Acid-Soluble Drilling Fluid in the Northern Carbonate Reservoir of the Yishan Slope in the Ordos Basin

Abstract

The carbonate reservoir in the northern Yishan slope of the Ordos Basin presents significant challenges to gas field exploration and development. With its low pressure, limited porosity, low permeability, and abundance of micro-fractures, the reservoir is resistant to acid dissolution. Once solid particles block these fractures during drilling and completion, serious reservoir damage ensues. To address these obstacles, we engineered an acid-soluble, solid-free drilling fluid system in the lab. This involved incorporating sodium carboxymethyl cellulose, heat-resistant starch, fungicides, and lubricants. Contrasted with the commonly used potassium ammonium based drilling fluid system, our innovative solution showed notable improvements. Specifically, the density decreased by 0.04 to 0.06 g/cm3, and the solid content decreased by 4.0% to 6.50%, while the acid-soluble rate surged from 8.50% to 95.45%. In addition, the reservoir permeability recovery value saw an increase from 51.50% to 95.88%. In practical field application, we employed this novel drilling fluid system in ten horizontal wells. Following acid fracturing and reconstruction, these wells registered a 75.94% increase in gas production compared to nearby wells. Our findings demonstrate that the proposed system effectively mitigates the incursion of solid-phase particles into the reservoir while enhancing acidification during acid fracturing. This results in the swift removal of plugging, restoration of formation permeability, and improved well production. Our research thus introduces a drilling and completion fluid system of high efficiency with superior reservoir protection performance, potentially offering substantial benefits to the development of carbonate rock salt gas reservoirs.