Mineralogical Impact on CO2-Rock-Fluid Interactions in High-Temperature Carbonate Reservoir: Case Study from Offshore Malaysia

ABSTRACT: Understanding the evolution of rock porosity and permeability due to geochemical interaction is vital for assessing the impact on CO2 storage performance in identified geological formations. This study focused on the influence of rock mineralogy on CO2-rock-fluid interactions and their effects on rock porosity and permeability for S field, which is high temperature, high in-situ CO2 content (∼70%) offshore carbonate field located in Malaysian waters. Two representative core samples, one from gas interval and another from the aquifer interval, were selected from the S field for unsteady-state core-flooding experiments conducted at reservoir conditions (35 MPa/5076 psi and 150° C). Pre- and post-flooding core characterization was carried out using Nuclear Magnetic Resonance (NMR), X-Ray CT-imaging and porosity-permeability measurement to examine any petrophysical alteration resulting from the experiment. The results show that the gas interval sample exhibited a dissolution fingering pattern at the inlet face, induced by CO2 flooding, whereas the aquifer interval sample showed very limited changes. These observations were supported by increased gas porosity-permeability in the gas interval, as indicated by NMR porosity measurements and X-ray CT images showing fingering features, whereas no significant changes were detected in the aquifer interval sample. 1. INTRODUCTION CO2-rock-fluid interaction could result in alterations of the petrophysical properties (Hao et al., 2013; Zekri et al., 2013; Garing et al., 2015; Khather et al., 2017; Khather et al., 2020) and the multiphase flow characteristics (El-Maghraby and Blunt, 2013; Bennion and Bachu, 2020; Seyyedi et al., 2020) of both rock formation and the subsurface system. The rates of surface reaction are very high in carbonate reservoirs, where calcite reaction rate is 1 × 10−5 mol/cm2/s and for dolomite is 1 × 10−7 mol/cm2/s, which usually lead to greatly uneven rock minerals dissolution (Wellman et al., 2003; Egermann et al., 2005). Even though the rock porosity and permeability are expected to increase due to the dissolution of rock minerals, the change in temperature/pressure conditions at downhole conditions could result in rock minerals precipitation, counteracting the dissolution mechanism. The net effect of this process will result in reduction of rock effective porosity and permeability (Oomole and Osoba, 1983; Izgec et al., 2005).