In this study, we identify recurring hydraulic fracture patterns among a series of laboratory experiments. Details for each of the individual experiments are detailed in the prior work (Frash 2012, 2014; Frash et al. 2013a, b, 2014a, b, 2015a, b, c; Haas et al. 2013; Hampton et al. 2013; Revil et al. 2015). Table 1 gives an overview of the hydraulic stimulation experiments evaluated for this study, detailing the various specimen materials, injection fluids, and boundary conditions. Specimen materials included concrete, cement, granite, acrylic, and carbonate-rich shale, representing a large range of material types. Hydraulic fracture breakdown pressures were typically high due to scale effects from using small boreholes at 10 mm and 6 mm for cased and uncased intervals, respectively (Haimson and Zhao 1991). Available mechanical properties for each of these materials are given in Table 2 with additional description in the Appendix. Injection fluids included water, oil, and epoxies, as detailed in Table 3. Additional details and data for these experiments are available by reasonable request to the corresponding author.
Multiple hydraulic injection stages were executed for experiments P01-00, E01-00, E02-00, G01-00, G01-90, G01-92, and G01-93. Each injection stage included the possibility of new fracture growth; therefore, acoustic emission monitoring and/or real-time visual observation was used to estimate the location and timing of any new fractures activated or created by fluid injection. Pre-fracture inspections and a sequential series of post-fracture saw-cut cross sections were acquired from each of these experiments to measure hydraulically stimulated 3D fracture geometries. We find this saw-cut method to be more informative than the common method in the previous studies, where a hammer and chisel were used to mechanically split specimens parallel to the hydraulic fracture plane. Epoxy injection yielded the best fracture growth control, and the most pristine examples of complex fracture network geometry because fractures were better preserved and the path of injected fluid penetration into fractures and matrix was easily traceable, even when branching occured. Other methods such as X-ray tomography are available and have been successfully used for fracture identification in rocks, but these methods have difficulty resolving small-scale fracture structures within large specimens (Frash et al. 2016). For example, X-ray tomography would not be able to resolve 0.1 mm aperture fractures within a 300 mm × 300 mm × 300 mm cubic specimen at the current state of the technology.
Here, we first present results showing complex fracturing in acrylic specimens. Results from fracturing this relatively homogeneous and transparent acrylic material provide useful insight into understanding the experiments in opaque rocks and cements. Next, we present examples of recurring 3D fracture patterns observed at the full specimen scale, focusing on the least ambiguous results. Then, we show smaller, sub-specimen scale, fracture patterns where 2D evidence for similar structures to the large scale is found.
Complex fractures in acrylic
The most self-evident examples of simple tensile and shear fractures were acquired from A01-05, as shown in Fig. 1. The included photomicrographs were taken from cut cross sections through the hydraulic-fractured acrylic specimen. Injected epoxy (off-white/orange color) infilled the fractures in this experiment, preserving the fracture aperture during cross sectioning and enhancing contrast between fractures and the acrylic matrix. Similar fracture geometries were observed by eye in all of the acrylic specimens, but the tight residual fractures in A01-03 and A01-04 could not be easily photographed. Longitudinal tensile opening (mode I) was the dominant fracture type in the acrylic experiments. Shear-oriented fractures in the propagation direction (mode II) and perpendicular to the propagation direction (mode III) were observed within the tensile-dominated fractures.
A video of the A01-05 experiment (Frash et al. 2013c) reveals a staged progression to the hydraulic fracture’s propagation. This progression starts with epoxy filling pre-existing drilling-induced fractures along the borehole wall. At the breakdown injection pressure (i.e., peak pressure), these fractures begin to extend along their initial orientations, possibly dominated by mode I (tensile) opening. Then, at a critical amount of extension, these small tensile fractures begin to coalesce via new interconnecting fractures that likely include a mixture of shear modes. The tips of the shear-coalesced fracture fronts lag behind the tensile front with respect to radial distance from the borehole. This detail indicates that more strain is required to create the coalescing shear fractures than what is required to create the tensile fractures. The progression of fracture growth demonstrates that offset tensile fracture strands can simultaneously propagate on what could appear to be competing planes (e.g., stress shadowing) and later coalesce into a common fracture by shear (Fig. 1: mixed-mode example). The geometry in these experiments provides examples of: (1) hydraulic fractures propagating by different modes along segments within a larger coalesced fracture and (2) multiple fracture strands sharing a common plane closer to a common origin, even when they appear disconnected or separate from a 2D perspective.
