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Abstract In this paper, the results of laboratory studies of hydraulic fracture in homogeneous sandstone blocks with man-made interfaces and heterogeneous shale blocks with weak natural interfaces are reported. Tests were conducted under similar stress conditions, with fluids of different viscosity and at different injection rates. The measurements and analysis allows the identification of fracture initiation and behavior. Fracturing with high viscosity fluids resulted in stable fracture propagation initiated before breakdown, while fracturing with low viscosity fluids resulted in unstable fracture propagation initiated almost simultaneously with breakdown. Analysis also allows us to measure the fluid volume entering the fracture and the fracture volume. Monitoring of Acoustic Emission (AE) hypocenter localizations, indicates the development of created fractured area including the intersection with interfaces, fluid propagation along interfaces, crossing interfaces, and approaching the boundaries of the block. We observe strong differences in hydraulic fracture behavior, fracture geometry and fracture propagation speed, when fracturing with water and high viscosity fluids. We also observed distinct differences between sandstone blocks and shale blocks, when a certain P-wave velocity ray path is intersected by the hydraulic fracture. The velocity increases in sandstones and decreases in shale.
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Sandstone (0.68)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Mudrock > Shale (0.68)
- North America > United States > Wyoming > Laramie Basin > Niobrara Formation (0.99)
- North America > United States > Nebraska > Laramie Basin > Niobrara Formation (0.99)
- North America > United States > Kansas > Laramie Basin > Niobrara Formation (0.99)
- North America > United States > Colorado > Laramie Basin > Niobrara Formation (0.99)
Steady State-Creep of Rock Salt - Improved Approaches for Lab Determination and Modeling to Describe Transient, Stationary and Accelerated Creep, Dilatancy and Healing
Günther, R.-M. (Institute for Geomechanics GmbH (IfG)) | Salzer, K. (Institute for Geomechanics GmbH (IfG)) | Popp, T. (Institute for Geomechanics GmbH (IfG)) | Lüdeling, C. (Institute for Geomechanics GmbH (IfG))
Abstract Actual problems in geotechnical design, e.g. of underground openings for radioactive waste repositories or high-pressure gas storages, require sophisticated constitutive models and consistent parameters for rock salt that facilitate reliable prognosis of stress-dependent deformation and associated damage from the initial excavation to long times. Fortunately in the long term the response of salt masses is governed by its steady state creep behavior. However, because in experiments the time necessary to reach true steady creep rates can last time periods of some few days to years, depending mainly on temperature, an innovative but simple creep testing approach is suggested. A series of multi-step tests with loading and un-loading cycles allow a more reliable estimate of stationary creep rates in a reasonable time schedule. In completion, the advanced strain-hardening approach of Günther/Salzer is used which describes all relevant deformation properties of rock salt, e.g. creep and damage induced rock failure, comprehensively within the scope of an unified creep approach. The capability of the combination of improved creep testing procedures and accompanied modelling is demonstrated by recalculating multi-step creep at different loading and temperature conditions. Thus reliable extrapolations relevant to in-situ creep rates (10 to 10 s) become possible.
- North America > United States (1.00)
- Europe (0.68)
- Geology > Mineral > Halide > Halite (1.00)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
Abstract The laboratory testing described here can be seen as the first step in investigating potential thermal effects leading to the creation of a leakage pathway at or in the vicinity of a CO2 injection well. The occurrence of thermal stresses in metal casing, cement and formation can lead to either one or more of these materials developing cracks, or debonding between pairs of materials at their interface. A first investigation is thus concentrating on the rock immediately above the injection reservoir; this sealing rock is most often some variant of shale formation. Here we look at the required temperature contrast between the injected CO2 (or for that matter any other liquid) and the shale formation, in order to initiate tensile fracturing due to the development of tensile stresses exceeding the rock's tensile strength. Finite element simulations suggest that significant fracturing may occur for a temperature contrast of 80° C. An accompanying series of laboratory tests showed that for the chosen shale specimen, fracturing should only be of concern for much higher temperature contrasts.
