Mapping the brittle-ductile transition in shales
Abstract
The Marcellus shale is the lowest unit of Devonian age in the Hamilton Group. It is an organic-rich shale located in the Appalachian Basin and contains an estimated ~1.4 trillion cubic meters of natural gas. The majority of gas is held in matrix pore space, with vertical fractures providing additional storage and acting as primary flow pathways. However, commercial production of the gas requires the use of directional drilling and hydraulic fracturing to generate additional permeable pathways. Understanding the response of the Marcellus to stresses created by horizontal drilling and more importantly by hydraulic fracturing is critical for wide-scale commercialization of the resource. We investigated the mechanical behavior of shales from the Marcellus formation, with an emphasis on understanding controls on its rheology and brittle-ductile behavior. Although black shale is the dominant lithology of the Marcellus, interbeds of low-density organic-rich shale and limestone are also present. We conducted experiments on three lithologies: 1) true “paper shale” (fissile, finely layered) with density ρ≈2.5 g/cm3 and porosity φ=6.3%, 2) a low density (ρ≈1.45 g/cm3) organic rich shale with porosity φ=39.2% (organic subunit 1), and 3) a lower density (ρ≈1.05 g/cm3) organic rich lithology with porosity φ=50.8% (organic subunit 2). We performed experiments on cylindrical samples 25-mm in diameter and 50-mm in length in a triaxial configuration (σ1≠σ2=σ3). Samples were deformed using both gas and water as pore fluid, using a displacement rate boundary condition (velocity of 0.1 to 10 μm/s corresponding to axial strain rates of 2.07e-4 s-1 to 1.63e-2 s-1), and under confining pressures ranging from 0 to 50 MPa. Additionally, we conducted permeability experiments with water (flow through) and helium gas (pulse) at an effective confining pressure of 10 MPa. Our experiments show brittle behavior for the fissile shale unit, including a peak in differential stress, failure, and a stress drop to a residual level. Failure stress shows a systematic increase with confining pressure, typical of brittle behavior. Post experiment visual inspection indicates localization of deformation along planes that are inclined 40°-50° from vertical. Experiments on organic subunit 1 show less brittle behavior; the stress-strain curves exhibit a peak stress and post peak strain hardening. With increasing confining pressure, the behavior evolves from brittle to ductile, with an evolution from localized deformation to cataclastic flow. Experiments on organic subunit 2 show a less brittle behavior, characterized by strain hardening, and we observe compaction and cataclastic flow along discrete zones with no evidence of localized fracture. Under isostatic conditions and with water as the permeant, measured permeabilities of organic subunit 1 is on the order of 5.5e-15 m2. Pulse-measured permeabilities to non-sorbing gaseous helium are similar to those for water in the order of 8.8e-15 m2.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.T41B2121S
- Keywords:
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- 8010 STRUCTURAL GEOLOGY / Fractures and faults;
- 8020 STRUCTURAL GEOLOGY / Mechanics;
- theory;
- and modeling;
- 8045 STRUCTURAL GEOLOGY / Role of fluids