Evaluation Of Hydraulic Fracture Performance in a Complex Stress Environment in Pakistan

Abstract

Propped fracture stimulation of gas wells has a long history of both successes and failures in Pakistan. The operator is actively developing tight gas reservoirs which include the Cretaceous Pab Formation. In recent years, this field has been subjected to an intense drilling and stimulation program with the objective to increase gas production from the tight Pab sandstone. One significant issue is the highly complex geo-mechanical situation, due to strike-slip stress regime which leads to very high fracturing pressures and the potential for fracture growth both vertically and horizontally. This paper is a case study of the evolution of propped fracturing strategy in an existing gas field, looking in detail at 4 wells that have been stimulated over the last seven years. In addition to analyzing the fracture treatment data, the pre- and post-frac welltest and long-term production data are also analyzed to better understand the fracture performance and the issues that affect post-fracture performance in a reservoir with such high stresses and normal (or depleted) reservoir pressure. The analysis uses an integrated approach, with history matching of both the fracture treatment data and the pre- and post-frac welltests and long-term production. The basis of the integrated approach is a reservoir description with one set of permeabilities that is used for both types of analysis, to ensure that the fracture analysis results and reservoir simulation results are consistent with each other. The fracture treatments in these four wells all share a pattern of very high closure stress gradients (close to overburden gradient), low fluid efficiency and a significant risk of screenout. Post-fracture production is close to expectation in most of the wells, although there are issues with the initial post-frac performance during the cleanup period, consistent with the effect of multi-phase non-Darcy flow. Resin Coated Proppant (RCP) was used in two of the wells, and may have had a negative effect on the initial production (choke or mechanical skin effect) as described in a previous publication by the authors (Shaoul, 2020). Two of the wells were vertical and two were deviated, which also has an effect on the post-fracture performance. There are very few published case studies of propped fracture stimulation from Pakistan, which has a unique and difficult geo-mechanical situation. This paper also compares the hydraulic fracturing results in Pakistan with a few other locations around the world where similar behavior has been seen, to give a better understanding of what can be done to improve hydraulic fracturing in these difficult reservoirs, either by trying to prevent some problems that can be avoided and by accepting that some problems are inherent in such a reservoir can only be partially mitigated by changing fracture design style and fracture design goals.