- North America > United States > New Mexico > San Juan Basin > San Juan Basin Field > Mancos Formation (0.99)
- North America > United States > Colorado > San Juan Basin > San Juan Basin Field > Mancos Formation (0.99)
Abstract Design, analysis and performance assessment of potential salt repositories for heat-generating nuclear waste require knowledge of thermal, mechanical, and fluid transport properties of reconsolidating granular salt. To inform salt repository evaluations, we have undertaken an experimental program to determine Bulk and Young’s moduli and Poisson’s ratio of reconsolidated granular salt as a function of porosity and temperature and to establish the deformational processes by which the salt reconsolidates. Tests were conducted at 100, 175, and 250°C. In hydrostatic tests, confining pressure is increased to 20 MPa with periodic unload/reload loops to determine K. Volume strain increases with increasing temperature. In shear tests at 2.5 and 5 MPa confining pressure, after confining pressure is applied, the crushed salt is subjected to a differential stress, with periodic unload/reload loops to determine E and ?. At predetermined differential stress levels the stress is held constant and the salt consolidates. Displacement gages mounted on the samples show little lateral deformation until the samples reach a porosity of ~ 10%. Interestingly, vapor is vented only for 250°C tests and condenses at the vent port. It is hypothesized the brine originates from fluid inclusions, which were made accessible by heating and intragranular deformational processes including decrepitation.
- Energy > Oil & Gas > Upstream (1.00)
- Government > Regional Government > North America Government > United States Government (0.94)
- Energy > Power Industry > Utilities > Nuclear (0.67)
Abstract A repository for heat-generating nuclear waste provides an engineering challenge far beyond general experience. Long-term evolution of repository performance is precluded from direct observation or measurement. Therefore, analogues and long-term predictions are necessary to establish enduring safety functions. Accurate prediction of salt repository response is enhanced by a thorough understanding of the mechanistic processes and application of valid models. In the instance of a salt formation providing the host medium, the scientific community has made great strides toward formulating and using models that capture observed physical phenomena in computational mechanics applications. For example, strain rates are often cast as functions of stress difference and temperature because salt deformation under repository conditions has been shown to be very sensitive to these parameters. Micromechanics help define appropriate relationships by virtue of a deformation mechanism map approach. Incorporation of micromechanics helps explain history effects, normal transient response, inverse transients, and dependence of creep rate on stress difference and temperature, which are a direct consequence of existing and evolving substructures. If one understands the physical processes then predictions can be made with greater confidence. A viable constitutive model should therefore provide a reasonable approximation of laboratory results and reflect the physical processes that account for deformation. Laboratory studies facilitate measurement of generic mechanical, thermal, hydrological, and chemical salt properties in a controlled environment. A large body of laboratory data has been generated on salt deformation, which describes phenomenology of salt across a range of temperatures expected in a heat-generating waste disposal system. Microstructural studies described here can be used to develop and interpret input parameters for models that predict material behavior. This overview emphasizes empirical evidence of isochoric deformation processes in salt and supports engineering and long-term performance evaluation of repositories for heat-generating nuclear waste.
- Geology > Mineral > Halide > Halite (0.70)
- Geology > Geological Subdiscipline > Geomechanics (0.68)
- Water & Waste Management > Solid Waste Management (1.00)
- Energy > Oil & Gas > Upstream (1.00)
- Government (0.94)
- Energy > Power Industry > Utilities > Nuclear (0.87)
- North America > United States > Nebraska > Salina Basin (0.99)
- North America > United States > Kansas > Salina Basin (0.99)
Impact of Discrete Fracture Network (DFN) Reactivation on Productive Stimulated Rock Volume: Microseismic, Geomechanics and Reservoir Coupling
Huang, Jian (Weatherford) | Safari, Reza (Weatherford) | Lakshminarayanan, Sunil (Weatherford) | Mutlu, Uno (Weatherford) | McClure, Mark (University of Texas)
Abstract In the absence of natural fractures, hydraulic fractures can open against the minimum principal stress and propagate along continuously through the formation. However, field observations and microseismic (MS) activity suggest that, natural fractures can reactivate upon stimulation: forming complex/sheared discrete fracture networks (DFNs). Current industry practice primarily utilizes MS events alone to determine the extent of reactivation within a DFN and consequently the stimulated rock volume (SRV). However, MS activity does not necessarily distinguish between dry (i.e. reactivation with non-conductive fractures) vs. wet events (i.e. reactivation with conductive fractures). Indeed, wet events are most likely to contribute towards a Productive-SRV as these conductive fractures have an increased chance to remain open during production. This paper presents a series of geomechanical sensitivity analysis that quantify the difference between fracture events within the framework of a complex discrete fracture network simulator coupled with fluid flow and fracture reactivation. Our results show that the extent of Productive-SRV is controlled primarily by a combination of reservoir, geomechanical and stimulation parameters. These parameters include: DFN geometry (e.g. patterns, spacing, density, and orientation), stress anisotropy/isotropy, frictional strength, the duration of stimulation, and the number of stages. Results are further coupled with an unconventional reservoir simulator to quantify ultimate recovery/production for different scenarios. This integrated study/workflow provides guidelines in the field to optimize stimulation operations and to better interpret microseismic field data.'ép.
