Study On Liquid Nitrogen Fracturing Technology In Coalbed Gas Wells

Hydraulic fracturing is an important method to reconstruct coalbed methane(CBM) reservoirs. CBM reconstruction technology of China is based on the development theory and experience of medium and low rank CBM reservoir, which is poorly adaptive to high rank CBM reservoir. Therefore, we are often faced with the problem of low well production during the development process. With poor porosity and permeability, during the stimulation treatment, high rank CBM reservoir should be increased coal reservoir fragmentation degree to enhance its desorption capacity and permeability. In this way, CBM well production can be significantly increased. Based on the concept of improving coal reservoir fragmentation degree, this paper provides liquid nitrogen fracturing technology for CBM wells. Based on the previous research work of CBM well liquid nitrogen fracturing stimulated mechanism, technique and related fields research, this paper carried out the following studies:temperature field calculation, the influence on coal rock mechanics, coal permeability, formation stress and coal rock stress sensitivity induced by liquid nitrogen injection, the following conclusion are obtained:(1) On the basis of investigation and analysis on the reservoir characteristics of Southern Qinshui Basin high rank reservoir, the technical advantage of liquid nitrogen fracturing and economic feasibility of liquid nitrogen as a fracturing fluid, it is concluded that liquid nitrogen fracturing technology is feasible for CBM wells. At the same time, technological process and five coal reservoir damage patterns of liquid nitrogen fracturing is purposed.(2) Wellbore temperature field model of liquid nitrogen tubing injection and nitrogen annulus of tubing and casing injection was established, then the temperature distribution of wellbore at different conditions was obtained by Matlab program. A model considering multiple fractures near wellbore, cleats and formation fluids phase change was developed by CMSOL Multiphysics. On this basis, the temperature distribution of near wellbore and fractures was calculated.(3) According to the results of experiments at different freeze-thaw temperature range, permeability variation behavior before and after thaw-freeze cycle of dry and saturated coal samples was obtained. At the same freezing time, the larger freeze-thaw temperature range, the higher permeability raised rate, and saturated coal samples have higher raised rate than dry coal samples.(4) Tensile strength at different freezing temperature, young’s modulus and Poisson ratio at different freeze-thaw temperature range of saturated coal or dry coal samples were obtained by experiments. The results showed that saturated coal samples tensile strength are increased with the reduction of freezing temperature during freezing process, and the tensile strength, young’s modulus reduced, and Poisson ratio increased with the increasing of freeze-thaw temperature range. The larger freeze-thaw temperature range, the more tensile strength reduced.(5) Based on present formation stress models, models considering temperature and freeze-thaw temperature range were established respectively, and horizontal formation stress results at different temperatures and freeze-thaw temperature range were calculated. With the reduction of temperature, the maximum horizontal principal stress first decreased then increased, the minimum principal stress gradually reduced but fluctuated, and the difference between maximum and minimum horizontal principal stress is increasing. Variation of horizontal principal stress induced by the low temperature can lead shear failure of coalbed cleats. Freeze-thaw cycle brings about the reduction of both maximum and minimum horizontal principal stress as well as the augment of minimum principal stress’s ascending degree. Coal rock stress sensitivity is improved with freeze-thaw and stress sensitivity is improved with the increasing of freeze-thaw temperature range.(6) At present, according to the problem of the lack of nitrogen fracturing field treatments, based on the research of this paper, technique improvement measures and theoretical guidance are put forward for the present liquid nitrogen stimulation techniques.