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Holland M.,RWTH Aachen | Holland M.,Geomechanics International Inc. | Urai J.L.,RWTH Aachen
Journal of Structural Geology | Year: 2010

We studied a special type of zebra carbonate in limestones of an overpressure cell exhumed from at 5. km depth, in outcrops on Jabal Shams, Oman Mountains. The rocks show anastomosing patterns of regularly spaced calcite veins in dark gray, fine-grained carbonate; microscopic observations reveal these as dense bundles of much finer veinlets, typically 10-50 μm thick. The vein bundles are up to 5. mm thick, they contain multiple sub-parallel arrays of host rock fragments embedded in the coarse-grained vein calcite. We interpret these structures as the result of numerous mechanically effective crack and reseal events together with strong growth competition or crystallization from sparse nucleation sites. Cementation produced mechanically strong veins so that new fractures were localized along the vein/rock interface or within the matrix itself. We present simple conceptional models relating the mechanical strength of the vein and the morphology of the resulting vein network. © 2009 Elsevier Ltd. Source

Fernandez-Ibanez F.,Geomechanics International Inc. | Perez-Pena J.V.,University of Granada | Azor A.,University of Granada | Soto J.I.,University of Granada | Azanon J.M.,University of Granada
Geology | Year: 2010

Fluvial piracy in the Guadix-Baza basin (southeast Spain) promoted erosion of a high volume of sediments during the late Quaternary, after the former internal drainage of this basin changed to external due to headward erosion by the Guadalquivir River. As a response to load release, this basin underwent uplift, which, in turn, enhanced fluvial incision and erosional processes. Differential erosion within the Guadix-Baza basin resulted in dissimilar isostatic rebound. We numerically model the lithospheric response to sediment unloading and show a total rising of 15 m in the Guadix subbasin compared to 2 m in the Baza subbasin. This differential uplift is likely to have been accommodated along the Baza normal fault, which has been active throughout the Quaternary. Modeling results suggest that ~13% of the total Quaternary fault throw directly arises from the differential isostatic readjustment between subbasins. This example is one of the first estimates of fault slip partitioning between tectonic and isostatic effects due to unloading driven by river incision. © 2010 Geological Society of America. Source

Park N.,Geomechanics International Inc. | Olson J.E.,University of Texas at Austin | Holder J.,University of Texas at Austin
SPE Drilling and Completion | Year: 2010

Wellbore stability in shale has been a crucial issue for drilling in all kinds of environments. The analysis of time-dependent wellbore stability in shales has largely concentrated on the influence of fluid chemistry and filtrate invasion into the formation to predict compressive failure using poroelasticity and continuum models. This paper presents another possible mechanism for time-dependent behavior-stress-corrosion cracking (subcritical crack growth). Using the discrete-element method (DEM) to simulate grainscale processes, we apply the concept of time-dependent cracking to hole enlargement for vertical wellbores. We use a published example from the North Sea to verify the stress-corrosion model and demonstrate the application to wellbore stability in shale. Laboratory results on rocks indicate a wide range of susceptibility to stress-corrosion cracking related to rock petrology and contact-fluid chemistry. Using laboratory calibrated rock properties, we run 2D, plane-strain simulations of vertical-wellbore stability in shale, where hole enlargement is tracked through time. As a result of stress-corrosion cracking, the numerical models show a time-dependent failure history, with an initial stable period of varying duration (influenced by mud weight, rock properties, and in-situ stress), followed by a brief period of combined shear and tensile failure, and ending with stabilization at an enlarged, elliptically shaped geometry. Time to failure increases with increasing mud weight. Enlarged-hole shape changes from elliptical to roughly circular with decreasing stress anisotropy. These behaviors simulated by the stress-corrosion model coincide with previously reported field experience. This new modeling approach for time-dependent wellbore failure can be readily constrained with straight forward fracture-mechanics tests on rock samples and has the potential to also be applied to time-dependent, intermittent sand or fines generation during production. © 2010 Society of Petroleum Engineers. Source

Trautwein-Bruns U.,RWTH Aachen | Schulze K.C.,Geomechanics International Inc. | Becker S.,RWTH Aachen | Kukla P.A.,RWTH Aachen | Urai J.L.,RWTH Aachen
Tectonophysics | Year: 2010

In 2004 the 2544. m deep RWTH-1 well was drilled in the city centre of Aachen to supply geothermal heat for the heating and cooling of the new student service centre "SuperC" of RWTH Aachen University. Aachen is located in a complex geologic and tectonic position at the northern margin of the Variscan deformation front at the borders between the Brabant Massif, the Hohes Venn/Eifel areas and the presently active rift zone of the Lower Rhine Embayment, where existing data on in situ stress show complex changes over short distances. The borehole offers a unique opportunity to study varying stress regimes in this area of complex geodynamic evolution.This study of the in situ stresses is based on the observation of compressive borehole breakouts and drilling-induced tensile fractures in electrical and acoustic image logs. The borehole failure analysis shows that the maximum horizontal stress trends SE-NW which is in accordance with the general West European stress trend. Stress magnitudes modelled in accordance to the Mohr-Coulomb Theory of Sliding Friction indicate minimum and maximum horizontal stress gradients of 0.019. MPa/m and 0.038. MPa/m, respectively. The occurrence of drilling-induced tensile failure and the calculated in situ stress magnitudes are consistent with a model of strike-slip deformation.The observed strike-slip faulting regime supports the extension of the Brabant Shear Zone proposed by Ahorner (1975) into the Aachen city area, where it joins the major normal faulting set of the Roer Valley Graben zone. This intersection of the inherited Variscan deformation grain and the Cenozoic deformation resulting in recent strike-slip and normal faulting activity proves the tectonically different deformation responses over a short distance between the long-lived Brabant Massif and the Cenozoic Rhine Rift System. © 2010 Elsevier B.V. Source

van Gent H.W.,RWTH Aachen | Holland M.,RWTH Aachen | Holland M.,Geomechanics International Inc. | Urai J.L.,RWTH Aachen | Loosveld R.,Royal Dutch Shell
Journal of Structural Geology | Year: 2010

We present analogue models of the formation of dilatant normal faults and fractures in carbonate fault zones, using cohesive hemihydrate powder (CaSO4·H2O). The evolution of these dilatant fault zones involves a range of processes such as fragmentation, gravity-driven breccia transport and the formation of dilatant jogs. To allow scaling to natural prototypes, extensive material characterisation was done. This showed that tensile strength and cohesion depend on the state of compaction, whereas the friction angle remains approximately constant. In our models, tensile strength of the hemihydrate increases with depth from 9 to 50. Pa, while cohesion increases from 40 to 250. Pa. We studied homogeneous and layered material sequences, using sand as a relatively weak layer and hemihydrate/graphite mixtures as a slightly stronger layer. Deformation was analyzed by time-lapse photography and Particle Image Velocimetry (PIV) to calculate the evolution of the displacement field. With PIV the initial, predominantly elastic deformation and progressive localization of deformation are observed in detail. We observed near-vertical opening-mode fractures near the surface. With increasing depth, dilational shear faults were dominant, with releasing jogs forming at fault-dip variations. A transition to non-dilatant shear faults was observed near the bottom of the model. In models with mechanical stratigraphy, fault zones are more complex. The inferred stress states and strengths in different parts of the model agree with the observed transitions in the mode of deformation. © 2009 Elsevier Ltd. Source

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