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Abstract In this case study, we apply a novel fracture imaging and interpretation workflow to take a systematic look at hydraulic fractures captured during thorugh fracture coring at the Hydraulic Fracturing Test Site (HFTS) in Midland Basin. Digital fracture maps rendered using high resolution 3D laser scans are analyzed for fracture morphology and roughness. Analysis of hydraulic fracture faces show that the roughness varies systematically in clusters with average cluster separation of approximately 20' along the core. While isolated smooth hydraulic fractures are observed in the dataset, very rough fractures are found to be accompanied by proximal smoother fractures. Roughness distribution also helps understand the effect of stresses on fracture distribution. Locally, fracture roughness seems to vary with fracture orientations indicating possible inter-fracture stress effects. At the scale of stage lengths however, we see evidence of inter-stage stress effects. We also observe fracture morphology being strongly driven by rock properties and changes in lithology. Identified proppant distribution along the cored interval is also correlated with roughness variations and we observe strong positive correlation between proppant concentrations and fracture roughness at the local scale. Finally, based on the observed distribution of hydraulic fracture properties, we propose a conceptual spatio-temporal model of fracture propagation which can help explain the hydraulic fracture roughness distribution and ties in other observations as well.
Multiphase flow through fractures is common in many fields, yet our understanding of the process remains limited. In general, this is because some factors which separate multiphase flow from single-phase flow (interfacial tension, wettability, residual saturation) are difficult to characterize and control in a laboratory setting, and are also challenging to implement in traditional numerical simulators. Here, we present a series of lattice Boltzmann simulations of CO2 displacing brine in rough fractures with heterogeneous wettability. This extended abstract focuses on the application of this technique to predict irreducible brine saturation within the fractures. We show that this irreducible brine saturation may be greater than 25%, which could have significant impacts on production estimates from unconventional reservoirs and is typically not accounted for in reservoir simulators. However, performing these simulations at the field scale is not possible due to their computational expense. Therefore, we present a machine learning technique based on deep neural networks to predict the fluid distribution within these fractures at steady state trained upon on the lattice Boltzmann simulations. To our knowledge, this is the first example of machine learning being used to predict the distribution of fluid within a subsurface media. Here we show that a trained network is able to accurately predict the fluid residual saturation and distribution based solely on the dry fracture characteristics. This proves that machine learning holds promise for upscaling these simulations to a relevant scale for application to the oil and gas industry.
Multiphase flow in fractures has implications to many fields including nuclear waste disposal, CO2 sequestration, geothermal energy, and the oil and gas industry. During multiphase flow, factors that do not play a role in single-phase flow become important. These factors include the interfacial tension between fluids, the fluids viscosity ratio, and the wettability between the solid surfaces and each fluid. Compared to porous media, where the effect of wettability has been extensively researched, the influence of wettability during fracture flow is relatively unstudied. This is partly due to the difficulty in characterizing the wettability of natural rock cores and conducting experiments as well as the difficulty in including wettability into numerical simulations. Therefore, the importance, or lack thereof, of wettability during multiphase fracture flow remains uncertain.
Uotinen, L. (Aalto University) | Torkan, M. (Aalto University) | Janiszewski, M. (Aalto University) | Baghbanan, A. (Aalto University /Isfahan University of Technology) | Nieminen, V. (Aalto University) | Rinne, M. (Aalto University)
Characterization of Hydro-Mechanical (H-M) properties of rock fractures is the initial and important step in modeling of fully H-M coupled processes in fractured rock masses. Fluid flow in the fractured rock mass is an important aspect when evaluating the safety of geological disposal of high-level nuclear waste. Many attempts have been taken to measure and model fluid flow in rock fractures in different stress field conditions. However, still study about the scale effect of fracture properties and confinement stress on the conductivity of rough rock fractures remains a challenging topic of research. As a part of an ongoing research project about fluid flow modeling in fractured rock mass (RAKKA), and as an initial step one rock slab pair with sizes of 250 mm x 250 mm of Kuru grey granite halves was prepared. It has a horizontal mechanically induced tensile fracture. The surface roughness of the fracture was mapped using a conventional profilometer and structure-from-motion photogrammetry before each fluid flow test. The fractures were subjected to different normal stress and then fluid flow within the fractures was conducted linearly from edge to opposite edge with perpendicular edges sealed, and conductivity of the fractures under steady-state condition was measured. Then the test is repeated with all three sides open. The results show anisotropic behaviour in permeability. The diagonal components of the permeability matrix are significantly stress-dependent. Together the new fracture digitization method and the new three-way fluid flow test allow the contactless characterization of hydro-mechanical properties of rock fractures and the validation of the results.
