Water block from hydraulic fracturing in low permeability rocks : experimental studies on causes and potential mitigation methods
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In the U.S., over half of the oil and gas production comes from hydraulically fractured wells in 2015; and the current trend is to inject more fracturing fluids (typically water) and proppants to create more complex fracture network and maximize the contact area with the formation. Hydrocarbon is mainly produced from the reservoir rock adjacent to the open fractures; therefore, any water left behind therein can block the flow of hydrocarbon and thus reduce the overall well productivity. In this study, it is proposed that matrix-fracture interaction is crucial to understanding the water block. Water can be retained in the matrix through this interaction, which is analogous to capillary end effect in laboratory measurements. This can be typically ignored in conventional reservoirs with long length scales and large pressure drawdown relative to capillary pressure. However, this should not be ignored in the fractured low permeability reservoirs where hydrocarbon production comes from short distances from the fractures and pressure drawdown is not significantly higher than capillary pressure. Additionally, water block in different wetting conditions needs to be studied so that mitigation methods can be wisely chosen to solve the right problem and enhance hydrocarbon production effectively and efficiently. This is experimentally achieved by using a three-step coreflood platform. This platform simulates the fracturing fluid invasion as well as the flowback occurring within the rock matrix adjacent to the fracture face. Under various mimicked reservoir/production conditions, regaining of rock permeability to hydrocarbon is obtained from measuring pressure drop versus time; this is further compared with the change of phase saturations in real-time either through flowback/effluent measurement or CT scans for the entire period of the coreflood. Based on the coreflood results, a more comprehensive understanding is achieved regarding the water block from fracturing in low permeability rocks. To mitigate water block, three major methods have been suggested based on field and/or laboratory studies. They are drawdown management, shut-in/soaking treatment, and surfactant or volatile additive treatment. Our experimental methods also provide a new avenue to compare the efficiency and effectiveness of various mitigation methods in different mimicked reservoir conditions, so that their governing mechanisms can be elucidated from the viewpoint of multiphase flow. For water-wet portion of the rock, matrix-fracture interaction dominates the early-time water block; the smaller the rock permeability, the longer its time-span. Once this interaction disappears, water block becomes the general form of capillary trapping. Shut-in/soaking is only effective on cleaning up the first form of water block and increasing the early-time production; however, it is unlikely to accelerate the spontaneous imbibition that mitigates such damage naturally, and shut-in does not increase the ultimate hydrocarbon production rate. Surfactant is very promising on cleaning up both forms of water block; among all tested formulations, the one generating Winsor type-I microemulsions with the mimicked reservoir oil shows the best performance. For oil-wet portion of the rock, water block is mainly in the general form of capillary trapping. Trapped water within pore bodies creates a more serious reduction on rock permeability to hydrocarbon comparing to the water-wet condition. To mitigate such water block, two typical surfactant treatments are mainly focused and compared, which are altering the rock wettability and achieving ultralow IFT with the reservoir oil. Synthesizing all the results, it is proposed that achieving ultralow IFT seems to be a better option for mitigating water block in oil-wet low permeability rocks.