Study On Annular Flow Performance Of Coal-bed Methane Pinnate Horizontal Wells' Under-balanced Drilling

Coal-bed methane (CBM), of which the dominant ingredient is methane, is the source of coal-mine gas explosion. It will pollute the environment if discharged to the air directly because the greenhouse effect of the methane is 20 times larger than the carbon dioxide. Besides, the resource of CBM is abundant in our country, which amount to nearly 36810 billions cubic meters, and the nature gas delivery line from the Western China to the Eastern China goes by six main CBM enrichment areas such as Ordos Basin and Qinshui Basin, so it is worthwhile to explore and develop the CBM.Pinnate horizontal well technology, a new under balanced drilling technology, has been recently researched and used in enhancing the coal bed methane recovery of low permeability reservoirs. By using this technology, it could implement the under balanced drilling in coal seams, so as to reduce the damage of coal seams. In this paper, the gas-injection models for the pinnate horizontal wells'under balanced drilling are dual- wellbore models, namely a horizontal well drilling in the coal seams for developing CBM while a vertical well injecting the gas after the two wells connected. The gas injected by the vertical well would decrease the density of the fluid so as to decrease the horizontal well's bottom hole pressure. Thus, it could implement the horizontal well's under balanced drilling in coal seams. There are only two phase in the horizontal annular from bit to the gas injection point, but three from the point to the wellhead just due to the gas injected. In this paper, with the establishment of physical model, the optimization of mathematical models and the field case, the annular flowing parameters of pinnate horizontal wells under different gas injection conditions are calculated and analyzed. The relationships between the annular flowing parameters and the well depth are also elaborated. At last, the optimal gas injection and drilling mud flow rate for the under balanced drilling in coal seams are also proposed in the paper. By comparing the calculated results with the field drilling parameters, the reliability of the established physical models, the selected mathematical models and the optimized results are proven true.