Laboratory evaluation of viscous slickwater and its field application in Jimsar shale oil reservoirs

Unconventional oil and gas reservoirs require a large-scaled fracturing to form dense and effective fracture network, so as to achieve the economic development. Slickwater (i.e., water with the friction reducer) has been widely applied in this field. However, it has low viscosity and poor sand-carrying performance, which increases the risk of sand plugging during fracturing. As for friction reducers, although increasing its concentration in water can slightly increase the viscosity of the slickwater, but this can dramatically decrease the friction reduction rate of the slickwater, which turns out to be detrimental to the stimulationnew. New viscous slickwater (i.e., water with the viscous friction reducer) was developed that can solve the above problems. It can meet the requirements of different viscosity by changing the concentration of the friction reducer (from 0.1wt% to 0.8wt%). With a simple operation process and lower friction reducer, it can effectively maintain a high friction reduction rate, and increase the efficiency of proppant carrying and fracturing fluid. In this study, through a series of laboratory experiments (including the rheological performance tests, friction reduction rate tests and sand-carrying tests), the friction reduction and sand-carrying performance of one chosen viscous slickwater were evaluated, and further compared with the conventional slickwater was conducted. Experimental results showed that a continuous viscosity change in a wide range could be achieved by controlling the concentration of the new viscous friction reducer. The viscous slick water with a concentration above 0.6 wt% could maintain the viscosity more than 50 mPa·s even with a high shear. While there was less different between the conventional slick water and the viscous slick water with the concentration of 0.1wt%, the viscosity of both was lower than 5 mPa·s. The friction reduction rate of the 0.1wt% slickwater could up to 77.5% with the flow rate in a pipe of 11 m/s. In the sand-carrying tests, no sand-settlement was observed. This allows sands to transport further into the hydraulic fractures and be uniformly distributed. The further tests were carried out in two horizontal wells in Jimsar shale oil reservoirs. Field data showed that the friction reduction rate was 81% in the fracturing, and the maximum daily oil production rate was 90 t/d, and the cumulative oil production was up to 4000 t/d in 80 days, which is 4 times of the production with the conventional fracturing fluid.