[1] When the power law equation, _ e / s n exp(ÀQ/RT ), which relates strain rate (_ e), stress (s), gas constant (R), and temperature (T ), is used to describe thermally activated dislocation creep of calcite rocks, the stress sensitivity (n) and temperature sensitivity or apparent activation energy (Q) differ greatly from rock to rock. To better constrain parameters of a mechanical equation of state, we performed triaxial deformation experiments on dense synthetic aggregates of polycrystalline calcite at temperatures of 873-1073 K and strain rates between 5 Â 10 À7 and 3 Â 10 À3 s À1 to strains <0.20. The strength of the marbles decreases with increasing temperature or decreasing strain rate. Combining microstructure analysis with mechanical data indicates that strength decreases with increasing grain size (d) following a Hall-Petch relation. A detailed analysis of the data revealed systematic dependence of n and Q on stress, grain size, and temperature. The variations in n and Q can be accommodated by using a Peierls law, _ e P = A P s 2 exp(s/ s P )exp(ÀQ P /RT). The resistance to glide, s P , is composed of an intrinsic Peierls stress and a grain size dependent back stress and is given by s P = (AE P,0 + Kd À0.5 )(T m À T ), where T m denotes the melting temperature. The following parameters seem to apply to all calcite rocks: A P % 10 ±0.5 MPa À2 s À1 , Q P % 200 kJ/mol, AE P,0 % 7.8 MPa kK À1 , and K % 115 MPa kK À1 mm 0.5 . Under some laboratory conditions, dislocation creep may operate simultaneously with grain size sensitive diffusion creep, complicating the quantification of the individual flow laws. More accurate flow laws will need to include the evolution of microstructure in composite flow laws, perhaps requiring a statistical description of a microstructure variable yet to be specified exactly.
Abstract. We investigated the effect of mechanical deformation on transport properties by deforming synthetic calcite/quartz aggregates to strains (e) up to 5øA at confining pressures (Pc) up to 300 MPa, and at temperatures (T) from 300 K to 873 K. Subsequently, we measured permeability (k) using a wide-range permeameter at effective pressures (Pc) of 15-155 MPa and room temperature. We then measured electrical conductivity (rr) in an impedancemeasuring assembly over the same pressure range at room temperature. The values of permeability and conductivity of the undeformed material at Pe=100 MPa were 4xl 0 -28 m 2 and 0.07 S/m. Samples deformed at 673 K and P½=200 MPa, or at room temperature and P½=50-300 MPa, show small variations in permeability and conductivity: k changed only by up to a factor of 3 and c• increased by up to 10%. But, when a sample was deformed at 873 K and P½=200 MPa, electrical conductivity dropped by 1 order of magnitude and permeability dropped by 2 orders of magnitude. To assess whether changes in length scales of the pore structure owing to deformation may account for large variation in transport properties, we counted cracks and pores, measured their lengths and widths, defined a damage parameter (•), and computed effective hydraulic and electrical conductivity using renormalization group methods. The undeformed rocks and the samples deformed at low confining pressure have severe damage, appear to be close to failure, and hence have high transport coefficients. Materials deformed at high pressures and temperatures have lower flaw densities, cormectivities, and transport coefficients after deformation. We found that renormalization methods are suitable to model connectivity loss and large changes in transport properties owing to changes in flaw density and length scales. Pore connectivity and transport properties vary strongly during semibrittle deformation.
[1] To quantify the effect of rigid inclusions on the flow behavior of a creeping matrix, we fabricated a suite of two-phase marbles by hot isostatically pressing mixtures of calcite and quartz powders and subsequently deformed these synthetic rocks at 300 MPa confining pressure, up to 100 MPa pore pressure, temperatures ranging from 873 to 1073 K, strain rates and stresses ranging from $10 À7 s À1 to $10 À3 s À1 and $4 MPa to 190 MPa, respectively. In constant displacement rate tests performed to axial strains of up to about 30%, we observed deformation at approximately constant stress for samples with 20% or less quartz. Even small additions of second phase significantly strengthen the aggregate, although the magnitude of the strengthening varies significantly with temperature and imposed strain rate. Investigation of the microstructure of starting samples reveals that initial porosity directly varies with quartz content and that matrix grain size inversely varies with the amount of second phase. Strength decreases modestly with increasing porosity, and consequently, strength increases as the samples compact. The effect of matrix grain size on the aggregate strength depends on the deformation conditions. The majority of tests exhibit an inverse relation between strength and matrix grain size (i.e., similar to a Hall-Petch relation), but fine-grained samples deformed at the highest temperatures exhibit decreasing strength with decreasing grain size. The addition of the quartz particles seems to result in both structural strengthening owing to decreased grain size and to transfer of load from the flowing matrix to the rigid quartz particles.
SUMMARY It is suggested that fluid injection in normal faulting stress regimes can stabilize a reservoir if the stress path is high enough. This stabilization is not seen when the reservoir is significantly cooled as a result of injection. Further, a new strategy is suggested for stimulating reservoirs in shear with a reduced chance of inducing a large magnitude seismic event. The version of this methodology presented here is applicable for reverse faulting stress regimes and involves an initial stress preconditioning stage where the reservoir is cooled and the pressure increase is limited. This process reduces the horizontal total stress and thereby also the differential stress. Next, the reservoir is stimulated with a rapid increase in pore pressure, resulting in shear failure at a lower differential stress than was initially present in the reservoir. Due to the connection seen between the Gutenberg–Richter b-value and differential stress, it is suggested that reservoirs stimulated in this fashion will exhibit higher b-values and thereby also have a reduced chance of hosting a large magnitude event. It is suggested that adaptations of this methodology are applicable to both normal and strike-slip faulting stress regimes.
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