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Potsdam, Germany

Backers T.,Geomecon GmbH
76th European Association of Geoscientists and Engineers Conference and Exhibition 2014: Experience the Energy - Incorporating SPE EUROPEC 2014 | Year: 2014

When stimulating a reservoir at depth, the existing fractures are propagated and coalesce. Whilst at low stresses Mode I (tensile) fracturing is mostly dominating, at large overburden stresses Mode II (shear) becomes the dominant mechanism for fracture extension. The contribution analyses the influence of fracture orientation and stress path during a stimulation at depth and discusses the implications for the fracture extension. It is shown that Mode II becomes the dominant mode during hydraulic stimulation at depth >3km.

Rybacki E.,German Research Center for Geosciences | Meier T.,Geomecon GmbH | Dresen G.,German Research Center for Geosciences
Journal of Petroleum Science and Engineering | Year: 2016

Successful stimulation of shale gas reservoirs by hydraulic fracturing operations requires prospective rocks characterized by high brittleness to prevent fast healing of natural and hydraulically induced fractures and to decrease the breakdown pressure required to (re-) initiate a fracture. We briefly reviewed existing brittleness indices (B) and applied several, partly redefined, definitions relying on composition and deformation behavior on various, mainly European black shales with different mineralogical composition, porosity and maturity. Samples were experimentally deformed at ambient and elevated pressures (P) and temperatures (T), revealing a transition from brittle to semibrittle deformation behavior with increasing pressure and temperature. At given composition and deformation conditions, B values obtained from different definitions vary considerably. The change of B with applied deformation conditions are reasonably well captured by most definitions based on the stress-strain behavior, which do not correlate with the fraction of individual phases, e.g., clay content. However, at given deformation conditions, most composition-based indices show similar variations with bulk composition as those derived from stress-strain behavior. At low P-T conditions (≲4 km depth), where samples showed pronounced post-failure weakening, B values determined from composition correlate with those calculated from pre-failure stress-strain behavior and both correlate with the static Young's modulus. In this regime, the brittleness concept can help to constrain successful hydraulic fracturing campaigns and brittleness maybe estimated from core or sonic logs at shallow depth. However, long term creep experiments are required to estimate in-situ stress anisotropy and the healing behavior of hydraulically induced fractures. © 2016 Elsevier B.V.

Moeck I.,German Research Center for Geosciences | Backers T.,Geomecon GmbH
First Break | Year: 2011

Hydraulic stimulation is frequently used to enhance reservoir productivity. The aim of hydraulic stimulation is to increase the formation pressure by fluid injection to create artificial fractures that act as additional fluid pathways. But large-scale fluid injection as applied in hydrocarbon and geothermal reservoirs can also induce seismicity and fault reactivation depending on the reservoir geomechanics and stress regime. Recent case studies in stimulation of geothermal reservoirs have shown induced seismicity as an undesirable side effect which needs to be understood prior to massive fluid injection. Slip tendency analysis has been successfully used to characterize fault slip likelihood and fault slip directions in any stress regime. In our study, we applied slip tendency analysis to assess the reactivation potential of shear and dilational fractures in a deep geothermal reservoir in the North-East German Basin, based on the notion that slip on faults is controlled by the ratio of shear to normal effective stress acting on the plane of weakness. The results from slip tendency analysis are supported by the spatial distribution of recorded microseismicity, which indicates slip rather than extension along a presumed NE-striking failure plane. © 2011 EAGE.

Backers T.,Geomecon GmbH | Stephansson O.,Geomecon GmbH | Stephansson O.,Helmholtz Center Potsdam
Rock Mechanics and Rock Engineering | Year: 2012

The suggested method for the determination of Mode II fracture toughness is reviewed. This so-called Mode II loading in fracture mechanics, the crack faces slide relative to each other and displacements of the crack surfaces are in the crack plane and perpendicular to the crack front. For any specimen preparation treatment appropriate high precision tools should be used. The specimens should be right circular cylinders having a height L to diameter D ratio of 1:1 and a diameter D equal to 50 mm. The minimum information on each specimen shall include dimensions, specimen preparation routines, special observations made during specimen preparation, moisture content, and macroscopic description of the surface. The report of each experiment should include source of specimen as precisely as possible; location and orientation. The shear stress at failure is reported to increase with confining pressure for various rock types.

Shen B.,CSIRO | Kim H.-M.,Korea Institute of Geoscience and Mineral Resources | Park E.-S.,Korea Institute of Geoscience and Mineral Resources | Kim T.-K.,SK Engineering and Construction SKEC | And 4 more authors.
Rock Mechanics and Rock Engineering | Year: 2013

This paper describes a boundary element code development on coupled thermal-mechanical processes of rock fracture propagation. The code development was based on the fracture mechanics code FRACOD that has previously been developed by Shen and Stephansson (Int J Eng Fracture Mech 47:177-189, 1993) and FRACOM (A fracture propagation code - FRACOD, User's manual. FRACOM Ltd. 2002) and simulates complex fracture propagation in rocks governed by both tensile and shear mechanisms. For the coupled thermal-fracturing analysis, an indirect boundary element method, namely the fictitious heat source method, was implemented in FRACOD to simulate the temperature change and thermal stresses in rocks. This indirect method is particularly suitable for the thermal-fracturing coupling in FRACOD where the displacement discontinuity method is used for mechanical simulation. The coupled code was also extended to simulate multiple region problems in which rock mass, concrete linings and insulation layers with different thermal and mechanical properties were present. Both verification and application cases were presented where a point heat source in a 2D infinite medium and a pilot LNG underground cavern were solved and studied using the coupled code. Good agreement was observed between the simulation results, analytical solutions and in situ measurements which validates an applicability of the developed coupled code. © 2012 Springer-Verlag.

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