Experimental Study on Hydraulic Fracture Initiation and Propagation in Hydrated Shale

Abstract

Shale reservoirs contain a certain amount of clay minerals, which can hydrate through imbibition when in contact with various water-based fluids during drilling and completion. Shale hydration can lead to structural changes in the shale such as the expansion of bedding planes and propagation of microfractures, consequently affecting the initiation and propagation of hydraulic fractures. However, the effect of shale hydration under confining pressure on hydraulic fracture propagation and stimulation effect is still unclear. To this end, a novel experimental method integrating shale hydration and hydraulic fracturing was proposed based on the laboratory triaxial hydraulic fracturing simulation system. This method enables a more realistic simulation of shale hydration and hydraulic fracturing process happening in downhole conditions. The experimental results show that under simulated reservoir conditions, water imbibition increases over time with the imbibition rate reaching its peak within 24 h. The breakdown pressure, number of fractures, and complexity of fractures are positively correlated with imbibition time. The increase in fracture complexity could be attributed to the increase in the number of fractures. In contrast, imbibition pressure (injection pressure for imbibition) has little influence on water imbibition. For specimens under different imbibition pressure, the breakdown pressure and the number of fractures are close, and the complexity of fractures does not change prominently; all are T-shaped fractures. It is believed that the closure of microfractures under confining pressure caused by hydration is the main reason for the increase in breakdown pressure. Higher breakdown pressure means higher net pressure in the wellbore, which facilitates fracture initiation where the breakdown pressure is higher. Therefore, shale hydration is conducive to the initiation of multiple fractures, thus increasing the number and complexity of fractures.

Review and Outlook on Fracturing Technology and Mechanism of Hydrate-Bearing Sediments

“…Hydraulic fracturing has become a prevalent technique in the oil and gas industry, − primarily leveraging technological interventions to induce artificial fractures within subterranean rock strata and natural gas reservoirs. , This process serves to enhance hydrocarbon recovery rates by facilitating increased fluid flow. ,,, …”

“…Hydraulic fracturing has become a prevalent technique in the oil and gas industry, 103−105 primarily leveraging technological interventions to induce artificial fractures within subterranean rock strata and natural gas reservoirs. 106,107 This process serves to enhance hydrocarbon recovery rates by facilitating increased fluid flow. 42,44,88,108 When examining the mechanical properties of HBS, the subsurface stress is a complex combination of radial, alonglayer, and vertical positions, and the triaxial instrument has the same peripheral pressure, which is unable to provide the displacement and deformation information related to other dominant stress directions, and cannot accurately simulate the real stress environment of the subsurface reservoirs.…”