HYDRAULIC PROCESSES IN FRACTURED RESERVOIRS
Résumé
Understanding hydraulic processes in fractured systems remains a puzzling activity as observations are most of the time and scale dependant. Moreover the so called REV can not be defined in many geological systems and quantitative approaches are often site specific. Dealing with shallow systems and groundwater management, research activity at EMP is now focussing on transfers through the non saturated zone in fractured/karstified carabonated formations that can be commonly found in the Paris Basin (chalk layers). The objective is to evaluate recharge processes as well as contaminant migration (nitrates/pesticides) from land fields toward aquifers, from the local to the global scale. Modelling hydraulic interference tests in multiple well systems at the field scale in fractured carbonates (Dogger age, SEH site, Poitiers, France)) was shown tractable using Discrete Fracture Network approaches. Similar work was performed for understanding short pulse and longer hydraulic tests in granitic basements (Mahswaram site, AP, India), submitted to massive over exploitation. The DFN numerical package is still in progress to address the functioning of perched aquifers and the sensibility to sea intrusion, in the basaltic context of volcanic islands (Galapagos ANR GWIIS project). Recent developments include two phase flow capabilities in fractures but double porosity is still missing. Regarding the behaviour of fractured rocks at greater depth (3-5 km), most of EMP experience comes from the analysis of enhanced geothermal systems, devoted to CO2 free base load electricity production. Heat extraction from deep 'engineered' fractured formations is currently under investigation at many places in the world. The challenge is to develop a reservoir in deep rock masses, to circulate a fluid and to recover heat for clean electricity production at the surface. In most cases, the promoted technology is to force cracks that pre-exist in deep rocks by injection of pressurised water as the effect of a pore pressure increase is to weaken fracture strength. Failure in fractures is explained by a linear relation in between shear stress at failure and normal stress, with two parameters, the internal friction μ and the internal cohesive strength C. When failure develops along particular fractures, frictional slip occurs. The triggered dislocations are accompanied by acoustic emissions which are recorded and processed for the evaluation of the success of the hydraulic treatment. However many uncontrolled events and late events with unwanted seismic magnitude are reported that may trouble public acceptance for this CO2 free source of energy (See Bale project, Switzerland). The experimental European site at Soultz sous forêts (1.5 MWe) is however unique. Research is therefore required on the understanding of hydraulic and geo-mechanical spatial and temporal interactions from the sample test at lab scale to in-situ the field tests requiring the injection of thousands cubic meter of fluid at elevated rate and high pressures. Roles of thermal processes have to be questioned. 3D stress distribution at depth, stress heterogeneity along pre-existing faults, fault architecture close to an exploratory well are also key question that have to be fixed early in a project, using new technologies. The evaluation of induced seismic risks has to be carefully evaluated regarding to the known/probable natural activity, if available. Some of these items are forming the core of the GEISER project (European project, FP7 call energy, 2010-2013). Responses are multidisciplinary but will be valuable for any other application involving massive injections in a geological formation (e.g.CO2 storage, heat storage, liquid gas storage,...)