Testing of drilling mud filter cake for low permeability micro-fracture plugging performance in shale rocks

Abstract

Despite the current demand for shale gas development, there is no means to evaluate nanoscale microfracture plugging technologies for ultra-low permeability formations. In this paper, we expand upon previous research and develop new means to create artificial fractures in model mud cake. By controlling the mud cake quality and strength, we developed mud cake with permeability that approached that of real mud shale. Low-permeability mud cake was prepared from barite powder, calcium carbonate, polyacrylamide, sodium polyacrylate, water, silica, a polycarboxylate comb macromolecule, a double sparse inhibitor, a film-forming agent and a mud cake curing agent. The mud cake permeability reached 5.9 × 10–4 mD and increased with soaking time to 1.3 × 10–3 mD (240 h). The equivalent opening of a single fracture did not change greatly and remained in the range from 3.93 × 10–5 m to 4.93 × 10–5 m. Plugging performance was evaluated by simulating microfractures and low-permeability environment of the formation. This method provides very important guidelines for the selection and development of nanoscale and microscale plugging agents and the evaluation of the plugging effect. It is also applicable to the evaluation of plugging performance in other fractured formations.