Regional Fractures I: A Mechanism for the Formation of Regional Fractures at Depth in Flat-Lying Reservoirs (1)

Lorenz, John C.; Teufel, Lawrence W.; Warpinski, N.R.

doi:10.1306/0c9b29e3-1710-11d7-8645000102c1865d

Cited by 48 publications

(7 citation statements)

References 89 publications

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“…Critical crack growth occurs when P f > σ n , i.e., when hydraulic fracturing takes place [Segall, 1984;Engelder and Lacazette, 1990]. This stress condition is, however, relatively rare in most subsurface rocks and does not explain the abundance of open natural fractures [Atkinson, 1984;Segall, 1984;Lorenz et al, 1991;Swarbrick et al, 2002].…”

Section: Models For Aperture Predictionmentioning

confidence: 99%

The impact of different aperture distribution models and critical stress criteria on equivalent permeability in fractured rocks

2016

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Predicting equivalent permeability in fractured reservoirs requires an understanding of the fracture network geometry and apertures. There are different methods for defining aperture, based on outcrop observations (power law scaling), fundamental mechanics (sublinear length‐aperture scaling), and experiments (Barton‐Bandis conductive shearing). Each method predicts heterogeneous apertures, even along single fractures (i.e., intrafracture variations), but most fractured reservoir models imply constant apertures for single fractures. We compare the relative differences in aperture and permeability predicted by three aperture methods, where permeability is modeled in explicit fracture networks with coupled fracture‐matrix flow. Aperture varies along single fractures, and geomechanical relations are used to identify which fractures are critically stressed. The aperture models are applied to real‐world large‐scale fracture networks. (Sub)linear length scaling predicts the largest average aperture and equivalent permeability. Barton‐Bandis aperture is smaller, predicting on average a sixfold increase compared to matrix permeability. Application of critical stress criteria results in a decrease in the fraction of open fractures. For the applied stress conditions, Coulomb predicts that 50% of the network is critically stressed, compared to 80% for Barton‐Bandis peak shear. The impact of the fracture network on equivalent permeability depends on the matrix hydraulic properties, as in a low‐permeable matrix, intrafracture connectivity, i.e., the opening along a single fracture, controls equivalent permeability, whereas for a more permeable matrix, absolute apertures have a larger impact. Quantification of fracture flow regimes using only the ratio of fracture versus matrix permeability is insufficient, as these regimes also depend on aperture variations within fractures.

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Section: Models For Aperture Predictionmentioning

confidence: 99%

The impact of different aperture distribution models and critical stress criteria on equivalent permeability in fractured rocks

2016

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“…Characterizing the DFN requires an access to the reservoir rock at reservoir related depth which is not present. However, we have reasonable concerns to expect comparable discontinuity orientations in our geological subsurface model as in our studied outcrops (Lorenz et al, 1991;Narr, 1991). For example, Narr (1991Narr ( , 1996 presented how to derive DFN from drill core samples of reservoirs.…”

Section: Discussionmentioning

confidence: 79%

“…Pre-existing discontinuities oriented parallel to the maximum horizontal stress could be observed openly, while discontinuities oriented perpendicular to this stress tend to be closed (Lorenz et al, 1991). It can therefore be assumed that higher fracture permeability can be expected in the N−S direction.…”

Section: Orientation Of the Fracture Networkmentioning

confidence: 99%

Characterization of fractures in potential reservoir rocks for geothermal applications in the Rhine-Ruhr metropolitan area (Germany)

Balcewicz¹,

Ahrens²,

Lippert³

et al. 2020

Preprint

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Abstract. The importance of research into clean and renewable energy solutions has increased over the last decade. Geothermal energy provision is proven to meet both conditions. Therefore, conceptual models for deep geothermal applications were developed for different field sites regarding different local conditions. In Bavaria, Germany, geothermal applications were successfully carried out in carbonate horizons at depth of 4000 to 6000 m. High permeability rates combined with sufficient thermal conductivities were mainly studied in karstified carbonates from the Late Jurassic reef facies. Similar to Bavaria, carbonates are located in the east of the Rhenohercynian Massif, in North Rhine-Westphalia (NRW), which quantification of the geothermal potential is still lacking. Compared to Bavaria, a supraregional carbonate mountain belt is exposed at the Remscheid-Altena anitcline (NRW) from late Devonian and early Carboniferous times. The aim of our study was to examine the potential geothermal reservoir by field and laboratory investigations. Therefore, three representative outcrops in Wuppertal, Hagen Hohenlimburg, and Hönnetal were studied. During field surveys, 1068 discontinuities at various spacial scales were observed by scanline surveys. These discontinuities were characterized by trace length, true spacing, roughness, aperture, and filling materials. Joint orientation analysis indicated three dominant strike orientations in NNW–SSE, NW–SE, and NE–SW directions within the target horizon of interest. This compacted limestone layer (Massenkalk) is approximately 300 m thick and located at 4000 to 6000 m depth, dipping northwards at a shallow dip angle of about 30 to 40°. An extrapolation of the measured layer orientation and dip suggests that the carbonate reservoir extends below Essen, Bochum, and Dortmund. Our combined analysis of the field and laboratory results has shown that it could be a naturally fractured carbonate reservoir. We evaluated the potential fracture network in the reservoir and its orientation with respect to the prevailing maximum horizontal stress before concluding with implications for fluid flow: We proposed to focus on discontinuities that are approximately N–S oriented for upcoming geothermal applications, because the geothermal potential of the characterized reservoir matrix is insufficient for deep geothermal applications. Our results indicate that even higher permeability can be expected for karstified formations related to the reef facies. Our compiled data set consisting of laboratory and field measurements may provide a good basis for 3D subsurface modeling and numerical prediction of fluid flow in the naturally fractured carbonate reservoir. Further studies have to be elaborated to verify, if the fractured reservoir could possibly be reactivated by, for instance, hydraulic stimulation and thus enable geothermal applications.

