Classification of drill-in fluids in coalbed methane formation based on research and application contents to Improve selection efficiency

Due to lack of systematic literature study on coal rock drill-in fluids for natural gas reservoirs, many coalbed methane producers overlooked suitable drill-in fluids as option, resulting in unsatisfied results in terms of research and application. Based on the research history of drill-in fluids in coal reservoirs and on-site requirements, 49 directly related literatures were obtained from over 100 international papers. According to the reported content, there are two types of drill-in fluids, namely research and application.Amongst of them, there are 31 literatures introducing laboratory research exclusive of field application, and 18 literatures with field application cases. According to the reported content, the literatures on coal rock drill-in fluid research and application can be further divided into three subdivisions: 17 papers on drill-in fluids controlling tough underground failures, 15 on drill-in fluids controlling formation damages, and 17 on drill-in fluids aiming at these two challenges above mentioned. There are 12, 10, and 9 research literatures, respectively, on these three types of fluids, as well as 5, 5, and 8 applied literatures, respectively. The key to drill-in fluid in coalbed methane formations is to ensure safe drilling and minimize formation damages. Literatures on drill-in fluids that can control downhole accidents and formation damages accounts for 34.7% (17/49), including 8 applied literatures, which further confirms that safe operations and maximizing resource extraction are the key to drill-in fluids selection. The findings indicate that the two major categories of research and application, as well as the three further subdivisions, classified by research content, provide theoretical research directions and application system reserves for the development of coalbed methane, especially deep coalbed methane.