Mechanical behaviour of Lixhe chalk partly saturated by oil and water: experiment and modelling (original) (raw)
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Oil reservoir rocks contain various immiscible fluids (oil, water and gas) and they are submitted to elevated temperatures in the oilfield. Hence, they exhibit a typical coupled behaviour where multiphysics and coupled Thermo-Hydro-Mechanical aspects are predominant. This paper presents some results from researches carried out within the PASACHALK 2 European funded collaborative research. In this research dedicated to subsidence problems in the North sea Ekofisk oilfield, the behaviour of a reservoir chalk containing two immiscible fluids (an organic non polar fluid and water) is considered within the framework of the mechanics of unsaturated soils (no temperature effects considered). In order to account for the mechanical coupled effects related to the two pore fluids, the oil-water suction was considered as an independent stress variable. The paper presents some experimental results describing the combined effect of suction and time on the isotropic compression behaviour of the chalk. A viscoelastoplastic constitutive model based on one hand on the Barcelona BBM elastoplastic model for unsaturated and on the other hand on Perzyna's approach of viscous behaviour of soils is also presented.
Constitutive modeling of chalk – application to waterflooding
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The compaction of chalky reservoirs during oil extraction and other important problems like the "casing collapse" or the "chalk production" are related to the mechanical properties of chalk. Controlling compaction is very important because reservoir deformations imply seabed subsidence that endangers the offshore stations. The first explanation of subsidence links the compaction to the pore pressure decrease in the reservoir. The solution was the injection of gas and water into the oilfield in order to repressurise the reservoir. But the waterflooding induced additional subsidence. Though many studies have been already performed on chalks, the basis mechanism of the water sensitivity was not defined. Obviously, no satisfactory constitutive law can be written without this deep insight of the phenomenon. This is the scope of the ongoing EC Research Program Pasachalk. The origin of the research is in the comparison of experimental results obtained on Lixhe chalk and...
Transport in Porous Media
Brines containing surface-active divalent ions such as Ca 2+ , Mg 2+ and SO 4 2− impact the stiffness, strength and time-dependent deformation of water wet Mons outcrop chalk. This study documents how stiffness and strength of wettability-altered oil and water-saturated (mixed wet) Mons chalk compare to water-saturated and water wet samples during hydrostatic loading and creep. During hydrostatic creep, the strain rate response to magnesium chloride (MgCl 2) brine injection is compared for water wet and mixed wet samples. For the mixed wet samples, the oil production was estimated during compaction and non-equilibrium MgCl 2 flow. The results presented here were then compared to a similar test series on Kansas outcrop chalk. The differences were interpreted in terms of difference in physical parameters such as porosity and pore size. Two Mons samples were wettability-altered and tested in parallel to two completely water wet samples from the same chalk block at hydrostatic conditions and 130 °C. It was found that beyond the experimental uncertainty, the stiffness and yield strength measurements showed insignificant differences for water wet and mixed wet Mons samples. This is contrary to Kansas chalk where mixed wet samples were systematically stronger and stiffer than water wet samples. In the following creep phase, both water wet and wettability-altered Mons chalk samples gave trends comparable to each other during a stagnant phase and a following MgCl 2 injection phase at varying flow rates. Similar observations were reported for water wet and wettability-altered Kansas chalks as well. Further, the same chemical reactions were observed for Kansas and Mons chalks and were found to be insensitive to the initial wettability and oil/water saturations. The oil production observations from Mons chalk showed that 43% of the total oil was recovered during early-stage compaction with no flow, whereas Kansas chalk did not produce any oil at no-flow conditions. After the oil recovery from the first 2-3 pore volumes of brine flow, no further oil production was observed due to compaction or non-equilibrium brine flow in any of the two chalk types.
