**4. Conclusions**

The performance of solid-free CBM drilling fluid in central Hunan have been examined. The results show that TX-10 and HSB1618 enhance the rheological properties of drilling fluid, such as yield point and gel strength. It was found that the minimum API filtration is only 0.3 mL. Experimental results show that the suspended cuttings capacity is successively from large to small: basic fluid + HSB1618, basic fluid + TX-10 and basic fluid + penetrant T in the drilling process of CBM reservoir in central Hunan. With the increase of surfactant concentration, the suspended cuttings capacity of basic fluid + HSB1618 and basic fluid + TX-10 increase. The residual amount of basic fluid + HSB1618 in the sample is the smallest. The longitudinal saturation changes from large to small was basic fluid + TX-10, basic fluid + penetrant T, basic fluid + HSB1618 and the water.

With the increase of surfactant concentration, the wettability of the three drilling fluids to samples increases in different degrees. When the surfactant concentration is 0.15%, the dynamic contact angle of the three drilling fluids fluctuates greatly, which greatly improves the water wettability of the samples. The basic fluid + permeant T has the best ability to improve the wettability of the reservoir surface in the drilling process of CBM reservoir in central Hunan. The hydrophilicity of samples decrease when the basic fluid + HSB1618 is at pH 8–10, while that of samples increase at pH 10–12.

In the process of CBM development in central Hunan, the solid-free drilling fluid formula 3%KCl + 0.1%Na2CO3 + 0.1%HSB1618 + 0.2%XC + 0.1%PAC has strong suspension performance and large moistening contact angle, which is helpful for the flowback of drilling fluid, and can reduce the residual amount of drilling fluid in CBM reservoir, and finally reduce the damage to reservoir. Only three kinds of surfactants are discussed in this paper, and the application of cationic surfactants in solid-free drilling fluid can be studied in the future. In the follow-up studies, it is planned to carry out comparative studies of CBM reservoirs in multiple regions and study the acoustic and mechanical properties of reservoir samples before and after the action of solid-free drilling fluid.

**Author Contributions:** Conceptualization, P.S., M.H. and H.C.; Methodology, P.S., H.C. and S.Z.; Software, M.H. and J.Z.; Validation, P.S., W.L., M.H. and J.Z.; Writing—Original Draft Preparation, P.S., M.H., S.Z. and H.C.; Writing—Review and Editing, P.S., M.H., S.Z., W.L., and H.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This study was supported by Hunan Provincial Innovation Foundation for Postgraduate (No. 2019zzts162), the National Natural Science Foundation of China (No. 41602372), the open fund of the State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation (Southwest Petroleum University, No. PLN201609 & No. PLN201607) and the fund of Ministry of Land and Resources Key Laboratory of Drilling Technology in Complex Conditions (No. DET201612).

**Acknowledgments:** All of the authors would like to direct their thanks to the professors and students in the Key Laboratory of Metallogenic Prediction of Nonferrous Metals and Geological Environment Monitoring (Central South University), Ministry of Education.

**Conflicts of Interest:** The authors declare no conflict of interest.
