Geologic perspectives on geophysical prospecting for natural fractures

Anisotropy and beyond: Geologic perspectives on geophysical prospecting for natural fractures RANDALL MARRETT, STEPHEN E. LAUBACH, and JON E. OLSON, The University of Texas at Austin, USA

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ecent geologic research on natural fractures challenges assumptions frequently made by geophysicists. Open fractures are not necessarily oriented parallel to the maximum horizontal stress, and fractures do not necessarily close when the fluid pressure within them is reduced. Even in the most mechanically favorable environment, precipitated cements can prop fractures open or seal fractures of any orientation. Fracture sets typically show dispersion in strike, and multiple sets of open fractures can coexist. More importantly, fractures comprise populations that commonly range over orders of magnitude in aperture and length and that occur in nonuniform clusters. Instead of iso- Figure 1. Rose diagrams of maximum compressive stress (SHmax, upper row, blue) and open fraclated, regularly spaced, large, equally ture strike (lower row, red) for study areas in Texas and Wyoming. (a) SHmax and (b) fracture compliant fractures, the Earth presents strike, East Texas, average ENE fracture strike is similar to SHmax trends, but open fractures have complex fractal clustering of fractures a spread of 130°; (c) SHmax and (d) fracture strike, four West Texas wells. (e) SHmax and (f) fracture strike, western Green River Basin; (g) SHmax and (h) fracture strike southern Powder River having a wide range of sizes and vari- Basin, horizontal well image log data (after Laubach et al., 2004). able compliance dictated by natural cements in the fractures and the rock mass. Going beyond anisotropy to document these essen- discarded. Among seismically important attributes, fractial fracture attributes in the interwell space is a key chal- ture openness can determine the extent of mechanical coupling across fractures, fracture orientation may control the lenge for geophysicists. direction of velocity anisotropy, and fracture sizes and abunMotivation for seismic characterization of fractures. dance can control the magnitude of seismic signature Fractures are notoriously challenging to study in the sub- (Marrett, 1997). Open fracture length may be related to the surface, where they can have dramatic effects on fluid flow. magnitude of velocity anisotropy, and could also affect difDirect study of subsurface fractures, using logs or core from fraction patterns. Here we outline some recent core, outcrop, and modelboreholes, is hampered by several sampling problems. Fractures commonly are nearly vertical, so vertical wellbores based findings on natural fracture populations that suggest are unlikely to intersect many fractures. Sampling proba- that subsurface fracture patterns are highly heterogeneous bility also is poor because the spacing between conductive on a range of scales and in a variety of ways. Thus, there fractures typically is large in comparison with borehole are first-order implications for the expected seismic response diameters. Additionally, heterogeneity of fractures com- of fractures that are typically not accounted for in current monly occurs on length scales that are a fraction of well spac- geophysical approaches. ing, so important lateral changes in fracture attributes can remain undiscovered. Heterogeneity can also be manifest Fracture geology. Outcrops commonly contain sets of large, as significant variation of fractures from one layer to the next. more-or-less planar, mostly evenly spaced, barren (no minBecause of the limitations in wellbore-based observations, eral fill), opening-mode fractures. Known since the early seismic detection and characterization of fractures poten- days of geology (Pollard and Aydin, 1988; Cosgrove and Engelder, 2004), these joints undoubtedly influence common tially offers valuable tools for subsurface prospecting. The presence of fracture signal in seismic reflection data conceptions of fractures. Yet, inherently sparse information is detected by studying anisotropic behavior of seismic about subsurface fractures suggests they differ from joints velocities and amplitudes (Queen and Rizer, 1990; in some important ways. Cores and well logs, particularly Schoenberg and Sayers, 1995). However, anisotropic attrib- image logs, provide direct samples of subsurface fractures, utes are challenging to measure, and their interpretation may but fracture sampling using wells is notoriously incomplete. rely on assumptions about fracture orientations, shapes, For example, prior to the advent of horizontal wells, subopenness, sizes, and spatial arrangement that are challeng- surface fracture size and spatial distributions were mostly ing to verify independently. Based on the geologic literature, conjectural. Consequently, geologists have long utilized outgeophysicists frequently assume that fractures are oriented crops containing exhumed fractures as a proxy source of parallel to the maximum horizontal stress, that fractures information on subsurface fractures. Outcrops, however, close when the fluid pressure within the fractures is reduced, are subject to uplift and weathering-induced fractures that and that there is a single set of parallel, evenly spaced, open are nonrepresentative of the subsurface. We do not believe that a consensus exists in the geologic fractures. Many of these assumptions need to be revised or 000

