Waterflooding performance of partially naturally fractured reservoirs

Abstract

Waterflooding is a well-established method frequently used in the oil industry to improve oil sweep efficiency and enhance ultimate recovery. Even though this method has been extensively scrutinized in homogeneous and naturally fractured reservoirs (NFRs), it is not fully understood in partially naturally fractured reservoirs (PNFRs) due to many complicated factors that affect fluid flow behavior in these reservoirs. This paper presents a thorough investigation of waterflooding performance in PNFRs. In this study, fracture intensity (FI) is considered the main parameter characterizing PNFRs. An FI value of 0% represents a homogeneous reservoir whereas an FI value of 100% represents a fully NFR. An advanced 3-D black oil simulator in dual-porosity/dual-permeability mode was used to scrutinize the effects of gravity, capillary, and viscous forces on waterflooding performance under a full spectrum of FI values (0–100%). A comprehensive sensitivity analysis was conducted at very high, high, intermediate, low, and very low water injection rates. Dimensionless parameters such as FI, recovery factor (RF), and water/oil ratio (WOR) were used for characterizing and predicting the waterflooding performance. In addition, the effect of gravity on co-current and counter-current imbibition phenomena has been investigated. The results of this research have demonstrated that FI values above the percolation threshold of 31.2% create high vertical permeability conduits which enhance the gravity effect and reduce the capillary pressure holdup and lead to water underride and early water breakthrough time. Furthermore, at high FI values, the viscous forces decrease in favor of counter-current imbibition phenomenon. Conversely, in low FI reservoirs, the waterfront shape approaches piston like displacement, and the counter-current imbibition phenomenon decreases due to limited fractures connectivity. Under these circumstances, the gravity and underride effects become negligible. The findings of this work have also illustrated that the water injection rate is not FI sensitive for FI values below the critical value (FI < 31.2%). In this case, high and low water injection rates yield a stable front and similar breakthrough time. At the critical FI value, the competition between viscous, capillary, and gravity forces becomes evident. Under this condition, the waterfront is piston like displacement and the breakthrough time is delayed. Furthermore, the findings of this work have demonstrated that the water underride phenomenon is not a function of FI only but also a function of water injection rate. At low water injection rate, as FI increases the severity of water underride increases. Conversely, at high water injection rate, the water underride decreases, and the breakthrough time is delayed. This work also illustrates that despite the existence of gravity underride and early water breakthrough time; the highest oil recovery is attained at the lowest water injection rates for FI values above the critical value at water pore volume injected (PVinj) higher than one.