A Modified Gassmann Fluid Substitution Theory for Carbonate Rock Physics Modeling

摘要

Research now shows that CO2 is a significant cause of the earth's rising temperature. Decreasing those elemental emissions has become a crucial requirement for environment protection. CO2 sequestration in depleted reservoirs is a technique which is widely used to reduce the problem of greenhouse gasses. To guarantee a successful sequestration process, we have to monitor CO2 migration in the reservoir to prevent any leakage, which may cause more pollution difficulties (Matsuoka and Azuma, 2014). 4D time-lapse seismic monitoring is the method by which we can achieve reliable monitoring results (Eiken et al, 2000). However, in some reservoirs, seismic techniques failed to monitor CO2 movement (Blonk et al, 1998). Ensuring our ability to accurately monitor CO2 is a crucial step in the CO2 sequestration project plan (Blonk et al, 1998). Utilizing rock physics modeling, and applying fluid substitutions are effective ways which we can use to examine the feasibility of monitoring CO2 movement within a reservoir. Building a rock physics model in elastic rock is less complicated than in carbonate rock because of the complex heterogeneity of carbonates (Xu and Payne, 2009). We should direct our attention towards developing rock physics models for carbonate reservoirs, as more than percent of the world's reservoirs are carbonates (SLB, 2007). For this thesis, we shadowed the work of Quanxiong et al (2014). Gassmann's fluid substitution can be applied based on Eshelby-Walsh theory to establish a relationship between the elastic properties of the rock (Jiang et al, 2011). Quanxiong et al (2014) called Gassmann-Eshelby-Walsh theory the GEW theory. Notably, we differed from Quanxiong et al (2014), in that we used Reuse-Voigt-Hill average (Hill, 1952) to estimate bulk modulus of the solid matrix. Dry bulk modulus was calculated using Eshelby-Walsh theory, which considers pore geometry effects on bulk modulus of dry rock. Quanxiong et al (2014) assumed that the pores filled with the same fluid and the rock sample consists of one pore system, which is spherical. We used three types of pore systems with different aspect ratios. The aspect ratios and pore geometry types could be determined using Xu-Payne (2009) cross plot and inversion. The sensitivity analysis was performed to examine the effect of CO2 injection on the elastic properties of the carbonate rocks. CO2 injection effects were mapped in 3D modeling using Sequential Gaussian Simulation function to better understand the possible CO2 impact over the entire reservoir.