- North America > United States > Texas > West Gulf Coast Tertiary Basin > Eagle Ford Shale Formation (0.99)
- North America > United States > Texas > Sabinas - Rio Grande Basin > Eagle Ford Shale Formation (0.99)
- North America > United States > Texas > Maverick Basin > Eagle Ford Shale Formation (0.99)
- (2 more...)
Abstract The discontinuum-mechanical modeling of salt rocks enables us to take the microstructure of polycrystalline salt rocks into account explicitly within geomechanical simulations. On a micromechanical scale, rock salts constitute a discontinuum of intergrown salt crystals. The macroscopic behavior of this system is determined by the interaction of intra- and intercrystalline properties. Essential processes like softening, fragmentation and pressure-driven percolation along grain boundaries can only be captured and explained physically by considering the microstructure of the material. Constitutive models developed at the IfG, such as the visco-elasto-plastic model for the salt grains themselves and an adhesive frictional shear model for the interaction between the grains, are used to describe the mechanical behavior, while the hydraulic description of fluid percolation is realized using the cubic law for laminar flow. Only after overcoming a percolation threshold the pressure-driven opening and interconnection of flow paths along grain boundaries is initiated in the salt rock and induces a directional percolation in the direction of the maximum principal stress. A consistent parameterization strategy based on macroscopically measured data has been developed, that is able to reproduce the deformation- and failure-behavior of salt rocks qualitatively and quantitatively based on the discontinuum-mechanical approach. The introduced method is successfully used and verified in numerous hydro-mechanical simulations of different boundary conditions, geometries and types of loading.
- Europe (0.69)
- North America > United States > California (0.28)
- Geology > Mineral > Halide > Halite (1.00)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
Abstract Crushed salt is used as a backfill and sealing material for mines and underground waste repositories, and as a byproduct of potash mining it is usually stored in large above-ground tailing heaps. For the analysis of strength, permeability and heap stability in these applications, compaction needs to be taken into account. We present a constitutive model for crushed salt which comprises viscous compaction as well as intact salt properties such as creep and viscoplastic deformation with strain hardening and softening. The model is a generalised combination of the salt rock model of Minkley and the WIPP crushed salt compaction model by Sjaardema and Krieg. We validate the model on laboratory experiments, using both analytical estimates and numerical simulations with UDEC. As an application, we investigate the behaviour of tailings heaps, where the model reproduces observed compaction and related effects such as subsidence and surface cracks. Furthermore, we find a natural mechanism explaining heap stability.
Abstract Underground cavities in rock salt mass for storage of crude oil and natural gas as well as compressed air and hydrogen are essential elements in energy supply management. These cavities are at the same time complex geotechnical constructions, characterized by special excavation procedure using solution mining technique and therefore no direct access for humans is existent. Nevertheless these underground cavities have to fulfil manifold requirements, especially static stability, tightness, third party protection and last but not least environmental safe abandonment after decades of storage operation. This paper at first gives some basic information about geotechnical characteristics of storage cavities in rock salt mass and the related design concepts guaranteeing long term safe as well as economic efficient storage operation. Essential elements of cavern design are presented such as physical modelling of rock salt behaviour based on laboratory investigations, constitutive models, numerical simulations to site-specifically validate simulation models, to identify prospective load-bearing behaviour as well as to quantify rock mass stresses and deformations related to proposed operation patterns. Special attention is focused on thermomechanically coupled processes induced in rock mass due to gas withdrawal and gas injection. Finally, some hints with respect to cavern abandonment and related physical modelling as well as numerical simulation of brine-filled closed cavities are given.
- North America > United States (0.28)
- Europe > Germany (0.28)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Mineral > Halide > Halite (0.81)
Abstract Dynamic loading methods promise new modes for stimulating geological resources, as the fracture patterns they produce can be tailored by the shape and nature of the pressure pulse employed. However, selecting the type of load is a difficult task: too slow and the stimulatory effect is reduced; too fast and the resource may be negatively impacted by wellbore damage, fines creation or permeability reduction. Moreover, modeling these systems proves challenging due to the myriad of length and timescales involved, combined with the need to accommodate both the generation of new fractures and propagation of preexisting fracture networks. GEODYN-L is a massively-parallel multi-material Lagrangian code that includes advanced contact models to simulate nonlinear wave propagation through heavily-jointed rock masses, along with material model libraries specifically developed to capture the dynamic response of geologic media. We present results using GEODYN-L to simulate dynamic stimulation of geologic resources with pre-existing fracture networks and discuss the implications of these results for enhancing fracture networks with dynamic loading techniques.
- Energy > Oil & Gas > Upstream (1.00)
- Government > Regional Government > North America Government > United States Government (0.69)