Subsidence and capillary effects in chalks
2008
Based on the concepts of the mechanics of unsaturated soils where capillary phenomena arise between the wetting fluid (water) and the non-wetting one (air), the subsidence of chalks containing oil (non-wetting fluid) during water injection (wetting fluid) is analysed. It is shown that the collapse phenomenon of unsaturated soils under wetting provides a physical explanation and a satisfactory prediction of the order of magnitude of the subsidence of the chalk. The use of a well established constitutive model for unsaturated soils allows a description of the hydro-mechanical history of the chalk, from its deposition to the oil exploitation.
Experimental investigation on the time dependent behaviour of a multiphase chalk
… Unsaturated Soil Mechanics, 2007
A series of high pressure oedometer compression tests were carried out on samples of Lixhe chalk (Belgium) containing various pore fluids: water, air (dry sample), oil (Soltrol) and oil/water under a controlled suction of 200 kPa. Strain curves with respect to time of tests carried out under a constant load were examined in a purpose of investigating the time dependent behaviour of chalk. A simple empirical exponential law able to account for the combined effects of stress and suction is proposed. Experiments show that it provides satisfactory results to predict a feature of multiphase chalk behaviour that is important in the study of the behaviour of oilfields.
The effect of suction on the hydro-mechanical behaviour of chalk rocks
Engineering Geology, 2009
The aim of this paper is to study the effect of suction on the hydro-mechanical behaviour of a partially saturated soft rock (a porous chalk) under low confining stress. Such conditions correspond to those of the vertical chalk cliffs of the English Channel, which are subjected to spatial and temporal variations of the water table under climatic variations. We apply to this material a method usually used in soils, whose hydromechanical behaviour is strongly modified by changes in matric suction (or capillary suction), according to the degree of water saturation. Under controlled matric suction conditions, wet chalk undergoes volume changes due to pore infilling and drainage. Relationships between matric suction and the mechanical behaviour of the chalk are analysed on the basis of drying-wetting cycles, variations of maximum strength, secant modulus during uniaxial unconfined compression and confined triaxial tests. Mechanical and dryingwetting tests were carried out on dry, saturated and moist natural chalk samples. Drying-wetting cycles result in two types of behaviours according to the range of imposed matric suction, with irreversible paths in the [s, S r ] plane for high gravimetric water contents (i.e. matric suction values between 0.1 and 6 MPa) and reversible paths for low water contents (i.e. high values of matric suction, larger than 6 MPa). For chalk samples initially at their natural water content, the behaviour is intermediate between those of the dry and saturated chalk samples. Comparison of the behaviour of the two chalk formations highlights differences on wetting, which could be linked to the different pore size distributions from one chalk formation to the other. The results of uniaxial and triaxial compressive strength tests at various water contents show elastic-brittle behaviour. Brittle failure occurs at very low water contents. The secant modulus reaches 600 MPa for dry chalk samples and decreases to 250 MPa for quasi-saturated samples. Concerning triaxial tests, the envelopes of maximum strength (failure criteria) show a noticeable shift, with an apparent cohesion varying by a factor of 2 to 3 from dry to saturated chalk. The effect of matric suction is interpreted using an effective stress approach, based on a simple model for granular soils, by considering the expression of the intergranular forces between two particles in an idealised medium at the microscopic scale. It makes it possible to describe the behaviour of chalk in relation to the variations of suction. The capillary effect is well marked at very low values of water saturation, whereas it disappears at high water content values.
Experimental investigation on the hydromechanical behaviour of a porous chalk
E3S web of conferences, 2023
This paper deals with the impact of climate change on stability of abandoned subsurface cavities in chalk due to its indirect effect on ground water levels. It is the first part of a general work on the hydromechanical behaviour of a partially saturated soft rock (a porous chalk) and the effect of changes in water saturation degree. An experimental investigation including a laboratory-testing program is presented. Different degrees of saturation are imposed by controlled relative humidity conditions. Conventional hydrostatic and triaxial compression tests are performed under drained conditions for saturation degree up to 100% under low confining pressures. The obtained results have allowed to show up fundamental aspects of the chalk behaviour. The high sensitivity of the extracted material to water is described and a water induced plastic deformation is observed.
An elasto-viscoplastic model for chalk including suction effects
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 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 interpret 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.