Multi-scale flow patterns during immiscible displacement of oil by water in a layer-inhomogeneous porous media

This paper presents the results of numerical study of the relationship between micro-and macroscale flows during immiscible displacement in a two-layer porous medium. A feature of the proposed approach is the allowance for large-scale capillarity induced flow due to curvature of the displacement front in macro-inhomogeneous porous medium. The physical mechanisms determining the development of viscous instability in a layer-inhomogeneous porous medium are considered, the methods for suppressing viscous fingers formation based on the stabilization of the displacement front due the action of capillary forces are proposed.

Numerical Simulation Study of Tracking the Displacement Fronts and Enhancing Oil Recovery Based on Ferrofluid Flooding

Water flooding is crucial means to improve oil recovery after primary production. However, the utilization ratio of injected water is often seriously affected by heterogeneities in the reservoir. Identification of the location of the displacement fronts and the associated reservoir heterogeneity is important for the management and improvement of water flooding. In recent years, ferrofluids have generated much interest from the oil industry owning to its unique properties. First, saturation of ferrofluids alters the magnetic permeability of the porous medium, which means that the presence of ferrofluids should produce magnetic anomalies in an externally imposed magnetic field or the local geomagnetic field. Second, with a strong external magnetic field, ferrofluids can be guided into regions that were bypassed and with high residual oil saturation. In view of these properties, a potential dual-application of ferrofluid as both a tracer to locate the displacement front and a displacing fluid to improve recovery in a heterogeneous reservoir is examined in this paper. Throughout the injection process, the magnetic field generated by electromagnets and altered by the distribution of ferrofluids was calculated dynamically by applying a finite element method, and a finite volume method was used to solve the multiphase flow. Numerical simulation results indicate that the displacement fronts in reservoirs can indeed be detected, through which the major features of reservoir heterogeneity can be inferred. After the locations of the displacement fronts and reservoir heterogeneities are identified, strong magnetic fields were applied to direct ferrofluids into poorly swept regions and the efficiency of the flooding was significantly improved.

In-Situ Combustion Frontal Stability Analysis

Summary Because of complex chemical reactions and multiphase flow physics, the displacement front stability for in-situ combustion (ISC) enhanced oil recovery (EOR) processes are not well understood. In this work, theory and numerical simulation validation are presented to establish an analytical frontal stability criterion for ISC processes. First, the four influencing factors for ISC displacement stability are analyzed: viscous force, heat conduction, matrix permeability changes caused by coke deposition, and gravity. A thorough analysis of the different zones and displacement fronts in a typical ISC process is conducted, and the most unstable front with the strongest tendency for gravity override is identified. Second, analytical solutions for judging the frontal stability and gravity override are established. Third, numerical reservoir simulation is performed to study the frontal stability and gravity override to validate the analytical theory. Carefully selected numerical schemes, as well as spatial and temporal discretization, are used to ensure the accuracy of these simulations. The four major zones and three displacement fronts (combustion front, leading edge of steam plateau, and oil bank leading edge) are identified in a typical 1D ISC process. The most unstable front with the largest pressure gradient contrast is the leading edge of the steam plateau. Gravity override also first takes place here with large fluid density differences across the front. By establishing material and energy balances and solving the wavy perturbation of the steam front, an analytical equation for deciding the ISC flood front stability in a 2D horizontal plane is achieved. Furthermore, the analytical solution for ISC gravity override is established. In numerical simulations, we are able to obtain results with sufficient accuracy to capture unstable ISC displacements and show fingering behavior under different conditions. The matrix permeability reduction caused by coke deposition has minimal impact on frontal stability. The simulation results are successfully validated with the analytical work for conditions in which the ISC process is stable or unstable and also for the degree of ISC gravity override. This demonstrates the predictive capability of the analytical method. In summary, a theoretical framework to analyze whether the displacement front of an ISC process is stable or not has been established. Numerical simulations confirm its predictive capability. This serves as a new reservoir engineering tool to aid the implementation and design of practical ISC projects.

