Refracturing is often required in shale and tight gas formations because of inadequate initial HF design or unexpectedly rapid production decline. Water blocking because of fracturing liquid incompatibility, unexpected proppant embedment and crushing, shorter or curved primary fracture length because of premature screen off, general pressure depletion, primary fracture mis-orientation from stress shadowing, unfavourable poroelastic effects limiting the performance of the stimulated volume, and, in general, formation permeability reduction from stress sensitivity may all contribute to unsatisfactory or rapidly declining production. We emphasize the role of geomechanics in candidate screening and review the major factors leading to production decline in unconventional reservoirs. Although the fracture geometry may be altered in a staged fracturing process, the primary focus should be given to the formation permeability enhancement either due to shear dilation or induced fractured network elongation.
The development of a stimulated volume during hydraulic fracturing in a naturally fractured rock mass is singularly challenging to simulate mathematically. At the discrete level there are strong fabric issues (oriented joint sets, perhaps faults, weak bedding planes), different joint properties, Biot coupling, advective-conductive heat transfer, and flow in joint arrays with changing apertures. Complex discrete interaction laws for joints involve sliding Mohr-Coulomb friction with joint dilation (i.e. aperture increase), cohesion loss related to sliding and to extensional displacements across joints, strongly non-linear block contact stiffness behavior that deviates from Hertzian behavior, and loss of contacts during hydraulic fracturing when some natural fractures become open. Furthermore, the rock blocks delineated by the joints in the stimulated volume may anisotropic properties, and large-scale heterogeneity also exists. A novel upscaled HF model for stimulated volume simulation in a naturally fractured rock mass has been developed using a typical Galerkin Finite Element Method approach with full Biot coupling, but using a non-local plasticity formulation with dilation to track the evolution of bulk stiffness and fluid transmission properties. First, the evolution of the stimulated volume is quantified for an example of 3D hydraulic fracturing propagation in a naturally fractured rock. After creation of a sufficiently large stimulated volume, the well is shut-in and pressure is allowed to fall-off. We use the pressure decline analysis to relate the post shut-in behavior of the stimulated volume to the geomechanics of fracturing. Preliminary results are promising: we present interesting results that show this approach should be highly useful in analyzing hydraulic fracturing data and incorporating damage mechanics approaches into geomechanical design in naturally fractured rock masses.
It is generally understood that Hydraulic Fracturing (HF) in Naturally Fractured Rock masses (NFRs) does not lead to creation of a single fracture. Rather, it creates a Stimulated Volume (SV) with enhanced permeability through opening of pre-existing natural fractures, tensile fracturing, irreversible plastic deformation through shearing, and general damaging of a volume of the rock mass (Cipolla et al., 2010; Mayerhofer et al., 2010). However, mathematical simulation of the evolution of the SV is an intricate task because of considerable uncertainty with respect to the shape, size, and conductivity of the SV, as these characteristics are strongly dependent upon the distribution of natural fractures, in situ stresses, and other reservoir and fluid parameters.
Yang, Jianfeng (Taiyuan University of Technology) | Liang, Weiguo (Taiyuan University of Technology) | Lian, Haojie (Taiyuan University of Technology) | Chen, Yuedu (Taiyuan University of Technology) | Li, Li (University of Waterloo)
Cylindrical sandy mudstone specimens were fractured by water, liquid carbon dioxide (L-CO2) and supercritical carbon dioxide (Sc-CO2) via the self-developed multi-phase fluid fracturing experimental equipment. Meanwhile, the fracture morphology of the specimens was documented by 3D morphology scanning after fracturing. The results show that compared to water and L-CO2, using Sc-CO2 as the fracturing fluid can help reduce the pressure needed to initiate fracture due to the increased percolation and pore pressure effects of using Sc-CO2, which is about 50% of the water and 70% of the L-CO2. From experimental results, Sc-CO2-induced fractures are irregular multiple cracks, and the surfaces of the fractures opened by Sc-CO2 are more complex and rugged. It means that Sc-CO2 fracturing can achieve better fracture networks to enhance the coal-bed methane. In addition, it has been studied that the influence of fluid viscosity of the different fracturing fluids on the hydraulic fracturing mechanism. With the reducing fluid viscosity, it leads to narrow fracture aperture, low breakdown pressure and complex or dispersion hydraulic fractures.
ABAQUS, finite element software, was used to simulate the fracturing process of the cylindrical sandy mudstone specimens. The initiation fracture pressure and multicrack propagation has been simulated under different fracturing fluid fracturing by using zero-thickness cohesive elements through the secondary development with python programming language in ABAQUS. The simulation results can consistent with the experiment results.
