A new optimization method of double-crack distributions for improving network fracture conductivity of natural gas exploitation

Abstract

Hydraulic fracturing is used for natural gas extraction and its purpose is to form fracture networks. The effective measure to enhance the productivity of hydraulic fracturing is to improve the conductivity of network fractures, which takes crack penetration length as evaluation index. In this paper, a new Mode I and Mode II SIF calculation formula for branch-crack tips of multi-crack was deduced by the complex function and the superposition method. A new multi-crack fracture criterion was established based on the criterion of maximum tensile-shear SIF ratio under hydraulic-mechanical (HM) load. The total double-crack propagation length L was selected to characterize the conductivity of network fractures. An infinite plate contains any double-crack AB (hydraulic crack) with unequal length and CD (natural crack) under hydraulic-mechanical biaxial compression is taken as an example. The effects of single factor (such as relative distance Ds and Dh, relative inclined angle α, crack length a1 and a2) on the total length L are calculated and the calculation formula of L is derived by the uniform design method. Take Lmax as the optimization objective, the optimization model of fracturing parameters was established to obtain the optimization results, which are verified by test results. Results show that peak load decreases (or increases) and decreases (or increases) more when the crack relative inclination angle and relative vertical distance decrease (or increase). The crack growth is restrained by the increase of hydraulic pressure.