Reservoir Mechanics

RESERVOIR MECHANICS

Is the measurement, correlation and interpretation of all factors related to the reservoir and its contents. An understanding of the nature of oil and gas, their occurrence in the earth, and the
mechanics by which they are produced from the sub-surface reservoir is materially helpful and even essential to those who are associated with the business of producing oil.  The association may be one of acquiring land, drilling wells, operating producing wells, participating in hearings before regulatory bodies or with the various legal entanglements which occasionally grow out of the many operations connected with the business of producing oil. 

WHICH DIRECTLY OR INDIRECTLY LEAD TO THE FOLLOWING DETERMINATIONS:

·         Volume of fluid in place

·         Availability of natural energy that can be utilized for the expelling of oil and/or gas from reservoir. optimum number of wells and their location to deplete the reservoir most economically.

·         Maximum rate of production from the reservoir that will permit the optimum utilization of the available reservoir energy.

·         Proportion of the total oil and gas in place that can be recovered with the utilization of the natural reservoir energy.

·         Feasibility of supplementing the natural energy with secondary source of energy or other processes directed toward the removal of oil and /or gas that cannot be removed by the application of the natural forces.

PHASE RULE

A rule used in thermodynamics stating that the number of degrees of freedom in a physical system at equilibrium is equal to the number of chemical components in the system minus the number of phases plus the constant 2.

ALSO CALLED GIBBS PHASE RULE.

 If F is the number of degrees of freedom, C is the number of components and P is the number of phases, then F = C - P + 2

Degree of freedom (f):

 It is the number of external variables that can be changed independently without disturbing the number of phases in equilibrium. These are pressure, temperature and composition.  The number of different independent intensive variables that we may change in this way.

The number of degrees of freedom is the maximum number of intensive properties of the equilibrium system we may independently vary, or fix at arbitrary values, without causing a change in the number and kinds of phases and species.

Component:

Is a chemically independent constituent of a system

Number of components (c):

Is he minimum number of independent species necessary to define the composition of all the phase present in the system.

Phase:

Is a state of matter that is uniform throughout in chemical composition and physical state.

Is a form of matter that is homogeneous in chemical composition and physical state, Typical phases are solid, liquid and gas, Two immiscible liquids (or liquid mixtures with different compositions) separated by a distinct boundary are counted as two different phases, as are two immiscible solids.

Number of phase (p):

– Gas or gaseous mixture – single phase

 – Liquid – one, two and three phases

• two totally miscible liquids – single phase

• a slurry of ice and water – two phases

– Solid

• a crystal is a single phase

• an alloy of two metals – two phases (immiscible)

• one phase (miscible)

The difference between (a) a single-phase solution, in which the composition is uniform on a microscopic scale, and (b) a dispersion containing two phases, in which regions of one component are embedded in a matrix of a second component.

PHASE DIAGRAM

Shows the regions of pressure and temperature at which its various phases are thermodynamically stable.

Is a graphical representation of the physical states of a substance under different conditions of temperature and pressure. A typical phase diagram has pressure on the y-axis and temperature on the x-axis. As we cross the lines or curves on the phase diagram, a phase change occurs. In addition, two states of the substance coexist in equilibrium on the lines or curves.

A phase diagram is a chart showing the thermodynamic conditions of a substance at different pressures and temperatures.

The regions around the lines show the phase of the substance and the lines show where the phases are in equilibrium.

SURFACE

The term surface is customarily used when referring to either a gas-solid or a gas-liquid interface. “Every surface is an interface.”

Is a region between a condensed phase (liquid or solid) and a gas phase or vacuum

In surface and colloid science ‘‘stable’’ is or can be a relative term (as willbecome apparent later). For that reason, one should always have clearly inmind just what is intended by the term in a given situation.

INTERFACES

Is a systems involving two condensed phases. Where complete generality is implied, ‘‘interface’’ is probably the better term.

That convention will generally be employed in the material to follow. However, no guarantee of complete consistency is given or implied. There are several types of interfaces that are of great practical importance and that will be discussed in turn.

