Direct Computation of Fracture Network Equivalent Porous Medium Properties using Digital Outcrop Models
P33 (m3/m3)
4.6 x 10-5
0
Dr Thomas Seers Petroleum Engineering, Texas A&M University at Qatar (
[email protected])
Digital Outcrop Geology Active (lidar) and passive (photogrammetry) optical remote sensing techniques producing 3D surface models of rigid scenes: digital outcrop models Produced datasets are fairly equivalent: vertices / tri-mesh / calibrated cameras Overlay of calibrated photography produces high degree of geological realism Flexible in terms of scale: kilometers to decimeters
Terrestrial lidar
UAVphotogrammetry photogrammetry (UAV): 1 of 30
Digital Outcrop Geology Produces fully interrogatable models… we can measure stuff Permits measurements away from the outcrop edge… we can measure stuff safer
Allows mapping of geology away from the field… we can measure more stuff Enables greater access to the sampling domain… we can measure even more stuff
Amenable to numerical techniques… we can measure stuff faster and smarter
Terrestrial lidar
UAVphotogrammetry photogrammetry (UAV): 2 of 30
Current Paradigms in Digital Fracture Characterization Digital outcrop based fracture characterization commonly consists of: • Use automated / semi-automated planar extraction to define orientation statistics: rapid / simple to implement but selective • Extract ‘hard to get’ statistics from outcrop sub-areas (length, density, connectivity) using manual tools: slow, little advantage over surveys
…can we do more?
Lyons-Baral, 2012: using Split FX
After Gomes et al. 2016: planar extraction using PCA (C&G)
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Outcrop Constrained Facies Models Since the adoption of terrestrial laser scanning in outcrop geology (c. 2005), facies models have been constructed using two point geostatistics and object models Provides insights in to the 3D distribution of sub-seimic scale sedimentary architecture
Can we do something similar with outcropping fracture networks? Outcrop constrained facies model: Rarity et al. (2014)
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EPM Properties from Outcrop Models Digital outcrops to reservoir equivalent porous medium properties ① Fracture trace extraction
② DFN modelling
③ Equivalent porous medium fracture network property modelling (DFN upscaling)
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EPM Properties from Outcrop Models
Extraction and analysis of 3D fracture trace networks from digital outcrop models
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Three Dimensional Trace Maps 3D Traces Extraction • • • •
Polyline interpretation from textured 3D meshes: truncation issues Orthophotos: e.g. Banjaree & Mitra 2005, Vasuki et al. 2014 Mesh curvature: Umili et al. 2013 Optical ray tracing: Seers & Hodgetts (2016a), Geosphere - Trace length, termination styles, orientation*
* Deterministic constraints on 3D structural architecture
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Three Dimensional Trace Maps 3D Traces Extraction • • • •
Polyline interpretation from textured 3D meshes: truncation issues Orthophotos: e.g. Banjaree & Mitra 2005, Vasuki et al. 2014 Mesh curvature: Umili et al. 2013 Optical ray tracing: Seers & Hodgetts (2016a), Geosphere - Trace length, termination styles, orientation*
* Deterministic constraints on 3D structural architecture
Pinhole camera model
Optical ray tracing using calibrated image sequences
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Pixel Based Fracture Characterization Why use pixel based approaches? • Allows native image resolution of the camera to be exploited (zero down-sampling) • Facilitates the use of a wide suite of pixel based image processing routines
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Pixel Based Fracture Characterization Why use pixel based approaches? • Allows native image resolution of the camera to be exploited (zero down-sampling) • Facilitates the use of a wide suite of pixel based image processing routines
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3D Trace Map Extraction • Routine tested using photogrammetric dataset: Lacey’s Caves, NW UK (Aeolian
sandstones hosting cataclastic shear bands) • ~ 800 3D trace objects extracted from 25 images • Features spanning contiguous images merged producing 645 trace objects
Input outcrop model 9 of 30
3D Trace Map Properties
Connectivity map
Areal Intensity (P 21)
3D trace maps enable numerous fracture properties to be extracted from digital outcrop model datasets: • 3D trace connectivity maps • Termination styles and topological characteristics • Trace length size distributions • Vertex attribute maps of fracture intensity (P21: areal intensity, extendable to P32: volumetric intensity using conversion factors: e.g. Wang, 2005) 10 of 30
