The main focus of this paper is to present experimental and simulation results that describe carbon dioxide (CO2) injection in a chalk sample with fracture/matrix interaction at reservoir conditions. On the basis of the experiments, simulation models were built to mimic the main transport phenomena, including diffusion, which was found to be particularly important.
The first experiment consisted of a vertically oriented Sigerslev outcrop chalk core, where a single “fracture” was represented by a centralized hole along the core. Both matrix and fracture were initially saturated with a North Sea stock-tank oil (STO) at reservoir conditions. Once the initial conditions were established, CO2 was injected from the top of the fracture and the oil was produced from the bottom.
Injected CO2 diffused into the oil in the matrix and swelled the oil. Once the oil in the fracture was drained, the matrix fed the fracture with oil at decreasing rates. The first experiment lasted up to approximately 24 pore volumes injected (PVinj). The second experiment is similar, but laboratory oil n-C10 was used instead of STO. Laboratory oil and CO2 have very similar densities at the chosen reservoir conditions, which minimizes gravity-driven convective (Darcy) transport and maximizes the effect of diffusion.
Our modeling was conducted with a compositional reservoir simulator. We developed and used a tuned equation-of-state (EOS) model that accounts for proper estimation of the phase and volumetric properties for CO2 mixtures in the STO and n-C10 systems. Automated history matching was used to fit the experimental data. A commercial reservoir simulator could reproduce laboratory results adequately.
Numerical simulations were conducted to match experimental oil-production data by tuning the oil- and gas-diffusion coefficients. Good agreement between the numerical model and the experimental data was obtained. For the n-C10 system, we found that the results were not sensitive to vertical permeability, confirming displacement was dominated by diffusion rather than convective flux.
Verifying the accuracy of modeling the diffusion-dominated processes in a fractured chalk system with CO2 at reservoir conditions has been accomplished. The lesson learned from the experimental and modeling work flow obtained from this study becomes an important step toward modeling an actual fractured chalk/reservoir-oil system undergoing CO2 injection.