Static and Dynamic Moduli of Malm Carbonate: A Poroelastic Correlation

Abstract

The static and poroelastic moduli of a porous rock, e.g. the drained bulk modulus, can be derived from stress-strain curves in rock mechanical tests and the dynamic moduli, e.g. dynamic Poisson's ratio, can be determined by acoustic velocity and bulk density measurements. As static and dynamic elastic moduli are different a correlation is often required to populate geomechanical models. A novel poroelastic approach is introduced to correlate static and dynamic bulk moduli of outcrop analogues samples, representative of Upper-Malm reservoir rock in the Molasse basin, southwestern Germany. Drained and unjacketed poroelastic experiments were performed at two different temperature levels (30 and 60• C). For correlating the static and dynamic elastic moduli, a drained acoustic velocity ratio is introduced, corresponding to the drained Poisson's ratio in poroelasticity. The strength of poroelastic coupling, i.e. the product of Biot and Skempton coefficients here, was the key parameter. The value of this parameter decreased with increasing effective pressure by about 56% from 0.51 at 3 MPa to 0.22 at 73 MPa. In contrast, the maximum change in Pand S-wave velocities was only 3% in this pressure range. This correlation approach can be used in characterizing underground reservoirs, and can be employed to relate seismicity and geomechanics (seismo-mechanics).

Stress sensitivity of porosity and permeability under varying hydrostatic stress conditions for different carbonate rock types of the geothermal Malm reservoir in Southern Germany

“…Pei et al (2018) observed an inelastic deformation during preconditioning even after 12 loading/unloading cycles before this effect disappears completely. Hassanzadegan et al (2016) made similar observations of time-dependent deformation and interpreted this behavior as creep, inelasticity or plasticity. The growth of subcritical cracks might be responsible for this time-dependent deformation under constant stress in a brittle regime (Nicolas et al 2017) that can be explained by a combination of enhanced plastic flow and pressure solution in carbonate Fig.…”

“…Regarding the investigations of carbonate rocks from the Upper Jurassic Malm reservoir of the Molasse Basin, studies have been rather limited so far. The poroelastic (static) and dynamic moduli of outcrop analogues samples were investigated by Hassanzadegan et al (2016) by the derivation of stress-strain curves in rock mechanical tests and measurement of acoustic velocities. The authors conducted drained and unjacketed poroelastic experiments at different temperatures and examined the strength of poroelastic coupling (product of Biot and Skempton coefficients), which helps to improve the link between seismicity and geomechanics in reservoir characterization.…”

“…For permeability, the dependency on initial pore volume is of secondary importance, since the development of the pore geometry and the interconnection through pore throats have a greater influence on the flow behavior (Lucia 2007). Penny-shaped pores with a low aspect ratio and crack-like pores can significantly reduce permeability if preferred flow paths are closed by compression, which might be independent of the total pore volume (Hassanzadegan et al 2016). These pore types were observed in the samples dL2 and D5 as channel pores along microfractures, both showing a relatively high stress sensitivity of permeability (Figs.…”

“…12b. The main cause of permeability loss is thus considered here to be the closure of penny-shaped and fracture pores with a low aspect ratio, micro-fractures, and small sized effective pore throats (Benson et al 2006;Xu et al 2018;Hassanzadegan et al 2016;Cant et al 2018).…”

The effect of Biot coefficient and elastic moduli stress–pore pressure dependency on poroelastic response to fluid injection: laboratory experiments and geomechanical modeling

“…A constant stress‐independent Biot coefficient as well as elastic moduli (i.e., Young's modulus and Poisson's ratio) is typically used to estimate poroelastic response of injection such as surface uplift 26 to predict reservoir's stress path 1,2,23–25,27 or model fault activation and induced seismicity 5–9,28 . However, rock physics experiments on different rock types (including sandstone, 29–35 carbonate, 29,34,36,37 and shale 38 ) show that Biot coefficient as well as elastic moduli could vary with either pore pressure or confining stress. Biot coefficient decreases with stress when pore pressure is constant and increases with pore pressure when stress is constant 30,38 .…”