Question: What are reservoir fluids?
Answer: They are the fluids contained within the reservoir rock.

Question: What do they do?
Answer: They are the primary mechanism driving the movement of hydrocarbons in the subsurface.

Question: Why do they flow through the subsurface?
Answer: Subsurface rock materials have porosity and permeability (Oil Formation).

Source rocks and reservoir rocks have two characteristics needed for fluid movement: porosity and permeability. They contain voids, or pores, in which to store fluid (porosity) and the pores are interconnected (permeability) in order to allow flow to occur. Hydrocarbons are a type of reservoir fluid. The volume of hydrocarbons stored in a reservoir depends upon the porosity of the reservoir rock. The rate and volume at which hydrocarbons are withdrawn depends upon the permeability of the reservoir rock. Nearly all pore space of the upper few miles of crust is filled with water. Thus hydrocarbons exist in a water environment with combinations of gas, oil, and water occurring in varying proportions.

The two dominant physical properties of oil and gas that enhance their movement through subsurface materials, are their relative immiscibility in water and their lower density than water which causes their buoyancy. Buoyancy of the hydrocarbons occurs because of differences in densities of respective fluids while flow through the reservoir is in response to differential pressures that exist in a reservoir rock.

Question: Where does this water come from?
Answer: Three sources.

There are three (3) sources for all free water found in the subsurface:

1. Meteoric water - falls as rain and enters into the subsurface via percolation;

2. Connate Water - sea water mixed in with the sediments when they were deposited; and

3. Mixed water - multiple origins.

Question: Oil-field waters contains all three types of water. How do we know this?
Answer: Meteoric water is that which originates as fallen rain and enters into subsurface via percolation through surficial materials eventually collecting in aquifers. Meteoric water contains substantial quantities of O₂ and CO₂ which is acquired as the precipitation falls through the atmosphere. The O₂ and CO₂ rich water produces sulfate (SO₄), carbonate (CO₃), and bicarbonate (HCO₃) rich solutions. Oil field water contains all three compounds and therefore at least part of these oil field waters have an origin as rainfall that fell during the geologic past. Connate water refers to sea water in which the original sediments were deposited. Sea water contains an average of 30,000 PPM Chloride (Cl^-) while most oil-field waters, called brines, contain an average of 80,000 PPM Cl^-. The presence of Chloride indicates a sea-water origin. But there is a problem. If the chemical concentration of the ancient seas were about same as present day sea water, then this "original" water has acquired additional mineral matter since it entered the rock and became associated with the hydrocarbons. Additional Chloride is probably added during the transformation phase of hydrocarbon formation. Mixed waters are those of multiple origin, containing both meteoric and connate water.

The distribution of reservoir fluids (gas, oil, water) trapped in a reservoir is determined by the laws of physics. Gas has the lowest density of the three mediums and is therefore trapped at the top of the reservoir when it occurs in free-phase. Gas may also be dissolved in the oil. If the gas is dissolved in the oil, the amount dissolved affects the viscosity of oil (viscosity being reduced as gas content in oil increases). Both temperature and pressure affect how much gas can be dissolved; the temperature and pressure at which gas comes bubbling out of solution is called bubble point. Gas volume is measured in cubic feet (cf) and written in multiples of 1,000 which is abbreviated M. For example, 3,540,000 cubic feet of gas would be 3,540 Mcf.

Click to view a typical anticline structure that holds oil, gas, and water and a cross-section of the structure.

Oil is most valuable of the three reservoir fluids, next comes natural gas.

Crude is trapped near the top of the reservoir between the water below and gas above. The volume of gas in solution with the oil for any given reservoir varies greatly, but generally it is greater especially near the oil-gas boundary interface.

Question: Oil is measured in barrels (bbs), one barrel containing 42 US gallons and weighing an average of 310 pounds. Why do we say ".... an average ....?"
Answer: Remember, the density of crude oil varies and therefore the weight of a barrel of crude will also vary.

Question: The volume of production (product) removed from a well is commonly measured in barrels per day (bbd). Initial production, measured per 24-hour period, is usually greater with volume declining once the well has been on-line. Why do you think this is?
Answer: Pressure drop: as product is removed, less upward pressure is exerted by the formation water below the oil (product).

Oil field reserves are estimated in millions total barrels (volumetric).

Water associated with the production of hydrocarbons has no commercial value and must be disposed of in an economically and environmentally safe manner. But these production waters, also referred to as oil-field waters (water associated with oil and gas pools), are often used to further promote the removal of hydrocarbons from the reservoir. More about this aspect in a later discussion.

Oil-field waters are generally trapped at the bottom of the reservoir with the oil floating above it or they may be mixed with the oil, especially near the water/oil boundary interface. Wells that produce only water or water with no commercial amounts of hydrocarbons are called dry holes or dusters. Product removal from producing hydrocarbon wells eventually ceases, either because the oil runs out, but more often because of economical reasons. Either the ratio of oil:water in solution decreases or the well begins to produce free water.

Question: Why would a well begin producing water when it never did before?
Answer: As hydrocarbons are removed from reservoir, the voids that were once filled with oil are now filled with water (water under pressure) from below.

Question: As hydrocarbons are removed from the reservoir, what happens to the pressure?
Answer: Drops because there is an increasing void space in which to insert some existing liquid (or gas).

Question: What effect does this pressure drop have on remaining hydrocarbons?
Answer: More difficult to pump (remove) from the subsurface because pressure assists in pushing the hydrocarbons out of the reservoir rock and up the well casing.

Question: Can pressure be artificially increased in a depleted reservoir?
Answer: Yes - but this is a later discussion dealing with secondary recovery methodology.

Click to view a movie showing the action of reservoir fluids in porous/permeable rock.

The nature of this water, and its affect on oil in the reservoir is an important concept. Originally, it was thought that the entire pore space of the petroleum reservoir was filled with only oil & gas, but now geologists realize this is incorrect as our discussion above has indicated. Current thinking is that interstitial water (water attached to rock surfaces), as opposed to free water (free to flow through permeable rock materials), coexists with oil/gas. Interstitial water exists as an adsorbed film of water surrounding the grains of rock and acts as a wetting agent (referred to as a "surfactant"). Hydrocarbons are not in contact with rock, but are in contact with water. As a result, there is an oil-water interfacial relationship between the two mediums in which the water "lubricates" the oil allowing the oil to flow more readily (less friction) through reservoir rock.

REFERENCES

DOTT, R. H., and M. J. REYNOLDS, 1969, Source book for Petroleum Geology: Am. Assoc. Petrol. Geol., Mem. No. 5, 471 pp.

JUDSON, SIDNEY A. and R. A. STAMEY, 1933, Overhanging Salt On Domes of Texas and Louisiana: Bull. Am. Assoc. Petrol. Geol., Vol. 17, pp. 1492-1520.

Other Materials Utilized in Preparation of this Section

BATES, R. L. AND J. A. JACKSON, 1980, Glossary of Geology: American Geological Institute, Falls Church, VA., 2nd Ed., 749 pp.

BERGER, B. D. and K. E. Anderson, 1992, Modern Petroleum - A Basic Primer of the Industry: PennWell Books, 3rd Ed., Tulsa, OK., 517 pp.

LEVORSEN, A. I., 1967, Geology of Petroleum: W. H. Freeman and Company, San Franciso, CA., 2nd Ed., 724 pp. SELLEY, R. C., 1985, Element of Petroleum Geology: W. H. Freeman & Co., New York, 449 pp.

NORTH, F. K., 1985, Petroleum Geology: Allen & Unwin, Inc., Winchester, MA., 607 pp.