Experimental study on hydraulic fracturing in clayey-silty hydrate-bearing sediments and fracability evaluation based on multilayer perceptron-analytic hierarchy process

Abstract

Hydraulic fracturing is considered a promising stimulation technology for low-permeability hydrate reservoirs. To date, only a few studies have focused on hydraulic fracturing in hydrate-bearing sediments. However, the numerous factors that affect fracture initiation and propagation are not clearly understood, and the fracability of non-diagenetic geo-materials has not been systematically evaluated. In this study, a series of true triaxial hydraulic fracturing experiments are conducted on clayey-silty hydrate-bearing sediments to investigate the effects of the key reservoir and engineering parameters on fracture initiation and propagation. Based on the resulting data, a fracability index (FI) that considers multiple factors is developed using a novel method. The results indicate that fracture initiation pressure does not always increase with increasing hydrate saturation. Moreover, a maximum value of 14.92 MPa for the initiation pressure is observed at 40% hydrate saturation. This value is increased by 20.51 MPa when the effective horizontal in-situ stress increases from 1 to 4 MPa, which is in contrast to the tensile crack initiation law of elastic rocks. Additionally, owing to the inhomogeneous hydrate in sediments, fractures expand unevenly, and double fractures are able to form in an isotropic horizontal stress state. The horizontal stress difference is the primary parameter (weight 0.4) that governs the FI, followed by the coefficient of earth pressure at rest (weight 0.31), fracture toughness (weight 0.18), and hydrate saturation (weight 0.05), and vertical in-situ stress (weight 0.05). Increasing the injection rate and fracturing fluid viscosity is an effective method to promote fracture propagation, particularly when Qμ‾> 0.33 (defined by the normalised injection rate and fracturing fluid viscosity) and FI> 0.4. In such conditions, a considerable reconstruction area can be obtained.