Trace Orientation • Seers & Hodgetts (2016b), Journal of Structural Geology • Obtaining reliable fracture orientation measurements and determining fracture sets is a fundamental analysis stage in any fracture characterization study • Most 3D traces are degenerate in terms of a least squares plane solution: high collinearity (K) • Obtain confidence intervals on least squares plane precision at different intervals of K
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Trace Orientation • Seers & Hodgetts (2016b), Journal of Structural Geology • Obtain confidence intervals on least squares plane precision at different intervals of K using Monte Carlo simulations of mesh-mesh intersection (plasma fractals) • Use to condition a numerical solver: simulated orientations for collinear traces • High quality orientation data (low collinear traces, extracted planes, compass clino measurements) used to condition the solver
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Trace Orientation • Structural traces reduce the rotational degrees of freedom of their associated discontinuity from a notional system of three to one. Assume K, cluster proximity and angular distance to the outcrop controls probability density characteristics of a given pole vector along this great circle
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EPM Properties from Outcrop Models
Near Deterministic Discrete Fracture Network (DFN) Modelling
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DFN Modelling Statistical representation of a fractured network comprising of discrete objects (lines – 2D / polygons – 3D). First order consideration should be whether the
DFN implementation is appropriate for modelling the studied network: • Unbounded fractures: Classic ‘Baecher DFN model’ (Baecher et al, 1977: Seers and Hodgetts, 2014) • Mechanically bounded and unbounded fractures honoring termination patterns: ‘modified Baecher model’ (e.g. Grenon et al. 2017) • Mechanically layered DFN models honoring mechanical units (Cottrell, 2014) • Mechanically driven models (Davy et al. 2013): conceptually attractive but difficult to
condition from field datasets
Stochastic DFN
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DFN Modelling Fracture Abundance (P21: m/m2 / P32: m2/m3)
Fracture Aperture / Transmissivity • • •
•
• • •
Image logs / core Well tests (pump tests) Lab based petrophysical measurements
Seismic attributes & seismic surfaces (field curvature /
Fracture Orientation
fault proximity based proxies) Well logs (FMI) &
geostatistical modelling Outcrop measurements Geomechanical modelling (strain magnitude analysis)
Fracture Size (2D: length / 3D: radius) •
•
Well logs (i.e. if mechanical layer thickness is the primary control) Outcrop datasets***
• •
• •
Well logs (FMI) Conceptual geological models (of fold / fault damage zone architecture) Measurements from outcrop Geomechanical modelling (Analysis of stress vectors)
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Outcrop Constrained DFN Modelling • (Near) deterministic implementation of the classic Baecher DFN • Trace map controls the location, orientation and size characteristics of an outcrop constrained DFN • Non-deterministic component modelled implicitly (grid based) • Currently used to estimate EPM porosity permeability (Oda tensor method)
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Outcrop Constrained DFN Modelling
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EPM Properties from Outcrop Models
Equivalent Porous Medium Fracture network Property Modelling (DFN Upscaling)
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DFN Upscaling: Porosity Equivalent Porous Medium (EPM) fracture porosity: The ratio between the volume of the intersected DFN and the total upscaled volume
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DFN Upscaling: Oda’s Tensor EPM fracture permeability (Oda’s fractured rock mass k tensor): The permeability tensor (Kijk) is viewed as a function of the crack tensor (Pijk) of the fractured rock mass
Nijk is the product of the directional cosines of 𝑓over the solid angle ɷ and the reciprocal of their resultant vector magnitude. Nijk and the pdf of fracture area 𝛼 are given by
with the EPM permeability tensor of the fracture system given by 21 of 30
DFN Upscaling: Spherical Kernel Kijk and Φfracture calculated across the outcrop surface Spherical kernel used to locate the intersection area/volume of the DFN around each vertex on the parent outcrop model mesh: numerical scheme generates a vertex attribute map of EPM properties across the outcrop model surface
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Mishrif Formation, UAE
ROI
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Mishrif Formation, UAE
ROI
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Trace Extraction and DFN 25 x 20 m section of the Rams quarry was mapped in detail for fracture property extraction. 215 traces were mapped using in-house developed tools and modelled using a bespoke deterministic DFN modelling code.
