Impact of Well Shut-in After Hydraulic-Fracture Treatments on Productivity and Recovery in Shale Oil Reservoirs

Abstract

Water invasion, associated with hydraulic fracturing, often causes hydrocarbon-mobility hindrance, known as water blocking. The effect on productivity is largely dependent on saturation profiles inside fractures and formation matrix. Enhancement to hydrocarbon recovery has been reported in some field cases after shutting in wells for long time periods. Here we conduct numerical-simulation studies to investigate the effect of well shut-in on initial productivity and long-term recovery. We replicate post-fracturing conditions with an extensive fracture network that intermeshes with formation matrix. The models are designed using either logarithmically spaced, locally refined grid or embedded discrete fracture model (EDFM). Starting with varying initial fluid distributions, we compare productivity and recovery of two cases: one that does not start production until after 32 days of shut-in, and another that starts immediately without soaking. For the initial conditions that favor shut-in, we carry out case studies in attempt to find the ideal shut-in conditions for maximum recovery improvement. Results confirm improvement in early productivity after shut-in for all the considered initial fluid-distribution cases. The majority of cases exhibit net gain in total oil recovery. We report improvement in recovery of as much as 5%, owing to spontaneous imbibition. Imbibition of the injected water into formation matrix causes fluid redistribution and favored mobility for the non-wetting phase, and hence enhanced hydrocarbon productivity. The simulations take into account spontaneous imbibition and gravity segregation, but do not consider geo-mechanical forces, water adsorption or chemical reactions. When capillary forces are neglected, well productivity and recovery decrease, even when the well is not shut in. Such observations underline imbibition counter-current flow as an important production mechanism that should not be neglected in shale-oil-reservoir simulations.

Experimental and Modeling Study of Water Imbibition and Flowback in Shale: Prediction of Relative Permeability and Capillary Pressure Curves

“…The well shut-in can increase gas flow rates while decreasing water flow rates in water-wet rocks. 62 Eltahan et al 63 investigated the impact of well shut-in on initial productivity and long-term recovery using numerical simulations. They concluded that injecting water into the formation matrix causes fluid redistribution and encourages non-wetting phase mobility, hence increasing hydrocarbon productivity.…”

Insight into the Methods for Improving the Utilization Efficiency of Fracturing Liquid in Unconventional Reservoirs

“…Well shut-in for a specific time after hydraulic fracturing is the technical way to improve the efficiency of liquid utilization. Shut-in time after fracturing is an important factor influencing the utilization efficiency of fracturing fluid [34][35][36]. In this study, the dualmedium single-stage fracturing model was used to study the utilization law of fracturing fluid during shut-in after fracturing and the effect of shut-in time on the efficiency of fracturing fluid utilization.…”

Study on Double Pressure Funnels and Gas-Liquid Two-Way Mass Transfer after Fracturing in Shale Gas Reservoirs

“…As shown in complex fracture (considering nature fractures) system, it has large drainage area and highdrainage efficiency, therefore the cumulative gas and water production will be larger than simple fractures (without considering nature fractures) system. Chen et al [24] used the structured grid near the fracture to simulate the unified flowback of multilayer fracturing; Eltahan et al [25] conducted 3D-reservoir simulation of complex fractured network that intermeshes with formation matrix and studied well shut-in impact on the productivity and recovery of shale oil reservoirs; Wijaya and Sheng [26] conducted a flow-geomechanical model to study the relationship between imbibition and well shut-in; it shows only if imbibition dominated during recovery in shale reservoir, well shut-in tends to improve both the ultimate oil recovery and net present value (NPV); Tao et al [27] designed a water spontaneous imbibition apparatus, though clay mineral content measurement and salt ion concentration diffusion experiment, the optimal shut-in time for type I and type II shale reservoir are 20 days and 15 days, respectively. Wu et al [28] proposed a two-phase flow model considering formation damage caused by clay expansion and gave a semianalytical-solution model based on Laplace transform, but this method is mainly used for production data analysis and prediction of tight gas wells.…”