Reasons for the low flowback rates of fracturing fluids in marine shale

摘要

In this paper, marine shale cores taken from Zhaotong, Changning and Weiyuan Blocks in South China were used as samples to investigate the interaction between fracturing fluids and shale and the retention mechanisms. Firstly, adsorption, swelling, dissolution pore, dissolution fluid mineralization degree and ionic composition were experimentally studied to reveal the occurrence of water in shale and the reason for a high mineralization degree. Then, the mechanisms of water retention and mineralization degree increase were simulated and calculated. The scanning electron microscopy (SEM) analysis shows that there are a large number of micro fractures originated from clay minerals in the shale. Mineral dissolution rates of shale immersed in ultrasonic is around 0.5–0.7%. The ionic composition is in accordance with that of formation water. The clay minerals in core samples are mainly composed of chlorites and illites with a small amount of illites/smectites, but no montmorillonites (SS), and its content is between 18% and 20%. It is verified by XRD and infrared spectroscopy that the fracturing fluid doesn't flow into the space between clay mineral layers, so it can't lead to shale swelling. Thus, the retention of fracturing fluids is mainly caused by the adsorption at the surface of the newly fractured micro fractures in shale in a mode of successive permeation, and its adsorptive saturation rates is proportional to the pore diameters. It is concluded that the step-by-step extraction of fracturing fluids to shale and the repulsion of nano-cracks to ion are the main reasons for the abrupt increase of mineralization degree in the late stage of flowing back. In addition, the liquid carrying effect of methane during the formation of a gas reservoir is also a possible reason. Based on the experimental and field data, fracturing fluid flowback rates and gas production rates of 9 wells were analyzed. It is indicated that the same block follows an overall trend, namely, the lower the flowback rates, the more developed the micro fractures, the better the volume simulation effect and the higher the gas production rates.