 These general classifications include, solid– vacuum, liquid–vacuum, solid–gas, liquid–gas, solid–liquid, liquid–liquid, and solid–solid. From a practical standpoint, solid– and liquid–vacuum interfaces are of little concern

CRICONDENTHERM

Cricondentherm (Tct) is the maximum temperature above which liquid can not be formed regardless of the pressure. The corresponding pressure is called cricondentherm pressure.

Is the maximum temperature at which two phases of a substance (for example liquid and vapour) can coexist. The maximum temperature for the formation of two phases (e.g., liquid and vapor) at a given pressure in a reservoir.

Cricondenbar is the maximum pressure above which no gas can be formed regardless of the temperature. The corresponding temperature is called cricondenbar temperature.

PHASE BOUNDARY

Is a boundary between regions, shows the values of p and t at which two phases coexist in equilibrium.

In thermal equilibrium, each phase (that is, liquid, solid etc.) of physical matter comes to an end at a transitional point, or spatial interface, called a phase boundary, due to the immiscibility of the matter with the matter on the other side of the boundary. This immiscibility is due to at least one difference between the two substances' corresponding physical properties

QUESTION 2:

OIL RESERVOIRS

If the reservoir temperature T is less than the critical temperature Tc of the reservoir fluid, the reservoir is classified as an oil reservoir.

petroleum system or petroleum reservoir is often thought of as being an underground " lake" of oil, but it is actually composed of hydrocarbons contained in porous rock formations.

Are pools of hydrocarbons, located beneath the earth’s surface in porous rock structures. The total estimate of petroleum reservoirs includes the total quantity of oil that be can be recovered and that which cannot be recovered. The fraction of crude oil reservoirs that can be extracted from the oil field is classified as reserves.

Depending upon initial reservoir pressure Pi, oil reservoirs can be subclassified into the following categories:

·         Undersaturated oil reservoir. If the initial reservoir pressure Pi, is greater than the bubble-point pressure Pb  of the reservoir fluid, the reservoir is labeled an undersaturated oil reservoir.

·         Saturated oil reservoir. When the initial reservoir pressure is equal to the bubble-point pressure of the reservoir fluid, the reservoir is called a saturated oil reservoir.

·         Gas-cap reservoir. If the initial reservoir pressure is below the bubble point pressure of the reservoir fluid, the reservoir is termed a gas-cap or two-phase reservoir, in which the gas or vapor phase is underlain by an oil phase. The appropriate quality line gives the ratio of the gas-cap volume to reservoir oil volume.

GAS RESERVOIRS

If the reservoir temperature is greater than the critical temperature of the ydrocarbon fluid, the reservoir is considered a gas reservoir.

A naturally occurring storage area, characteristically a folded rock formation such as an anticline, that traps and holds natural gas. The reservoir rock must be permeable and porous to contain the gas, and it has to be capped by impervious rock in order to form an effective seal that prevents the gas from escaping upward or laterally.

There are two main types of Gas Reservoirs, that is, lithologic structural gas reservoir and lithologic gas reservoir. The primary purpose of the reservoir is to explore the natural gas formed in the porous rock formations. Thus, it is used in the hydrocarbon exploration methods

Gas reservoirs have been divided into three groups; dry gas, wet gas, and retrograde-condensate gas.

·         A dry-gas reservoir is defined as producing a single composition of gas that is constant in the reservoir, wellbore, and lease-separation equipment throughout the life of a field. Some liquids may be recovered by processing in a gas plant.

·         A wet-gas reservoir is defined as producing a single gas composition to the producing well perforations throughout its life. Condensate will form either while flowing to the surface or in lease-separation equipment.

·         A retrograde-condensate gas reservoir initially contains a single-phase fluid, which changes to two phases (condensate and gas) in the reservoir when the reservoir pressure decreases.