Abstract The propped fracture dimension and fracture conductivity are controlled by the proppant placement during the hydraulic fracturing stimulation treatment. Proppant placement has traditionally been studied by flowing proppant slurries between smooth parallel plates, which is a simplification of rough fracture walls in the subsurface. In this study, for the first time we propose a simulation workflow to study proppant transport in realistic rough-walled rock fractures to optimize the proppant placement efficiency. The simulation domain was extracted from a micro-CT image of an induced fracture in a sandstone core. The fracture has many contact points (zero aperture) and all apertures on the order of a millimeter and less. Taking advantage of image processing technique, two fracture surfaces were separated and moved to obtain a wider fracture and to mimic the coupling effect of geomechanics and proppant transport. Proppant transport in the rough-walled rock fractures was simulated by coupling computational fluid dynamics with a discrete element model (CFD-DEM) and this is the first time this was done in a detailed image-based fracture geometry for an extensive range of proppant and Newtonian fluid parameters as well as non-Newtonian (shear-thinning). Proppant placement, proppant concentration profile along the flow direction, pressure difference build-up across the fracture and total proppant volume retained within the fracture are quantified under various flow conditions. Two fracture orientations including horizontal fracture without gravity influence and vertical fracture with gravity influence are investigated in this study. Proppant placement visualization shows that proppant particles distribute quite uniformly over the horizontal fracture without considering gravity effect, while for the vertical fracture an obvious proppant settling bank is observed at the bottom of the fracture except when using very viscous or non-Newtonian fluids. For the case of horizontal fracture without gravity influence, proppant placement inside the rough fracture is mainly controlled by the fracture aperture field, and the proppant size plays a crucial role in proppant placement. Fracturing fluid viscosity has to get rather high (100cP) for the fluid drag force to have a larger effect in preventing particle accumulation within the fracture and carry the proppant deeper into the fracture. The effect of fracturing fluid viscosity is much more prominent in the vertical fracture case with gravity effect. The cross-linked gel (modeled as a shear-thinning fluid with apparent viscosity between 10cP and 100cP in this study) can achieve a quite spatially uniform proppant placement. The horizontal fracture case with a higher proppant concentration shows a quite clear proppant displacement front, a slower proppant displacement speed into the fracture and a larger pressure difference build-up across the fracture. Larger fracture aperture can induce larger retained proppant volume inside. Less proppant settling and further proppant transport into the fracture are observed for a lighter proppant. For vertical fracture case a larger fluid injection velocity carries the proppant way deeper into the fracture and also gives a more even proppant placement within the fracture, while the influence of injection velocity for the horizontal fracture case is quite complicated. This study presents the first 3D simulation that tracks detailed, dynamic proppant placement in realistic rough-walled rock fractures under various flow conditions. Results from this study provide a completion engineer with directional guidance for proppant and fracturing fluid selection to optimize final proppant placement in the complex fracture networks.
