Seismic characterization of multiple fracture sets: Does orthotropy suffice?

Geophysics ◽  
2006 ◽  
Vol 71 (3) ◽  
pp. D93-D105 ◽  
Author(s):  
Vladimir Grechka ◽  
Mark Kachanov
Keyword(s):  
Finite Element ◽  
Fractured Reservoirs ◽  
Naturally Fractured Reservoirs ◽  
Multiple Fracture ◽  
P Waves ◽  
Fracture Characterization ◽  
Surface Reflection ◽  
Reflection Data ◽  
Effective Media ◽  
Independent Parameters

As geophysicists rely increasingly on effective media theories to characterize naturally fractured reservoirs, they become more and more interested in evaluating the accuracy of different theories, estimating their limits of applicability, and assessing their usefulness for practice. With this in mind, we compare two popular seismological theories of Hudson and Schoenberg with the theory of Kachanov developed in the context of mechanics of materials. By performing finite-element simulations of effective media for models that contain several sets of nonintersecting, circular, vertical fractures embedded in otherwise isotropic host rock, we examine the accuracy of these theories. Our numeric study reveals that predictions of both the first- and second-order Hudson's theories are typically inferior to those of others, especially when the fractures are dry. While, on average, the theories of Schoenberg and Kachanov fit finite-element computations with comparable accuracy, the latter appears to be more useful for fracture characterization. The reason is thatit correctly predicts the proximity of crack-induced anisotropy to orthotropy, whereas the other theories do not. Kachanov's results not only yield approximate effective orthotropy regardless of the number of fracture sets, their crack densities, and orientations, but they also lead to a substantially reduced number of independent parameters that govern the effective stiffnesses. This number is only four for dry cracks, compared to nine required for general orthorhombic media. These four quantities can be chosen as two Lamé constants of the isotropic background and two principal crack densities. If fractures are filled with a compressible fluid, the number of independent parameters increases. After numeric verification of the accuracy of crack-induced orthotropy, we invert the NMO ellipses and zero-offset traveltimes of P-waves and two split shear waves for the parameters characterizing multiple fracture sets. Our inversion reveals the fracture parameters that can be unambiguously estimated from multiazimuth, multicomponent surface reflection data.

Feasibility of seismic characterization of multiple fracture sets

Geophysics ◽  
2003 ◽  
Vol 68 (4) ◽  
pp. 1399-1407 ◽  
Author(s):  
Vladimir Grechka ◽  
Ilya Tsvankin
Keyword(s):  
Host Rock ◽  
Fractured Reservoirs ◽  
Transversely Isotropic ◽  
Naturally Fractured Reservoirs ◽  
Multiple Fracture ◽  
Stiffness Tensor ◽  
Fracture Detection ◽  
Estimation Of Parameters ◽  
Fracture Characterization ◽  
Underlying Assumption

Estimation of parameters of multiple fracture sets is often required for successful exploration and development of naturally fractured reservoirs. The goal of this paper is to determine the maximum number of fracture sets of a certain rheological type which, in principle, can be resolved from seismic data. The main underlying assumption is that an estimate of the complete effective stiffness tensor has been obtained, for example, from multiazimuth, multicomponent surface seismic and vertical seismic profiling (VSP) data. Although typically only a subset of the stiffness elements (or some of their combinations) may be available, this study helps to establish the limits of seismic fracture‐detection algorithms. The number of uniquely resolvable fracture systems depends on the anisotropy of the host rock and the rheology and orientation of the fractures. Somewhat surprisingly, it is possible to characterize fewer vertical fracture sets than dipping ones, even though in the latter case the fracture dip has to be found from the data. For the simplest, rotationally invariant fractures embedded in either isotropic or transversely isotropic with a vertical symmetry axis (VTI) host rock, the stiffness tensor can be inverted for up to two vertical or four dipping fracture sets. In contrast, only one fracture set of the most general (microcorrugated) type, regardless of its orientation, is constrained by the effective stiffnesses. These results can be used to guide the development of seismic fracture‐characterization algorithms that should address important practical issues of data acquisition, processing, and inversion for particular fracture models.

scholarly journals Integrated fracture characterization of Asmari reservoir in Haftkel field

