Slug Size and Mobility Requirements for Chemically Enhanced Oil Recovery Within Heterogeneous Reservoirs
- R.J. Wright (Imperial C.) | M.R. Wheat (Imperial C.) | R.A. Dawe (Imperial C.)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Engineering
- Publication Date
- February 1987
- Document Type
- Journal Paper
- 92 - 102
- 1987. Society of Petroleum Engineers
- 5.4.5 Conformance Improvement, 5.1.5 Geologic Modeling, 4.1.5 Processing Equipment, 5.4.7 Chemical Flooding Methods (e.g., Polymer, Solvent, Nitrogen, Immiscible CO2, Surfactant, Vapex), 5.3.2 Multiphase Flow, 1.10 Drilling Equipment, 5.5 Reservoir Simulation, 5.6.5 Tracers, 4.3.4 Scale, 2.5.2 Fracturing Materials (Fluids, Proppant), 5.1.1 Exploration, Development, Structural Geology, 5.4.1 Waterflooding, 5.2 Reservoir Fluid Dynamics
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Summary. The influence of fluid mobilities on channeling and crossflow is demonstrated and quantified in systems containing thin high-permeability layers. Flow visualization of continuous and slug-mode displacements was performed under conditions in which capillary pressure, gravity, and dispersion were insignificant. The data from numerous experiments were correlated by a mathematical model and the viscous crossflow effects for layered media quantified. The analytical approximation, although simple, is novel and gives good agreement with the data. Such a model is essential for the initial design of stable slugs before computer simulation of the oil recovery process. For heterogeneous reservoirs, prevention of slug disintegration caused by chase fluid bypassing clearly requires oil recovery agent plus mobility buffer quantities of around 1 PV and stringent mobility balancing. The slug design criteria are more demanding than previously thought necessary for reservoirs with normal degrees of previously thought necessary for reservoirs with normal degrees of heterogeneity.
The initial assessment of chemical EOR schemes, especially polymer flooding and surfactant/polymer methods, should be aided by our design criteria, which also provides guidelines for optimization of the slug size and its chemical concentration.
Chemical slug processes that use, for instance, polymer diverting agents, surfactants, or alkalis may be considered to improve oil recovery. In the initial stages of a feasibility study, analytic calculations are extremely useful for comparing the different schemes. 1 Expensive computer simulation exercises must follow. However, these may have a poor physical basis.
Dual-layer models have been used in many previous experimental and numerical investigations. These can represent the stratification that is a common feature of a majority of petroleum reservoirs and that can dominate the flow patterns. High-permeability layers or streaks give rise to very early displacing-fluid breakthrough and the breakup of injected chemical slugs, possibly leading to undesirable degrees of mixing of chemicals with reservoir fluids. In this paper, we focus on the common permeability/porosity variations that occur in practically all permeability/porosity variations that occur in practically all reservoirs, rather than gross thief zones or fractures that can preclude consideration of chemical injection anyway. We consider the question, "What combinations of slug mobility and size are required for the technical success of chemical oil recovery?" We do not discuss the problems of coreflood-scale displacement efficiency and gross problems of coreflood-scale displacement efficiency and gross reservoir modeling for which methodologies are well developed, but concentrate on the problem of flow instabilities within layered sections.
Continuous Fluid Displacement Within Stratified Models
A stratified two-dimensional (2D) cross-sectional model was used in our experimental work (Fig. 1). Layers of various permeabilities were formed by packing glass beads of specific size ranges. Displacements were performed between miscible fluids for which gravity effects were negligible and fluid mixing of little importance. We first determined the relative speeds at which fronts of one fluid displace another within two communicating layers. For isotropic media, there are the two variables, mobility ratio, M, which for simple miscible systems is the viscosity of displaced fluid divided by the viscosity of the displacing fluid, and conductance ratio of the layers, Rc (conductance is permeability divided by porosity), plus the model dimensions. The layer conductance ratios of our models were determined directly by equal mobility displacements. The measurements showed the relationship between the relative rate of displacement through the layers and both the conductance ratio of the layers and mobility ratio of the fluids.
A symmetry half of the physical model is shown in Fig. 1; Layer a is relatively thin and of high conductance, and Layer b is of lower conductance. The ratio, R, of frontal displacement rates within Layers a and b was determined from measurements of the front positions (Xa and Xb) at various instants. Plots of Xa vs. Xb were usually approximately linear.
Viscous crossflow effects were estimated successfully by calculation.
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