Glossary of Common Terms and Concepts in Reservoir Simulation
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| Oilfield units | Volumes in oilfield units are barrels (bbl or B) 1bbl= 5.615ft3 or 0.159m3 | |
| Stock tank barrel (STB) | Same volume defined at some surface standard conditions, which are usually 600F and 14.7psi. | |
| Reservoir barrel | Same volume defined at some reservoir conditions, which can range from - 900F and 1500psi. for shallow reservoirs to > 3500F and 15.000psi for very deep (high-pressure- high-temperature) reservoirs | |
| Oil types | Dry gas, wet gas, gas condensate, volatile oil, “black” oil, heavy oil (viscous), tar | |
| Phase | Chemically homogeneous region of fluid which is separated from another phase by an interface, aqueous phase (mainly water), gas phase, solid phase (rock). There is no particularly symbol but frequently subscripted o, w, g; phases are immiscible | |
| Component | A single chemical species that may be present in a phase; in th aqueous phase there are many components – water (H2O), sodium chloride (NaCl), dissolved oxygen (O2) | |
| Viscosity | A measure of the energy dissipated when it is in motion resisting an applied shearing force; dimensions – force \ area, time and units are Pa.s (SI) or poise (metric). The most common unit in oilfield applications is centipoises (cP or cp) | |
| Formation volume factor | Factor describing the ratio of volume of a phase in the “formation” to that at the surface (Bw, Bo) | |
| Gas solubility factors ( solution gas\oil ratios) | Factors describe the volume of gas (usually ft3, SCF) per volume of oil (usually stock tank barrel, STB) (Rso, Rsw) | |
| Compressibility | Compressibility (c) of a fluid (oil, gas, water) or rock formation can be defined in terms of the volume (V) change or density (p) change with pressure | |
| Material balance equation | applied to a reservoir is simply a volumetric balance. It is expressed as an equation which relates: the quantities of oil, gas and water produced; the reservoir (average) pressure; the quantity of water influx initial oil and gas content of the reservoir | |
| Permeability | Conducting capacity of a rock; symbol k; units Darcy (D) or milliDarcy (mD); dimensions L2 | |
| Porosity | Fraction of a rock that is pore space | |
| Pores and pore throats | Tiny connected passages that exist in permeable rocks; typically of size 1mm or 200mm; easily visible. Pores may be lined by diagenetic minerals (clays). The narrower constrictions between pore bodies are referred to as pore throats. | |
| Darcy Law | A law for single phase flow that relates the total volume flow rate (Q) of a fluid through a porous medium to the pressure gradient and the properties of the fluid (μ = viscosity) and the porous medium can be used to define permeability. | |
| Pore velocity | Fluid velocity | |
| Relative permeability | A quantity (fraction) that describes the amount of impairment to flow of one phase on another | |
| Capillary pressure | Difference in pressure between two (immiscible) phases; defined as the non-wetting phase pressure minus the wetting phase pressure. | |
| Saturation | Fraction of the pore space that it occupies | |
| Residual saturation | The amount of that phase (fraction pore space) that is trapped or is irreducible | |
| Mobility (mobility ratio) | The effective permeability of that phase divided by the viscosity of that phase. | |
| Fractional flow | The volumetric flow rate of the phase under a given pressure gradient, in the presence of another phase. | |
| Wettability | Measure of the preference of the rock surface to wetted by a particular phase-aqueous or oleic or some mixed or intermediate combination. | |
| Water -wet | Where water is the preferential wetting phase. Water occupies the smaller pores and forms a film over all of the rock surface-even in the pores containing oil. | |
| Oil-wet | Where the oil is the preferential wetting phase. Oil occupies the smaller pores and forms a film over all of the rock surface-even in the pores containing water. | |
| Intermediate -wet | Where some degree of water wetness and oil wetness is shown by the same rock. | |
| Drainage displacement | When the non-wetting phase is increasing | |
| Imbibition | When the wetting phase is increasing | |
| Spontaneous imbibition | This process occurs when a wetting phase invades a porous medium in the absence of any external driving force. The wetting fluid is “ sucked in” under the influence of the surface forces. | |
| Primary\secondary recovery processes | Refer to the stage in the fluid displacement when one phase displaces another. | |
| Black oil model | Which cam simulates primary depletion and most secondary processes. A black oil simulation model is one of the most common approaches to modelling three phases (o, w, g) flow in porous media; it treats the phases rather like components; it does not model full compositional effects. | |
| Grid structure | Geometry of the grid being used in the numerical simulation of the system. This grid may be Cartesian, radial or distorted and may be 1D, 2D or 3D. | |
| Transmissibility | Between two grid blocks is a measure of how easily fluids flow between them | |
| Spatial discretisation | The process of dividing the grid in space into divisions of ∆x, ∆y, and ∆z. In reservoir simulation, we always “chop up” the reservoir into blocks. | |
| Temporal discretisation | This is the process of dividing up the time steps into divisions of ∆t. | |
| Mass conservation | Used in many areas of computational fluid dynamics: mass flow rate into a block – mass flow rate out = rate of mass accumulation in that block. | |
| Local grid refinement | When the simulation grid is made fine in a region of the reservoir where LGR quantities (pressure or saturation) are changing rapidly. The idea is to increase the accuracy of the simulation in the region where it matters, rather than everywhere in the reservoir. | |
| Hybrid grid LGR | Mixed geometry combinations of girds which are used to improve the modelling of flows in different regions. | |
| Distorted grid | Grid structure that is “bent” to more closely to follow the flow lines or the system geometry in a particular case. | |
| Corner point geometry | In some simulators the option exists to enter the geometry of the vertices of the grid blocks. | |
| History matching | The process of adjusting the simulator input in such a way as to achieve a better fit to the actual reservoir performance. Ideally, the changes in the simulation model should most closely reflect change in the knowledge of the field geology. | |
| Finite differences | When the derivative in a differential equation is approximated as a difference equation. | |
| Linear equation | When finite difference methods are applied to the differential equations of reservoir simulation, a set of linear results. | |
| Direct solution of linear equation | A method when linear equations are solved by an algorithm which has a fixed number of operations. | |
| Iterative solution of linear equation | Is when the linear equations are solved by an algorithm which ha a variable number of operations |
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