A hydro-thermo-mechanical numerical model for HDR geothermal reservoir evaluation

Hossain

Num Anal Meth Geomechanics

2002

SUMMARYThe role of shear dilation as a mechanism of enhancing fluid flow permeability in naturally fractured reservoirs was mainly recognized in the context of hot dry rock (HDR) geothermal reservoir stimulation. Simplified models based on shear slippage only were developed and their applications to evaluate HDR geothermal reservoir stimulation were reported. Research attention is recently focused to adjust this stimulation mechanism for naturally fractured oil and gas reservoirs which reserve vast resources worldwide. This paper develops the overall framework and basic formulations of this stimulation model for oil and gas reservoirs. Major computational modules include: natural fracture simulation, response analysis of stimulated fractures, average permeability estimation for the stimulated reservoir and prediction of an average flow direction. Natural fractures are simulated stochastically by implementing 'fractal dimension' concept. Natural fracture propagation and shear displacements are formulated by following computationally efficient approximate approaches interrelating in situ stresses, natural fracture parameters and stimulation pressure developed by fluid injection inside fractures. The average permeability of the stimulated reservoir is formulated as a function of discretized gridblock permeabilities by applying cubic law of fluid flow. The average reservoir elongation, or the flow direction, is expressed as a function of reservoir aspect ratio induced by directional permeability contributions. The natural fracture simulation module is verified by comparing its results with observed microseismic clouds in actual naturally fractured reservoirs. Permeability enhancement and reservoir growth are characterized with respect to stimulation pressure, in situ stresses and natural fracture density applying the model to two example reservoirs.

Section: Evolution Of Shear-dilation-based Stimulation Modelmentioning

confidence: 99%

“…In subsequent laboratory experiments, shear displacements of 0.5-5 mm [13] were observed. According to linear elastic theory, the change in shear stress (called excess shear stress) is proportional to the shear displacement [15], U s (shown in Figure 1) that can be calculated as…”

Section: Fracture Dilation By Shear Slippagementioning

confidence: 99%

A shear‐dilation‐based model for evaluation of hydraulically stimulated naturally fractured reservoirs

Hossain

Num Anal Meth Geomechanics

2002

Oil & Gas Science and Technology - Rev. IFP

“…Numerical approaches developed in two dimensions using a regular fractured pattern (Hicks et al, 1996) with perpendicular fracture sets already showed the perverse effects of stress perturbations. Impedance reduction is obtained but it is likely that the coldest flow paths will carry growing proportions of the injected fluid volume.…”

Section: Thermal Behavior Of the Rock Massmentioning

confidence: 99%

“…To address these coupled effects at relevant time scales, other numerical schemes must be proposed. Sub blocks of rock should exchange heat and stress changes calculated at a given fracture should induce displacements at other locations as it is suggested in Hicks et al (1996). Alternative boundary element methods based on the superposition of elastic solutions in infinite rock masses could be helpful to evaluate, in non regular fracture patterns, the critical flow rate required for the thermal induced stresses to trigger shear propagation along fractures at a further date.…”

Section: Figurementioning

confidence: 99%

“…Since fractures are unevenly distributed in space but rather assemble into clusters and are widely variable in size-see Genter et al, (2000) for this particular faulted area-and since the heat to mine out is stored in the matrix surrounding the fractures, continuum models based on equivalent porous materials are not adequate. Conceptual models based on deterministic regular fracture patterns (Hicks et al, 1996;Kolditz and Diersch, 1993;Kolditz and Clauser, 1998) or randomly distributed networks (Robinson, 1990;Bruel, 1995;WillisRichard et al, 1996) embedded in the rock mass have been developed for this particular HDR research area. Similar discrete fracture network approaches were recently proposed for the evaluation of oil recovery techniques in fractured reservoirs using injection of steam .…”

Section: The Discrete Fracture Network Approach -Theoritical Backgroumentioning

confidence: 99%

See 1 more Smart Citation

Impact of Induced Thermal Stresses During Circulation Tests in an Engineered Fractured Geothermal Reservoir: Example of the Soultz-Sous-Forêts European Hot Fractured Rock Geothermal Project, Rhine Graben, France

Bruel

2002

Résumé -Impact des contraintes d'origine thermique lors d'un essai de circulation dans un réservoir géothermal développé en milieu fracturé : application au site de Soultz-sous-Forêts, Alsace, France, dans le cadre du projet européen sur la géothermie en milieu fracturé profondLe concept de géothermie en roches profondes fracturées est introduit comme une généralisation du concept classique de géothermie sur nappes aquifères, par nature limitées dans l'espace. Un outil numérique bâti selon une approche de réseaux de fractures discrètes est présenté, avec certaines des spécificités et couplages requis pour l'étude des écoulements en milieu fracturé et profond. L'application s'appuie sur les données acquises dans le cadre du projet européen mené à Soultz-sous-Forêts (Alsace, France), dans le graben du Rhin. Les conditions géologiques semblent bonnes, puisqu'un grand nombre de fractures se trouvent favorablement orientées vis-à-vis des contraintes actuellement en place. Un modèle des principales structures géologiques a été construit à partir des données acquises de 1993 à 1997, entre 2850 et 3600 m de profondeur. Le jeu de données comprend de la microsismicité induite, de la géophysique en forage, des essais hydrauliques en puits unique ou entre forages, ainsi qu'un test de circulation de quatre mois pendant lequel deux tests de traçage ont été réalisés. La modélisation numérique montre que les effets de refroidissement dans le voisinage de la zone de réinjection peuvent se manifester durant cette période, ce qui pourrait expliquer certaines observations, comme la lente diminution de l'impédance hydraulique du réservoir, la modification des débimétries en forage, ainsi que les allures sensiblement différentes des courbes de percées des traceurs. Abstract -Impact of Induced Thermal Stresses during Circulation Tests in an Engineered

Geophysical Research Letters

Postinjection Normal Closure of Fractures as a Mechanism for Induced Seismicity

Uçar

Berre

Keilegavlen

2017

Understanding the controlling mechanisms underlying injection‐induced seismicity is important for optimizing reservoir productivity and addressing seismicity‐related concerns related to hydraulic stimulation in Enhanced Geothermal Systems. Hydraulic stimulation enhances permeability through elevated pressures, which cause normal deformations and the shear slip of preexisting fractures. Previous experiments indicate that fracture deformation in the normal direction reverses as the pressure decreases, e.g., at the end of stimulation. We hypothesize that this normal closure of fractures enhances pressure propagation away from the injection region and significantly increases the potential for postinjection seismicity. To test this hypothesis, hydraulic stimulation is modeled by numerically coupling flow in the fractures and matrix, fracture deformation, and matrix deformation for a synthetic reservoir in which the flow and mechanics are strongly affected by a complex three‐dimensional fracture network. The role of the normal closure of fractures is verified by comparing simulations conducted with and without the normal closure effect.