Force Balance Analysis and Efficient Measures to Improve Vertical Sweep Efficiency in Oil-Wet Carbonate Reservoirs

摘要

Many carbonate reservoirs in Middle East under water or gas injection exhibit poor or limited vertical sweep efficiency due to channeling of the injected fluid through high permeability streaks (high-k streaks) and/or gravity segregation. In this paper, causes of the limited sweep efficiency and its possible mitigation measures are discussed through conceptual reservoir simulation and the force balance (FB) analyses of their results. The analyses are based on convention-diffusion-gravity (CDG) formulation, and applied to five-spot waterflood, line drive waterflood and line drive gas flood schemes to provide insights to improve reservoir development strategy. In waterflood either in five-spot or line drive scheme, the injected water moves fast in a high-k streak in oil-wet reservoirs where the effect of convection decreases rapidly and the diffusion has no or limited contribution to diverting the water from the high-k streak to neighboring layers, leading to early water breakthrough and poor sweep efficiency. Gravity force works to slump the water into the high-k streak, thus accelerates water breakthrough especially in case the high-k streak is located at the bottom of the reservoir. For such reservoirs, the practical mitigation strategy will be to delay the decline of the convection effect. Mobility control is the effective mitigation measure in this regard. In-depth conformance control is another mitigation measure in line with this strategy. If the blocking is implemented sufficiently away from the wellbore and with sufficient blockage width, it can make similar level of positive impact on waterflood performance as that expected by mobility control with minimal impairment of injectivity. In gas flood under unfavorable mobility ratio, gravity override is the key risk even in homogeneous reservoirs. Gas breakthrough is further accelerated in case the high-k streak is located in the upper part of the reservoir. In such reservoirs, the principal mitigation strategy will be to reduce the relative effect of gravity force in addition to increasing the gas front saturation. Enhancement of the viscosity of the injectant will realize both, thus lead to significant improvement of vertical sweep and oil recovery. On the other hand, in-depth conformance control will have positive but limited impact on gas flood performances. Considering the study results mentioned above and the state-of-the-art of the related technologies, mobility control is considered to be the principal solutions to the poor vertical sweep both for waterflood and gas flood, and in-depth conformance control can be another cost effective solution for waterflood in the oil-wet carbonate reservoirs associated with high-k streaks.