Effect of Multi-Stage Hydraulic Fractures on Performance of Naturally Fractured Tight Oil Reservoirs with Bottom-Water

Abstract

Abstract Water coning is a major concern in any oil reservoirs with bottom-water. Stimulation techniques such as multi-stage hydraulic fracturing in a horizontal well can accelerate water coning, but uncertainty prevails over the increased recovery under bottom-water support. This paper studies the effect of hydraulic fracture (HF) design properties combined with the matrix and natural fractures properties in tight reservoirs on water-cut and recovery. Ultimately, the results can be applied in deciding the feasibility and design parameters of multi-stage hydraulic fracturing in these reservoirs. The paper presents a 3-D Cartesian simulation model, which is run for different fracture half-length, fracture height, HF conductivity, natural fractures permeability and matrix permeability, to represent wide variety of hydraulically fractured reservoir systems. Dual porosity-dual permeability (DPDP) model is used to simulate natural fractures inside water-wet reservoir matrix. In a constant pressure bottom-water reservoirs, where the reservoir pressure remains more or less constant and so there is a minimal change in effective stresses with production time, any effect of these stresses on the variation of fracture conductivity or productivity can be ignored. Results show that although there is a severe water-cut problem associated with hydraulic fracturing a tight oil reservoir with bottom-water, recovery increase is much higher than that without bottom-water, demonstrating the viability of the project. Based on the simulation results on the sensitivity of HF conductivity on water-cut and recovery, lower HF conductivity with smaller fracture width should be preferred which would further reduce the cost associated with the proppants required. Higher matrix permeability enhances the ultimate recovery, whereas tighter zones brings more water-cut problem. Further, studies showed that HF half-length should not be too low to result in low recovery (at10 years) or should not be too high to give rise to low ultimate recovery. Operators should be concerned about the height of HF, as they should not allow it to propagate beyond the upper oil-zone either to transition-zone or water-zone that can severely mitigate the recovery rate as well as ultimate recovery with severe water-cut problems. This study which is based on sensitivity analysis, can be used to design the fracture half-length, height, and conductivity in bottom-water-tight reservoirs. The study can provide a boost to operators who are reluctant to hydraulically fracture a bottom-water reservoirs.