Coalbed methane (CBM) is referred to as the hydrocarbon gas existing in coal strata. As a kind of important clean energy, the exploitation and utilization of CBM is of great significance to increasing the supply of clean energy, reducing greenhouse gas emissions and ensuring safe production of coal mines. According to the statistics from the State Administration of Coal Mine Safety in China, there exists 36.8 trillion cubic meters of CBM buried within a depth of 2000 m, ranking China the third in the world in terms of resource amount. However, the efficiency of the CBM exploitation in China has not matured to be a satisfactory level. By the end of 2015, CBM cumulative production was 17.1 billion cubic meters, only accounting for 57% of the total target volume suggested by the government. One of the major obstacles to the exploitation of CBM is the low permeability of coal seams (generally lower than 1mD). Hydraulic fracturing is the main technology to increase the permeability and enhance CBM production. However, a major concern with hydraulic fracturing is the severe water consumption and contamination problems, especially considering the fact that most coalbed methane reservoirs in China are located in water-deficient areas, for example Shanxi province. In addition, some coal seams in China have the mudstone interbed with high clay contents, so hydraulic fracturing will lead the clays to swell and thus obstruct gas transport channels, i.e. so-called water block effect. To overcome these drawbacks, some researchers are exploring non-aqueous fracturing technologies (Middleton et al., 2015), in which supercritical carbon dioxide (Sc-CO2) is considered to be an ideal fracturing medium due to its low viscosity (similar to gas) and high density (similar to aqueous fluids). When CO2 sequestration takes place at deep coal seams, where the pressure can exceed 7.38 MPa and temperature might exceed 31.1 °C, the CO2 reaches the supercritical state. Fig. 1 is the phase transition diagram of CO2, from which it can been seen that the critical conditions for CO2 phase transition can be satisfied in the deep geological environment. Compared to aqueous fluids, Sc-CO2 is able to induce complicated fracture networks, enhance coalbed methane recovery by displacing adsorbed methane in coal, reduce the water footprint, and minimize environmental impacts (Pei et al., 2015, Stauffer et al., 2011).
ABSTRACT: In fractured crystalline rock settings, the presence and distribution of fracture zones in the geosphere will strongly influence groundwater system behaviour. Dual continuum computational models that include both porous media and discrete fracture zones are valuable tools in assessing groundwater flow and pathways in fractured rock systems. A methodology and algorithm are presented to incorporate a discrete, complex and irregular fracture zone network, represented as a triangulated two-dimensional mesh, within an orthogonal three-dimensional finite-element mesh. For this study, numerical groundwater models were used as a means to assemble, integrate and illustrate the role of geosphere parameters and properties. The numerical groundwater modelling was performed using HydroGeoSphere. A discrete fracture zone network model, generated using MoFrac and delineated from surface features, was superimposed onto a three-dimensional mesh. The MoFrac code enables the generation of geostatistically and structurally possible 3D fracture network models at the tunnel, site and regional scale.
Kazemi, Alireza (K. N. Toosi University of Technology) | Mahbaz, SeyedBijan (University of Waterloo) | Soltani, Madjid (K. N. Toosi University of Technology / University of Waterloo) | Yaghoubi, Ali A. (Tarbiat Modares University) | Dusseault, Maurice B. (University of Waterloo)
ABSTRACT: One of the basic design factors in porous media enhanced geothermal systems exploiting hot (warm) saline liquids is the time that the system starts to operate until significant thermal breakthrough occurs, i.e., the reservoir lifetime. This study is focused on a sedimentary enhanced geothermal system (SEGS) located in the Williston Basin, a sub-basin of the Western Canada Sedimentary Basin (WCSB). Variables such as the doublet distance and the injection/production flow rate are investigated to assess the effect of the growth of the heat transfer area with and without a high-permeability fracture. For instance, the modeling results illustrate that with short doublet distances, the production temperature of a fractured reservoir with high injection rates is higher than that of a nonfractured system with very same properties, whereas decreasing the injection rate leads to opposite outcomes. The stress changes are also estimated as fluid flows over time at the reservoir scale, since they impact fracture aperture in a highly non-linear manner. The dominant phenomenon is tension development in the entire domain, except in the injection well. The computational platform used is a finite element based model and a 30-year project timescale is considered.
