Study On Underbalanced Drilling Technology Of DFS-M85 Cbm Wells

Coal-bed gas is a clean and efficient energy source, China has rich coal resources, and coalbed gas reserves is also very abundant, its exploitation can reduce coal mine disasters, increase energy supply, meanwhile, reduce the pressure of environmental protection. however, due to the characteristic of " three low" for coal reservoirs in our country, coal-bed methane faces the difficulty of high-efficient mining, conventional overbalance drilling can cause the formation damage, this affected the gas production seriously, thus the widespread use of underbalanced drilling technology maturity for the oil industry in coal bed gas mining would be significant.The coal-bed methane resources in the studied area(Binchang Dafosi) is abundant, but always with the method of conventional overbalance drilling vertical well drilling, results in great pollution to the reservoir, low coal bed methane production and incensement of mining cost. To the present situation of area coal rock characteristics and coal bed methane development in Dafosi mining area, in the present work, we studied the DFS- M85 coal bed methane well group, The main study work is shown as follows:(1) underbalance technology adaptability in the study area;(2) the determination of under voltage value;(3) the implementation and control of the underbalance method and actual application effect. Combined with theoretical analysis and field application, in the study of technology adaptability, the underbalance adaptability evaluation was carried on formation pressure, fluid and the characteristics of mineral rock sensitive for the study area, and then the effect of underbalance adaptability evaluation on the studied coal bed was analyzed. Finally the overall technology economy analyses were performed. The results show that the features of coal and rock strata in the studied area are suitable to use aerated underbalance drilling way for coal bed methane exploration, and this method is economical, available. In order to ensure the stability of borehole wall in the process of determining under voltage value, a model was built to calculate the collapse pressure, which results in the limit under voltage value to ensure the safety of drilling; the minimum design values of the bottom negative pressure was obtained according to horizontal section length and the effect of annulus pressure loss on negative pressure value. Then the stress sensitive reservoir damage was analyzed, the optimal combination of injection of gas and liquid flow was calculated by software, the core design of under voltage value was completed for under-balanced drilling, in addition, we also elaborated the implement method of underbalanced drilling process. Eventually the DFS-M85 – H Wells level docking DFS- M85- V Wells and infusing air in vertical wells was adopted to achieve weight loss of drilling fluid, which can reach the bottom of underbalance.The rationality of the design and good results for real drilling is verified by field application, DFS- M85 well group finished drilling safely and smoothly,, and we completed drilling design in high quality. Compared to the adjacent overbalance drilling, gas production increased 5-9 times, the effect of yield promotion was significant. The successful application of under-balanced drilling technology in DFS- M85 well group not only prove the feasibility of underbalance drilling technology on the research area, but also shows the obvious advantages of this technology on the economic rationality, it has a great value to application.