Analysis of factors affecting productivity of CBM in horizontal wells in southern Qinshui Basin

In the past 20 years,coalbed methane industry in China has steadily achieved commercial development,but the bottleneck of industrial development has become increasingly prominent: the proportion of inefficient wells is large,the production rate is low,and the scale of storage and production is far from the expected pla nning goals. In order to find out the factors affecting the productivity of coalbed methane wells,the appropriate geological selection evaluation,engineering process mea sures and drainage management and control technology were studied. Focusing on the practice of large-scale production of horizontal wells in the southern Qinshui Ba sin,based on the analysis of geological characteristics and gas well production performance,combined with structure,engineering factors and drainage management,the main controlling factors affecting the productivity of CBM horizontal wells in this area are discussed. The countermeasures and suggestions to improve the productivity of horizontal wells were put forward to achieve large-scale and efficient development of coalbed methane horizontal wells. Studies have shown that: affected by the gra vity differentiation of gas and water,horizontal wells in the high part of the structure produce gas earlier and produce faster,and the effect is significantly better than that of horizontal wells at low positions;the upturned horizontal wells in low positions produce gas later and produce more water with slow production; affected by the hetero geneity of permeability,the correlation between the effective length of the horizontal section in the area and the peak production is weak,but the sidetracking can easily connect with adjacent water layers and cause local wellbore stress concentration,resulting in large water production in gas wells and pulverized coal and other issues; t here is a close relationship between the continuity of drainage and gas production. Long and frequent stoppages will cause frequent disturbances in reservoir pressure, destroy the continuity of reservoir pressure reduction,and affect the rate of recovery of gas production and the overall effect. In addition,the excessive pressure reductio n rate in the early stage of drainage can easily cause pressure sensitivity and velocity sensitivity effects, which will damage the permeability of coal reservoirs,and then affect the expansion of the pressure drop funnel of the gas reservoir and the overall pressure reduction range, and ultimately affect the gas-water production effect. Ther efore,it is recommended to optimize the location of horizontal wells and increase the drainage intensity of low-level horizontal wells; and regular annulus water injection can be employed to dilute the bottom pulverized coal,reduce the frequency of pump shutdown and maintenance,and improve the continuity of drainage and make the rat e of pressure drop in the early stage of drainage within 0.02 MPa/d.