Déplacements polyphasiques en milieu poreux. Injection de vapeur en conditions adiabatiques Multiphase Displacements in Porous Media. Steam Flooding under Adiabatic Conditions

Abstract

Dans un article antérieur [1], un dispositif expérimental permettant l'étude en milieu poreux des déplacements par fluides chauds, en conditions isothermes, a été présenté, ainsi que les résultats obtenus. Ce mode d'écoulement, qui ne fait pas intervenir A previous paper [1] described an experimental device designed for the study of multiphase displacements by hot fluids in porous media under isothermal conditions together with the results obtained. This type of flow, which does not take into account the progress of temperature fronts, is not representative of what actually happens in the field when a thermal enhanced oil recovery method is applied. In fact in this case, flows may be considered as quasiadiabatic. To gain a better understanding of the phenomena induced by such adiabatic displacements, new equipment was designed to reproduce conditions close to those in the field. Various experiments were modeled with a simulator developed at Institut Français du Pétrole (IFP), using results obtained under isothermal conditions (for instance, relative permeability curves). There is good agreement between experiments and computation. These experimental results were then compared to those obtained under isothermal conditions. Some hypotheses are put forward to explain the differences observed between the two types of flows. Experiments were carried out in unconsolidated cores made of packed sand. This sand mainly consisted of silica (over 99 weight %). Grain size was between 60 and 100 microns; the corresponding permeability was about 4. 10 to the power of (-12) m². The fluids consisted of distilled water and Albelf C-68 oil. New equipment was designed because of problems related to heat losses. For slow displacement rates at high temperatures, a small heat loss results in a decrease in temperature and therefore in steam condensation. Use of nonmetallic parts for the core-holder strongly reduces heat losses radially and longitudinally so that no heating collars are necessary. The experimental device shown in Figs. 1 and 2 can work under either isothermal or adiabatic conditions. It is designed to operate up to 250°C and 20 bar. The residual oil saturations after steam injection are calculated from the mass balance. Some of these results are shown in Table 1. The parameters modified were the injection flow rate (between 2 and 5 g/min. ) and the absolute pressure at the inlet of the porous medium (between 4 and 7 bar). Figures 3 to 6 correspond to an experiment performed under the following conditions :(a) Initial condition: irreducible water saturation. (b) Injection flow rate = 2 g/min. (c) Injection temperature = 220°C. (d) Absolute pressure at outlet = 3 bar. (e) The injected steam was slightly overheated (this allows to know what is really injected and, due to the rather low enthalpy added compared to the enthalpy of saturated steam, this does not have any significant influence on the heat balance and therefore on the displacement). This experiment shows a very clearcut steam front moving all along the tube. The entire displacement occurs under conditions corresponding to vaporization/condensation equilibrium of steam (Table 2). Experiments were modeled by the TSAR (Thermal Simulator Applied to Reservoir) simulator for thermal methods developed at IFP [2]. This 3-D model can be used to either simulate field processes or laboratory experiments. Relative permeabilities in function of saturations and temperatures obtained from previous isothermal experiments [1] were used in the simulator. Three-phase permeabilities were computed by the Stone method [4 to 6]. For computing, the following assumptions were made :(a) The porous medium is considered to be as homogeneous and isotropic. (b) There is no interaction between fluids and the mineral phase. (c) Steam is considered to be a perfect gas. (d) Steam is injected at a constant weight rate. (e) Contact time between phases is long enough to assume that local thermodynamic equilibrium is reached. The results of the simulation of the previous experiment are shown in Figs. 8 to 12. The decrease in temperature in the core (in the experiment - Fig. 6 - as well as in the simulation - Fig. 8) corresponds to a pressure decrease along the core as the steam moves towards the outlet. This decrease does not correspond to any heat losses. In fact, as the differential pressure decreases due to oil production, the temperature of the vaporization/condensation equilibrium is reduced. This induces an evolution of the residual oil saturations along the core (Fig. 12). Steam condensation at the steam front level causes an increase in water saturation just ahead (Fig. 10). The results of isothermal [1] and adiabatic displacements in terms of residual oil saturation are given in Fig. 14. The comparison is made on the basis of the temperature corresponding to the steam front. Residual oil saturations are lower under adiabatic conditions than under isothermal conditions. This is due to the fact that the mechanisms are not the same. For our experimental conditions, the characteristics of the oil (Table 3) are such that no stripping effect of the hydrocarbon phase has to be taken into account. The most important effect is assumed to be due to the steam vaporization/condensation effects. Oil recovery is a function of the spreading coefficient for the gas/oil system on the interface with the water phase [18]. One can consider that these properties are very different for a gas/water/oil system. It is then possible that phase changes can modify the displacement and hence the oil recovery. Displacement by hot water is less stable than displacement by cold water. On the contrary, displacement by steam is far less stable than displacement by hot water [19, 20] : in the case of a two-phase displacement, viscous fingering is stabilized essentially by the capillary pressure, in the case of steam injection, condensation occurs rapidly for a gas finger entering cold oil. The main conclusions are as follows :(1) Residual oil saturations to steam after adiabatic displacements are lower than residual oil saturations after isothermal hot-water displacements. (2) Displacements by steam occur under conditions corresponding to steam/water thermodynamic equilibrium. (3) Reduction of oil saturation for adiabatic displacements by steam is mainly due to:- Phase changes inducing modifications in the capillary equilibrium;- Three-phase flow;- Displacement stabilities. (4) All the experiments were simulated very satisfactorily. This cross-checking validates both the physical and numerical models.