An elasto-viscoplastic model for chalk including suction effects

in Toll, David (Ed.) Unsaturated Soils: Advances in Geo-Engineering (2008)

During the six years long Pasachalk project devoted to the mechanical behaviour of high porosity chalks from North Sea oilfields, the constitutive model Pasachalk (Collin et al., 2002) was proposed based ... [more ▼]

During the six years long Pasachalk project devoted to the mechanical behaviour of high porosity chalks from North Sea oilfields, the constitutive model Pasachalk (Collin et al., 2002) was proposed based on the Barcelona Basic Model (BBM) (Alonso et al., 1990). The approach was based on the similarities found between the oil-water interactions (oil and water being the non wetting and wetting fluid respectively) in oil reservoir chalk and the air-water interactions in unsaturated soils. This approach appeared to be relevant to in-terpret the subsidence of the seafloor during waterflooding operations for enhanced oil recovery that has been observed in North Sea oilfields (e.g. Ekofisk oilfield). Another important component of subsidence was then related to the creep behaviour of the multiphase chalk (De Gennaro et al., 2003). A modified Pasachalk model was proposed to account for time effects using the framework of Perzyna’s viscoplasticity (1964) but without considering suction effects. Based on available experimental results (Priol et al., 2007), a modified version of the viscoplastic Pasachalk model including suction effects is proposed in this paper. [less ▲]

A deterministic/stochastic model to predict the variation in bulk modulus of chalk

in Geotechnique (2005), 55(2), 135-141

Ekofisk, located 200 km west of the Norwegian coast, is one of the main oilfields in the North Sea. Since the early 1980s severe compaction of chalk reservoir layers has been observed as a consequence of ... [more ▼]

Ekofisk, located 200 km west of the Norwegian coast, is one of the main oilfields in the North Sea. Since the early 1980s severe compaction of chalk reservoir layers has been observed as a consequence of reservoir depletion during oil production. Subsequently, this compaction has been amplified by assisted oil recovery using seawater flooding. The development of our understanding of the inherent mechanisms of this phenomenon has been the objective of extensive experimental investigations in the last two decades. Owing to the very high cost of cored material from the reservoir, experiments are usually performed on chalk samples from an outcrop in Belgium lying at the same stratigraphic level as the Ekofisk reservoir chalks. However, even at the laboratory sample scale,. experimental variability of material response is observed. From a theoretical and numerical point of view, the determination of the mechanical properties of the material is of utmost importance for accurate modelling at both sample scale (laboratory tests) and reservoir scale. The aim of this paper is to introduce a stochastic approach within a deterministic constitutive model of chalk to enable the influence of material heterogeneity to be included in analyses for the range of observed mechanical responses. In modelling the random distribution of material parameters, the time-consuming Monte Carlo simulation method is replaced by a more efficient stochastic modelling technique. The results are given in the form of statistical parameters for the experimental laboratory test responses. The parameters of the distribution law (mean value, range of variation, spatial correlation structure) are fitted to reproduce the range of experimental responses observed at sample scale. Interest is focused on the variability of the chalk bulk modulus, as observed during isotropic compression tests. The proposed methodology provides a satisfactory explanation for the variability of response observed at the sample scale. The potential for extending the proposed approach to reservoir scale is briefly discussed. [less ▲]

Multiphase time dependent compression behaviour of two chalks

Conference (2005)

In this paper, the multiphase time dependent compression behaviour of two chalks are considered within a fully coupled general hydro-mechanical framework derived from the mechanics of unsaturated soil. In ... [more ▼]

In this paper, the multiphase time dependent compression behaviour of two chalks are considered within a fully coupled general hydro-mechanical framework derived from the mechanics of unsaturated soil. In the former case, reservoir chalks contain oil and water and subsidence appeared once enhanced oil recovery by waterflooding started. In the latter case, water saturated chalk is periodically desaturated by water evaporation, resulting in air penetration. In this paper, the fluid retention curves of both samples are determined and the data obtained are used in the interpretation of suction controlled compression tests where time effects appear to be significant. The experimental data obtained show the relevance of the approach adopted. [less ▲]

Time-dependent behaviour of oil reservoir chalk: A multiphase approach

in Soils and Foundations (2003), 43(4),

In the North Sea Ekofisk oilfield, oil is located in a 300 m thick layer of porous chalk (n = 40-50%) at a 3000 m depth. After the initial phase of depletion an enhanced oil recovery procedure was carried ... [more ▼]

In the North Sea Ekofisk oilfield, oil is located in a 300 m thick layer of porous chalk (n = 40-50%) at a 3000 m depth. After the initial phase of depletion an enhanced oil recovery procedure was carried out by injecting sea water (waterflooding). An unexpected consequence of this waterflooding has been the occurrence of a seafloor subsidence, corresponding up to now to a decrease of the seafloor level of approximately 10 m. It is now well recognised that hydro-mechanical coupling involving multiphase fluid interactions (oil and water) is determinant for the interpretation of the phenomenological aspects associated with the chalk compaction and the related subsidence observed in the North Sea oilfields (Ekofisk reservoir) when water flooded. The subsidence due to waterflooding is interpreted as a collapse phenomenon due to suction decrease, typical of loose and low plasticity unsaturated soils when wetted under load. On the other hand, time-dependent stress-strain behaviour of geomaterials is one of the major concerns in soil mechanics and, in effect, subsidence includes creep effects. A multiphase approach, including creep effects under controlled suction levels, is proposed in this paper; the preliminary results of this study are presented and discussed. Attention will be focused at first on the theoretical approach, supplying the essential elements for the work, and on the interpretation of the experimental results. This will provide secondly the base for formulation and validation of the constitutive law proposed for the description of the time-dependent mechanical behaviour of the chalk. [less ▲]

Constitutive modeling of chalk. Application to waterflooding

in Auriault, J. L.; Geindreau, C.; Royer, P. (Eds.) Poromechanics II (2002)

Subsidence of chalk oil reservoirs in North Sea is related to the chalk compaction induced by fluid depletion and by the water – chalk interaction. A constitutive model is developed in order to take into ... [more ▼]

Subsidence of chalk oil reservoirs in North Sea is related to the chalk compaction induced by fluid depletion and by the water – chalk interaction. A constitutive model is developed in order to take into account these two effects. It is based on frictional – cap elastoplasticity and on the Barcelona unsaturated soil model. Oil – water – chalk interaction is modeled through the suction variable. In a second step, viscous effects are added to the model. After implementation into a finite element code, these tools allow to simulate a waterflooding experiment on a chalk sample. The model appears to reproduce qualitatively and quantitatively the experimental results. [less ▲]