Dynamics of hydrofracturing and permeability evolution in layered reservoirs

Abstract

International audienceA coupled hydro-mechanical model is presented to model fluid driven fracturing in layered porous rocks. In the model the solid elastic continuum is described by a discrete element approach coupled with a fluid continuum grid that is used to solve Darcy based pressure diffusion. The model assumes poro-elasto-plastic effects and yields real time dynamic aspects of the fracturing and effective stress evolution under the influence of excess fluid pressure gradients. We show that the formation and propagation of hydrofractures are sensitive to mechanical and tectonic conditions of the system. In cases where elevated fluid pressure is the sole driving agent in a stable tectonic system, sealing layers induce permutations between the principal directions of the local stress tensor, which regulate the growth of vertical fractures and may result in irregular pattern formation or sub-horizontal failure below the seal. Stiffer layers tend to concentrate differential stresses and lead to vertical fracture growth, whereas the layer-contact tends to fracture if the strength of the neighboring rock is comparably high. If the system has remained under extension for a longer time period, the developed hydrofractures propagate by linking up confined tensile fractures in competent layers. This leads to the growth of large-scale normal faults in the layered systems, so that subsequently the effective permeability is highly variable over time and the faults drain the system. The simulation results are shown to be consistent with some of the field observations carried out in the Oman Mountains, where abnormal fluid pressure is reported to be a significant factor in the development of several generations of local and regional fracture and fault sets

The importance of fracture-healing on the deformation of fluid-filled layered systems

“…This is consistent with the findings of Ghani et al (2013 and2015) who also pointed out that unless the fractures are formed exclusively by fluid pressure gradients, they will not look fundamentally different to purely tectonic ones. Consequently a range of different fracture sets can develop due to the interplay between the applied driving forces.…”

“…where is the compressiblity of the fluid, the permeability of the solid and the viscosity of the fluid. A detailed derivation of the underlying computations can be found in Ghani et al (2013 and2015) and references therein. The solid lattice evolution is calculated using an over-relaxation algorithm and the fluid pressure evolution is calculated using an Alternating Direction Implicit algorithm.…”

“…These limitations provided inspiration for further work in terms of numerical modeling. This paper is a continuation of the work by Ghani et al (2013 and2015) who presented a 2D coupled hydro-mechanical model through which the dynamics of layered porous systems were examined. In this contribution, we focus on the influence of the changing material properties (e.g.…”

“…Both the importance of hydrofracturing and the lack of detection of the process in nature have provoked the development of several numerical models (Tzshichholz et al, 1994;Dahm, 2000, McNamara et al, 2000Flekkøy et al, 2002;Johnsen et al, 2006;Olson et al, 2009;Goren et al, 2010;Niebling et al, 2010a and2010b;Goren et al, 2011;Wangen, 2011, Niebling et al, 2012Aochi et al, 2013;Ghani et al, 2013 and2015). The different models face the same complexity as hydrofracturing involves (1) elastic deformation of the matrix triggered by fluid pressure, (2) fluid flow inside matrix and fracture and (3) fracturepropagation (Adachi et al, 2007).…”

“…Our large temporal and spatial scale simulations demonstrate how faults evolve that drain the system and how the system reaches a steady-state by accommodating strain on pre-existing fractures. Ghani et al (2013 and2015) proposed a numerical scheme which was built on the work of McNamara et al (2000), Flekkøy et al (2002), Koehn et al (2005), Vinningland et al (2007), Niebling et al (2010a and2010b) and Goren et al (2010 and2011). Here we extend the model of Koehn et al (2005) and Ghani et al (2013 and2015) to study the interplay between gravitational, tectonic and fluid forces as well as healing of fractures.…”