Study on Methods of Evaluating Performance of Shale to Adsorb Fracturing Fluid and Swell

In large-scale SRV fracturing in shale gas development, flowback rate is generally 10% to 60%, resulting in a mass of water that is trapped in the reservoir. Using XRD analysis and IR spectrometry to study the swelling property of shales taken from Zhaotong, Changning and Weiyuan. Gravimetric analysis was used to characterize the water absorption mechanisms and pore generation by dissolution. These methods reveal the state of existence of different working fluids in shales. The experimental results show that the clay contents of the shale samples are 18%-20%, mainly chlorite, illite and some illite/smectite mixed layer. The shale samples were soaked with distilled water, slick water, water solution of cleanup additive and water solution of swelling inhibitor for 1 d, 3 d, 5 d and 7 d. There is no obvious change at 1 nm on the XRD graph. IR spectrometry shows that there is no absorption peak for interlayer water. The volume of absorbed water and rate of absorption are proportional to particle size and size of pore throat. Percent of matters dissolved out of shale samples through ultrasonic soaking is about 0.6%, mainly KCl and NaCl, with minor amount of CaCl₂. Specific surface area measured with BET method shows that pore generation is not obvious. A core with particle size of 0.154 mm was soaked in fresh water, and the sizes of pore throat were decreasing with time. The amount of water absorbed by the core was 2.37%-2.85% at saturation, equivalent to the volume of pores of the core. Using the width and height of the peak at 1 nm on the XRD graph as indicators, and ratio of interlayer water peak value over Si-O peak value on IR spectrometry graph as verification, a method for the evaluation of clay swelling property is established. Evaluation of cores taken from south Sichuan shows that the soaking of shale in working fluid does not cause clays to swell. Shales taken from south Sichuan have lower contents of soluble salts, hence the pore generation by the working fluids is not obvious. The working fluids can enter into the nanometer-sized micro pores in time is enough. The working fluid will occupy the micro pores, but will not enter into the spaces between crystal layers of clay, and will not cause the permeability of formations to be impaired by swelling.