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159 Initial Gas Saturation-Final Gas Saturation Initial Gas Saturation where Sw^ is the initial water content of the rock (i.e., the connate water) and Sgr is the residual gas saturation, both expressed as fractions of pore space. For a reservoir rock with a connate water of 25 percent and a residual gas saturation of 30 percent, the recovery factor for the invaded portion is 1-0 25.- 0.30 Recovery Factor 1 - 0.25 = 0.60 or 60% Obviously, the higher the connate water and the higher the residual gas saturation, the lower the recovery factor. Unless wells are fortuitously located in the very highest structural position, when the last well has been invaded by water, there remains a volume of the reservoir which is unswept by water and which contains unrecoverable gas. If the original volume of the reservoir is say 1,000 units and 800 units are swept by invaded water with a recovery factor of 60 percent, then the overall recovery factor is Recovery Factor = 0.60 x = 0.48 or 48% In reservoir engineering terminology, for this example, 800/1000 is a sweep efficiency of 80 percent. In the Federal Power Commission report, estimates of the physically recoverable gas assume that the entire reservoir volume is swept by invading water. This in turn assumes that wells are fortuitously located in the
160 highest structural position and that water does not prematurely reach and "drown" such wells before the entire reservoir is flooded with water. Three ways that water can enter a well prematurely are by edge water cusping, bottom water coning, and bypassing. Reservoirs underlain wholly or largely by water are called bottom water reservoirs. Even where a well is completed (perforated) only in the top few feet of the reservoir, pressure reduction in the vicinity of the well, as a result of gas production, causes the bottom water to cone up into the well and drown it. This may occur when as little as 5 percent of the gas has been produced. Edge water cusping is a similar phenomenon in the horizontal plane. Bypassing occurs, for example, where a producing stratum is composed of two or more layers of stringers of different permeability. Water moves up more rapidly to the well in the high permeability stringer and drowns out the gas well, leaving the less permeable stringers only partially swept by invading water. There are also other kinds of reservoir heterogeneity which contribute to bypassing and lower recovery. Associated Gas Reservoirs In oil reservoirs without initial gas caps, the depletion mechanism results in oil recoveries usually in the range of 10 to 25 percent. However, recovery of the dissolved gas is essentially 100 percent. This is because pumps are usually installed in the later stages of production to help recover more oil. Pumping reduces reservoir pressure to very low levels causing the release from solution and production of essentially all of the gas.
161 Active water drive in oil reservoirs without initial gas caps is essentially the same as active water drive in gas reservoirs. In the oil reservoir, residual oil saturation is used in place of residual gas saturations, and together with sweep efficiency, it is used to estimate the oil recovery factor. Because pressure is maintained, no gas is released from solution in the reservoir. The gas recovery factor, there- fore equals the oil recovery factor, which usually lies in the range of 30 to 60 percent, but may be much lower in cases such as bottom water drives. In oil reservoirs with initial gas caps, essentially all of the gas will be produced by the depletion mechanism, either by wells completed in the gas cap or vial wells completed in the oil zone. Here again, the use of oil pumps causes reservoir pressure to reach very low levels. Where active water drive occurs with this type, the proper practice is to produce the oil zone first and then the gas cap. Gas recovery factors are therefore computed from recovery in the oil zone by water invasion, as explained in the preceding paragraph, and by water invasion of a gas reservoir, also explained previously. In depletion oil reservoirs, with or without initial gas caps, actual gas recovery approaches 100 percent, therefore physically recoverable gas also approaches this value. Under active water drive, physically recoverable gas, both dissolved and in caps, may considerably exceed actual recovery by a factor determined by the sweep efficiency in the oil zone and overlying cap.