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Figure 2. Subsurface and outcrop data on open and sealed fractures. (a) Rocks may have many sets of open fractures or none (modified from Laubach and Ward, 2006). (b) Even in the most mechanically favorable environment, precipitated cements can seal any orientation fracture (modified from Laubach, 2003).

accumulating cement deposits. Core studies show (Figure 2) and modeling studies predict that in otherwise open fractures, isolated deposits of cement are common. Laboratory tests show that partial mineral fill can make fracture aperture insensitive to static changes in effective stress. The prevalence of strong, spatially isolated mineral bridges that resist fracture closure is not widely appreciated. Cement precipitation in the host rock during or immediately after fracture formation is another mechanism that can increase the resistance of natural fractures to closing. This process, which is probably widespread, essentially freezes fractures open. Lander et al. (2002) showed that, without sealing fractures, as much as 20% whole rock volume of quartz cement can precipitate in a rock’s pore space after fractures form. Thus, fractures do not necessarily close when the fluid pressure within them is reduced, even if they lack cement bridges (Figure 2). A simple calculation demonstrates how host rock stiffening can affect fracture aperture compressibility (Olson et al., 2007). Assuming linear elasticity and plane strain, the compliance of fracture aperture (opening per unit driving stress) for a fracture of a given total length, L, can be written as

where v is Poisson’s ratio, Δσ is the driving stress, and E is Young’s modulus. If after initial fracture opening (Figure 3, A to B), diagenetic cementation in the host rock increased Young’s modulus by a factor of 5, for instance, fracture compliance would be reduced by that same factor and it would take five times the stress change to close the Figure 3. Plot of fracture opening (aperture) versus driving stress. When driving stress is zero fracture (Figure 3, B to C) as it took to (A), the fracture is closed. Increasing driving stress by increased pore pressure or reducing crackopen it. normal compression causes the crack to open (A to B). If diagenesis occurs at time B, crack closing takes more stress because of host rock stiffening effect (after Olson et al., 2007). Of course, fracture aperture can be closed without any kinematic aperture community about the general attributes of subsurface frac- change by the diagenetic process of cement precipitation, tures. Nevertheless, geologic observations do test the valid- which is insensitive to fracture compliance or fracture-stress ity of assumptions about fractures that geophysicists accept orientation relationships. Empirical evidence shows that as generally true. For example, Figure 1 shows that open heterogeneous patterns of infilling of large fractures by fractures are not necessarily oriented parallel to current-day cements is a common occurrence. Core demonstrates that maximum horizontal stress. In these examples, stress ori- sealed and open fractures having identical strike can be entation data from reliable measurements are consistent interspersed over vertical distances that range from a few with regional stress maps. Yet observations from extensive meters or less to decimeters and over lateral distances of m core collections show that open fractures can have arbitrary to km. Production and core data demonstrate that it is the strike relative to SHmax (maximum horizontal compression). degree of cement fill in fractures rather than fracture oriProduction data show that these fractures also govern fluid entation that limits fluid flow. Flow occurs only where fracflow. tures are not sealed with cement. In the absence of reliable Chemical processes can account for resistance of frac- measurements of both open fracture strike and SHmax, these tures to closure. Fractures at depth in sedimentary basins features should not be presumed to be parallel. Even in the are exposed to hot (>80°C), mineral-laden water. In this most mechanically favorable environment, precipitated environment, reactive fracture surfaces are susceptible to cements can seal any orientation fracture (Laubach, 2003). 000

