The paradigm change of hydraulic fracture stimulation is to artificially create reservoirs in unconventional source rocks, where production was previously unimaginable. Effective stimulation requires either the connection to existing natural fractures or the presence of geomechanical brittleness capable of supporting extensive induced fractures. As very few tight formations are thick, continuous and homogenous, so any given well can be uneconomic to the point of never paying out (negative cash flow). The variation between wells in economic ultimate recovery in these tight formations is due to the heterogeneity of stimulated fracture potential or fracability, porosity and saturation that are critical to mapping “sweetspots” for development drilling and optimizing completion parameters. Consequently 3D, AVO and AVO variation with azimuth (AVAZ) to detect anisotropy due to fractures, stress or over-pressure are almost routinely applied for the development success of such plays.
The Wiki definition of fractures (http://en.wikipedia.org/wiki/Fracture_(geology)) is “any local separation or discontinuity plane in a geologic formation, such as a joint or a fault that divides the rock into two or more pieces. Fractures are commonly caused by stress exceeding the rock strength, causing the rock to lose cohesion along its weakest plane.” The simplicity of this definition confirms the tri-axial geomechanical models used in the description of the subsurface, thereby enabling azimuthal reflection seismic inversion to remotely sense fractures despite the usual underdetermined condition of remote sensing methods. However excepting post-stack spatial volume attributes such as curvature, propagating seismic waves do not directly sense fractures, but instead the anisotropy arising from the directional heterogeneity of fractures or the differential stresses that produce them in the first place.
These controlling stresses are relatively simple consisting of normal and tangential components of a second order tri-axial tensor that determines various anisotropic symmetries. The seismic signature of these stresses can be measured through an anisotropic fourth order stiffness tensor composed of λ’s and μ’s relating stress to elastic strain in Hooke’s law. The anisotropy in these tensors are described as fractional relative changes in terms of Thomsen parameters ε(v), δ(v) and γ as well as Schoenberg’s “linear slip model” ΔN and ΔT for a simple HTI medium, with ε(v), δ(v), ΔN and γ, ΔT being the respective natural extensions to the isotropic AVO attributes of λρ and μρ from P-wave and S-wave impedance.
The three articles in this special RECORDER edition on fractures describe and introduce the potential of interesting new methods such as 4D and AVO variation with azimuth (AVAZ) to detect anisotropy due to natural fractures or in-situ stress that would ideally corroborate the mapping of optimal fracture prone zones from the standard isotropic AVO.
The first paper “A Math-free Look at Azimuthal Surface Seismic Techniques” by Franck Delbecq et al is an excellent tutorial on pre-stack azimuthal AVO methods to estimate the confusing variation of the attributes briefly described above. The article covers some unique new methods such as azimuthal Fourier coefficient decomposition that resolves fundamental ambiguities in conventional AVAZ arising from its limitation that estimates a single combined P- and S-wave anisotropy and thereby is unable to correctly detect the orientation and degree of anisotropy. The authors pessimistically point out that the interest in characterizing fractured reservoirs using a variety of azimuthal techniques may be diminished once the results from two or more methods are shown to differ and may even be opposed! Despite the tutorial nature of the article it is refreshingly and deliberately mathfree. This is in response to an oft-heard request: "Please, no equations!" The authors hope that the loss of precision that comes from describing these ideas without using equations is compensated by a fresh perspective on their significance. One conclusion that is drawn as to why the estimates of anisotropy are different depending on the azimuthal technique used is that the methods basically sense varying dimensions of the rock in different directions. The authors point out that travel time methods such as S-wave splitting and P-wave Velocity Variation with Azimuth (VVAz) measure layer anisotropic properties, while reflectivity based azimuthal AVO methods measure interface properties giving rise to scale or resolution complexity and varying combinations of the Thomsen and Schoenberg parameters.
The second paper “Microseismic, 3D and 4D Applications and its Relation to Geomechanics and Completion Performance” by Iverson et al, is a case study describing the use of passive, active and time lapse seismic to investigate the large variation in completion’s performance and consequent production from a series of multi-stage multi-well pads. The study establishes empirical relationships between microseismic, 3D inversion attributes and 4D seismic and production in the unconventional shale of the Horn River Basin. Production variations between pads are explained through the use of these methods and a careful integration of stimulated rock volume and are shown to correlate with a highly stressed fault zone which intersects the northern portion of the main pad in the study. Estimates of stimulated rock volume are shown to result from differentiable frack fluid and proppant effects that are ascertained by microseismic event locations as well as the 4D mapped response. Finally it is shown that high b values occur in brittle rock and in a lower stress regime while low b-value stages occur in ductile rock in a relatively higher stress regime.
The final paper “4D attenuation analysis for permeability estimates in hydraulically induced fractures” by Cho et al, is a companion paper to Iverson et al, and shows that the effects of wave induced fluid motion between fractures and pores that result in complex elastic stiffness coefficients can produce measurable frequency-dependent attenuation. Based on this theory, spectral analysis of the seismic wavefield is used to obtain direct permeability estimates. The case study analyzes the time-lapse attenuation response from a 4D acquired before and after hydraulic fracturing to investigate the permeability within the stimulated zone. The results obtained from this analysis qualitatively confirm the microseismic observations and 4D seismic amplitude and traveltime anomalies, indicating the presence of a permeable fault/fracture system that diverted the hydro-fracture energy from the treatment to re-stimulate isolated portions of the in-situ rock.
I would like to acknowledge and thank the primary authors for their contribution to this special RECORDER edition on fractures. All the articles are technically illuminating with a clear exposition of a complicated topic and also describe some novel methods that have progressed seismic applications to estimate fractures in the subsurface.