Experimental study of hydraulic fracture initiation and propagation in highly saturated methane-hydrate-bearing sands

Abstract

A series of tri-axial hydraulic fracturing experiments were performed on highly saturated methane-hydrate-bearing sands (MHBS) with different fluid viscosities. Experiments were also conducted on unconsolidated sands as the solid skeleton of gas hydrate for comparison. Furthermore, the breakdown pressures of fractured samples are investigated. In contrast to the unconsolidated sands, a simple and straight hydraulic fracture can be found while no radial infiltration zones were readily observed for MHBS samples, indicating low fluid leak-off rate and a good fracability of highly saturated MHBS. The breakdown pressure of the MHBS sample is higher than that of the unconsolidated sands when low- or medium-viscosity fracturing fluid is used, whereas the breakdown pressure of the unconsolidated sands sample is higher than that of the MHBS sample with a high-viscosity fracturing fluid. The high-viscosity fracturing fluid can create tensile fractures and local shear fractures in MHBS samples, while a large number of natural gas bubbles are found on the rock surface and released gas can be ignited. However, low- and medium-viscosity fracturing fluids were found to generate a clear tensile hydraulic fracture, and no microfractures were readily observed on the rock surface. This outcome occurs because the high-viscosity fluid flow increases the thermal exchange in the fracture channel; this drag flow can rapidly disrupt the thermodynamic balance and induce gas hydrate decomposition and rock strength reduction. Unlike conventional sedimentary rocks, high-viscosity fluid is beneficial to microfractures creation while mixed fracture failure can be observed. Thus, both fracturing fluid and pressure and temperature (PT)-dependent mechanical properties dominate the fracturing mechanisms in MHBS fracturing. This study can provide considerations regarding the implementation of hydraulic fracturing in natural gas hydrate stimulation.