Research On Secondary Stimulation Technologies Of Coalbed Methane Stripper Wells In A Block Of The Eastern Ordos Basin

The development of coalbed methane(CBM)has experienced more than 40 years in China,and more than 22,000 coalbed methane wells have been constructed.Although it has formed two CBM industrial bases in the Qinshui Basin and Ordos Basin,the commercial production of CBM wells is still less than one-third.The CBM stripper wells account for about 70% in a block of the eastern Ordos Basin,and the CBM development has been seriously hindered.Therefore,it is imperative to explore a set of effective secondary stimulation technologies for the stripper wells.For this purpose,the geological and engineering factors for affecting CBM production are systematically discussed,the main factors leading to low yield are analyzed in this paper,through the method of literature research,laboratory test and numerical simulation.On this basis,the corresponding secondary stimulation technologies for the stripper wells are put forward,and the application effect is evaluated.The main conclusions are as follows:(1)Through the statistical analysis of the CBM stripper wells in the research area,it is found that the main geological factors affecting the CBM production are resource abundance and ground stress,while the engineering factors include hydraulic fracturing and drainage control.First,the CBM resources abundance in some areas is insufficient.Secondly,the overall medium-high stress zone is the mainstay,and more than 90% of the wells adopt conventional hydraulic fracturing methods of continuous perforation,low displacement and fluid volume in the coal seams,resulting in poor stimulation effect and serious reservoir damage.In addition,some CBM wells have the problem of too fast or discontinuous drainage,which induces serious reservoir stress sensitive and velocity sensitive damages,meanwhile,the fracturing fluid flowback rate is generally low,indicating the reservoir water blocking damage is serious.(2)According to the division of the causes of CBM stripper wells,it is proposed to change the active water fracturing fluid into the low damage fracturing fluid to mitigate reservoir damages;change the two-stage support agent into a multi-stage support agent to support the multi-scale fractures;change the conventional hydraulic fracturing into large-scale fracture network stimulation to improve the overall permeability of the reservoir;change the CBM development into the integrated development of coal measure gas to maintain the conductivity of fractures and increase the amount of recoverable resources in the well control area;change the single-stage fracturing into multi-stage fracturing to promote fracture diversion and increase the volume of fracture network.On the basis,the adaptability of the secondary stimulation technologies for different types of stripper wells are proposed.Specifically,for the resource-controlled stripper wells,the large-scale seam network remouldfracture network stimulation of coal seam and coal measure gas integrated development can be implemented according to the differences in resource abundance and coal seam space distribution.For the ground stress-controlled stripper wells,facture network stimulation of the surrounding rock can be adopted to construct more stable fluid migarion channels.For the fracturing-controlled stripper wells,the largescale fracture network stimulation can be implemented for coal reservoirs to improve the overall reservoir permeability.In addition,for the drainage controlled stripper wells,it is only necessary to unblock and reopen the fractures in the coal seams.Furthermore,all stripper wells should adopt low damage fracturing fluid and multi-stage proppants to mitigage reservoir damages and support multiscale and multi-class fractures,also,the drainage rate should be controlled to mitigate stress and velocity sensitive damages.(3)For the stripper wells of different genesis types in the study area,the resource-controlled(MCQ-01 well),fracturing-controlled(CBM-02 well)and the drainage-controlled stripper wells(MCQ-07 well)were selected,representatively,and the corresponding secondary stimulaiton technologies as well as the stimulation designs were proposed.Also,numerical simulation was used to analyze the fracture extension before and after the secondary stimulation of the resource-controlled and fracturing-controlled stripper wells,and the stimulation effect was pre-evaluated.The results showed that the semi-long,the width of the seams,the height of the seams,and the total surface area of the hydraulic fractures created by the secondary stimulaiton were greater than those of the first stimulation,indicating that the effect of the secondary stimulation was better.