Ahmadi-Joughi, Atefeh (K. N. Toosi University of Technology) | Ziabasharhagh, Masoud (K. N. Toosi University of Technology) | Mahbaz, SeyedBijan (University of Waterloo) | Soltani, Madjid (K. N. Toosi University of Technology / University of Waterloo) | Yaghoubi, Ali A. (Tarbiat Modarres University) | Dusseault, Maurice B. (University of Waterloo)
ABSTRACT: Over 80% of energy used in the Canadian residential sector is for space heating and provision of domestic hot water; natural gas provides much of this primary energy, with attendant GHG issues related to climate change. Aquifer thermal energy storage (ATES) is one way to increase heating efficiency. ATES systems in shallow geothermal configurations have the ability to reduce heating and cooling costs of buildings by 20-40%, with as associated reduction in GHG emissions. The efficiency of an ATES system depends in part on the thermal front growth, which can be investigated to a first-order approximation by thermo-hydraulic modeling. In this study we addressed parameters such as the well-spacing and water flowrate that affect the thermal front to avoid premature thermal breakthrough by defining a dimensionless ratio η. The area of study is located in the Williston Basin, as a sub-basin of the Western Canada Sedimentary Basin (WCSB). Stress and strain changes due to fluid flow are enhancements of this model, executed using a finite element computational platform. Results show that increasing flowrate increases the length of heat propagation and stress/strain magnitude but increasing well spacing has the opposite effect.
Normani, Stefano D. (University of Waterloo) | Sykes, Jonathan F. (University of Waterloo) | Sykes, Eric A. (Nuclear Waste Management Organization) | Jensen, Mark R. (Nuclear Waste Management Organization)
During a glaciation cycle, the entire Canadian land mass has been covered by a series of continental ice-sheets whose maximum thickness reached 4 km (Peltier, 2002). This paper undertakes a sensitivity analysis of a coupled hydromechanical model that is used to assess the impact of mechanical loading due to glaciation on fractured crystalline rock at a hypothetical site in the Canadian Shield. Although the data represent a hypothetical Shield site, the information is consistent with reported values obtained from site-specific investigations during the Canadian Nuclear Fuel Waste Management Program on the Canadian Shield (Garisto et al. 2010, Sykes et al. 2004, 2009, Normani et al. 2007). Climate change and glaciation are not only a concern for the Canadian nuclear fuel waste disposal concept, but also for the Swedish (Provost et al., 1998; Boulton et al., 2001) and Finnish concepts. Cedercreutz (2004), Peltier (2002) and Marshall et al. (2000) have developed glaciological reconstructions of the Laurentide Ice-Sheet using numerical models.
ABSTRACT: Hydraulically stimulating dense reservoirs to extract Oil and Gas (O&G) will remain the preferred technology in shale resource development. Fluid is injected into a well and as the pressure increases, the rock mass fractures, leaving residual increased fluid conductivity along the stimulated natural fractures, increasing formation permeability so that slow diffusive O&G escape from tiny pores can take place at commercially interesting rates. The Hydraulic Fracturing (HF) design process is a complex mechanical interaction analysis based on the Geology of the rocks and the Geometry of the HF fractures or stimulated volume - G&G interactions. Stress is the primary control (work minimization) and if the fracture size (height) is large, or if fracturing is taking place in multi-layered strata with lateral stress inhomogeneity, the geometry of the stimulated zone for a HF stage - height, length, and spatially averaged aperture changes - is impacted by the initial and induced stress profile. This paper attempts to somewhat clarify the outcomes for a single fracture stimulation in a homogeneous 2D elastic medium with non-uniform stresses at the boundaries.
Hydraulic Fracture Stimulation (HFS) is the most effective technique to extract Oil and Gas (O&G) from low-permeability formations. Similar to all other stimulation techniques, the goal of HFS is to increase the reservoir permeability, connect natural and induced fractures and ultimately, increase the productivity of the reservoir to enhance O&G profitability (Daneshy, 2010).
Hydraulic Fracturing (HF) is a multi-disciplinary process (Taleghani et al, 2015), and is acclaimed as the most effective reservoir-scale stimulation technique. To attain economical production rates from tight strata such as milliDarcy range sandstones to microDarcy range shale oil and shale gas (Al-Kanaan, 2014), HF is applied along long horizontal wells in stages. A full well stimulation may cost one to two million dollars per well, more for exceptional cases (high pressures, large volumes, complex treatment schedules, many stages). The design and operational processes involved in HF are complex and include hydraulic, mechanical, geomechanical and logistics aspects, requiring a comprehensive workflow. HF design must address geomechanical (in-situ state) and production aspects (experience) in advance, and for optimization, it is also necessary to monitor and evaluate the success of the HFS (Cantagliari et al., 2010). Furthermore, preliminary workflows for HFS can be enhanced by including other concerns and recommendations arising from environmental, economic, social, and related issues (Oyarhossein & Dusseault, 2016).
