Multi-index synergistic optimization of drilling-fluid inhibition and reservoir protection based on RSM and the entropy weight method

The hydration and dispersion of clay minerals in deep coal reservoirs can readily induce wellbore instability, whereas the adsorption and retention of certain treatment agents on coal surfaces and within pore throats may impair reservoir permeability. In this study, deep coal reservoir rocks from the Ordos Basin were investigated. The mineral compositions and microstructural characteristics of the coal and associated gangue were characterized, and potassium chloride (KCl), polymeric alcohol (SPI), and a polymeric compound (TC-DBY) were selected as the key treatment agents. An entropy-weighting method was used to integrate the rolling recovery, linear swelling, and permeability recovery rates into a comprehensive inhibition–reservoir protection index (IRPI). Response surface methodology was then employed to establish a model relating the dosages and interactions of the treatment agents to the IRPI. Based on this model, a formulation optimization map was developed, delineating three formulation domains: strong inhibition–weak reservoir protection, balanced inhibition–reservoir protection, and weak inhibition–weak reservoir protection. The results indicated that the response surface model predicted an optimal formulation containing 5.914% KCl, 3.421% SPI, and 0.563% TC-DBY. Among the investigated interactions, the interaction between TC-DBY and SPI exerted the strongest effect on the IRPI. Three representative formulations selected from the optimization map exhibited rheological properties, lubricity, plugging performance, and system stability that satisfied the laboratory evaluation criteria. In particular, the balanced inhibition–reservoir protection formulation achieved a rolling recovery rate of 86.3%, a linear swelling rate of 1.17%, and a permeability recovery rate of 88.6%, demonstrating a favorable balance between inhibition and reservoir protection. During field application, the strong inhibition–weak reservoir protection system was applied to coal-gangue interbed intervals, whereas the balanced inhibition–reservoir protection system was applied to coal-seam intervals, thereby enabling interval-specific regulation of drilling-fluid performance. This strategy shortened the drilling cycle by 15.89%, reduced the downhole complication rate to 3.5%, and increased initial production by 18.72%. These findings provide a quantitative basis for evaluating drilling-fluid performance and optimizing formulations for different intervals in deep coalbed methane horizontal wells.