Study and application of stimulation technology for low production CBM well through high pressure N2 Injection-soak

In China,coal reservoirs are complex and under-performance CBM wells are widely distributed. To develop a novel theory and technology for stimulating these wells again for increasing gas rate is very important for CBM enterpri- ses and the industry. After a long time water drainage and gas production,the reservoir is characterized as low gas pressure and low water pressure,even in dry condition. For re-stimulating these low productive CBM wells meanwhile protecting the sensitive low pressure or dry reservoir, a nitrogen ECBM technology named nitrogen injection-soak (NIS) was developed and tested in two CBM wells located in Yuwu coal mine of Luan mining area,Shanxi Province, China,and some desired results were obtained. The objective reservoir coal No. 3 has low pressure and low permeabili- ty,and the gas production is generally low in the study area. The two experimental wells LA-011 and LA-016 started pump and production in 2008. Four years production result shown that the average gas rate was only 31 m3 / d and 20 m3 / d. The two wells have been re-stimulated with hydraulic fracturing in 2010,but no desired result reached. The reservoir pressure gradient of the two wells before the current experiment was only 1. 0 kPa / m,which were two typical low pressure and low productivity wells. The stimulation trial of NIS was carried out in October 2012. The nitrogen amounted to 34 800 m3 and 44 960 m3 were pumped into LA-011 and LA-016,and shut-in times was 92 h and 112 h respectively,then the wells were opened for drainage when the wellhead pressure was lower than 1. 0 MPa. During the period of NIS,the casing pressure of adjacent CBM wells was measured for monitoring the nitrogen migration direction in the coal seam. After the NIS experiment,the wells were continually in production for 1 to 3 years. The experiment result shown that:① The casing pressure increase of adjacent wells during injection stage indicates that NIS has a re- gional stimulation effectiveness,which clearly indicates the migration direction of high pressure nitrogen. The influ- enced area is mainly controlled by current in-situ stress that highly influenced by water drainage and gas production. ② The NIS has regional production improvement,that is “ one well stimulated,multiple wells improved”,or 2 wells stimulated and 5 wells improved. ③ The gas rate of the 2 NIS wells increased 1. 2-8. 9 times,as well as 3 adjacent wells increased 1． 4-3． 7 times. It is concluded that N2 ECBM is an advanced technology for improving gas rate in un- der-performance CBM wells,and can be applied in other CBM wells in China.