Impact of Stress-Dependent Matrix and Fracture Properties on Shale Gas Production

Abstract

Unconventional shale gas production is often characterized by a short period of high production followed by a rapid decline in the production rate. Given the high costs of hydraulic fracturing and horizontal drilling, it is critical to identify the mechanisms behind the production loss. The existing shale gas production models often assume constant matrix permeability. However, laboratory observations show that matrix permeability can decrease significantly with increasing effective stress, which highlights the necessity of considering the stress-dependent properties of shale matrix in production analysis. Moreover, the compaction of pore space will also increase the matrix permeability by enhancing the gas-slippage effect. In this paper, a matrix permeability model which couples the effect of pore volume compaction and non-Darcy slip flow is derived. Numerical simulations are conducted to understand the role of matrix permeability evolution during production. Changes of fractures' permeability and contact area during depletion process are also taken into account. The results indicate that the loss of fracture permeability has a greater impact at the early stage of the depletion process, while matrix permeability evolution is more important for the long-term production.

Permeability Change Caused by Stress Damage of Gas Shale

“…If the shale is only loaded with confining pressure without axial stress, the pores of shale will be composted under confining pressure, resulting in a decrease in the permeability of shale, which is commonly called stress sensitivity [27]. In the triaxial compressive experiments, if the axial stress is applied under high confining pressure, the failure mode is mainly shear failure, which is very different from the axial splitting under low confining pressure and uniaxial conditions, so the stress damage characteristics are different.…”

“…They focused primarily on sandstone and marble and seldom on shale. The study on the relationship between stress and shale permeability mainly focuses on the influence of confining pressure on shale permeability [27], and the effect of stress damage caused by axial stress on permeability is lacking. Brittle rock exhibited axial splitting in the uniaxial compressive test [19,[28][29][30][31] and the primary cause of failure is lateral damage from crack extension [32,33].…”

A Novel Analytical Method to Calculate the Amounts of Free and Adsorbed Gas in Shale Gas Production

“…Porosity and Permeability Equations. In this work, the permeability and the effective porosity vary with the pressure, and the relations are investigated by many scholars [33][34][35][36]. Raghavan and Chin presented the following relations, which is one of the most common relationships for shales [36]:…”