Development and application of dense multi-cluster fracturing in horizontal wells for low permeability and low pressure coal reservoir

Coal reservoirs are geologically complex and gas-related coal mine incidents and disasters are getting worse in China. as Also,some underperformance coalbed methane (CBM) wells are widely distributed. Effective gas production rate enhancement from low permeability and low pressure CBM seams is one of the key technical challenges for the CBM industry and coal mine safety. In this study,a novel technology was developed and high gas production rate was achieved in No. 3 coal seam of Shanxi Formation in Daning coal mine,Qinshui basin,Shanxi province. The out- comes of this study include four key points. Firstly,the No. 3 coal seam is recognized as a typical low permeability and low pressure reservoir with low production in the studied coal field. Based on the observed data from six testing wells, the average permeability was estimated only 0. 112 mD,and the reservoir pressure gradient was 3. 7 kPa / m. Average gas production rate was 362 m3 / d from 120 vertical wells,and average gas production rate was only 800-1 000 m3 / d from a multi-lateral horizontal wells. Secondly,the horizontal well with multi-clusters fracturing technology was recom- mended for increasing gas production in this area. The key technical parameters were numerically optimized through a numerical modeling using COMET3. The optimized cluster spacing was found to be 15 -25 m,and four clusters in a single stage fracturing were recommended. Fractures with 64. 1 m semi-length can be effectively induced through pum- ping the well (in one stage) fluid of 1 000 m3 ,sand of 50 m3 with an injection rate 10 m3 / min. Thus,the coal seam gas content can be reduced to 8 m3 / t at the middle of two clusters fractures end after 3. 85-5. 95 years production. Thirdly,a U-shape horizontal wells named LDP-02 was completed for a pilot trail. The pilot horizontal well with total length of 1 445. 6 m connected with a directional well. The horizontal section in coal seam was 920 m and was fully cased and cemented. The hydraulic fracturing was completed for the coal seam section of 920 m with 38 clusters in 10 stages. Each stage was designed as 3-4 clusters and cluster spacing was as close as about 11-23 m. Finally,this pilot well reached a continually stable daily gas production rate as high as 16 000 m3 / d and an average of 12 955 m3 / d de- pending a drainage period of 893 days and gas production period of 687 days. The cumulative gas production was 890 ×104 m3 . This production was 20-36 folds of vertical wells and 10 times of the multiple lateral horizontal wells in the same area.