Experimental study on microscopic formation damage of low permeability reservoir caused by HPG fracturing fluid

Hydraulic fracturing has become an important stimulation technique for low/ultra-low permeability reservoirs. Research on formation damage caused by fracturing fluid has mainly focused on permeability, sensitivity, fracturing fluid and clay minerals. However, little attention has been devoted to the microscopic changes in pore structure and properties. In this paper, nitrogen gas-adsorption techniques were introduced to characterize the microscopic changes in pore structure. Meanwhile, contact angle testing and Scanning Electron Microscope (SEM) observations were also used to show the changes in wettability and surface morphology of cores, respectively. The results showed that the pore size distributions (PSDs) of low permeability cores were wide, with approximately 75% macropore volume. The obvious changes in PSDs of damaged samples occurred in the range of 2-5 nm and 30-80 nm. The specific surface area decreased, and the adsorbability of damaged samples declined, as well. Concentrated gel and solid residues were confirmed to be the main damage mechanisms for gel breaking liquids, which resulted in changes in composition and roughness, making the cores more hydrophilic. Based on the results of the analysis, the N2 adsorption technique is deemed appropriate for the study of reservoir formation damage when a certain amount of mesopores are present. Additionally, a new method of determining contact angles for porous media is also recommended.