Wang, Chaoyi (The Pennsylvania State University / Purdue University) | Elsworth, Derek (The Pennsylvania State University) | Fang, Yi (Institute for Geophysics / Jackson School of Geosciences / The University of Texas at Austin) | Zhang, Fengshou (Underground Engineering of Ministry of Education / Tongji University )
ABSTRACT: Subsurface fluid injection can disturb the effective stress regime by elevating pore pressure and potentially reactivate faults and fractures. Laboratory studies indicate that fracture rheology and permeability in such reactivation events are linked to the roughness of the fracture surfaces. We construct discrete element method (DEM) models to explore the influence of fracture surface roughness on the shear strength, slip stability, and permeability evolution during such slip events. For each simulation, a pair of analog rock coupons (3D bonded quartz-particle analogs) representing a mated fracture are sheared under a velocity-stepping scheme. The roughness of the fracture is defined in terms of asperity height and asperity wavelength. Results show that (1) samples with larger asperity heights (rougher), when sheared, exhibit a higher peak strength which quickly devolves to a residual strength after a threshold shear displacement; (2) these rougher samples also exhibit greater slip stability due to a high degree of asperity wear and resultant production of wear products; (3) long-term suppression of permeability is observed with rougher fractures, which is plausibly due to the removal of asperities and redistribution of wear products, which locally reduces porosity in the dilating fracture. This study provides insights into the understanding of the mechanisms of frictional and rheological evolution of rough fractures anticipated during reactivation events.
Subsurface fluid injections such as hydraulic fracturing, carbon sequestration and geothermal stimulation involve injecting large volumes of fluid at high overpressures, therefore disturbs the stress field by elevating pore pressure and altering far-field stress (Elsworth et al., 2016), potentially resulting in the reactivation and potential seismic rupture of pre-existing faults and fractures. These hydraulic fractures, while creating possibility to extract hydrocarbon resources from tight shale, can be extremely vulnerable to seismic failure upon stress perturbation (Ellsworth, 2013; Walsh & Zoback, 2015; Zoback & Gorelick, 2012), causing hazardous consequences. One key question in understanding the seismic cycle is in unraveling the evolution of shear strength, stability, and permeability of fault and fractures that may contribute to dynamic slip events.
Chen, Yuedu (Taiyuan University of Technology) | Liang, Weiguo (Taiyuan University of Technology) | Lian, Haojie (Taiyuan University of Technology) | Yang, Jianfeng (Taiyuan University of Technology) | Xiao, Ning (Taiyuan University of Technology)
ABSTRACT: The hydraulic and mechanical behaviors of a rough fracture largely depended on the fracture geometries formed between two rough surfaces, especially for deformable fractures under the normal stress. To investigate the variations of fracture geometric characteristics with normal stresses, a total of six fractured rock specimens, including cubic cement and cylindrical sandstone specimens with different grain sizes, were chosen to conduct the normal compression tests. The 3D scanner was used to capture the surface characteristics of the rough fracture, and the point-cloud data matching method was adopted to calculate the fracture apertures distribution variation with increasing normal stress. Then, the geometric characteristics of the fracture, including the areas of distributions of the contact and interconnected void, and the size of apertures of the interconnected voids, were determined. Test results shows that the required normal stress to achieve the maximum facture closure are different for various specimens, which are largely related to the joint compression strength and the matching degree between two surfaces. Besides, the distribution of the void and contact regions shows heterogeneous under different normal stress, and the frequency of apertures variation approximately follows a normal distribution function. With the increase of the normal stress, the contact area ratio shows a rapid increase at low stress and then tend to stable at higher level, while the variation of the interconnected void area ratio is just the opposite. Meanwhile, both the effective mechanical and mechanical aperture show rapid decrease at low stress and then tend to stable at high stress,. The gap between these two apertures are larger for high normal stress than for low normal stress, mainly due to the rapid increase of the isolated void spaces at high stresses. The test results is helpful to the knowledge of the hydro-mechanical behavior of the rough fracture.
The mechanical and hydraulic behaviors of the rock mass largely depended on that of fractures or joints in them, which gained wide attention in various underground geotechnical engineering, including CO2 geological sequestration (Noiriel et al., 2013; Radilla et al., 2013), hydraulic fracturing for exploiting shale gas (Chen et al., 2015) and geothermal extraction. However, the natural joints are intrinsically heterogeneous and easily modified by natural and human activities (Pyrak-Nolte and Nolte, 2016), which in turn affect the coupled hydro-mechanical behaviors. Thus, an adequate knowledge of the geometric characteristics evolution in deformable rough fractures is necessary to understand the mechanical and hydraulic behaviors of the fractured rock mass.
