Study On Recovery Improvement Of Cbm Vertical Wells Via Hydraulic Fracturing With Nitrogen Injection

The characteristics of low reservoir pressure and permeability of coal reservoir require some necessary enhancing operations on permeability increase in coal seams for the recovery of coalbed methane(CBM), of which hydraulic fracturing with nitrogen injection is an important approach in improving the fluid/gas flow rate within coal seams. The determinations of matrix deformation, permeability variation and recovery improvement after nitrogen injection into coal here in this study were intended to provide a theoretical reference for fracturing with nitrogen injection.In this paper, two coal samples with different particle sizes collected from Yuwu colliery were used for isothermal adsorption/desorption experiments with N2 injection for replacing CH4, and the results indicated that the adsorption capacity of small particle-size/pulverized coal was higher than that of big particle-size/lump coal. Based on the theories of surface free energy and adsorption/desorption, a mathematical model was established for discussing the changes of coal matrix. As pressure changed, the coal matrix swelling-by-adsorption deformation and shrinkage-by-desorption deformation were interrelated logarithmically, and pulverized coal saw a bigger deformation than lump coal did. The coal matrix swelling deformation increased exponentially with the adsorption of CH4, while the shrinkage deformation decreased logarithmically with the desorption of mixed gases. By conducting the mercury injection and low-temperature nitrogen adsorption, the changes of pores in coal before and after nitrogen injection were investigated. The results showed that the total specific surface area of coal decreased but the total pore volume of coal increased after nitrogen injection. According to the geometric model of coal reservoir and conventional gas flow theory, a mathematical permeability model of coal reservoir before and after nitrogen injection was established. The results suggested that the permeability decreased with the adsorption of CH4 and further increased with the injection of N2 and desorption of mixed gases, finally resulting in a higher permeability value. Considering the initial pressure gradient, two prediction models of CBM recovery before and after nitrogen injection were established. The field results showed that the recovery rate of CBM after nitrogen injection increased by 0.26%~10%, which was greatly affected by the values ofcritical desorption pressure and permeability. With the enhancement of permeability or the increase of critical desorption pressure, the recovery rate saw a logarithmic growth.The research results of the paper can provide a theoretical basis with nitrogen injection coal reservoir property selection.