The damage mechanism of oil-based drilling fluid for tight sandstone gas reservoir and its optimization

Abstract

The B block is a typical tight sandstone gas reservoir in Tarim basin in the Xinjiang Uygur Autonomous Region, China. In this block, the skin factors of each well drilled by oil-based drilling fluids were high and the reservoir damage was serious. The permeability recovery rates of two kinds of oil-based drilling fluids used in site were both less than 60%. To investigate the damage mechanism, the particle size distribution was tested to analyze solids damage incorporating the pore throat radius and micro-fracture width in this block. The contact angles of deionized water on reservoir cores before and after polluted by oil-based filtrate were measured to analyze the wettability alteration. Core flow experiment was applied to evaluate oil phase trapping, and the emulsion droplets formed by mixing oil-based filtrate with formation water were observed to analyze emulsion plugging damage. Based on the test results, the factors causing damage to the tight sandstone gas reservoir included solid phase invasion to fractures, wettability alteration, oil phase trapping and emulsion plugging.Therefore, the oil-based drilling fluids were optimized from aspects of reduction of the filtration loss, improvement of the sealing ability of the micro-fractures and reduction of the oil-water interfacial tension. A temporary plugging agent was compounded by using the Ideal Packing Theory, which could effectively plug the micro-fractures. The surfactant was optimized, and it could destroy the oil-based filtrate cake effectively and break emulsion in a short time. The permeability recovery rate of optimized oil-based drilling fluids was evaluated and the results showed that permeability recovery rate was 87.40% and 86.74%. Furthermore, cores early contaminated by oil-based drilling fluid were treated by optimized surfactant, and the tested permeability recovery rate reached more than 100%.