Recurring fracture patterns at the specimen scale
It may not be immediately apparent how varied laboratory hydraulic fracture experiments can provide insight into complex fracturing processes in the subsurface. It is also common knowledge that natural rocks contain significant complexity (e.g., heterogeneity, discontinuity, anisotropy, nonlinearity, and variable stresses) that cannot be fully characterized across the relevant scales even using the most advanced available methods. In consequence and by subjective argument, it is useful to be able to identify recurring fracture geometries that occur across a range of material types subjected to differing boundary conditions. In this study, we categorize three characteristic 3D hydraulic fracture patterns of a higher order than simple tensile and shear modes. These higher-order patterns are proposed as constituents of even more complex fracture networks. Each higher-order fracture pattern hints at the origins of tortuosity, roughness, and branching and, in turn, has implications for hindered fluid flow and increased fracture surface area relative to more idealized fracture geometries. The three categorized fracture patterns include (1) offset branching, (2) traversing coalescence, and (3) smooth reorientation, as depicted in Fig. 2. Note that the ‘3D perspectives’ column in Fig. 2 shares a common boundary stress orientation with the x-axis being the minimum principal stress axis. The geometry shown for the complex network was produced from direct measurement (Frash et al. 2015a, b, c), while the other geometries are simplified from actual observations which tended to each include multiple fracture structures.
Simple tensile fractures are the simplest hydraulic fracture geometry and are a convenient assumed shape for modeling (de Pater et al. 1994; Perkins and Kern 1961; Nordgren 1972; Geertsma and de Klerk 1969). This geometry results from tensile-dominated fracture propagation radially away from the borehole and often exhibits an elliptical aperture profile (de Pater et al. 1994; Frash et al. 2014a, b). This was also the most commonly observed macro-scale fracture geometry with examples identified in all specimens except for E01-00—true-triaxially confined high-strength cement with brine injection and G01-92—triaxially confined granite with water injection. These laboratory results support the expectation that rock stresses, borehole orientation, and fluid parameters dominate over the influence of rock structure, particularly when high-viscosity fluids were used (de Pater et al. 1994; Ishida et al. 2004; Warpinski et al. 1982a, b).
Discontinuity shear was observed at some scale in all experiments with the most prominent examples identified in E01-00—true-triaxially confined high-strength cement with brine injection, E02-00—true-triaxially confined high-strength cement with oil injection, and S01-00—true-triaxially confined shale with epoxy injection. In each of these, large and prolific pre-existing discontinuities were hydrosheared by fluid injection because these fractures were weaker than the intact matrix rock (Mokhtari et al. 2014). The result from S01-00 was particularly interesting because a large shearing event occurred suddenly during low-rate constant-pressure injection at what was thought to be an injection pressure below the hydraulic fracture propagation pressure (Frash et al. 2015a, b, c). This indicates a transient effect where slow pressurization of a natural fracture eventually resulted in shear slip.
Offset fracture branching was observed in all specimens. At the field scale, similar hydraulic fracture branching has been described as a hydraulic fractured zone (Warpinski and Teufel 1987) or as a fracture band (Aydin et al. 2006). This structure often appears in cross sections as a set of unconnected or weakly connected sub-parallel hydraulic fractures. However, in many cases, these branches could be traced back to a common fracture plane closer to the injection well. Considering observations from the acrylic specimens, it is evident that these branches can propagate simultaneously, in parallel, overlapping, and in close proximity without significant competition from stress shadowing. The offset branching structure is significant for fluid flow. Lateral offsets in vertical fractures are likely to hinder proppant settling. Overlapping parallel branched fractures are likely to each have reduced aperture relative to a single fracture plane which will reduce both individual and cumulative hydraulic conductivities. Increased surface area from branched fractures will likely improve flow from matrix pores to the more conductive hydraulic fractures per length of the fracture, as well as enhance reactivity because new fracture surfaces tend to be highly reactive. The number of fracture branches does not appear to be predictable, but it is possible that they have some relation to distance from the well.