Abstract The understanding of proppant flow through fractures is critical in evaluating the hydraulic fracturing performance. As a continuation of our experimental efforts devoted to understanding how proppant flows in rough vertical fractures, in this paper, we examine the effect of injection parameters on the proppant transport in rough vertical fractures. The effects of polymer concentration, injection rate, proppant concentration, and type of proppant were investigated in detail. Experimental results show that a sufficiently high polymer concentration is needed to enable effective proppant flow in rough fractures. In general, the relative coverage of proppants increased dramatically as the polymer concentration increased, implying that the higher viscosity of fracturing fluid could enhance the slurry's ability to place more proppant vertically into the fracture and help to maintain a better conductivity after fracturing treatment. A sufficiently high injection rate of the slurry is also needed to enable effective proppant flow in rough fractures. At certain low injection rate, the proppants carried by a low polymer solution might not exhibit a tree-like settling pattern, diminishing the effect of roughness effect on the proppant transport. This means that even in rough fractures, the tree-like settling pattern of the proppants did not necessarily occur for sure; the injection rate should be properly selected to enable such phenomenon. With other condition being kept constant, a higher proppant loading led to a higher final relative coverage of the proppants in the rough fractures. But if the injection rate used for delivering the proppants is not sufficiently high, we may encounter injectivity issues; in our lab experiments, this caused the choking of the pump. The heavier proppant (ceramic proppants) in the rough fracture models tended to suppress the tree-like settling pattern that was experienced by the lighter proppant (silica sands). This is attributed to the larger density of the ceramic proppants, leading to a larger settling velocity. In order to maximize the spreading of a given proppant over a rough fracture model, we should determine the proper values of all the essential injection parameters (including polymer solution, injection rate, proppant concentration) by striking a good balance among them. The conclusions obtained in this study shed light on how to optimize slurry injection parameters to achieve an optimal proppant-filling ratio during hydraulic fracturing.
Huang, Hai (Xi'an Shiyou University and Shaanxi Key Laboratory of Advanced Stimulation Technology for Oil & Gas Reservoirs) | Babadagli, Tayfun (University of Alberta) | Andy Li, Huazhou (University of Alberta) | Develi, Kayhan (Istanbul Technical University)
Abstract The fracture-surface characteristics (such as roughness and fractal dimensions) may greatly affect the proppant transport during hydraulic fracturing operation. Few researches have focused on investigating the proppant transport in vertical fracture with actual surface characteristics. As a continuation of our previous study (Huang et al. 2017), we qualitatively investigatethe migration of proppants in rough and vertical fractures by considering the effects of surface characteristics and rock type on the instantaneous transport and areal spreading of proppant in the fractures. We fractured different types of tight rocks (including limestone, marble, tight sandstone, and granite) with Brazilian test and molded them to manufacture 20×20cm transparent replicas with an aperture of 1 mm. We characterized the surface characteristics of these rock samples with different fractal dimensions. Subsequently, dyed fracturing fluid with or without proppant loading was injected into the rough vertical fracture. In each test, we monitored the inlet pressure continuously while the proppants were being transported in the fracture. The process was videotaped to monitor the proppant distribution in the rough fracture. Different from our previous study (Huang et al. 2017), a higher injection rate is used in this present study. The experimental results obtained in this study further consolidate the many findings reported in our recent study (Huang et al. 2017): in rough and narrow fracture, the surface roughness plays a pivotal role in affecting how proppants settle in the fracture as well as where the proppants settle in the fracture. Roughness of the vertical fractures tends to significantly enhance the vertical placement of proppants in the fracture, leading to a much higher proppant-filling ratio in a rough fracture than in a smooth fracture. Interestingly, in addition to the bridging effect observed in Huang et al. (2017), a previously formed proppants cluster can be broken up under a higher-rate slurry flow. The bridging of proppants and its subsequent breaking up can recursively occur during the high-rate slurry flow, resulting in fluctuations in the proppant filling ratios as well as fluctuations in the pressure profiles recorded in the inlet of the fracture model. The roughness of fracture models not only affects how much area of the fracture is being occupied by the proppants in the fracture, but also affects how tightly the proppants are filling up the fracture. Different types of rock have different surface characteristics, leading to the observed differences with regard to how the proppants migrate, settle down and fill up the fractures. No definite correlation could be established between any of the fractal numbers and the relative coverage of proppants in the fracture. More experiments, however, need to be conducted to reach more concrete conclusions in this regard.