ABSTRACT: It is widely recognized that hydraulic fracturing of naturally fractured rocks leads to creation of a stimulated zone of increased conductivity, realized through shear reactivation of natural fractures, irreversible plastic deformation, and induced bulk damage. The characterization of the Stimulated Volume (SV) can improve the fracture design and subsequent well performance. However, there still remain considerable uncertainty with respect to the shape, size, and conductivity of the stimulated volume, as these characteristics are strongly dependent upon the distribution of natural fractures, in-situ stresses, and other reservoir, fluid, and treatment parameters. In this work, we introduce a non-local poro-elasto-plastic zone model of enhanced permeability for the SV, with a characteristic length scale controlled by the density of the fracture network. We first quantify the evolution of the SV and pressure change in the reservoir for a typical example of hydraulic fracture stimulation in a tight formation. After the creation of a sufficient stimulated volume, the well is shut-in for an extended period of time and the wellbore pressure is allowed to fall-off. Using the existing analytical solution for the finite conductive fracture, the flow capacity of’ the stimulated zone created by a prescribed stimulation plan is evaluated.
Development of the shale reservoirs involves the use of horizontal wells stimulated via hydraulic fracturing (HF) (Economides and Nolte, 2000). During HF a working fluid is injected into a target formation at high rates and pressures causing deformation and failure (fracture) of the rock mass and increasing the permeability in a region surrounding the injection point creating a Stimulated Volume (SV) (Cipolla et al., 2010 and Mayerhofer et al., 2010).
Microseismic imaging is a primary tool to estimate the extent of the complex hydraulic fracturing and SV evolution in hydraulically stimulated tight formations (Maxwell, 2014, Dusseault et al. 2011) (Fig. 1a). Microseismic events are usually collected on the nearsurface geophones to measure the seismic energy release associated with shear breakage of the rocks along the pre-existing natural fractures. (i.e., Mode II and Mode III fracturing). The shear fracturing effects (seismic and aseismic deformations) result in a stimulated region of naturally self-propped fractures (i.e., a shear dilation zone) spanning a large volume of the target reservoir formation (Fig. 1a) (Dusseault, 2013). The size of the micorsiemic clouds is often considered as the effective SV and shear dilation zone. However, microseismic rarely provides any information on the dynamic of fracturing, fluid flow and stimulated permeabilty. In addition, the total energy of microseismic waves is often negligible compared to the total hydraulic fracturing input energy suggesting that most of the shear slip events are aseismic (Boroumand and Eaton, 2012) Hence, the development of a comprehensive geomechanical model which can explain the complex fracturing of Mode II and Mode III deformation can constrain the size of the SV and provide a more consistent microseismic interpretation.
ABSTRACT: Non-destructive ultrasonic evaluation (NDE) is commonly used for assessment of civil infrastructure and characterization of construction materials. Among the acoustic methods the impact echo, ultrasonic pulse velocity (UPV), and surface waves can be distinguished. In this paper, we focus on the UPV method as an ASTM standard test method for concrete specimens. UPV method has different applications such as the assessment of the relative quality of concrete, the detection of voids and cracks, and the evaluation of the effectiveness of repairs. UPV measurements can be also used for monitoring changes in the condition of a specimen. In spite of the simplicity of the method, its results highly depend on the type of transducers, the coupling transducer-specimen, and the specimen dimensions. In this article, we investigate the effects of the sensor and the specimen dimensions. The results for UPV tests on 9 mortar specimens of different heights and diameters are presented. The specimens are tested with 54 kHz and 850 kHz resonant frequency (fc) transducers and a state-of-the-art 5MHz laser vibrometer.
Concrete is a popular structural material used in Civil Engineering applications. As for any material, condition of concrete may be affected by the quality of design, manufacturing, loads applied to a structure, character of the loads, environmental deterioration, or aging. Condition of concrete plays a key role for safety of structures (Kim et al., 2005). Non-destructive ultrasonic evaluation (NDE) is commonly used for assessment of civil infrastructure and characterization of construction materials. Among the acoustic methods the impact echo, ultrasonic pulse velocity (UPV), and surface waves analysis can be distinguished (McCann and Forde, 2001), (Popovics, 2003). The latest trends focuses more on attenuation of wave front (Aggelis et al., 2005), (Kirlangiç et al., 2015) and more sensitive methods for detecting changes in velocity (e.g. Coda Wave Interfefometry (Dai et al., 2013), (Planes and Larose, 2013), (Snieder, 2006)). Wave velocity depends on the medium properties, therefore UPV method is a very popular technique used in NDE in Civil Engineering. Propagation velocity of the longitudinal (P-wave) through the material (VP) can be calculated as: