Optimising Brine Production for Pressure Management During CO2 Sequestration in the Bunter Sandstone of the UK Southern North Sea

Abstract

Abstract This paper focuses on pressure management via brine production optimisation to reduce reservoir pressure buildup during carbon dioxide (CO2) sequestration using a geocellular model representing a sector of the Bunter Sandstone Formation. The Bunter Sandstone is a deep saline aquifer with high reservoir quality and is a leading candidate for potential CO2 capture and storage (CCS) in the UK. Brine production optimization during CO2 sequestration is necessary because it helps minimize brine waste and well construction and operational costs. In this paper, various sensitivity analyses were performed investigating well geometry, injection and production well spacing, pressure management and boundary condition effects. Two scenarios were investigated and development plans were proposed for annual injection of 7 MT/yr CO2 (Scenario 1), which is equivalent to the CO2 emissions of a 1.2 GW coal-fired power plant, and for scenario 2, where we aim to utilize the maximum storage capacity of the reservoir model. Three pressure management schemes were compared for each scenario: no pressure management or no brine production, passive pressure management where pressure relief holes are drilled and brine passively flows to seafloor without external energy, and active pressure management where brine is actively pumped out. Brine production rate and relief well patterns were evaluated and optimised. The results show that well perforation length and the use of deviated wells have a significant impact on injectivity improvement whereas well radius has little impact on injectivity. Symmetrical well placements between injection and production wells yields higher storage capacity than asymmetrical ones, and increasing the number of relief wells improves CO2 storage capacity. In the case of open boundary conditions, no pressure management is required because the reservoir quality enables pressure dissipation, resulting in a pressure buildup of less than 5 bars. In the case of closed boundary conditions, either passive or active pressure management is required to prevent seal failure from overpressurization of the reservoir and it also increases storage capacity. The cases with open boundaries, as expected, yield higher storage capacity than the cases with closed boundaries. In scenario 1, or assumed annual injection of 7 MT, storage capacity is 344 MT without pressure management and with open boundaries. This is compared to 332 and 328 MT for cases with closed boundaries and passive and active pressure management, respectively. In scenario 2, the maximum storage capacity of the model is 684 MT with no pressure management and open boundaries, and 504 and 683 MT with closed boundaries with passive and active pressure management, respectively. The storage efficiency ranges from 1 to 6% in scenario 1 to the highest at 12% in the maximum storage capacity case. In addition, three aquifer sizes; open boundary aquifer size of 2.43×1012m3, an aquifer size of 1.8×1010m3 based on pressure recharge studies of the Esmond Gas Field, and closed boundary aquifer size of 1.22×1010m3, were compared for the optimised cases. The study shows that aquifer size has an impact on estimation of CO2 storage capacity. The storage capacities of the three aquifer size cases ranging from the largest to smallest without pressure management are 344, 105 and 74 MT, respectively.