Effect of fracturing fluids on the seepage properties of coalbed gas reservoirs

Hydraulic fracturing is a major stimulation measure for industrial exploitation of coalbed gas reservoirs. During the fracturing of coalbed gas reservoirs occur serious lost circulation and low flowback rate, which have a direct effect on the seepage properties of formation water and coalbed gas in the process of gas recovery by water drainage. In view of this, damage evaluation experiment of low-damage active water fracturing fluid on coalbed gas reservoir seepage properties was carried out with No. 15 coal of Taiyuan Formation, Upper Carboniferous-Lower Permian in the Qinshui Basin as the study object. Then, the surface characteristics and pore structure characteristics of coals are compared before and after fracturing fluids are used by means of microscopic analysis technologies (infrared spectrum and Scanning electron microscope). And finally, further study was performed on the microscopic effect mechanisms of fracturing fluids on coalbed gas reservoir seepage properties. Experimental results show that gas permeability damage rate is remarkably higher than liquid permeability damage rate no matter whether it is a macrofracture coal sample or a microfracture coal sample, and seepage property damage ratio of the macrofracture coal sample is slightly higher than that of the microfracture coal sample. After fracturing fluids are used, the amount of free hydroxyl and carboxyl functional groups on the surface of coals rises, water wettability increases, fracturing fluids are adsorbed and detained seriously, and pore diameter and fracture width of the coals decrease. These phenomena reveal, from the point of microscope, the reasons for the decline of coalbed gas reservoir seepage properties. These research findings provide basic parameters for fracturing fluid formula optimization and deliverability prediction of coalbed gas reservoirs. ©, 2015, Natural Gas Industry Journal Agency. All right reserved.