Research And Application Of CBM Cementing Leakage Prevention Technology In Southern Yanchuan Area

ASa new clean energy sources,coal bed methane(CBM)is one of the important strategic alternative energy sources in China.The efficient extraction of CBM will not only effectively alleviate the shortage of natural gas supply in China,but also provide guarantee for the safe mining of coal resources which has great development potential and value.However,CBM is chracterized by low permeability of matrix,low pore pressure,natural fractrures development and low mechanical strength,which brings a series of technical problems for drilling,completion,and exploration for CBM.Spontaneously,the cementing in CBM also faces great challenges.As the abundant cleat system in coal rocks,fluid leakage is very likely during cementing.As a result,the integrity of CBM wellbore is seriously weakened,and the reservoir reconstruction measures such as hydraulic fracturing are greatly restricted.Therefore,for the sake of safe,effective and long-term exploration of CBM,the anti-leakage cemmenting technology based on geological characteristics of coal rockhas became an urgent problem to be resolved.In the south area of Yanchuan,which is a typical CBM block in China,the leakage of coalbed methane wells is mainly caused by fractures and cracks.In this paper,the low-cost low-density cement slurry system is mainly considered for the leaky and fractured coal seam.In terms of reducing agent,the low-density slurry system with lower cost fly ash reducing material as reducing agent is selected from natural floating beads,artificial floating beads and fly ash reducing materials.Through the early strength evaluation of conventional density system and low density system,the D4 fly ash with smaller particle size and narrower particle size distribution has better activity.The fine fly ash fills the voids in the cement grains,improves the compactness and compressive strength of the cement paste,and the early strength of the low density cement slurry system reaches more than 8 MPa within 48 hours at low temperature,which meets the requirements of field application.According to formation pressure data,in order to reduce annulus fluid column pressure,reduce leakage during cementing operation and reduce pollution of coal seam caused by cement slurry,a binary cement slurry system of low density lead slurry and conventional tail slurry was adopted to optimize the density of cement slurry,and the final density of cement slurry was 1.60g/cm 3 and 1.85g/c respectively.M3.The modified fly ash low density cement slurry system was optimized and its performance was evaluated.The results show that the new low-density cement slurry system has better settlement stability.In terms of thickening characteristics,the thickening time of the new low density cement slurry is obviously affected by the early strength agent(activator)under the experimental conditions of 45?×10MPa×25min.The thickening time of the new low-density cement slurry is about 210-305min when the temperature is 45 degrees,and it can also be adjusted.The successful implementation of the anti-leakage cementing technology in the south of Yanchuan provides a good application example and reference for the improvement of CBM cementing technology in China.