2022 ◽  
Author(s):  
Kourosh Khadivi ◽  
Mojtaba Alinaghi ◽  
Saeed Dehghani ◽  
Mehrbod Soltani ◽  
Hamed Hassani ◽  
...  
Keyword(s):  
Fractured Reservoirs ◽  
Numerical Algorithms ◽  
Fracture Network ◽  
Naturally Fractured Reservoirs ◽  
Development Plan ◽  
Production Data ◽  
Efficient Manner ◽  
Fracture Characterization ◽  
Future Production ◽  
Fracture Modeling

AbstractThe Asmari reservoir in Haftkel field is one of the most prolific naturally fractured reservoirs in the Zagros folded zone in the southwest of Iran. The primary production was commenced in 1928 and continued until 1976 with a plateau rate of 200,000 bbl/day for several years. There was an initial gas cap on the oil column. Gas injection was commenced in June 1976 and so far, 28% of the initial oil in place have been recovered. As far as we concerned, fracture network is a key factor in sustaining oil production; therefore, it needs to be characterized and results be deployed in designing new wells to sustain future production. Multidisciplinary fracture evaluation from well to reservoir scale is a great privilege to improve model’s accuracy as well as enhancing reliability of future development plan in an efficient manner. Fracture identification and modeling usually establish at well scale and translate to reservoir using analytical or numerical algorithms with the limited tie-points between wells. Evaluating fracture network from production data can significantly improve conventional workflow where limited inter-well information is available. By incorporating those evidences, the fracture modeling workflow can be optimized further where lateral and vertical connectivity is a concern. This paper begins with the fracture characterization whereby all available data are evaluated to determine fracture patterns and extension of fracture network across the field. As results, a consistent correlation is obtained between the temperature gradient and productivity of wells, also convection phenomenon is confirmed. The findings of this section help us in better understanding fracture network, hydrodynamic communication and variation of temperature. Fracture modeling is the next step where characteristics of fractures are determined according to the structural geology and stress directions. Also, the fault’s related fractures and density of fractures are determined. Meanwhile, the results of data evaluation are deployed into the fracture model to control distribution and characteristics of fracture network, thereby a better representation is obtained that can be used for evaluating production data and optimizing development plan.

A Cut-Cell Polyhedral Finite Element Model for Coupled Fluid Flow and Mechanics in Fractured Reservoirs

2021 ◽  
Author(s):  
Igor Shovkun ◽  
Hamdi A. Tchelepi
Keyword(s):  
Finite Element ◽  
Fluid Flow ◽  
Fractured Reservoirs ◽  
Computational Cost ◽  
Mechanical Deformation ◽  
Naturally Fractured Reservoirs ◽  
Fracture Model ◽  
Single Fracture ◽  
Cut Cell ◽  
Flux Approximation

Abstract Mechanical deformation induced by injection and withdrawal of fluids from the subsurface can significantly alter the flow paths in naturally fractured reservoirs. Modeling coupled fluid-flow and mechanical deformation in fractured reservoirs relies on either sophisticated gridding techniques, or enhancing the variables (degrees-of-freedom) that represent the physics in order to describe the behavior of fractured formation accurately. The objective of this study is to develop a spatial discretization scheme that cuts the "matrix" grid with fracture planes and utilizes traditional formulations for fluid flow and geomechanics. The flow model uses the standard low-order finite-volume method with the Compartmental Embedded fracture Model (cEDFM). Due to the presence of non-standard polyhedra in the grid after cutting/splitting, we utilize numerical harmonic shape functions within a Polyhedral finite-element (PFE) formulation for mechanical deformation. In order to enforce fracture-contact constraints, we use a penalty approach. We provide a series of comparisons between the approach that uses conforming Unstructured grids and a Discrete Fracture Model (Unstructured DFM) with the new cut-cell PFE formulation. The manuscript analyzes the convergence of both methods for linear elastic, single-fracture slip, and Mandel’s problems with tetrahedral, Cartesian, and PEBI-grids. Finally, the paper presents a fully-coupled 3D simulation with multiple inclined intersecting faults activated in shear by fluid injection, which caused an increase in effective reservoir permeability. Our approach allows for great reduction in the complexity of the (gridded) model construction while retaining the solution accuracy together with great saving in the computational cost compared with UDFM. The flexibility of our model with respect to the types of grid polyhedra allows us to eliminate mesh artifacts in the solution of the transport equations typically observed when using tetrahedral grids and two-point flux approximation.

