547 research outputs found
Geomechanical impacts on flow in fractured reservoirs
No full textFlow responses in fractured reservoirs are difficult to predict. Apparent success in predicting flow has been achieved by developing simple rules of thumb based on (i) alterations of effective stress associated with pore pressure changes or (ii) concepts about fracture aperture alterations due to stress changes. Here it is argued that the assumptions underlying these explanations of flow are flawed, as they are based on ideas about stress that are physically wrong. It may be that these simple ideas can be fitted to some observations, but their use in this fashion is highly risky. The role of geomechanics in fractured reservoirs is more complex than suggested by the simple rules of thumb, as illustrated by numerical simulations that demonstrate the occurrence of strong non-linear interactions between the fluids, the geomechanics of blocky systems, and thermal changes. The resulting movements within fractured rock masses can cause major alterations of the upscaled flow properties. Flow performance discrepancies that are often associated with the operation of fractured reservoirs can, and often should, be seen as a consequence of motions occurring within the fractured rock mass. The explanations developed here are phenomenologically correct, and are more holistic than existing simple rules of thumb, improving the reliability of predictions
Introduction:the relationship between damage and localization
No full textThe papers that appear in this Special Publication were assembled to address a topic that was the subject of a conference entitled ‘Damage and Localization’, one of a series of three Euroconferences on rock mechanics and rock physics that were supported by European Commission funding. Some of papers contained herein were derived from the contributions presented at that meeting, but others were solicited subsequently in order to create a coherent volume that illustrates some key facets of the topic as it is now understood. However, the subject is sufficiently broad that a single collection of papers cannot hope to do justice to the whole theme. This Introduction outlines the conceptual threads that underpin the selection of papers that are included in this volume and introduces the cross-scale relationships that are addressed by the individual contributons. We hope that the reader will find these contributions to be stimulating and informative
Gary Hart, Jesse Douglas, and Julian Dixon, August 1985
No full textFrom left to right, Senator Gary Hart, Reverend Jesse Douglas, and Congressional Black Caucus Chair Julian Dixon sit during an event held at the 28th Annual Southern Christian Leadership Conference Convention in Montgomery, Alabama.The Atlanta University Center Robert W. Woodruff Library acknowledges the generous support of the Joseph & Evelyn Lowery Institute for Justice and Human Rights, the Joseph Echols Lowery Irrevocable Trust, and other donors in supporting the processing and digitization of Morehouse College's Joseph Echols and Evelyn Gibson Lowery Collection
Idealised Discrete Pore-Scale Model of Poro-Elasticity via Closed-Form Analytical Expressions
No full textThis paper describes a simple (idealized) lattice-type model of porous materials, and uses that model to derive analytical expressions that determine interactions between the pore fluid and the material framework. The examples included here address the change in horizontal stress associated with fluid depletion, and also the way that fracture apertures are changed due to alterations of fluid pressure. The predictions agree in magnitude with observations and large-scale inferences. Analysis of the model’s behavior leads to questions concerning the validity, or not, of the scale-independence of stress, along with posing questions about the use of continuum ideas for porous materials. The notion of effective stress is considered within the framework of the model, which reveals that the so-called poro-elastic coefficient is not a material property
Phenomenological Understanding of Poro-Elasticity via the Micro-Mechanics of a Simple Digital-Rock Model
No full textPoro-elasticity is a material concept that expresses the reversible, macro-scale process interactions that occur in a porous material, such as rocks. These process interactions take place between the pore fluids, and the rock framework (or ‘skeleton’) which contains the pores. The phenomenological basis of poro-elasticity is examined via a micro-mechanics analysis, using a simplified digital-rock model that consists of solid elements in a lattice arrangement, and which hosts a well-connected, lattice-like network of simply-shaped pore elements. The quasi-static poro-mechanical bulk response of this model is defined fully by closed-form equations that provide clear understanding of the process interactions, and which allow key effects to be identified. Several external boundary conditions (non-isotropic strain and stress) are analyzed, with drained and un-drained pore-fluid conditions, along with arbitrary pore pressure states. The calculated responses of the pore-scale model, when translated into continuum-scale equivalent behaviors, indicate significant problems with the existing theories of