Study on the main controlling geological factors of high yield deep CBM in Southern Yanchuan Block

The Southern Yanchuan Block belongs to a deep, high-rank coalbed methane(CBM) reservoir. Due to the influence of geological conditions, the single well productivity varies greatly. Researching on the main controlling geological factors and their changing mechanism of CBM accumulation, combined with the dynamic data of CBM development, the main controlling geological factors of the CBM well enrichment and high production are analyzed. The research results show that the productivity of the gas field is controlled by the geological factors of “tectonic, hydrology, coal structure”. Tectonic restricts the accumulation of CBM; the degree of salinity characterizes affects the preservation of CBM; and the coal structure restricts the reformation of the reservoir. The high-yield wells are mainly located in the wing parts of the local micro-uplift belt with a buried depth of 800 to 1 200 meters where the formation water salinity is 30 000 to 100 000 mg/L, with thickness of primary-fractured coal greater than 2.5 m, and the daily gas production is greater than 1 000 m³. The middle-production wells are located in the gentle tectonic in the western part of Wanbaoshan with a buried depth of more than 1 200 m, where the water salinity is greater than 100 000 mg/L, with a daily gas production volume of 500-1 000 m³; The low-yield wells are mainly distributed in the fault-sag zone near the middle level Ⅲ faults and local level Ⅳ faults where the salinity is less than 3 000 mg/L, with thickness of primary-fractured coal less than 2.5 m, and the daily gas production is less than 500 m³. The plane change of the block productivity confirms that the tectonic is the main controlling factor for the high production of deep CBM. Under the condition of underdeveloped tectonic activity in deep CBM reservoirs, the permeability of the reservoir is extremely low with the poor rebuildability, so it is difficult to obtain high production. The enhancement of tectonic activity achieves the goals of improving the reservoir, and the formation of fissures near the axis of the local anticline increases the permeability of the reservoir. Therefor, it is beneficial to CBM enrichment and reservoir reformation, and local small faults form micro-fractures, which are conducive to the desorption and seepage of CBM. However, the faults with violent tectonic activity and the vicinity of the depression zone form CBM escape channels, which has a negative effect on the enrichment and high yield of CBM.