Katrine Alling Andreassen

Biot critical frequency applied to description of failure and yield of highly porous chalk with different pore fluids

Injection of water into chalk hydrocarbon reservoirs has led to mechanical yield and failure. Laboratory experiments on chalk samples correspondingly show that the mechanical properties of porous chalk depend on pore fluid and temperature. In case of water-saturated samples, the concentration and nature of dissolved salts have an effect. Water has a significant softening effect on elastic properties of chalk as calculated from wave data, and the softening increases with increasing critical frequency as defined by Biot. The critical frequency is the highest frequency where elastic wave propagation is controlled by solid-fluid friction. The reference frequency is thus a measure of this friction, and we propose that the fluid effect on mechanical properties of chalk may be the result of liquid-solid friction. We reviewed 622 published experiments on mechanical properties of porouschalk. The data include chalk samples that were tested at temperatures from 20 °C to 130 °C with the following pore fluids: fresh ...

Effective stresses and shear failure pressure from in situ Biot's coefficient, Hejre Field, North Sea

ABSTRACT We propose a combination of Biots equations for effective stress and the expression for shear failure in a rock to obtain an expression for minimum pore pressure in a stable vertical well bore. We show that a Biots coefficient calculated from logging data in the Hejre Field, North Sea, is significantly different from 1. The log‐derived Biots coefficient is above 0.8 in the Shetland Chalk Group and in the Tyne Group, and 0.6–0.8 in the Heno Sandstone Formation. We show that the effective vertical and horizontal stresses obtained using the log‐derived Biots coefficient result in a drilling window for a vertical well larger than if approximating Biots coefficient by 1. The estimation of the Biots coefficient is straightforward in formations with a stiff frame, whereas in formations such as shales, caution has to be taken. We discuss the consequence of assumptions made on the mineral composition of shales as unphysical results could be obtained when choosing inappropriate mineral moduli.

Induration and Biot’s Coefficient of Palaeogene Limestone

Induration and Biot’s Coefficient of Palaeogene Limestone In engineering geology and classification of rock masses for civil engineering purposes, the degree of induration for a rock serves as a useful classification parameter. Induration is a measure of how well the grains of a sedimentary rock are cemented together from loosely cemented/soft rock to very competent/slightly metamorphic rock. The Biot coefficient links to the degree of cementation in the capacity of how it relates the elastic deformations with the change in pore pressure. A hypothesis is that the degree of induration could be correlated to the magnitude of the Biot coefficient. This is tested on 11 Copenhagen Limestone specimens of varying porosity and densities obtained from one borehole with a limestone interval of 30 m. Their induration varies from H2 to H5. Elastic wave propagation measurements are used to establish the Biot coefficient and determination of the mineralogy for H5 specimens aids in performing fluid substitution with Gassmann’s equation. For the soft H2 specimens the shear wave could not be obtained and isoframe modelling is applied. A correlation is found; although, the Biot coefficient for the intermediate indurations H3 and H4 overlap. The dry density is found to adequately correlate with the Biot coefficient.

Poroelasticity of high porosity chalk under depletion

The theory of poroelasticity for the elastic region below pore collapse by means of three different loading paths gives the possibility to compare the static and dynamically determined Biot coefficient for a set of experimental data with uniaxial loading on outcrop chalk performed with different levels of pore pressure. The chalk is oilsaturated Lixhe chalk from a quarry near Liege, Belgium, with a general porosity of 45%. Additionally, we compare the theoretical lateral stress to the experimentally determined lateral stress at the onset of pore collapse. The static Biot coefficient based on mechanical test results is found to be lower than the pretest dynamic Biot coefficient determined from elastic wave propagation for the loading path and with less deviation under depletion. The calculated lateral stress is lower than the experimentally measured lateral stress depending on loading path. An explanation to this behaviour is pore pressure build up.

Creep of Highly Porous Chalk and Biot Critical Frequency

Chalk behaves with time dependent deformation when subjected to a load. We review a previously published data set from high pressure oedometer tests and apply a friction factor corresponding to the friction between solid and pore fluid. The friction factor is the Biot critical frequency used in acoustic theory to discriminate between the high and low frequency domains. By doing this we show how test results from dry, oil, and water saturated chalk can be combined in one simple expression.