WET-GAS RESERVOIR

Temperature of wet-gas reservoir is above the cricondentherm of the hydrocarbon mixture. Because the reservoir temperature exceeds the cricondentherm of the hydrocarbon system, the reservoir fluid will always remain in the vapor phase region as the reservoir is depleted isothermally, along the vertical line A-B.

Wet gas reservoirs are characterized by the following properties:

•         Gas oil ratios between 60,000 and 100,000 scf/STB.

•         Stock-tank oil gravity above 60° API.

•         Liquid is water-white in color.

•         Separator conditions (that is separator pressure and temperature) lie within the two phase region.

DRY-GAS RESERVOIR

The hydrocarbon mixture exists as a gas both in the reservoir and the surface facilities.

The only liquid associated with the gas from a dry gasreservoir is water. A phase diagram of a dry gas reservoir. Usually, a system that has a gas/oil ratio greater than 100,000 scf/STB is considered to be a dry gas. The kinetic energy of the mixture is so high and attraction between molecules so small that none of them coalesce to a liquid at stocktank conditions of temperature  nd pressure.

RETROGRADE GAS-CONDENSATE RESERVOIR

If the reservoir temperature, T, lies between the critical temperature, Tc, and cricondentherm,Tct, of the reservoir fluid, the reservoir is classified as a retrograde gas- condensate reservoir.

This category of gas reservoir has a unique type of hydrocarbon accumulation, in that the special thermodynamic behavior of the reservoir fluid is the controlling factor in the development and the depletion process of the reservoir.

When the pressure is decreased on these mixtures, instead of expanding (if a gas) or vaporizing (if a liquid) as might be expected, they vaporize instead of condensing.

Consider that the initial condition of a retrograde gas reservoir is represented by point 1 on the pressure-temperature phase diagram.

Because the reservoir pressure is above the upper dew-point pressure, the hydrocarbon system exists as a single phase (that is, vapor phase) in the reservoir.

As the reservoir pressure declines isothermally during production from the initial pressure (point 1) to the upper dew-point pressure (point 2), the attraction between the molecules of the light and heavy components move further apart.

As this occurs, attraction between the heavy component molecules becomes more effective, therefore, liquid begins to condense. This retrograde condensation process continues with decreasing pressure until the liquid dropout reaches its maximum at point 3. Further reduction in pressure permits the heavy molecules to commence the normal vaporization process.

This is the process whereby fewer gas molecules strike the liquid surface and more molecules leave than enter the liquid phase. The vaporization process continues until the reservoir pressure reaches the lower dew-point pressure. This means that all the liquid that formed must vaporize because the system essentially is all vapor at the lower dew point.

Reference

Charles R. Smith, G. W. Tracy, R. Lance Farrar. 1999 "Applied Reservoir Engineering" (Oil & Gas Consultants International)

Craft, B.C. & Hawkins, M. Revised by Terry, R.E. 1990 "Applied Petroleum Reservoir Engineering" Second Edition (Prentice Hall).

Dake, L.P., 1978, "Fundamentals of Reservoir Engineering" (Elsevier)

Frick, Thomas C. 1962 "Petroleum Production Handbook, Vol II" (Society of Petroleum Engineers).

Kotz, John C., and Paul Jr. Treichel. Chemistry & Chemical Reactivity. N.p.: Saunders College Publishing, 1999.

Oxtoby, David W., H. P. Gillis, and Alan Campion. Principles of Modern Chemistry. Belmont, CA: Thomson Brooks/?Cole, 2008.

Petropedia. (2009, 02 16). Gas Reservoir. Retrieved from Petropedia: https://www.petropedia.com/definition/6454/gas-reservoir

Petrucci, Ralph, and William Harwood. F. Geoffrey Herring. Jeffry Madura. General Chemistry: Principles and Modern Applications. 9th ed. Upper Saddle River, NJ: Pearson, 2007.

Slider, H.C. 1976 "Practical Petroleum Reservoir Engineering Methods" (The Petroleum Publishing Company).

Vollmer, John J. "Out of "Thin Air": Exploring Phase Changes.' J. Chem. Educ. 2000: 77, 488A.