Richard Giot

Experimental observations of mechanical dilation at the onset of gas flow in Callovo-Oxfordian claystone

Abstract Understanding the mechanisms controlling the advective movement of gas and its potential impact on a geological disposal facility (GDF) for radioactive waste is important to performance assessment. In a clay-based GDF, four primary phenomenological models can be defined to describe gas flow: (i) diffusion and/or solution within interstitial water; (ii) visco-capillary (or two-phase) flow in the original porosity of the fabric; (iii) flow along localized dilatant pathways (micro-fissuring); and (iv) gas fracturing of the rock. To investigate which mechanism(s) control the movement of gas, two independent experimental studies on Callovo-Oxfordian claystone (COx) have been undertaken at the British Geological Survey (BGS) and LAEGO–ENSG Nancy (LAEGO). The study conducted at BGS used a triaxial apparatus specifically designed to resolve very small volumetric (axial and radial) strains potentially associated with the onset of gas flow. The LAEGO study utilized a triaxial setup with axial and radial strains measured by strain gauges glued to the sample. Both studies were conducted on COx at in situ stresses representative of the Bure Underground Research Laboratory (URL), with flux and pressure of gas and water carefully monitored throughout long-duration experiments. A four-stage model has been postulated to explain the experimental results. Stage 1: gas enters at the gas entry pressure. Gas propagation is along dilatant pathways that exploit the pore network of the material. Around each pathway the fabric compresses, which may lead to localized movement of water away from the pathways. Stage 2: the dendritic flow path network has reached the mid-plane of the sample, resulting in acceleration of the observed radial strain. During this stage, outflow from the sample also develops. Stage 3: gas has reached the backpressure end of the sample with end-to-end movement of gas. Dilation continues, indicating that gas pathway numbers have increased. Stage 4: gas-fracturing occurs with a significant tensile fracture forming, resulting in failure of the sample. Both studies clearly showed that as gas started to move through the COx, the sample underwent mechanical dilation (i.e. an increase in sample volume). Under in situ conditions, the onset of dilation (micro-fissuring) is a necessary precursor for the advective movement of gas.

Fracturing around radioactive minerals: elastic model and applications

A simplified mechanical model of a metamict crystal swelling in a host mineral shows that enough stress is generated in the host to trigger its failure and the appearance of radial fractures, in agreement with geological observations. The maximum stress, which combines radial compression and concentric tension, occurs along the host–inclusion interface and should create radial fractures. The maximum stress mainly depends on the amount of swelling, on the inclusion stiffness, and on the resistance to shearing of the host mineral. The external pressure, the host bulk modulus, and the inclusions/host radius ratio are parameters of minor relevance. The influence of the inclusion does not extend beyond about five times its radius. The application of published experimental data shows that a swelling metamict mineral may create significant fracturing in the host mineral. However, many parameters for both host and inclusion minerals are not available, even for the application of such a simple model. We think that in some circumstances, swelling of metamict mineral and fracturing of host mineral may significantly affect the physical properties of rocks at a larger scale by creating a network of fractures connecting metamict (i.e. uranium and thorium bearing) minerals to the geological environment.

A transversely isotropic thermo-poroelastic model for claystone: parameter identification and application to a 3D underground structure

ABSTRACT A transversely isotropic thermo-poroelastic constitutive law is developed and implemented in the finite element code Code_Aster (EDF, France). It is then validated using an analytic solution for an inclined borehole in a transversely isotropic medium. A strategy for identifying the parameters of the transversely isotropic thermo-poroelastic model based on an inverse method is proposed on the basis of different laboratory tests. To demonstrate the efficiency and applicability of the model, it is then applied in a three-dimensional numerical model of an underground structure in a parameter sensitivity study. The results of the modelling highlight the importance of accounting for anisotropic phenomena when determining the dimensions of underground facilities. The whole approach is presented in the paper, from model development to application to 3D numerical modelling to an engineering case study.

Stress estimation in reservoirs using an integrated inverse method

Estimating the stress in reservoirs and their surroundings prior to the production is a key issue for reservoir management planning. In this study, we propose an integrated inverse method to estimate such initial stress state. The 3D stress state is constructed with the displacement-based finite element method assuming linear isotropic elasticity and small perturbations in the current geometry of the geological structures. The Neumann boundary conditions are defined as piecewise linear functions of depth. The discontinuous functions are determined with the CMA-ES (Covariance Matrix Adaptation Evolution Strategy) optimization algorithm to fit wellbore stress data deduced from leak-off tests and breakouts. The disregard of the geological history and the simplified rheological assumptions mean that only the stress field, statically admissible and matching the wellbore data should be exploited. The spatial domain of validity of this statement is assessed by comparing the stress estimations for a synthetic folded structure of finite amplitude with a history constructed assuming a viscous response.

Stability, accuracy, and efficiency of numerical methods for coupled fluid flow in porous rocks

In this chapter, numerical methods for modeling of fluid-driven cracks in rocks are presented. The modeling of the coupled fluid flow in continuous media is first reminded, with presentation of several applications. Then the modeling of the fractured rock is considered. A review of the most common methods for coupled fluid flow in fractured porous rocks is presented and the manuscript focuses on the extended finite element method (XFEM) method combined with hydromechanical couplings. Several problems in relation with fluid-driven crack in porous rocks are foreseen, such as partial saturation, anisotropy, or crack initiation. Some details about numerical issues are also given.

Integrated Inverse Method to Estimate Virgin Stress State in Reservoirs and Overburden

Summary Stress estimation in reservoirs and overbuden has become a key point during the exploration and the exploitation of the oil ans gas fields. We propose in this abstract an integrated method to compute a physically admissible (i.e. satisfying the equilibrium equations) 3D stress field in whole geological models. Stress field is computed using a simple elastic behavior and it is constrained to the wellbore data using an inverse approach. Forward problem is solved using a Finite Element Analysis. The model parameters to invert are the Neumann conditions which are assumed to be piecewise linear functions along the vertical direction. The data parameters are stress observations which came from the hydraulic fracturing and the borehole breakouts. Misfit between the computed stress and the observed stress is minimized using the CMA-ES algorithm. The method is tested with a synthetic case by taking a stress field computed with the Limit Analysis method as reference. The inversion results show that the method is able to well retrieve the stress variation in the geological model.