poro-elasticity that are rooted in an enriched-continuum perspective. Specifically, the results show that the principle of effective stress (and the Biot coefficient alpha) is wrongly attributed to a deficiency in the role of pore pressure. Instead, the micro-mechanics-based phenomenological understanding identifies the change of effective stress, in a characteristically-confined setting, as being the result of changes in the stress components, with a key dependency on the specifics of the far-field constraints. Poro-elasticity is thus not a material characteristic; instead, it is a description of a non-linear system operating at the pore scale. The analysis reveals a discrepancy between the stress states within the model domain and the external stress state. This yet remains to be addressed, in order to translate the micro-scale behavior into an equivalent material law
Some thoughts on the geometric/mechanical evolution of basins; new avenues for 2-D and 3-D basin models
No full text
Induced and natural fractures in shales:a geomechanical perspective
No full textShales can serve as pressure barriers in basins, as top seals, and as reservoirs in shale gas plays. This paper emphasises the role of geomechanics in governing shale fracturing. In many basins, the fluid pressure of the aqueous system becomes significantly elevated, leading to the formation of a hydrofracture, and fluid bleed-off. Natural hydrofracture is an unlikely process in the circumstances that exist in most basins. The ideas that underpin hydrofracture thinking are briefly summarised as: a given state of stress such that two in-plane (normally a 2D analysis) principal stresses are almost equal in magnitude; an existing flaw in the material contains a highly pressurised fluid, and a stress concentration develops at the sharp tip of the flaw (which is normally assumed to be slit-like); the stress concentration locally causes a tensile stress to develop in a small region (on the order of mm) in front of the crack tip, causing the material to fail, and hence lengthening the crack; in the elastic equations, the stress concentration depends on the crack length, so the process can continue by feedback. In a P-Q diagram, the hydrofracture conditions plot in a tiny region near the origin. Those states can be reached in Nature, but only by peculiar paths. It seems likely that the conditions of fluid-related yielding (in low effective stresses) are not those of hydrofracture, but instead are associated with dilational, shear-related deformations. This type of deformation increases the pore volume of the material, and, locally, the fluid pressures will be decreased (at least temporarily) as a result. Fluids will flow into the dilated region, and may leave evidence in the form of veins or sand-filled intrusion swarms. Such physical features are widely observed, but usually attributed to hydrofracture. My analysis suggests that they may be better interpreted as dilational yielding of basin geomaterials. Shale gas plays require the manufacture of the reservoir by inducing hydraulic fractures within the shale. Experience suggests that the outcome can be a classical bi-wing, single hydraulic fracture or the creation of a fracture network. Geomechanical simulations, involving approaches that are based on discontinuum methods, help to understand these processes
Kinematic and dynamic considerations in the forced folding process as studied in the laboratory (experimental models) and in the field (Rattlesnake Mountain, Wyoming)
No full textTypescript (photocopy).The concept of forced folding (Stearns, 1971,1978) forms the basis for the research reported here on Rattlesnake Mountain Anticline. Field observations address fold formation with respect to: causative basement blocks, the role of translations, and the relationship of geometric details to underlying basement configuration. A newly recognized thrust feature transports rocks from the gentle limb of the fold across the steep limb late in the deformation history. Physical models using rock layers and a steel forcing assembly add significantly to the research. A new sequence of fold formation is proposed based largely on the models. Analysis of displacement markers in the models reveals that translation occurs in this system without external "pushing." The issue of line-length balance is clarified by the models in that the assumption or' local balance often used by others for forced-fold arguments is shown to be unnecessary. Based upon both field and laboratory examples, I propose that translation and the late thrust are driven by pressure gradients which are themselves a product of the folding. Many bedding -plane slip horizons are present in the field. Deformation fabrics in the field and in the models suggest that these slip surfaces both distribute strains and limit their magnitudes. The folding process at Rattlesnake Mountain can be described as the relative motion of material response elements of low internal strain which are defined by bedding-plane slip horizons and surfaces which are called fold-segment boundaries. Stearns' (1971, 1978) model appears essentially correct, although this research has added details which improve our understanding of the kinematic and dynamic processes associated with the folding
- …