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“…This analysis showed that the most common orientation of faults in the boreholes was nearly vertical, with a WNW-ESE to NW-SE strike Lim et al, 2010]. Typically, where fracture orientations are widely distributed in folded sedimentary rocks, but concentrated in a specific orientation in unfolded strata, such fractures are likely to have been formed by regional tectonic stress prior to folding [e.g., Lorenz et al, 1991;Silliphant et al, 2002;Mynatt et al, 2009]. Consequently, the characteristic orientation of faults in both the Wakkanai and Koetoi formations strongly suggests they formed as vertical faults just before the area was folded.…”

Section: Tectonic History Of the Mudstonesmentioning

confidence: 99%

“…However, such an increase in pore pressure is difficult to estimate accurately. In this study, when the stress state changed from that favoring normal faulting to that favoring strike slip, in response to tectonic compression near the time of maximum burial, it is assumed that only the maximum horizontal effective stress (s′ H ) increased and that the minimum horizontal effective stress (s′ h ) decreased under the influence of pore pressure increase, according to the classical model of Lorenz et al [1991]. In addition, the increase of pore pressure (DP p ) is assumed to be below s′ h , as determined from s′ Vmax in equation (6) and the following equation (8), because pore pressure does not usually exceed s′ h .…”

Section: Bri At the Maximum Burial Depthmentioning

confidence: 99%

The relationships among brittleness, deformation behavior, and transport properties in mudstones: An example from the Horonobe Underground Research Laboratory, Japan

et al. 2011

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[1] Mudstones are low-permeability sedimentary rocks; however, when shear stresses induced by tectonic movement or nonhydrostatic stresses exceed the shear strength of the rock, brittle or ductile deformation occurs. The nature of this deformation is controlled by the brittleness of the mudstone. If brittle deformation occurs, the resulting dilatant structures may increase the permeability and change the transport properties of the strata. This paper addresses the relationships among brittleness, deformation behavior, and transport properties in mudstones at the Horonobe Underground Research Laboratory, Japan. Geological, mechanical, and hydrogeological data from borehole investigations and laboratory tests were systematically interpreted using a brittleness index (BRI), which is the ratio of the unconfined compressive strength to the effective vertical stress. For mudstones under natural strain rates and low temperatures, ductile deformation occurs when BRI <2, brittle/ductile deformation occurs when BRI is 2-8, and brittle deformation occurs when BRI >8, although semibrittle behavior may also occur at the brittle-ductile boundary. When BRI >8 and faulting is well developed, the mudstone behaves hydrogeologically as a fractured medium at the mesoscopic scale, whereas for BRI <8 the mudstone behaves hydrogeologically as a porous medium, even if faulting is extensive. The BRI concept is a useful tool for systematically characterizing the hydromechanical behavior of mudstones; for example, when assessing the effectiveness of mudstone as a long-term barrier in disposal repositories for radioactive waste.Citation: Ishii, E., H. Sanada, H. Funaki, Y. Sugita, and H. Kurikami (2011), The relationships among brittleness, deformation behavior, and transport properties in mudstones: An example from the Horonobe Underground Research Laboratory, Japan,

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Regional Fractures I: A Mechanism for the Formation of Regional Fractures at Depth in Flat-Lying Reservoirs (1)

Cited by 48 publications

References 89 publications

The impact of different aperture distribution models and critical stress criteria on equivalent permeability in fractured rocks

The impact of different aperture distribution models and critical stress criteria on equivalent permeability in fractured rocks

Characterization of fractures in potential reservoir rocks for geothermal applications in the Rhine-Ruhr metropolitan area (Germany)

The relationships among brittleness, deformation behavior, and transport properties in mudstones: An example from the Horonobe Underground Research Laboratory, Japan

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