Research Of Permeability Changes In Coal And Gas Wetting Reversal Method To Enhance Coalbed Methane Production

Absolute permeability is used to describe the flowing capacity in rock, and also a critical parameter that determine the coalbed methane production. However, the situation is different for coalbed because of its unique adsorption ability. The coal matrix permeability is very low, so the fluid flowing capacity is mainly depend on the natural fracture. Methane will undergo desorption, diffusion and flow stage during production, accompanied by complex stress changes on coalbed formation. Therefore, change rule of coalbed permeability can’t be described as traditional method. The research of coalbed permeability change rule is crucial for production study and forecast. The method of enhance methane production includes hydraulically fracturing, carbon dioxide injection and pinnate horizontal multilateral well, however, above methods has many defects and its increasing production mechanism seems to be not suitable for Chinese coalbed. Based on above analysis, the research with enhance methane production is meaningful to improve single well production and recovery, and the main work is as follows:Based on the predecessor’s achievement, influences of effective stress, matrix shrinkage and slippage effect on coalbed permeability were expounded. In addition, experiments were conducted to investigate the change rule of coalbed under different influencing factor, such as effective stress, pore pressure and absorbed gas type.Features and shortage of commonly used coalbed permeability calculation method were discussed in this paper. According to the force analysis, the concept of swelling stress was introduced in the vertical direction and effective stress of coalbed was corrected. A new coal permeability model with consideration of swelling stress was developed for uniaxial strain conditions. The new model and current models were evaluated using logging data from San Juan Basin and published data from laboratory tests. Model comparing results shows that new model is verified to be more precise in coal-permeability prediction. In addition, our research shows that the prediction result is larger than the real permeability value without consideration of swelling stress.Based on the investigation of current methods of enhance methane production and recovery at home and aboard, the pros and cons of each method are summarized. The article suggests enhancing methane production and recovery by alternating the wettability of coalbed. The mechanism of gas wetting reversal method to enhance coalbed methane production is as follows:in the process of water displacement by gas, coalbed wettability is changed from water-wet into gas-wet and capillary pressure is changed from resistance to power, which will promote the flow of fluid and enhance coabed methane production and recovery.According to the measurement of contact angle of natural coal and artificial coal before and after wettability alteration, gas reverse wetting agent can change the wettability of coal, in other words, the water wettability will be decreased or water-wet will be changed into gas-wet.Based on imbibition experiment, the variation of contact angel leads to the variation of capillary pressure and great decrease in the amount of imbibition water. Gas-water relative permeability curve and feature points are obtained by experiments of water displacement by gas. The gas phase relative permeability values was increased after the wettability of the coal core was altered from preferential water-wet to gas-wet by the chemical treatments, especially under residual water saturation. In addition, the residual water saturation decreases after wettability alteration in the process of water displacement by gas. The experiment shows that gas wetting reversal method can enhance coalbed methane production and recovery.