Damaging Mechanism of Tight Sandstone Gas Reservoirs and Optimization of Drilling Fluids for Reservoir Protection

The tight sandstone gas reservoir in the Block B in Tarim Basin has small pore throats, low permeability, high bound water saturation, severe heterogeneity and highly developed micro fractures. The reservoir formations are easy to be damaged and the formation damage is difficult to eliminate. To address these problems, tests on the high temperature sensitivity of reservoir formation and water block damage were performed under simulated formation conditions. The mechanisms of formation damage taken place in tight sandstone reservoir are revealed as follows:Reservoir formations are mainly damaged by stress sensitivity, water block and invasion of solids into the micro fractures in reservoir rocks. A temporary plugging agent and a surfactant that effectively reduce surface tension were selected through laboratory experiments for use in polymer sulfonate potassium drilling fluid, which is commonly used in the block of interest. A low damage drilling fluid with "synergetic action" in tight sandstone gas reservoir protection was developed using ideal packing theory and the surfactant selected. Evaluation of the drilling fluid in reservoir protection through compatibility test and dynamic contamination test showed that the drilling fluid is compatible with the formation water in Block B, and the percentage of rock core permeability recovery is at least 85%, indicating that the drilling fluid has good reservoir protection ability. The optimized drilling fluid has been used successfully on the well B2 in Block B in drilling the Bashijiqike formation. The skin factor of the reservoir rocks was greatly reduced after drilling, meaning that drilling fluid-induced reservoir damage was greatly reduced.