Parameters selection for experiment on aqueous phase trapping damage in shale gas reservoirs

Abstract: Large amounts of fracturing fluid retained in shale gas reservoirs would result in formation damage through aqueous phase trapping (APT). The APT experiment is to evaluate the permeability reduction considering the occupation of aqueous phase in gas flow space. However, the setting of APT experimental parameters is still based on what is used for conventional sandstone gas reservoirs, which does not consider the shale characteristics. In this work, the main controlling factors of APT in shales are analyzed at first. It is shown that sensitive minerals, wettability, pore structure, initial water saturation and hydraulic fracturing scale are the main factors controlling damage degree of APT. Therefore, it is necessary to eliminate the interference of these factors from evaluation results of shale APT damage. In this work, the following four critical parameters of APT experiments for shale samples are determined as follows: (1) the brine salinity is determined as 5.5×104 ppm through the analysis of pore soluble salt and flowback fracturing fluid; (2) the pretreatment confining pressure is selected as 10 MPa through stress sensitivity measurement; (3) the displacement pressure difference is set as 0.22MPa/cm through velocity sensitivity measurement; (4) the backpressure is determined as 1 MPa through the gas slippage measurement. This work provides a workflow to set the experimental parameters and is beneficial for accurately evaluating APT in shale gas reservoirs.