Tight sandstone reservoir sensitivity and damage mechanism analysis: A case study from Ordos Basin, China and implications for reservoir damage prevention

Analysis of reservoir sensitivity to velocity,water,salt,acid,alkali and stress is critical for reservoir protection.To study the tight sandstone reservoir sensitivity at different formation depths(effective stress)and formation water conditions(pH,salinity,and fluid velocity),a series of dynamic core flow tests under different pH,salinity,acid,and effective stress conditions were performed on samples from tight sandstone reservoirs of the Upper Triassic Yanchang 8(T_(3)y^(8))Member and conventional reservoirs of the Middle-Lower Jurassic Yan'an 9(J_(1-2)y^(9))Member in the Ordos Basin.The results indicate that,compared with the conventional reservoirs,the tight sandstone reservoirs are more sensitive to velocity and stress,less sensitive to water,alkali and salinity,and respond better to acid fracturing.In addition,the critical conditions(salinity,velocity,pH,and stress)for pumping drilling,completion,and fracturing fluids into tight sandstone reservoirs were investigated.A combination of scanning electron microscopy coupled with energy-dispersive spectrometry(SEM-EDS),cathodoluminescence(CL),casting thin section(CTS)and nuclear magnetic resonance(NMR)images,high-pressure mercury injection capillary pressure(MICP)measurements as well as X-ray fluorescence spectral(XRF)analyses were employed to analyze the damage mechanisms of the conventional reservoirs(J_(1-2)y^(9))and tight sandstone reservoirs(T_(3)y^(8))caused by fluid invasion.The results suggest that reservoir sensitivity is primarily conditioned by the composition of detrital components and interstitial fillings,petrophysical properties,pore-throat structure,and diagenetic facies.All these factors control the sensitivity types and extent of the reser-voirs.Our results indicate that the poorer the reservoir physical properties,the stronger the reservoir heterogeneity and sensitivity,implying that tight sandstone reservoirs are more susceptible to changes in fluids than conventional reservoirs.This study offers insights into the reservoir damage types and helps to improve the design and implementation of protection measures for tight sandstone reservoir exploration.