Determining diffusion coefficients for CO₂ injection in oil saturated chalk using a constant volume diffusion method

This paper presents a novel technique to determine multi-component diffusion coefficients for CO₂ injection in a North Sea Chalk Field (NSCF) at reservoir conditions. Constant volume diffusion (CVD) method is used, consisting of an oil-saturated chalk core in contact with an overlying free-space, which is filled with the CO₂ . The experimental data are matched with an EOS-based compositional model. Transport by diffusion controls the dynamics of the constant-volume system, together with phase equilibria, allowing a consistent estimation of diffusion coefficients needed to describe the observed changes in system pressure. We conduct two experiments at reservoir condition: one utilizes a core plug saturated with live-oil, and the other with stock tank oil (STO). Once the experiments are completed, EOS-based compositional simulation is performed to match the experimental data using the oil and gas diffusion coefficients as history matching parameters. The modeling work is conducted with a commercial reservoir simulator using a two dimensional radial grid model to describe the experimental setup. The experiment utilizes a vertically-oriented core holder with a height of 92 mm and 37.6 mm in diameter. An outcrop chalk core with a sealing sleeve is mounted in the core holder, which has the same diameter and a height of 64.6 mm, thus resulting in an overlying void space. The system is initially saturated with oil at reservoir condition. CO₂ is then injected from the top, forming an overlying CO₂ chamber, and displacing oil towards the bottom of the core holder. Once CO₂ fills the overlying bulk space, the system is isolated with no further injection or production. The CO₂ and oil reach and remain in equilibrium locally at the gas-oil interface throughout the test, initiating and maintaining the diffusion mechanism. Diffusion of CO₂ into the oil results in a decreasing pressure, which is the main history matching parameter. The multi-component diffusion coefficients are found to match the model pressure-time prediction to the experimental data. This suggests the modelling workflow incorporates a representative EOS model and the main transport dynamics controlled by diffusion are being treated properly. The two main challenges in the modeling are (1) the limitation on setting an appropriately-high permeability for the CO₂ chamber, and (2) the reservoir simulator neglects compositional dependency of diffusion coefficients. Proper simulation of CO₂ injection in fractured chalk reservoirs requires the ability to model multicomponent diffusion accurately. The proposed CVD-method provides such modeling capabilities. Our modeling and experimental work indicate the novelty of the CVD method to determine the diffusion coefficients of a system where diffusion is the dominant displacement mechanism. The fact that the oil is contained within a low-permeability chalk sample reduces density-driven convection that could result due to non-monotonic oil density changes as CO₂ dissolves into the oil.