A significant body of published work has developed establishing fracture-related seismic anisotropy as an observable effect. To further the understanding of seismic birefringence techniques in characterizing natural fracture systems at depth, an integrated program of seismic and geologic measurements has been conducted at Conoco's Borehole Test Facility in Kay County, Oklahoma. Birefringence parameters inferred from the seismic data are consistent with a vertical fracture model of density 0.04 striking east-northeast. That direction is subparallel to a fracture set mapped both on the surface and from subsurface data, to the in situ maximum horizontal stress, and to the inferred microfabric. INTRODUCTION Natural fractures control fluid flow in many subsurface hydrocarbon reservoirs. Fractures can also strongly influence groundwater flow in the shallow subsurface [Dreier et al., 1987; Evans and Nicholson, 1987]. For those reasons, techniques such as shear wave birefringence for characterization of natural fracture systems may have significant impact on hydrocarbon production, waste disposal, and hydrology. Natural fractures can impart a significant permeability anisotropy to hydrocarbon reservoirs. For example, a permeability anisotropy attributed to fractures is documented in many subsurface reservoirs in eastern Oklahoma [Carnes, 1966; Szpakiewicz et al., 1986]. Fractures mapped in Mesaverde core from the multiwell experiment (MWX) site in the Piceance Basin enhance reservoir permeability by 1-2 orders of magnitude, with permeability anisotropy calculated at 100:1 [Lorenz et al., 1986].Characterizing fracture anisotropy from shear wave birefringence may also have application to earthquake prediction [Crampin, 1978 if the technique proves sufficiently sensitive to detect and monitor minor changes in stresssensitive fracture and microfracture systems. It has been suggested that such changes are precursory to some earthquakes [e.g., Aggarwal et al., 1973; Peacock et al., 1988].A significant body of published work has developed, establishing fracture-related seismic anisotropy as an observable effect. Shear wave anisotropy in earthquake data is
We recorded high‐resolution (1 to 10 kHz), crosswell and single well seismic data in a shallow (15 to 35 m), water‐saturated, fractured limestone sequence at Conoco's borehole test facility near Newkirk, Oklahoma. Our objective was to develop seismic methodologies for imaging gas‐filled fractures in naturally fractured gas reservoirs. The crosswell (1/4 m receiver spacing, 50 to 100 m well separation) surveys used a piezoelectric source and hydrophones before, during, and after an air injection that we designed to displace water from a fracture zone. Our intent was to increase the visibility of the fracture zone to seismic imaging and to confirm previous hydrologic data that indicated a preferred pathway. For the single well seismic imaging (a piezoelectric source and an eight‐element hydrophone array at 1/4 m spacing), we also recorded data before and after the air injection. The crosswell results indicate that the air did follow a preferred pathway that was predicted by hydrologic modeling. In addition, the single well seismic imaging using vertical common depth‐point (CDP) gathers indicated an anomaly consistent with the anomaly location of crosswell and hydrologic inversion results. Following the field tests, a slant well was drilled and cored to confirm the existence and nature of the rock associated with the seismic anomalies. A vertical fracture was intersected within less than 1 m of where the seismic results had predicted.
S U M M A R YThe behaviour of shear wave polarizations and shear wave splitting observed at the surface suggesting propagation through parallel vertical cracks has been the stimulus for many recent investigations, both in earthquake and exploration seismology. Cracks in surface outcrops, however, frequently display multiple sets of parallel vertical intersecting cracks. This paper examines seismic shear wave propagation in media with two sets of parallel vertical cracks (biplanar cracks) to determine whether the behaviour of shear waves can distinguish between the effects of multiple crack sets and the effects of single sets of parallel cracks (monoplanar cracks). This study shows that the difference between the overall patterns of polarizations of biplanar and monoplanar systems of vertical cracks within the shear wave window in many circumstances is marginal, and unlikely to be easily recognized in the field. We conclude that it is frequently not possible, from analysis of surface observations of shear wave polarizations alone, to distinguish between the effects of biplanar sets of parallel vertical cracks and those of a single parallel set. The difference can usually be recognized if an accurate estimate of both polarizations and time delays between the split shear waves is available over a wide range of azimuths and angles of incidence within the shear wave window. However, in areas with complex fracture and stress systems, time delays may be much harder to estimate than the polarization angles of the leading split shear waves, and it may not be easy to distinguish, from seismic data alone, the difference betwekn parallel and multiplanar sets of vertical fractures.
We discuss two inverse approaches to construction of fracture-flow models and their application in characterizing a fractured limestone formation. The first approach creates "equivalent discontinuum" models that conceptualize the fracture system as a partially filled lattice of conductors that are locally connected or disconnected to reproduce the observed hydrologic behavior. An alternative approach-i.e., "variable aperture lattice" models-represent the fracture system as a fully filled network composed of conductors of varying apertures or hydraulic conductivities. The fracture apertures are sampled from a specified distribution, usually log-normal, which is consistent with field data. The spatial arrangement of apertures is altered through inverse modeling to fit the available hydrologic data, such as transient pressure and/or tracer data.Unlike traditional discrete fracture-network approaches that rely on fracture geometry to reproduce flow and transport behavior, the inverse methods directly incorporate hydrologic data in deriving the fracture networks and thus naturally emphasize the underlying features that impact fluid flow and transport. However, hydrologic models derived by inversion are nonunique in general. We have addressed such nonuniqueness by examining an ensemble of models that satisfy the observation data within acceptable limits. We then determine properties that are shared by the ensemble of models and their associated uncertainties to create a conceptual model of the fracture system. We show the fracture-flow model to be consistent with geophysical imaging.
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