Abstract
Chalk shows a pronounced water-weakening effect, which can be attributed to the microscopic physicochemical interactions between the pore fluid and the chalk matrix. In this paper a conceptual elastoplastic model is developed for describing the chemo-mechanical behaviour of chalk based on the framework of the modified Cam Clay model. Central to the model is the concept of intergranular stresses, introduced as the substitute for Terzaghi's effective stress to describe the chalk behaviour. In this context, the microscopic physicochemical interactions are characterised by introducing a macroscopic surface force potential. The composition and significance of pore water pressure are analysed, and the intergranular stress tensor is explicitly defined. To address the hardening effect of the microscopic physicochemical interactions, the preconsolidation pressure function is modified by introducing the intrinsic intergranular pressure as an additional hardening variable that accounts solely for the chalk–fluid interaction. It is shown that the proposed concept of intergranular stresses can address the effects of osmosis, adsorption and capillarity on the chalk behaviour in a consistent and systematic way. Comparisons between the theoretical simulations and experimental results demonstrate that the proposed model is capable of capturing the main features of the chemo-mechanical behaviour of chalk saturated with an individual or two (miscible or immiscible) fluids.
Highlights
Seawater injection used to be an important approach to re-pressurising the oil reservoirs in the North Sea, in order to alleviate the seabed subsidence problem induced by oil production
Evaluation of constitutive parameters In the constitutive model, both the capillary effect and the physicochemical chalk–fluid interaction are taken into account
Papamichos et al (1997) conducted a series of experiments on Pietra Leccese chalk with different degrees of water saturation, and the results revealed that some material parameters depend strongly on the water saturation and the capillary suction
Summary
Seawater injection used to be an important approach to re-pressurising the oil reservoirs in the North Sea, in order to alleviate the seabed subsidence problem induced by oil production. The capillary and physicochemical effects are first characterised, and an explicit expression for the surface force potential is developed; the concepts of pore water pressure, the generalised osmotic pressure and the intergranular stress are explicitly defined In this context, a chemical– mechanical coupling constitutive model is developed for describing the mechanical behaviour of chalk. It is assumed that (a) the fixed charge density cfix, which is defined as the total number of fixed charges per unit mass of the solid matrix, remains constant (b) the solutions are ideal, so that the activity of a species is equal to its molar faction (c) plastic pore collapse and plastic shear strain are independent properties (d ) in addition, the de-cementation effect, which may result in the softening behaviour of chalk at low confining pressure, is not taken into account in the cohesion. For a dilute sodium chloride solution, alH2 O where cmlαlAH2iOs
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