Abstract
The objective of this study is to develop a technique to identify the optimum water-soaking time for maximizing productivity of shale gas and oil wells. Based on the lab observation of cracks formed in shale core samples under simulated water-soaking conditions, shale cracking was found to dominate the water-soaking process in multi-fractured gas/oil wells. An analytical model was derived from the principle of capillary-viscous force balance to describe the dynamic process of crack propagation in shale gas formations during water-soaking. Result of model analysis shows that the formation of cracks contributes to improving well inflow performance, while the cracks also draw fracturing fluid from the hydraulic fractures and reduce fracture width, and consequently lower well inflow performance. The tradeoff between the crack development and fracture closure allows for an optimum water-soaking time, which will maximize well productivity. Reducing viscosity of fracturing fluid will speed up the optimum water-soaking time, while lowering the water-shale interfacial tension will delay the optimum water-soaking time. It is recommended that real-time shut-in pressure data are measured and shale core samples are tested to predict the density of cracks under fluid-soaking conditions before using the crack propagation model. This work provides a shut-in pressure data-driven method for water-soaking time optimization in shale gas wells for maximizing well productivity and gas recovery factor.
Highlights
Multi-fractured shale gas/oil wells with high water recoveries after hydraulic fracturing are normally low-productivity wells, while those with 10% to 40% water recoveries are normally high-productivity wells
The result presented in this paper demonstrates that the cracking of shale rocks during the “soaking” period is responsible for further stimulation of shale rocks in the fractured regions and for increasing well productivity
The shale cracks developed in soaking process provide flow paths gas/oil to flow from shale matrix to hydraulic fractures, which is expected to improve for well inflow gas/oil to flow while from shale matrix hydraulicfluid fractures, is expected to improve well inflowclosure of performance, the loss of to fracturing into which the cracks results in the partial performance, while the loss of fracturing fluid into the cracks results in the partial closure of hydraulic fractures, which reduces well inflow performance
Summary
Multi-fractured shale gas/oil wells with high water recoveries after hydraulic fracturing are normally low-productivity wells, while those with 10% to 40% water recoveries are normally high-productivity wells. The water loss into the formation does not hinder the gas/oil flow into the wellbore. Ehlig-Economides and Economides [1] attributed this effect to the, “water as proppant”. Some shale gas and oil wells undergo month-long shut-in times after multi-stage hydraulic fracturing well stimulation. Such shut-in episodes increase the gas and oil flow rate. The mechanism behind it is not well understood
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