Experimental and numerical investigation of tertiary-CO₂ flooding in a fractured chalk reservoir

This paper describes tertiary CO₂ recovery mechanisms observed in two distinct core flooding experiments. In both experiments, water flooding followed by CO₂ injection into an outcrop chalk with matrix-fracture system at reservoir conditions. The objective of this paper is to study the main mechanisms that control the rate of oil recovery during tertiary-CO₂ flooding. A series of experiments as well as compositional numerical simulations have been carried out to evaluate the efficiency of the process. In the first experiment (Exp-1), the fractured core is initially saturated with North Sea Chalk Field (NSCF) stock tank oil (STO) and the connate water. Whereas in the second experiment (Exp-2), the core is saturated with NSCF live-oil and the connate water. Once the reservoir condition is established (258 bara, 110 °C), sea water is injected from the bottom of the fracture and the oil is recovered from the top. Once no more oil recovery is observed, the water flooding (WF) is stopped. A “shut-in” period follows which allows preparing the rig for the CO₂ flooding (CF). Right after the “shut-in”, CO₂ is injected from the top of the fracture and the oil is produced from the bottom. A compositional reservoir simulator with a tuned equation-of-state (EOS) is utilized to model the experimental work. An automated history matching algorithm is developed to fit the experimental data of WF and CF periods. Good agreement between the model and experimental data is obtained. We observe a strong impact of hysteresis on fitting the fluid production during CO₂ flooding in Exp-2. The sensitivity analysis shows that tertiary CO ₂ recovery is affected by the water saturation in the core after the secondary WF. Moreover, the fracture-matrix transport function during tertiary-CO₂ flooding is dominated by the diffusion rather than the convective flux or viscous forces. The outcome of this work is an important step towards modeling the tertiary-CO₂ flooding in an actual fracture-chalk system. Proper modeling of imbibition and diffusion dominated processes in a chalk system at reservoir conditions has been accomplished.