PVT analysis for oil reservoirs

Pressure- Volume-Temperature, PVT, is the study of the physical behavior of oilfield hydrocarbon systems. Information on fluid properties as volumes and phases in the PVT analysis, and how they change with temperature and pressure, is essential to many aspects of Dynamic Flow Analysis.

The need for valid PVT data cannot be over-stressed. Reservoir fluid sampling should be a major objective of any test, particularly during drill-stem testing, as PVT analysis should be performed as early as possible in the life of the field. Once production has started, it may never again be possible to obtain a sample of the original reservoir fluid, which may be continually changing thereafter.

Production analysis engineer, PVT will be used to:

  • Assess which phases are present at sandface and at surface.
  • Calculate fluid phase equilibrium and phase gravities to correct pressures to datum.
  • Calculate fluid viscosity to get from mobility to permeability.
  • Calculate pseudopressures and pseudotime to linearize the equations in order to use analytical models.
  • Input PVT Properties in a numerical model.

PHASE EQUILIBRIUM:

SINGLE COMPONENT FLUIDS:

Hydrocarbon fluids can exist in two or more separate phases, typically gaseous and liquid, which have different properties. Water may also be present as a separate phase in the reservoir. Reservoir types are classified by their phase behavior, which depends upon the composition, the pressure and temperature. It is the phase behavior that determines the economic recovery in most cases, that makes fluid sampling difficult, and that can sometimes complicate Dynamic Flow Analysis.

The simplest form of phase behavior is for a pure substance, such as methane or water, and ot can be represented on this graphic.

Phase diagram as a product of PVT analysis
PHASE DIAGRAM FOR A PURE SUBSTANCE

The boundary lines between the solid, liquid and gas phases represent values of pressure and temperature at which two phases can exist in equilibrium. There is no upper to the solid-liquid equilibrium line, but the liquid-gas line or vapor pressure curve, terminates at the critical point. At pressures or temperatures above this point only one phase can exist, referred to only as fluid because it has properties similar to both gas and liquid close to the critical point.

Starting in a single-phase liquid state, the volume is increased which causes a sharp pressure reduction due the low liquid compressibility. The point at which the first gas bubble appears is the bubble point. When the volume is further increased the pressure remains the same until the last drop of liquid disappears; this is the dew point. Past the point only gas exists and as the volume increases, the pressure is reduced.

Multi-components fluids in PVT Analysis

As soon as a mixture of at least two components is considered, phase boundaries become areas rather than lines, due to the combination of the physical properties of two components with different compositions.

Instead of a single vapor pressure curve there are separate lines to represent the bubble points and dew points of the mixture. The two-phase boundary of the system can extend beyond the critical point. With most reservoirs systems it is normal to concentrate only on the liquid-gas equilibrium behavior,

PHASE DIAGRAM FOR A MULTI-COMPONENT

although some hydrocarbons do exhibit solid phases, such as wax precipitation (solid-liquid) and gas hydrate formation (solid-gas). Natural hydrocarbon fluids can contain number of phase loops where liquid and gas phases can exist in equilibrium over a wide range of pressures and temperatures.

When it is above the bubble point, the oil is said to be under-saturated. At the bubble point, or anywhere in the two phase region, the oil is said to be saturated.

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