Productivity Improvement by Reperforation of Multistage-Fractured Wells in High-Pressure/High-Temperature Tight Gas Reservoirs: A Case History

Abstract

Productivity of multistage-fractured gas wells is possibly degraded by conductivity impairments and non-Darcy flow during long-term production. Such degradations are pronounced by flow convergence to short perforated intervals, while it is challenging to identify degraded stages for remediation. Moreover, remedial actions can be expensive under a high-pressure/high-temperature (HP/HT) environment. A field case demonstrates successful application of reperforation as a cost-effective way to mitigate the flow convergence by prioritizing targets with multirate production-logging (PL) results. This work presents theoretical investigations using numerical simulations and field execution of reperforation for a well with six-stage fracturing treatments in a HP/HT volcanic gas reservoir onshore Japan. Apparent conductivity reduction was suspected during more than 15 years of production, and it was pronounced by non-Darcy flow effects associated with flow convergence to short perforated intervals. Multirate PL was used to identify impaired stages by quantifying the inflow-performance relationship (IPR) of each stage under transient flow-after-flow (FAF) testing. The impaired stages were reperforated, adding perforation intervals with wireline-conveyed perforators. Pressure-buildup (PBU) tests before and after the job and post-job PL were used to validate productivity improvements. Target zones for reperforations were identified and prioritized with results of the multirate PL conducted. The stage IPRs were drawn, and relatively large non-Darcy effects were identified in three stages by shapes of the IPRs and/or decreasing inflow contributions as the surface rate increased. Also, a temperature log showed steep temperature change at the bottom of the fourth stage; the fracture might propagate below the perforated interval. Ranges of production increment were estimated using a numerical model calibrated against the estimated stage IPRs. The estimated increment was in the range of 15 to 30% with the planned reperforation program, while its magnitude depended on the connection between new perforations and existing fractures. Afterward, the reperforation job was performed and the gas rate was confirmed to be increased by 26% with the same wellhead pressure after 1 month of production. The post-job PL was conducted 3 months after the reperforation. The well IPR was improved, implying reduction of the non-Darcy effects. Results of PBU tests also indicated reduction of skin factor. The stage IPRs were redrawn with the post-job PL, and they suggested clear improvements in two stages where screenout occurred during fracturing treatments and a stage where significant non-Darcy effect was suspected. The workflow and strategy in this paper can be applied for productivity restoration in a cost-effective way to multistage-fractured gas wells with short perforated intervals and impaired apparent conductivity during long-term production. Especially, the interpreted results suggested effectiveness of the proposed approach for productivity improvement in stages where screenout occurs during fracturing treatments. Moreover, lessons learned on the importance of careful test designs for PL were discussed because they are keys for success.