Traversing coalescence of branched fractures is a structure that sometimes occurred as branched fractures extended. It appears that this structure was the most common when branched fractures propagated in close proximity or that this structure caused branched fractures to propagate close together. Prominent examples of traversing coalescence were identified in A01-05—unconfined acrylic with epoxy injection and S01-00—true-triaxially confined shale with epoxy injection. The effects of traversing coalescence are likely to be the opposite of branching with a tendency to decrease fracture surface area, increase fracture aperture, and decrease tortuosity, especially as fracture size increases. The sequence of traversing coalescence occurring after branching is indicative of discontinuous processes in fracture propagation. This discontinuous nature of propagation is further supported by a known tendency for irregular pressure peaks during continuous injection stimulation (Frash et al. 2015a, b, c).
Smooth reorientation was most commonly observed as fractures transitioned from longitudinal with the wellbore in the near-well zone to be perpendicular to the minimum principal stress further from the well. This curvilinear geometry was less apparent in our experiments than in previous works that used larger wellbores and more homogeneous materials (Abass et al. 1996; Hallam and Last 1991; Romero et al. 1995; Weijers et al. 1994). Our most prominent examples of smooth reorientation were observed in E01-00—true-triaxially confined high-strength cement with brine injection and G01-90—true-triaxially confined granite with oil injection. Fracture branching was common amidst smooth reorientation. These branches generally arose from multiple tensile fracture initiation points along the well or by interaction with pre-existing fractures during propagation. Tortuosity due to smooth reorientation is likely to affect flow by increasing frictional pressure losses, especially near the wellbore.
Combinations of these recurring fracture structures yield complex fracture networks. A prominent example of a complex fracture network was observed in S01-00—true-triaxially confined shale with epoxy injection. This experiment produced a simple tensile planar bi-wing fracture, shear activation of a pre-existing discontinuity, and fluid penetration into bedding planes. Among these primary structures was evidence for offset fracture branching followed by traversing coalescence. No significant smooth reorientation of fractures was observed, likely because the injection well was drilled parallel to the maximum principal stress. Most fracture branching occurred at the small scale, but some branching also occurred at the macro-scale with influence from shear activated discontinuities. When offset fracture branches were traced back to the injection borehole, they were found to share a common plane. It is clear that the complex hydraulic fractures stimulated in this experiment penetrate a larger fracture volume and greater surface area than a simple tensile fracture alone would have.
Small-scale fracture patterns
Microscopic inspection of the cut cross sections from the laboratory hydraulic fracture experiments revealed fracture complexity similar to the macroscopic scale, as shown in Fig. 3. At the macroscopic perspective, this complexity is often lumped into the description of roughness and generated fines (i.e., loose particles). Experiments using epoxy injection provided the best detail on microscopic fracture structures and produced the examples shown in Fig. 3. From observation, simple tensile fractures included propagation by void nucleation and coalescence and by transgranular fracturing. Shear activation was typically observed with intergranular fracturing; however, microscale intergranular fracturing was also observed within macro-scale simple tensile fractures. A special case of microscale fracturing included granular fracture bifurcation which is a branching process likely related to offset branching. Another special case included transportable grain release where fracture propagation was found to mobilize fine-sized particles. Without a detailed analysis or support from granular fracture models, evaluations of the mechanisms by which these structures arise would be merely hypothetical. The implications of these mechanisms are more self-evident, where void nucleation, fracture coalescence, and intergranular fracturing all contribute to fracture roughness which in turn causes increased frictional pressure losses in flowing fluids. Transportable grain release is an interesting special case, in that transportable fines could act as natural proppant or flow-blocking particles. Granular fracture bifurcation is another interesting special case, in that it creates increased surface area within macroscopic fractures and also potentially contributes to macroscopic branching.