SUMMARYThe wettability behavior of the matrix block is one of the major factors controlling the effectiveness of the employed EOR methods in NFRs. Water injection in NFRs with mixed-wet or effectively oil-wet matrix blocks usually results in low oil recoveries. In this case, gas injection is considered to be an alternative process, where the process benefits from the gravity forces and the process is called gas-oil gravity drainage. In this study, the effect of matrix wettability on the efficiency of gravity drainage by CO2 injection is addressed. Laboratory experiments and numerical simulation were performed to analyze the process under different wettability conditions of the matrix. It is concluded that for a system with an effectively oil-wet matrix, water is the most non-wetting phase while CO2 is the intermediate-wetting phase. In the three phase setting, which includes carbon dioxide, this is considered favorable for oil production. However, with a strongly water-wet matrix, CO2 is always the least wetting phase. For this condition, it turns out that when water is displaced by the gravity drainage process part of the oil is also produced. It is observed that higher oil recoveries are achieved by CO2 injection in an oil-wet matrix block.

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SUMMARYMiscible solvent injection has received increasing attention in recent years as an efficient method to improve oil recovery from fractured reservoirs. Due to the large permeability difference between fracture and matrix, the success of this method depends to large extent on the degree of enhancement of the mass exchange rate between the solvent flowing through the fracture and the oil residing in the matrix. A series of experiments have been conducted to investigate the mass transfer rate between the fracture and the matrix. Different scenarios have been considered to examine the effect of flow rate, matrix permeability, fracture aperture, and oil properties. To this end a porous medium (fully saturated with oil) is placed in a vertical core holder that can be used in a CT scanner, to simulate the matrix. A small slit between the porous medium and the core holder simulates the fracture. The interaction between the matrix and fracture is visualized for solvent flooding by means of CT-Scanning, which can be used to validate theories of enhanced transfer in fractured media. The experimental data are compared with a simulation model that takes diffusive, gravitational and convective forces into account.

Carbon dioxide injection, either miscible or immiscible, has been known as an efficient strategy to enhance oil recovery as well as reduce CO2 emission. Wettability has a significant effect on the performance of enhanced oil recovery techniques because of its effect on fluid saturation distribution and flow behavior in porous media. For oil-wet rock, CO2 must overcome a capillary barrier to invade the rock matrix in order to be able to displace the oil in a secondary drainage process. If the rock wettability alters from oil-wet to gas-wet, a positive value of capillary pressure is established. Therefore, the injected CO2 will spontaneously imbibe from the fractures into the matrix blocks and oil will be expelled. This study describes the contact angles, i.e., wettability, in systems with water, an oil-saturated rock, carbon dioxide and/or synthetic flue gas. Experiments were carried out at pressures varying between 0.1 and 16.0 MPa in a pendant-drop cell that can determine bubble shapes and contact angles. Two situations are considered: rock-system I is partially water-wet, whereas rock-system II is effectively oil-wet. The experiments show that contrary to results obtained previously with CO2 (Ameri et al. 2013), flue gas cannot become the wetting phase.