Abstract The roughness of fractures may play an important role in affecting the migration and placement of proppants during hydraulic fracturing operations. Previous studies focused on investigating the proppant transport in smooth vertical fractures, which did not consider the effect of the fracture-surface roughness. We examine the migration of proppants in rough and vertical fractures and then quantitatively reveal the effect of roughness on the instantaneous proppant transport and final proppant placement. Two types of rock samples (marble and granite) are fractured with the Brazilian test and molded to manufacture 20 × 20 × 5 cm transparent replicas. The surface roughness of these rock samples was first characterized by fractal dimensions. Then, the dyed fracturing fluid with a given proppant loading was injected into the rough vertical fracture. In each test, the inlet pressures were continuously monitored in order to obtain the differential pressure across the fracture model while the proppants were being transported in the fracture. The process was videotaped to real-time track the proppant distribution in the rough fracture. The proppant-transport behavior in the rough and vertical fracture was observed to be totally different from that in the smooth fracture. The major experimental findings include the following: 1) The proppant in a rough vertical fracture does not progress as a regular sand bank that commonly occurs in the smooth fracture, but rather an irregular-shape sand clusters with fractal characteristics; 2) In the rough and vertical fracture, the phenomenon of proppant bridging is visually observed, and such phenomenon is more likely to occur in the location with a larger roughness height. This implies rough fracture could promote a wider spreading of the proppant in the fracture compared to smooth fractures, and; 3) The existence of roughness enhances the vertical displacement of fluid containing proppants. These effects are also favorable for obtaining a better filling of the proppants in the fracture. Our experimental study reveals the mechanisms of proppant transport and distribution in real vertical fractures under the influence of roughness effect.
A closure model is developed for rough fractures along with a fluid flow model to predict fracture conductivity decline under normal closure stress. The closure model is based on surface asperity and half-space deformation considering the effect of mechanical interaction among asperities and inelastic deformation. Fracture aperture profile that is obtained from the closure model is then used in the fluid flow model to predict fracture conductivity. Hydraulic conductivity of synthetic surface profiles are compared to investigate the impact of surface pattern on conductivity. Simulation results indicate that considering half-space deformation and mechanical interaction among asperities affect conductivity decline behavior. Different aperture averaging methods are found to result in noticeably different conductivity results. Narrower and deeper channels undergo less conductivity decline compared to wider and shallower channels.
Rough fractures tend to close due to farfield stresses acting on the plane of fracture. Quantification of fracture closure is crucial to predicting hydraulic conductivity behavior. It is, therefore, necessary to study the closure mechanisms in rough surfaces. The concepts of fracture closure and contact mechanics can be applied to acid fracturing and unpropped fracturing which are of great interest in the petroleum industry.
Several attempts have been made to investigate the closure of rough surfaces in contact. The problem of rough surface closure is addressed in many analytical (Greenwood & Williamson 1966, Brown & Scholz 1985), numerical (Hopkins 1991, Pyrak-Nolte & Morris 2000), and experimental (Bandis et al. 1983, Marache et al. 2008) works. It is worth mentioning that the literature is mainly concerned with the elastic contact of rough surfaces.
Hydraulic conductivity of rough surfaces is extensively studied because of its important implications in different branches of science and engineering. The hydraulic conductivity of rough fractures is studied in many experimental and analytical works (Witherspoon et al. 1980, Tsang and Witherspoon 1981, Gong et al. 1998). It is our aim to investigate the impact of fracture closure on fracture conductivity and also study the importance of fracture surface pattern in conductivity decline behavior.