Productivity Improvement by Re-perforation of Multistage-fractured Wells in HP/HT Tight Gas Reservoirs: A Case History
Authors: Kidogawa Ryosuke , Yoshida Nozomu , Fuse Kei , Morimoto Yuta , Takatsu Kyoichi , Yamamura Keisuke 
Published: November 2019

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 high-pressure and high-temperature (HP/HT) environment. A field case demonstrates successful application of re-perforation as a cost-effective way to mitigate the flow convergence by prioritizing targets with multi-rate production logging (PL) results.

This work presents theoretical investigations using numerical simulations and field execution of re-perforation 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. Multi-rate PL was employed to identify impaired stages by quantifying inflow performance relationship (IPR) of each stage under transient flow-after-flow testing. The impaired stages were re-perforated adding perforation intervals with wireline-conveyed perforators. Pre/post pressure build-up tests and post-job PL were used to validate productivity improvements.

Target zones for re-perforations were identified and prioritized with results of the multi-rate 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 surface rate increased. Also, temperature log showed steep temperature change at bottom of the 4th 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 range of 15% to 30% with planned re-perforation program while its magnitude depended on connection between new perforations and existing fractures. Afterwards, re-perforation job was done, and, the gas rate was confirmed to be increased by 26% with the same well-head pressure after one month of production. The post-job PL was conducted three months after the re-perforation. The well's IPR was improved implying reduction of the non-Darcy effects. Results of pressure build-up 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 screen-out 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 multi-stage 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 importance of careful test designs for PL were discussed because they are keys for success.