Rheology Study and Quantification of Dynamic Leak-Off Coefficient of Slickwater for Tight Oil Reservoirs
Authors: Wei Yun , Zheng Leyi , Wang Ziqiang , Liu Sai , Wang Bin , Zhang Xing , Liang Tianbo 
Published: September 2023

ABSTRACT
Fracturing fluid carries proppants to hydraulic fractures, and the performance of fracturing fluid is the key to hydraulic fracturing. The conventional slickwater has a large friction reduction rate of over 70% that allows a high pumping rate during fracturing. The new viscous slickwater has a higher viscosity that enhances the proppant-carrying capacity while maintaining a large friction reduction rate, and it has been widely used in the field. However, the increase of slickwater viscosity can form a filter cake that affects the leak-off of fracturing fluid and hinders the closure of propped fractures. In this study, after rheological properties of the conventional and viscous slickwater are studied, their friction reduction rates and leak-off coefficients are quantified in the lab and from field operation curves. Laboratory measurements show that at high shear rates, the 0.4% viscous slickwater can maintain a viscosity of above 10 mPa·s; its leak-off coefficient is about 5 times lower than that of the 0.1% conventional slickwater according to corefloods under in-situ CT scans. Calculation results from field tests agree with the above results, which ensures the performance of the viscous slickwater and can further guide the optimization of shut-in treatment in the field.

INTRODUCTION
Hydraulic fracturing is a technique used to enhance the production and recovery rate of unconventional reservoirs, such as shale formations, which are typically characterized by low permeability, high flow resistance, and poor connectivity (Xu et al., 2004). This involves injecting fracturing fluid, a pressurized fluid, into the wellbore to create and extend fractures in the rock, which facilitates the flow of oil and gas into the well (Zhou et al., 2019). One of the significant challenges of hydraulic fracturing is efficiently delivering the fracturing fluid to the target formation, which usually includes the flow of viscous fluid, creation of solid surfaces, and leak-off of fracturing fluid (Emmanuel et al., 2016). The high pressure and complex geometry of the wellbore and fracture network lead to significant leak-off, which reduces flow velocity, increases the cost, and prolongs the time required to complete the operation (Yang et al., 2016). To overcome this, various additives have been developed to reduce leak-off and enhance the ability of the fluid to carry proppants and other solids into the fractures. However, the fracturing fluid can also leak off into the surrounding rock under pressure, which accumulates in the formation, makes it challenging to form wider fractures, and there is a potential for well productivity impairment due to the loss of fracturing fluids into the rock matrix (Sharma & Agrawal et al., 2013). Currently, slickwater is the most widely used fracturing fluid system. However, it has several limitations, including low viscosity and poor sand-carrying capacity. To address these issues, the viscous slickwater has been developed. It is a fracturing fluid that exhibits different viscosities at different concentrations, which allows it to maintain relatively high viscosity even at high shear rates (Liang et al., 2022). This property reduces leak-off and enhances the ability of the fluid to carry proppants.