Numerical modelling of the mechanical and fluid flow properties of fault zones - Implications for fault seal analysis

Existing fault seal algorithms are based on fault zone composition and fault slip (e.g., shale gouge ratio), or on fault orientations within the contemporary stress field (e.g., slip tendency). In this study, we aim to develop improved fault seal algorithms that account for differences in fault zone composition as well as deformation conditions under which the fault zone developed. The influence of composition and deformation conditions on the fluid flow properties of fault zones is investigated using discrete element simulations and laboratory experiments (cf. companioning paper by Giger et al.) of samples consisting of a low-permeability clay or shale layer, embedded in porous sandstone. A combination of discrete element and finite difference models is used to upscale the results and investigate the evolution of fault zone architecture and fluid flow properties of outcrop-scale faults. The fault seal algorithms are tested in a case study using finite element models of reservoir-scale faults. ExxonMobil; Igeoss; RDR; Shell; Total; Wintershal

Numerical model for flow in rocks composed of materials of different permeability

In the oil and gas industry permeability measurements on rock samples give an indication of the capacity to produce the output (oil/gas etc). Permeability of small samples can be derived from x-ray Computed Tomography (CT) scans which yields a three-dimensional (binary) digital image of the sample. Then using suitable numerical tools, one can use this digital data to compute a velocity field and hence the permeability of the sample. Up to now, this has been done on the assumption that fluid can only flow in pores (with no flow in solid regions). However, if the sample is made up of different materials, each material can have a different permeability to fluid flow. Hence, here we consider numerical modelling of flow through such a material. We use the Lattice Boltzmann method to model this flow, but need to change the usual streaming and collision steps to account for the partial permeability of voxels. We first implement this new algorithm on some well-known test cases, with excellent agreement with analytic results and then use our algorithm on some real CT digital data. Our results clearly show the effect of increasing the local fraction of a high permeability material within a sample on the global permeability.publishedVersio

Evolution of Clay Smears and Associated Changes in Fault Transmissibility Using a New Direct Shear Fluid Cell

A new type of fluid cell has been developed to allow for direct shear deformation of very large and cohesive rock samples under sealed conditions. Rock samples consist of a low-permeability clay or shale layer, which is embedded in porous quartz sandstone to mimic a reservoir-seal pair. The cell is specifically designed to monitor changes of fault permeability, both across and along the evolving rupture surface, to displacements equivalent to several times the thickness of the argillaceous layer (i.e. SSF>6, c.f. Lindsay et al., 1993), and under stress conditions typical for burial depths of up to 2000 m. We present the general concept of the new testing equipment, and provide structural and flow data of evolving clay smear structures at controlled physical conditions. The results of the analogue experiments are integrated into a numerical modelling study (cf. companion paper by Ter Heege et al.) in an attempt to upscale our findings to reservoir conditions. ExxonMobil; Igeoss; RDR; Shell; Total; Wintershal