scholarly journals Seismic characterization of multiple fracture sets at Rulison Field, Colorado

Geophysics ◽  
2007 ◽  
Vol 72 (2) ◽  
pp. B19-B30 ◽  
Author(s):  
Ivan Vasconcelos ◽  
Vladimir Grechka
Keyword(s):  
Velocity Model ◽  
Multiple Fracture ◽  
Fracture Characterization ◽  
Seismic Reflection Data ◽  
Characterization Methods ◽  
S Waves ◽  
Reflection Data ◽  
Seismic Characterization ◽  
Inversion Procedure

Conventional fracture-characterization methods assume the presence of a single set of aligned, vertical cracks in the subsur-face. We relax this assumption and demonstrate the feasibility of seismic characterization of multiple fracture sets. Our technique relies on recent numerical findings indicating that multiple, differently oriented, possibly intersecting planar cracks embedded in an otherwise isotropic host rock result in a nearly orthorhombic (or orthotropic) effective medium. Here, the governing parameters of crack-induced orthotropy are estimated from 3D, wide-azimuth, multicomponent seismic reflection data acquired over the tight-gas Rulison Field in Colorado. We translate strong azimuthal variations of the normal-moveout velocities intointerval crack densities, fracture orientations, type of fluid infill, and velocities of P- and S-waves in an unfractured rock. Our inversion procedure identifies a set of cracks aligned in approximately west northwest-east southeast direction in the western part of the study area and multiple, likely intersecting fractures in its eastern part. We validate both our underlying theoretical model and the obtained estimates by two independent measurements: (1) the estimated fluid-infill parameter indicates dry cracks as expected for the gas-producing sandstones at Rulison; and (2) the obtained crack orientations are supported by well observations. As a by-product of fracture characterization, we build an anisotropic velocity model of the Rulison reservoir which, we believe, is the first orthorhombic velocity field constructed from surface seismic data.

Seismic inversion for the parameters of two orthogonal fracture sets in a VTI background medium

Geophysics ◽  
2002 ◽  
Vol 67 (1) ◽  
pp. 292-299 ◽  
Author(s):  
Andrey Bakulin ◽  
Vladimir Grechka ◽  
Ilya Tsvankin
Keyword(s):  
Seismic Data ◽  
Fractured Reservoirs ◽  
Crack Density ◽  
Transversely Isotropic ◽  
Seismic Inversion ◽  
Naturally Fractured Reservoirs ◽  
Multiple Fracture ◽  
Effective Parameters ◽  
Wide Range

Characterization of naturally fractured reservoirs often requires estimating parameters of multiple fracture sets that develop in an anisotropic background. Here, we discuss modeling and inversion of the effective parameters of orthorhombic models formed by two orthogonal vertical fracture sets embedded in a VTI (transversely isotropic with a vertical symmetry axis) background matrix. Although the number of the microstructural (physical) medium parameters is equal to the number of effective stiffness elements (nine), we show that for this model there is an additional relation (constraint) between the stiffnesses or Tsvankin's anisotropic coefficients. As a result, the same effective orthorhombic medium can be produced by a wide range of equivalent models with vastly different fracture weaknesses and background VTI parameters, and the inversion of seismic data for the microstructural parameters is nonunique without additional information. Reflection moveout of PP‐ and PS‐waves can still be used to find the fracture orientation and estimate (in combination with the vertical velocities) the differences between the normal and shear weaknesses of the fracture sets, as well as the background anellipticity parameter ηb. Since for penny‐shaped cracks the shear weakness is close to twice the crack density, seismic data can help to identify the dominant fracture set, although the crack densities cannot be resolved individually. If the VTI symmetry of the background is caused by intrinsic anisotropy (as is usually the case for shales), it may be possible to determine at least one background anisotropic coefficient from borehole or core measurements. Then seismic data can be inverted for the fracture weaknesses and the rest of the background parameters. Therefore, seismic characterization of reservoirs with multiple fracture sets and anisotropic background is expected to give ambiguous results, unless the input data include measurements made on different scales (surface seismic, borehole, cores).

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