Enhanced Gas Recovery by CO2 Sequestration vs. Refracturing Treatment

Abstract

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 172083, “Enhanced Gas Recovery by Use of CO2 Sequestration vs. Refracturing Treatment in Unconventional Shale-Gas Reservoirs,” by Mohammad O. Eshkalak, SPE, Emad W. Al-Shalabi, SPE, Alireza Sanaei, SPE, Umut Aybar, SPE, and Kamy Sepehrnoori, SPE, The University of Texas at Austin, prepared for the 2014 Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, 10–13 November. The paper has not been peer reviewed. In this paper, shale-gas-field data have been evaluated and incorporated in simulations for both carbon dioxide (CO2) enhanced-gas-recovery (EGR) and refracturing-treatment purposes. This comprehensive sensitivity study helps in understanding the key reservoir and fracture properties that affect production performance and EGR in shale-gas reservoirs. The findings of this study recommend initial hydraulic refracturing of shale reservoirs for enhancing gas production, followed by CO2 injection at a later time. Introduction In this paper, two different EGR methods are investigated and systematically compared in terms of efficiency. The first method is a refracturing treatment in order to revive a shale-gas well that has failed to achieve economic gas-production rates. The second proposed method is CO2 injection for both storage/ sequestration and EGR. Discussions and definitions of each of these methods, as well as governing equations, are provided in the complete paper. Basic Reservoir Model With Multifractured Gas Wells The numerical-simulation technique is a technology that enables characterization, development, and management of a producing reservoir. It integrates geologic, geochemical, and petrophysical properties and examines their effects on production. The ability to use simulation to investigate the sensitivity of production to various reservoir properties is critical for the decision-making process in unconventional shale formations. Fig. 1 shows the configuration of Wells UT-1 and UT-2 in the shale-reservoir model. As seen in Fig. 1b, in order to maximize methane-gas production by a refracturing treatment, the new fractures are placed in the middle of each primary fracture. CO 2 Huff ’n’ Puff Scenario. In this scenario, after 5 years of production from both wells, one of the producers is converted into a CO2 injector for 5 years. Also, another 5 years is given as a soaking time. Afterward, wells are put back on production. Methane (CH4) recovery for this scenario is almost less than 4%. On the basis of the observed mass fraction of production after soaking time in the 15th year, it is observed that 96% of CO2 is produced back while only 4% of CH4 is produced at the well. This finding shows that the huff ’n’ puff process is not a viable option for CO2 EGR because it delays 5 years of profitable gas production. Hence, this scenario was excluded as an option for CO2 EGR, and, for the rest of the study, the authors compare CO2 flooding with refracturing treatment.