**1. Introduction**

Compared with conventional oil and natural gas, Coal bed Methane (CBM) is a kind of highly efficient and clean energy which has obvious advantages in safety, economy and environmental protection [1,2]. China is rich in CBM reserves, although having less than Russia and Canada. However, due to the complex geological conditions of domestic CBM reservoirs and development technology is somewhat behind, China's CBM exploration and development is still in the initial stage [3–6]. The deep layer in the middle Hunan area is dominated by conventional natural gas exploration, but the development of CBM plays an important role in the shallow gas development [7–9].

In CBM development, it is very important to use proper drilling fluid system for safe, e fficient and environmental protection drilling. First of all, solid phase particles in drilling fluid tend to block the fractures and pores of the CBM reservoir, and then block the gas production channel. To reduce the intrusion of solid phase particles into CBM reservoir, it is advisable to drill with solid-free drilling fluid with good suspension performance [10–12]. It has the characteristics of low density, good suspension performance and small filtration and maximize the control of solid intrusion which can reduce reservoir leakage and damage and is a perfect for low density drilling fluid system [13–15]. Secondly, the reservoir is easy to absorb or adsorb liquids and gases. When drilling fluid invasions into CBM reservoir, it is easy to change its surface wettability, thus a ffecting the desorption and percolation of CBM, and thereby affecting CBM productivity [16–20].

Foam drilling fluid has a special network structure, which can carry cuttings well. Cai et al. [21] improved the foaming volume of foamed drilling fluid up to 50% by using chemically treated nano-SiO2 dispersions. By studying the foam's properties, Su et al. [22] found that the appropriate temperature was 40–100 ◦C and that the foaming performance of hard foam could maintain within 120 ◦C. Fractures and minerals a ffect the permeability of CBM reservoirs [23–28]. Cui et al. [29] combined FESEM with X-ray mu-CT together with EDS to quantify the mineral and fracture characteristics. Coal reservoir characteristics may be related to coal rank and maceral as well as mineral content [30]. Yang et al. [31] used NMR tests and go<sup>t</sup> the scattergram of coal pore size at a variety of strain rates. It was found that the crest value in the fractures rose and the crest value for the meso-macropores declined with the fortify of strain rates. The T2 spectrum obtained by NMR can be converted to the pore throat distribution. Adebayo et al. [32] mentioned T2 is connected with the surface relaxivity and the pores surface to volume ratio.

Huang et al. [33] mentioned that SWIA could adjust the core wettability. Li et al. [34] and Shen et al. [35] mentioned that the surfactant can increase the wettability of coal, and further enhance the desorption of methane; it can advance the recovery factor in theory and critical desorption pressure of CBM. Drilling fluid and coal surface contact, its pH value directly a ffects the wettability, further influencing the permeability of CBM. Through a lot of experiments to study the influence of drilling fluid pH value on the coal wettability, Zheng et al. [36] found that wettability and drilling fluid pH value is related, first reduced, then increased, and finally reduced. The above research results have played an important role in accelerating the development of CBM drilling fluid. However, there is a lack of special evaluation on the rheological, wettability of drilling fluid and the amount of additives for the practical characteristics of CBM drilling, and the relevant research lacks a systematic approach and depth.

Therefore, this study takes this as the starting point to analyze the rock-carrying capacity of the solid-free drilling fluid and study the influence of surfactant concentration and pH in the solid-free drilling fluid on the wettability of the CBM reservoir in central Hunan. This study optimizes the indoor evaluation method of the suspension performance of the solid-free drilling fluid, optimizes the formulation of the solid-free drilling fluid for the central Hunan CBM reservoir, and reveals the wettability mechanism of the central Hunan CBM reservoir when the solid-free drilling fluid penetrates. It is of grea<sup>t</sup> significance to optimize the formulation of the solid-free drilling fluid during the development of CBM in central Hunan, which further contributes to the rational development of CBM resources and the improvement of CBM productivity in central Hunan.
