the upper sand underwent reservoir depletion. The target reservoir was subsequently perforated, confirming the virgin pressure status as predicted by the pulsed-neutron logging appraisal. The single-well opportunity increased productivity by approximately 15,000 barrels of oil equivalent per day (BOE/D). Technological advancements, resulting in increased logging speeds, extended the operator's operating window at high temperatures, allowing for a greater appraisal scope. Cased-hole depletion analysis presents a low-risk, cost-effective screening tool for in-fill production opportunities in late-life field production.