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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 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.
Abstract Proppants are one of the essential parameters in fracking design. They not only provide fracture permeability but do prevent “healing” of fractures. Hence, the quantification of proppant transport characteristics is highly critical in a sustainable production from hydraulically fractured wells. Previous attempts were limited to smooth (parallel) fracture surfaces, to a great extent. The consensus reached in the literature, however, is that the roughness of fractures may play a crucial role on proppant transport affecting the permeability of hydraulic fractures. In this paper, an experimental scheme to visually and quantitatively investigate the hydraulic characteristics of rough fractures in the presence of proppants is presented. Seven rock samples of different kinds (i.e., granite, marble, and limestone) were fractured under the Brazilian test and molded to manufacture 20x20 cm transparent replicas. Propping agents were injected at a constant rate into perfectly mating (joint) and sheared fractures in water and polymeric solutions representing typical rheological properties of hydraulic fracturing fluids. During these 2-D experiments, the inlet pressure was continuously monitored to quantify the permeability changes due to proppant distribution caused by the roughness of fracture surfaces. Simultaneously, corresponding images were collected to trace the transport of proppants and their behavior was correlated to the measured permeability change. For a better visualization of proppants, the injected fluid was dyed with a fluorescent material. The proppant behavior in joint and shear type fractures were different. In both cases, fracture closure areas existed, which controlled the proppant movement and permeability change significantly. The injection rate, proppant size, and fracture roughness controlled by lithological properties of the rocks were the other critical factors affecting the permeability and proppant transport. After quantifying the roughness characteristics through different fractal methods (e.g., variogram analysis, power spectral density, etc.), correlations between fracture permeability in the presence of proppant and rock types were presented. The quantitative and visual data collected for a wide range of rock types with original roughness characteristics are expected to be useful in fracking design and selection of proper proppants for different reservoirs. Key words: Proppant transport, fracture roughness, joint and shear fractures, fracture permeability, fractal fracture surfaces. Introduction The main goal of hydraulic fracturing is to provide permeable flow path for hydrocarbons in tight formations. The stability of this permeable flow path can be achieved by propping agents that are injected with treated water. Design of fracturing fluid treatment together with selecting proper proppant type critically impacts the hydrocarbon recovery from the formation (Coulter et. al.2004; Terracina et al. 2010; Kassis et al. 2010; Ribeiro and Sharma 2012, 2013). The mechanism of proppant transport in rough-walled fractures and its effect on permeability should be understood clearly in the assessment of recovery performance, as well. Proppant transport depends on the distribution of asperities, surface roughness, and contact area, which are all controlled by lithological properties of the rocks (Fredd et al. 2000). In addition, rough surface coupled with shear displacement causes closures of the fracture at some points and this eventually affects the proppant transport (van Dam and de Pater 1999).
Summary Proppants are one of the essential parameters in fracturing design. They not only provide fracture conductivity but also prevent “healing” of fractures. Hence, the quantification of proppant-transport characteristics is highly critical in a sustainable production from hydraulically fractured wells. Previous attempts in this regard were limited to smooth (parallel) fracture surfaces to a great extent, but the roughness of fractures may control the conductivity of hydraulic fractures in the presence of proppants. This paper focuses on experimental measurements to visually and quantitatively investigate the hydraulic characteristics of rough fractures in the presence of proppants. Transparent models of the fractures of different origin rocks (granite, marble, and limestone) were prepared. Water and polymeric solutions representing typical rheological properties of hydraulic-fracturing fluids were injected through the models (joint and sheared fractures) with and without propping agents. The conductivity changes caused by proppant distribution caused by the roughness of fracture surfaces were quantified and correlated to different fractal characteristics of surface roughness. Qualitative and quantitative analyses were supported by images collected through the experiments. Proppant behaviors in joint- and shear-type fractures were observed to be different. In both cases, fracture-closure areas existed, which controlled the proppant transportation and fracture conductivity. The qualitative and quantitative data provided on the degree of conductivity change in a single fracture (in the presence and absence of propping agents) are expected to be useful in accurate performance estimation of oil/gas production from fractured systems.
Abstract The consensus reached in the literature is that the roughness of fractures plays a crucial role on proppant transport affecting the aperture sustainability of hydraulic fractures. In this paper, an experimental scheme to visually and quantitatively investigate the hydraulic characteristics of rough fractures in the presence of proppants was presented. Rock samples of different kinds (i.e., granite, marble, and limestone) were fractured under the Brazilian test and molded to manufacture 20x20 cm transparent replicas. Propping agents were injected in a similar fashion and were introduced into the well with fracking fluid at a constant rate. Two types of fracture models were used: (1) perfectly mating (joint) and (2) sheared fractures in polymeric solutions. During the experiments, the inlet pressure was continuously monitored to quantify the permeability changes due to proppant distribution caused by the roughness of fracture surfaces. Meanwhile, corresponding images were collected to trace the transport of proppants and their behavior was correlated to the measured permeability change. For a better visualization of proppants, the injected fluid was dyed with a fluorescent material. In both joint and shear type fractures, existing closure areas controlled the proppant movement and permeability change significantly. The fracture roughness controlled by the lithological properties of the rocks was a critical factor affecting the permeability and proppant transport. After quantifying the roughness characteristics through the variogram fractal dimension, relationship between fracture permeability in the presence of proppant and rock types were presented. Also provided was a semi-quantitative analysis of the stability (or settlement) of proppants during injection with respect to the roughness type (and lithology). The quantitative and visual data collected for a wide range of rock types with original roughness characteristics are expected to be useful in fracking design and selection of proper proppants for different reservoirs.