Investigation of Stress Field and Fracture Development During Shale Maturation Using Analog Rock Systems

Abstract

The emergence of hydrocarbons within shale as a major recoverable resource has sparked interest in fluid transport through these tight mudstones. Recent studies suggest the importance to recovery of microfracture networks that connect localized zones with large organic content to the inorganic matrix. The paper presents a joint modeling and experimental study to examine the onset, formation, and evolution of microfracture networks as shale matures. Both the stress field and fractures are simulated and imaged. A novel laboratory-scale, phase-field fracture propagation model was developed to characterize the material failure mechanisms that play a significant role during the shale rock maturation process. The numerical model developed consists of coupled solid deformation, pore pressure, and fracture propagation. Benchmark tests were conducted to validate model accuracy. Laboratory-grade gelatins with varying Young’s modulus were used as scaled-rock analogs in two-dimensional Hele-Shaw cell setups. Yeast within the gelatin generates gas in a fashion analogous to hydrocarbon formation as shale matures. These setups allow study and visualization of host rock elastic-brittle fracture and fracture network propagation mechanisms. The experimental setup was fitted to utilize photoelasticity principles coupled with birefringence properties of gelatin to explore visually the stress field of the gelatin as the fracture network developed. Stress optics image analysis and Linear Elastic Fracture Mechanics (LEFM) principles for crack propagation were used to monitor fracture growth for each gelatin type. Observed and simulated responses suggest gas diffusion within and deformation of the gelatin matrix as predominant mechanisms for energy dissipation depending on gelatin strength. LEFM, an experimental estimation of principal stress development with fracture growth, at different stages was determined for each gelatin rheology. Synergy between diffusion and deformation determines the resulting frequency and pattern of fractures. Results correlate with Young’s modulus. Experimental and computed stress fields reveal that fractures resulting from internal gas generation are similar to, but not identical to, type 1 opening mode. The novelty of our work is that microfracture networks are imaged and modeled as they form rather than measured after the fact. Host rock elastic-brittle fracture and fracture network propagation mechanisms are triggered by internal gas generation, microfracture frequency, connectivity, and topology are linked to material properties in a direct fashion.

Numerical Modeling of Fracture Network Evolution in Organic‐Rich Shale With Rapid Internal Fluid Generation

“…Experiments with heated shale indicate that fracture initiation on the microscale starts from small preexisting flaws that promote stress concentration, such as elliptic pores or kerogen patches in shale (e.g., Figueroa Pilz et al, 2017;Kobchenko et al, 2011;Panahi et al, 2019). Similar fracture initiation mechanisms were observed in analogue experiments using brittle, elastic gelatin-yeast mixtures, where a drainage fracture network developed from elliptic gas bubbles (Kobchenko et al, 2014;Vega et al, 2018). Fracture propagation is thought to be driven by continuous liquid and gas production, and several factors may influence the propagation direction: (1) initial orientation of kerogen flakes or thin pores, for example, due to sedimentary lamination, (2) external stress field configuration, and (3) internal stress interactions (Kobchenko et al, 2011;Panahi et al, 2018;Vega & Kovscek, 2019;Vernik, 1994).…”

“…Journal of Geophysical Research: Solid Earth network developed from elliptic gas bubbles (Kobchenko et al, 2014;Vega et al, 2018). Fracture propagation is thought to be driven by continuous liquid and gas production, and several factors may influence the propagation direction: (1) initial orientation of kerogen flakes or thin pores, for example, due to sedimentary lamination, (2) external stress field configuration, and (3) internal stress interactions (Kobchenko et al, 2011;Panahi et al, 2018;Vega & Kovscek, 2019;Vernik, 1994).…”

“…Journal of Geophysical Research: Solid Earth upon coalescence (Kobchenko et al, 2014;Vega et al, 2018). However, our numerical results add the benefit of providing a quantitative analysis of the stress state during the coalescence process (cf.…”

“…We added 25 fractures of 1 cm length in an evenly spaced grid throughout the inner model domain, that is, we omit a simulation of the fracture initiation process and assume that initial, small flaws have already formed. Laboratory studies show that such initial microcracks could form from ellipsoid pores or kerogen flakes (Kobchenko et al, 2011(Kobchenko et al, , 2014Vega et al, 2018).…”

Phase‐field modeling of rate‐dependent fluid‐driven fracture initiation and propagation

“…This is important in a geological context where propagating fractures have the potential to merge with natural fractures. Phase‐field models have been successfully applied to hydraulic fracturing, 16–20 coupled with two‐phase flow in fractured poroelastic media, 21 applied to dynamic fracturing in poroelastic media, 22 and used to describe fracture initiation during processes with internal pressure generation 23 . In this work, we develop a thermodynamically consistent model for fluid‐driven fracture growth in rocks with a rate‐dependent response.…”