Regularities of Fractured Tight Sandstone Gas Reservoirs Damaged by Work Fluids

The fractured tight sandstone gas reservoir in Block Keshen, Tarim Basin, is deeply buried, and has those features such as low porosity and permeability, developed fractures and strong heterogeneity. The reservoir, if not stimulated by fracturing operation, is of no commercial value. Contact of the reservoir with work fluids during drilling/completion operations and stimulation process results in reduction in formation permeability because of the invasion of the solid particles in the fluids into the formation, thereby causing decrease in the productivity of the reservoir. In laboratory experiments, reservoir cores with artifcial fractures were used to evaluate reservoir damage caused by drilling/fracturing fluids, and the width of the fractures was determined by imaging logging data of the reservoir. Experimental results showed that when the confning pressure was less than 4.5 MPa, the permeability of the cores with artifcial fractures, which has cubic relation with fracture width, remained constant. As the width of the fractures increased, the permeability damage caused by fracturing fluid decreased. However, permeability damage caused by drilling fluid frst increased and then decreased when the fracture width was increasing, showing a peak value of permeability damage. Furthermore, acid can be used to remarkably increase the permeability of the fractures, thereby removing damage caused by drilling/fracturing fluids. This study is useful in guiding the development of new low damaging work fluids and the optimization of reservoir protection measures.