A new type of solid-free and acid-soluble drilling fluids suitable for carbonate reservoirs

In the Daniudi Gas Field of the Ordos Basin, there are developed microfractures in the Lower Paleozoic carbonate reservoirs. The solid phase of the potassium ammonium-based drilling fluid can easily enter the microfractures to damage reservoirs, and can be hardly removed. In practical exploration and development, the return permeability after acidizing is only 51.40%, having a direct influence on natural gas exploration and development in this area. In view that the carbonate reservoir of the fifth Submember of fifth Member of Majiagou formation (O1m55) in the Daniudi Gas Field has a low clay content, good wellbore stability and unsatisfactory stimulation effect after completion and acid fracturing, natural polymer treatment agents such as acid soluble carboxymethyl cellulose sodium salt and high-temperature starch are selected, and together with bactericides and lubricants, a new kind of low-density, solid-free and acid-soluble drilling fluid was prepared. Ccompared with potassium ammonium-based drilling fluid, the low-density, solid-free and acid-soluble drilling fluid has the characteristics of solid free, acid soluble and fast flowback, with actual drilling density decreasing by 0.04–0.06 g/cu cm and solid content less than 2%. Also, it is well compatible with reservoirs, so as to effectively prevent and reduce the damage of drilling fluid to reservoirs in the drilling process. The polymer acid solubility of this drilling fluid is 95.45%, the mud cake acid solubility is 100%, and the return permeability is increased 43.89% higher than that of potassium ammonium-based drilling fluid, which is favorable for the dredging and recovery of microfractures as natural gas seepage channels in the Lower Paleozoic carbonate gas reservoirs.