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In this work, the methane desorption isothermal curves at different water contents on deep sampled from Western Chongqing of China were measured at pressures up to 65 MPa and at 130°C by the volumetric method. In the first instance, the desorption increases with the decrease of pressure, the adsorbed gas desorbs slightly with decreasing pressures from 65 to 30 MPa. When the pressure drops to 30–20 MPa, the desorption rate increases rapidly with the decrease of pressure and the desorption curve begins to separate from the adsorption curve, resulting in desorption hysteresis. At last, when the pressure is lower than 20 MPa, the desorption increases almost linearly with the further decrease of pressure, but eventually there will be some adsorbed gas which cannot be desorbed to form residual adsorbed gas. After that, the isotherm desorption data of CH4 was fitted using the improved desorption model. The fitting results showed that the improved desorption model can be used to describe the desorption process of deep shale gas containing water and has a strong applicability. In addition, the critical desorption pressure increases with increasing water content. When the water content is lower than 1%, the effect of the water content on the desorption of deep shale gas increases rapidly with increasing water content, as well as when the water content is greater than 1%, the impact changes slowly. [ABSTRACT FROM AUTHOR]

A large portion of slickwater is trapped in gas shale formations after the hydraulic fracking process. To address the effect of slickwater retention on methane desorption, the adsorption and desorption isotherms for methane before and after slickwater treatment were measured based on the volumetric method using shale samples from the Longmaxi formation, China. We found the adsorption and desorption data for the methane before and after treatment fit well with the Freundlich model. We compared the effect of slickwater volume on desorption isotherms, desorption efficiency, adsorbed methane content and cumulative gas production. Slickwater treatment significantly reduces methane adsorption. The Freundlich constant k reduction decreases nearly linearly with the slickwater volume, and constant n increases nearly linearly first and then becomes slower with slickwater volume. Test results also reflected that slickwater treatment could improve methane desorption efficiency. The curve of desorption efficiency after treatment is significantly lower than before. Under 30 MPa pressure, the desorption efficiency increases by 50.93% when slickwater volume increases to 10.40 ml from 2.02 ml. A closed loop exists between two curves of adsorbed gas content before and after treatment, and its area increases with slickwater volume. Slickwater treatment improves cumulative gas production. The cumulative production of methane before treatment is smaller than after. 8.16 ml of slickwater is the best option for 130 g of shale powders to obtain the most cumulative gas production in the lab-scale condition. These results can be used to optimize the slickwater volume during hydraulic fracturing. • Methane adsorption/ desorption isotherm can be well fit by Freundlich modell. • Slickwater promote methane desorption from gas shale powders. • Slickwater volume significantly affects the desorption isotherm model and desorption efficiency. • Slickwater volume has an obvious impact on adsorbed methane content and cumulative gas production. [ABSTRACT FROM AUTHOR]

During and after multi-stage hydraulic fracturing, the spontaneous imbibition of water-base fracturing fluid in shale formation is considered as the main mechanism responsible for the retention of large amounts of fracturing fluid. Slickwater is widely applied in Fuling shale gas field, the largest shale gas field in China. To characterize the spontaneous imbibition of slickwater and its four major components, experiments were carried out by using modified imbibition cells. Results show that all of the imbibition curves can be divided into three stages: linear imbibition stage, transition stage and stable imbibition stage. Their imbibition capacities are in the order of cleanup additive, clay stabilizer, slickwater, defoamer and friction reducer from highest to lowest. We have found that the imbibed volume is larger than the initial pore volume. Nuclear Magnetic Resonance (NMR) tests, core images and porosity comparison before and after imbibition all indicate the secondary microfractures were created during the imbibition process. The calculated volume of secondary microfractures which is the difference between the pore volume before and after the imbibition is very close to the imbibed volume, indicating the imbibition mainly through the microfractures. We also found that the imbibition rate of slickwater and its components in the order from highest to lowest is cleanup additive (0.087 cc/d), slickwater (0.058 cc/d), clay stabilizer (0.035 cc/d), defoamer (0.022 cc/d) and friction reducer (0.014 cc/d), indicating the synergy effect of slickwater components on the imbibition process and the role to optimize the cleanup additives during slickwater design. • Spontaneous imbibition tests were performed with modified imbibition cells. • The imbibition characteristics of slickwater and its major components were investigated. • All of the imbibition curves can be divided into three stages in this study. • The secondary microfractures were created during the imbibition process. • The imbibition is mainly through the microfractures. [ABSTRACT FROM AUTHOR]