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be less velocity anisotropy than from either of the fracture sets alone. The case of nonorthogonal fracture sets might be diagnosed if fractures have different normal and shear compliances. That rock may contain fracture sets having differing orientations has long been appreciated, but recent studies show that rocks may have many sets of fractures that are open concurrently. And even given only one Figure 4. Fracture patterns generated using randomly oriented starter cracks and a subcritical set of fractures (formed by one deforpropagation model for increasingly anisotropic prefracture strain states (from a to c) followed by mation event), the assumption that all isotropic biaxial extension. individuals within that set are parallel may not always be reasonable. Some core data sets show substantial dispersion in strike for nominally coeval fractures that cannot be ascribed to core orientation errors. Locally, fracture sets in outcrop also show wide strike dispersion. This variable fracture orientation in some cases can be attributed to perturbations of stress fields caused by the presence of large faults (Rawnsley et al., 1992) or to nearly isotropic loading conditions that may promote random or orthogonal fracture patterns to develop during a single loading event. Geomechanical modeling results (Figure 4) show how differences in strain anisotropy for a given deformation event can significantly affect the orientation of natural, opening mode fracture sets. Beyond orientation and fracture cementation, another crucial fracture attribute is size. Subsurface fractures in a single set commonly have apertures and lengths that range over orders of magnitude in size, with small fractures far more abundant than large fractures (Marrett et al., 1999). In such cases, fractures of different size share orientations, kinematics, and timing relative to diagenetic events, so they are most simply interpreted as differFigure 5. Abundance of fractures as a function of fracture size. Fractures commonly have aperent size fractions of a single fracture tures and lengths that range over orders of magnitude in size and follow power-law frequency set with a common genesis. One condistributions. Fractures from the Marble Falls Limestone, Texas, follow a common power-law distribution of apertures, even though one subset of the data was collected at outcrop scale using sequence of the broad spectrum of a hand lens whereas other data were collected petrographically in a microscope (Marrett et al., fracture sizes is that fracture intensity, 1999). The power-law regression to data from microfractures is extrapolated for comparison with the abundance of fractures in space, is data collected along a 60-m line of observation at outcrop scale. inherently scale-dependent, and varies as a function of minimum observed Recognition of, and distinction among, uncemented frac- fracture size. As the threshold for counting fractures is tures, partially cemented fractures containing cement decreased, fracture intensity increases rapidly. Another conbridges, and completely cemented fractures is essential in sequence is that average fracture size is poorly defined and effective reservoir characterization and is a challenge for seis- depends sensitively to detection threshold. mic methods. This might be feasible if mineral fill within a These aspects of fracture size distributions are probfracture (e.g., mineral bridges) affects shear compliance lematic, because models of seismic velocity anisotropy variacross the fracture differently than normal compliance ation with fracture characteristics (e.g., Thomsen, 1993) are (Sayers and Dean, 2001). defined in terms of average fracture size and fracture intenGiven that open fractures exist in the subsurface, their sity. To the extent that fracture size distributions follow sysorientation is often surmised from velocity anisotropy. tematic patterns, theory can be modified to account for However, multiple sets of fractures are common, and if a realistic fracture parameters (e.g., Marrett, 1997). For examsecond set forms at a high angle with the first set, then ple, fracture apertures commonly follow power-law distrivelocity is decreased in all azimuths and the net effect can butions (Figure 5), which can be characterized by two 000

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about 20 m apart. Fracture clustering may affect seismic response in at least two ways. First, because the largest fractures tend to be clustered, the probability for them to be connected is much higher that it would otherwise be. Mechanical connectivity among fractures should magnify compliance and enhance velocity anisotropy compared with isolated fractures of the same size and abundance. Second, velocity anisotropy may be heterogeneous on length scales that are long compared with individual fractures but short compared with seismic wavelengths. The extent to which such heterogeneity can be teased from the seismic signal remains to be addressed.

Figure 6. Arrangement of fractures in space. Instead of regularly spaced fractures of comparable size, complex clustering of fractures having a wide range of sizes is common. (a) Fractures in the Marble Falls Limestone, Texas, are arranged in clusters, where the largest-aperture fractures tend to lie. (b) Autocorrelation function (Davis, 2002) of indicator series (value of 1 within fractures, value of 0 between fractures) documents concentration of fractures inside of clusters that are ~5 m wide and spaced ~20 m apart. This is the same data set as used in Figure 5 to represent outcrop-scale observations.

parameters that replace fracture intensity and average fracture size in formulations of the magnitude of velocity anisotropy. Models of seismic velocity anisotropy typically presume a statistically uniform arrangement of fractures in space (e.g., Thomsen, 1995), but subsurface fractures commonly are clustered (Figure 6a). Moreover, the largest fractures tend to occur in clusters. For example, Figure 6b shows the autocorrelation function (Davis, 2002) using logarithmically graduated lags for the data shown in Figure 6a. Positive autocorrelation characterizes the fractures for almost all lags less than a few meters, indicating the fractures occur in meterscale clusters. Negative autocorrelation dominates lags of 5–15 m, and a spike of positive autocorrelation occurs for lags of 15–25 m. This pattern of autocorrelation suggests that ~15 m wide domains of unusually low fracture intensity lie between fracture clusters, the centers of which are spaced 000

