Analysis on the difference of production characteristics and main factors for horizontal well in M coalbed methane field of Northern Bowen Basin

The coalbed methane is rich in Permian coal seams of the northern Bowen basin,but the degree of exploration and development is low.In order to select the favorable development area and to clarify the production characteristics and the main influential factors,based on the static and dynamic production data in M coalbed methane gas field,and according to the peak gas rate and gas rate curve of the development well,four gas production types are established.For typeⅠ,the peak gas rate is higher than 4×104 m3/d and the gas rate curve has a sharp rise-steep drop-slow decline trend.The peak gas is high and the peak gas time is short about three to seven months.The early decline phase has a high annual decline rate about 30%-35%,and the decline rate becomes slower in the later decline phase.For typeⅡ,the peak gas rate is 2×104-4×104 m3/d and the gas rate curve has a sharp rise-slow decline trend.The peak gas is medium and the peak gas time is relatively long about ten to twelve months,and the decline is relatively slow with annual decline rate about 8%-10%.For type Ⅲ,the peak gas rate is lower than 2×104 m3/d and the gas rate curve has a slow rise-steady trend.The gas rate is relatively low but steady,so there is no apparent decline.For type Ⅳ,the peak gas rate is lower than 1×104 m3/d with a discontinuous production.Moreover,the main influential factors for the above different production characteristics are specified.Firstly,under the control of coal seam depth and geological structure,the area with relatively shallow depth (smaller than 350 meters) and high permeability is the major region for the migration of desorbed and diffused coalbed methane,and it has a high peak gas rate,mainly in the west of the gas field with gas production type Ⅰ.As the increase of coal seam depth,in the depth range of 350 to 500 meters,at the lower location of the geological structure,the gas production type Ⅱ distributes in the middle of the gas field.When the depth of coal seam is deeper than 500 meters with permeability lower than 1×10-15 m2,the productivity is poor,and it lies in the east of the gas field with gas production type Ⅳ.Secondly,under the control of local low micro-structures,the peak gas rate is high,and the area is only in some region of the eastern gas field with gas production typeⅠandⅡ.Thirdly,with a poor caprock and the split thin or stoned-band coal seam,the area has a relatively lower peak gas rate and it is mainly in the north and south of the gas field with gas production type Ⅲ and Ⅳ.Based on the above analysis,the selected favorable area is located in the slope region with shallow depth and high structure of the northern Bowen basin，and the horizontal well has the priority.Meanwhile,the well pattern and spacing should be optimized with a reasonable production strategy to avoid the big fluctuation of the gas rate and to strive to reach a high and steady production.For the regions with a depth deeper than 500 meters,the gas resource is abundant and the fracturing technology should be carried out aiming for the breakthrough of the development for low permeability coal seams in relatively deep areas.