Pixel resolution 3D render
3D trace map 24 of 30
Trace Extraction and DFN 25 x 20 m section of the Rams quarry was mapped in detail for fracture property extraction. 215 traces were mapped using in-house developed tools and modelled using a bespoke deterministic DFN modelling code.
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Fracture Intensity and Porosity Vertex attribute maps of the Rams-Mishrif outcrop displaying volumetric fracture intensity (P32) and porosity (P33). Both attribute maps were calculated using a spherical kernel with a 1 m radius. Maps indicate low storavity / intensity within thw mapped fracture array.
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Fracture Intensity and Porosity Vertex attribute maps of the Rams-Mishrif outcrop displaying volumetric fracture intensity (P32) and porosity (P33). Both attribute maps were calculated using a spherical kernel with a 1 m radius. Maps indicate low storavity / intensity within thw mapped fracture array.
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Equivalent Fracture Permeability Per-vertex measures of equivalent fracture permeability (max, mdedial and min principle permeability: k1 k2 k3). Note that the mean vector of k 1 is approximately NE for the Mishrif outcrop model, attributable to the dominance of NW-SW sub-vertical fractures. Attribute maps generated using a spherical kernel with a 2.5 m radius.
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Equivalent Fracture Permeability Per-vertex measures of equivalent fracture permeability (max, mdedial and min principle permeability: k1 k2 k3). Note that the mean vector of k 1 is approximately NE for the Mishrif outcrop model, attributable to the dominance of NW-SW sub-vertical fractures. Attribute maps generated using a spherical kernel with a 2.5 m radius.
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Equivalent Fracture Permeability Per-vertex measures of equivalent fracture permeability (max, mdedial and min principle permeability: k1 k2 k3). Note that the mean vector of k 1 is approximately NE for the Mishrif outcrop model, attributable to the dominance of NW-SW sub-vertical fractures. Attribute maps generated using a spherical kernel with a 2.5 m radius.
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Outcrop Constrained DFN Applications Use vertex attribute maps to condition geostatistical simulations Condition DFNs with property cubes derived from outcrop data to obtained improved realism in fracture distributions Directly generate EPM models using vertex attribute maps of upscaled EPM properties
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Outcrop Constrained DFN Applications Investigate the representative elementary volume of natural fracture networks Grow kernel progressively and estimate EPM properties within at each volume Are continuum-like assumptions appropriate for natural fracture networks?
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Conclusions • Digital outcrop models are numerical objects: we can use them for
more that crude planar extraction and ‘digital tape measure’ based studies • An approach has been presented which enables near deterministic reconstructions of fracture architecture to be obtained from digital outcrop models: amenable to the computation of EPM properties •
Can be used to gain insights into the petrophysical architecture of natural fracture systems - are continuum assumptions appropriate? - direct modelling of EPM from outcrop - flow simulation using deterministic fracture networks 29 of 30
Acknowledgements • University of Manchester: David Hodgetts • Texas A&M University at Qatar: Yuhe Wang and
Mohamed Fadlelmula The financial assistance of Qatar Foundation, Total E&P UK, ExxonMobil and the AAPG is gratefully acknowledged
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