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Discussion. The well-known challenges of obtaining meaningful geologic samples of fractures using boreholes (see, for example, Narr, 1996; Mauldon and Mauldon, 2005) underline the need for seismic information on hard-to-measure attributes. Unfortunately, most seismic data analysis techniques currently practiced are based on equivalent or effective media theories which assume unrealistic fracture geometries and distributions. Nonetheless, seismic methods offer the hope of measuring key fracture attributes between boreholes and on length scales that are most meaningful to fluid-flow simulation. Appreciating the complexity of fracture systems as illustrated by outcrop and subsurface geologic investigations will be essential for creating new seismic analysis tools and improved processing and analysis of seismic data.

Suggested reading. The Initiation, Propagation, and Arrest of Joints and other Fractures by Cosgrove and Engelder (Geological Society of London Special Publication 231, 2004). Statistics and Data Analysis in Geology by Davis (Wiley, 2002). Predicting and Characterizing Fractures in Dolostone Reservoirs: Using the Link between Diagenesis and Fracturing” by Gale et al. (Geological Society of London Special Publication 235, 2004). “Feasibility of seismic characterization of multiple fracture sets” by Grechka and Tsvankin (GEOPHYSICS, 2002). “Lithologic and structural controls on natural fracture distribution and behavior within the Lisburne Group, northeastern Brooks Range and North Slope subsurface, Alaska” by Hanks et al. (AAPG Bulletin, 1997). “Interaction between quartz cementation and fracturing in sandstone” by Lander et al. (AAPG Annual Convention, 2002). “Practical approaches to identifying sealed and open fractures” by Laubach (AAPG Bulletin, 2003). “Are open fractures necessarily aligned with maximum horizontal stress?” by Laubach et al. (Earth and Planetary Science Letters, 2004). “Coevolution of crack-seal texture and fracture porosity in sedimentary rocks: Cathodoluminescence observations of regional SEPTEMBER 2007

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fractures” by Laubach et al. (Journal of Structural Geology, 2004b). “Diagenesis in porosity evolution of opening-mode fractures, Middle Triassic to Lower Jurassic la Boca Formation, NE Mexico” by Laubach and Ward (Tectonophysics, 2006). “Permeability, porosity, and shear-wave anisotropy from scaling of open-fracture populations” by Marrett (RMAG Fractured Reservoirs: Characterization and Modeling Guidebook, 1997). “Extent of power-law scaling for natural fractures in rock” by Marrett et al. (Geology, 1999). “Fracture sampling on a cylinder: From scanlines to boreholes to tunnels” by Mauldon and Mauldon (Rock Mechanics and Rock Engineering, 2005). “Estimating average fracture spacing in subsurface rock” by Narr (AAPG Bulletin, 1996). Combining Diagenesis and Mechanics in Natural Fracture Network Characterization by Olson et al. (Geological Society of London Special Publication 270, 2007). “Progress in understanding jointing over the past century” by Pollard and Aydin (GSA Bulletin, 1988). “An integrated study of seismic anisotropy and the natural fracture system at the Conoco Borehole Test facility, Kay County, Oklahoma” by Queen and Rizer (Journal of Geophysical Research, 1990). “Joint development in perturbed stress fields near faults” by Rawnsley et al. (Journal of Structural Geology, 1992). “Azimuth-dependent AVO in reservoirs containing non-orthogonal fracture sets” by Sayers and Dean (Geophysical Prospecting, 2001). “Seismic anisotropy of fractured rock” by Schoenberg and Sayers (GEOPHYSICS, 1995). “Weak elastic anisotropy” by Thomsen (GEOPHYSICS, 1993). “Elastic anisotropy due to aligned cracks in porous rock” by Thomsen (Geophysical Prospecting, 1995). TLE Acknowledgments: We gratefully acknowledge comments from S. Fomel and support by the U.S. Department of Energy Office of Basic Energy Sciences and the Fracture Research and Application Consortium. Corresponding author: [email protected]

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