Study of hydraulic fracturing processes in shale formations with complex geological settings

Abstract

Hydraulic fracturing has been applied to extract gas from shale-gas reservoirs. Complicated geological settings, such as spatial variability of the rock mass properties, local heterogeneities, complex in situ stress field, and pre-existing bedding planes and faults, could make hydraulic fracturing a challenging task. In order to effectively and economically recover gas from such reservoirs, it is crucial to explore how hydraulic fracturing performs in such complex geological settings. For this purpose, numerical modelling plays an important role because such conditions cannot be reproduced by laboratory experiments. This paper focuses on the analysis of the influence of confining formations and preexisting bedding planes and faults on the hydraulically-induced propagation of a vertical fracture, which will be called injection fracture, in a shale-gas reservoir. An elastic-brittle model based on material property degradation was implemented in a 2D finite-difference scheme and used for rock elements subjected to tension and shear failure. A base case is considered, in which the ratio SR between the magnitudes of the horizontal and vertical stresses, the permeability k c of the confining formations, the elastic modulus E p and initial permeability k p of the bedding plane and the initial fault permeability k F are fixed at reasonable values. In addition, the influence of multiple bedding planes, is investigated. Changes in pore pressure and permeability due to high pressure injection lasting 2 hours were analysed. Results show that in our case during the injection period the fracture reaches the confining formations and if the permeability of those layers is significantly larger than that of the shale, the pore pressure at the extended fracture tip decreases and fracture propagation becomes slower. After shut-in, the pore pressure decreases more and the fracture does not propagate any more. For bedding planes oriented perpendicular to the maximum principal stress direction and with the same elastic properties as the shale formation, results were found not to be influenced by their presence. In such a scenario, the impact of multiple bedding planes on fracture propagation is negligible. On the other hand, a bedding plane softer than the surrounding shale formation leads to a fracture propagation asymmetrical vertically with respect to the centre of the injection fracture with a more limited upward fracture propagation. A pre-existing fault leads to a decrease in fracture propagation because of fault reactivation with shear failure. This results in a smaller increase in injection fracture permeability and a slight higher injection pressure than that observed without the fault. Overall, results of a sensitivity analysis show that fracture propagation is influenced by the stress ratio SR, the permeability k c of the confining formations and the initial permeability k p of the bedding plane more than the other major parameters. 103tween the horizontal S h and vertical S v boundary stresses is 0.7 (Fig. 1). Further, a se...

Estimating Ultimate Recoveries of Unconventional Reservoirs: Knowledge Gained from the Developments Worldwide and Egyptian Challenges

“…The production of both oil and natural gas from unconventional resources has grown dramatically over the last two decades, especially in North America, through improving techniques of exploration and development [8], [15]. In 2018, The US production of petroleum and natural gas increased by 16% and 12%, respectively, and these totals combined established a new production record [50].…”

Fault slippage and its permeability evolution during supercritical CO2 fracturing in layered formation

“…A considerable number of numerical investigations have been carried out trying to understand the responses of hydraulic fracture-fault interactions (Figueiredo et al, 2017;Rutqvist et al, 2013;Zoback et al, 2012). The research works studied can be categorized into two main groups (Khoei et al, 2016): the first category is concerned with the mechanical interaction between hydraulic fracturing and natural faults.…”

“…Khoei et al (2016) adopted an enriched extended finite element method to expose the mechanism of interaction between the hydraulic fracture and a pre-existing natural fault considering several conditions, including the varying levels of friction and fault angles, but the fault was described by a simple discontinuity without fault gouge, and the hydromechanical behavior within the fault was not discussed. Based on the degradation of material properties, Figueiredo et al (2017) studied the influence of pre-existing faults on the vertical propagation of a hydrofracture with an elastic-brittle model, and the fracture propagation was found to decrease with the shear failure of the fault.…”

Numerical Study of Fracture Network Evolution during Nitrogen Fracturing Processes in Shale Reservoirs

“…The concept of stress ratio is introduced to define the ratio of vertical geostress to horizontal geostress, which gradually decreases with the stratum depth [38]. It is found that the stress ratio plays an important role in fracture behavior during hydraulic fracturing [34,39,40]. Fractures generally initiate and propagate normal to the maximal principal stress.…”

“…However, most of these studies focused on the hydraulic fracturing and the impact of stress ratio on nitrogen fracturing has not been well studied.Furthermore, anisotropy is one of the most distinctive characteristics of the shale due to the existence of bedding planes [42,43]. Compared to the shale matrix, the bedding plane has weak cementing force and high compressibility, which affects the mechanical deformation, fracture parameters, failure strength, and fracture behavior in bedding shales [21,27,39,40,[44][45][46]. The relative properties between the matrix and the bedding plane play an important role in fracture propagation [44].…”