Assessment of the Influence of Water Saturation and Capillary Pressure Gradients on Size Formation of Two-Phase Filtration Zone in Compressed Low-Permeable Reservoir

The paper examines the influence of capillary pressure and water saturation ratio gradients on the size of the two-phase filtration zone during flooding of a low-permeable reservoir. Variations of water saturation ratio s in the zone of two-phase filtration are associated with the pressure variation of water injected into the reservoir; moreover the law of variation of water saturation ratio s(r, t) must correspond to the variation of injection pressure, i.e. it must be described by the same functions, as the functions of water pressure variation, but be subject to its own boundary conditions. The paper considers five options of s(r, t) dependency on time and coordinates. In order to estimate the influence of formation and fluid compressibility, the authors examine Rapoport – Lis model for incompressible media with a violated lower limit for Darcy’s law application and a time-dependent radius of oil displacement by water. When the lower limit for Darcy’s law application is violated, the radius of the displacement front depends on the value of capillary pressure gradient and the assignment of s function. It is shown that displacement front radii contain coefficients that carry information about physical properties of the reservoir and the displacement fluid. A comparison of two-phase filtration radii for incompressible and compressible reservoirs is performed. The influence of capillary pressure gradient and functional dependencies of water saturation ratio on oil displacement in low-permeable reservoirs is assessed. It is identified that capillary pressure gradient has practically no effect on the size of the two-phase filtration zone and the share of water in the arbitrary point of the formation, whereas the variation of water saturation ratio and reservoir compressibility exert a significant influence thereupon.

One of the feasibilities of oil recovery increase in embedded-inhomogeneous reservoir

The paper reviews the feasibility of oil recovery increase in embedded-ingomogeneous reservoir with hydrodynamically isolated layers of various permeability applying chemical agents. As the chemical agents, electrical-chemical modified natural water (catolyte) and catolyte-based polyacrylamide agent (PAA) are used. For oil recovery increase of the inhomogeneous reservoirs, an ecologically safe catolyte with all the properties characteristic for the alkali is applied. For the alignment of displacement front in the layers of different permeability and the flooding decrease of high-permeable reservoir, the polyacrylamide is used. This combined method was previously conducted in homogenous model and showed high efficiency. It allowed testing the method of oil displacement in embedded-ingomogeneous reservoir model. The effectiveness is achieved using catolyte and catoly- te-based PAA solutions in embedded-inhomogeneous reservoir in oil displacement process. Due to this, the low-permeability la-yer is much more involved into the process and the oil recovery factor increases.

Mutual Dynamics of Heat Dissipation and Oil Displacement Fronts during In-Situ Oil Combustion. One-Dimensional Simulation

One-dimensional axis-symmetrical and plane-symmetrical problem of propagation of the combustion and displacement fronts in oil-containing layer in situ has been considered numerically. Two combustible components, viz. liquid (oil) and solid (kerogen, oil sorbate), were considered. The influence of the blast rate, liquid component viscosity, oxygen concentration in blasted air and heat losses (the width of the oil-containing layer) on the dynamics of the heat dissipation and displacement fronts is investigated. In the cylindrical system the oxidizer flow to the combustion front is reducing over time; and the shift-down of the maximum temperature from the solid combustion front to the oil displacement front takes place (the combustion front “jump”). The time of the “jump” may vary from tenths to hundreds of days and the distance of the shift, – up to 10 or more meters, depending on the parameters of the system. After the “jump”, the combustion rate and maximum temperature continue to deteriorate and after the period of time close to the time lapse before the “jump” the chemical reaction ceases. Herewith the transition of combustion to the liquid phase after the “jump” doesn’t influence notably on oils displacement front speed. The time of the “jump”, as well as the velocity of the mutual combustion (maximum temperature) front and displacement front removal nearly linearly depends on incoming gas blast rate and non-linearly – on oil viscosity. When viscosity is low, the displacement front rapidly runs away from the combustion front, time of the “jump” retards and the distance between the fronts at the instance of the “jump” may reach 10 m or more. The oxygen concentration in the gas being blasted influences significantly on the mutual dynamics of the combustion and displacement fronts since combustion front velocity is proportional to oxygen concentration and displacement front velocity is independent on it. Oxygen enrichment of the gas being blasted just after the “jump” may help localize the area of heat release (combustion) near the oil displacement front. The mentioned manipulation may be utilized for sustainability control of the displacement front. However for its practical implementation it is necessary to have information on concentration and temperature fields inside the layer, which may be obtained from indirect data and via modeling. The results of investigation may be utilized for development of technical projects of oil recovery via in-situ combustion.