162 RESERVOIR CHARACTERISTICS AND GLOSSARY OF TERMS Charles J. Mankin Reservoir Characteristics A gas reservoir is a rock unit that contains gas and has the ability to transmit it to a producing well. Because reservoirs are formed by a variety of geological processes, they display a wide range of characteristics. For example, deposits formed by streams tend to be long and narrow, whereas those formed by certain shoreline processes tend to have a more equant shape. Therefore, a particular reservoir is described by its thickness, stated in feet, and by its geographic or areal extent, stated in acres. The volume of a reservoir is thus definable in terms of acre-feet. The essential characteristics of a reservoir are its abilities to contain and to transmit a fluid. The ability to contain a fluid, in the form of either a liquid or a gas, is related to a property known as porosity. This property is the ratio of void space to rock material in the reservoir and is normally described as a percentage. The transmission of a fluid through a reservoir is related to a property described as permeability, a function of the degree to which the void spaces in the reservoir are connected. Thus a rock unit with a high porosity (i.e., a large amount of void space) but a low permeability (i.e., only a few interconnections of void spaces) would be a poor reservoir because of its inability to transmit its fluid to a producing well.
163 A reservoir containing petroleum in the form of crude oil and/ or natural gas has some additional limiting characteristics. Because liquid petroleum tends to be lighter than water, and because natural gas cannot accumulate except in a closed, or at least partially closed, system, petroleum reservoirs must be bounded by impermeable layers or traps. Reservoirs with their impermeable bounding layers occur under a variety of gological conditions. It is these conditions that the exploration geologist attempts to identify by using a variety of geological and geophysical methods. Petroleum in the form of a liquid and/or a gas occupies the void space between the reservoir-rock particles. The voids may be completely filled with petroleum, or they may contain some nonpetroleum ingredients, such as water. The degree to which a petroleum-bearing reservoir is filled with water is described as the water saturation. Reservoirs with a high water saturation will normally produce less recoverable petroleum. In a similar manner, these voids may also contain some gas instead of liquid petroleum. The degree to which the voids are filled with gas is described as the gas saturation. Some gas may also be found in the reservoir in solution with the contained liquid in a manner similar to the solution of carbon dioxide in a carbonated drink. This gas is commonly described as dissolved gas. To determine the amount of natural gas contained in a reservoir, it is necessary to be able to accurately describe its volume in terms of areal extent and thickness. Such a determination is only possible if a sufficient amount of accurate geological information is available. Except
164 in those areas with substantial development drilling, the amount of information available is not adequate to determine accurately the reservoir volume. Thus the volume of a reservoir having an areal extent of 500 acres with an average thickness of 100 feet may be subject to an error of almost 30 percent if the areal extent is estimated to be 640 acres. At the stage of early exploration of a reservoir, the amount of geological information is not adequate to determine the areal extent even to this degree of accuracy. Thus one obvious error in determining reserves of natural gas steins from faulty (inaccurate) determination of the volume of the reservoir.
165 GLOSSARY OF TERMS Most definitions in this glossary are generalized. An attempt has been made to simplify them for the layman. Abnormal pressure - formation or reservoir pressure in excess of nor- mal pressure for a given depth. During drilling abnormal pressures require added mud weight to prevent blowouts. Acreage - surface area, in acres. Acre-feet - a term used to describe volume (i.e., of reservoirs); for example, 1 acre-foot indicates the volume of an area of 1 acre which is 1 foot thick (43,560 feet3). Associated gas - gas which occurs with liquid hydrocarbons; it often contains some heavier hydrocarbons, in which case it is called "wet". Bottle test - a test of a formation or reservoir for fluid and/or gas content by means of establishing flow from the reservoir into a closed chamber lowered into the well on a wire line. Carbonate - a rock type which consists of limestone or dolomite. Carbonates and sandstones are the two major rock types of petroleum reservoirs. Condensate vaporizing volume ratio - the volume of gas vapor obtained from a unit volume of liquid condensate under pressure when the pressure of the condensate is reduced to atmospheric.
166 Critical pressure/temperature - the pressure or temperature at the critical state wherein all properties of coexisting vapor and liquid b ecome iden t ica1. Depletion drive mechanism - pools of oil or gas with no active water drive which produce solely as the result of expansion of natural gas. Dip log - graph giving data produced by three electrodes oriented at 120° to each other in a horizontal plane to define the slope of specific layers within the bore hole, oriented with respect to compass directions. Dissolved gas - gas in solution with the contained liquid in a reservoir. Drilling mud - mud formed of clay particles suspended in water or oil used in well drilling for the purpose of lubricating the hole, preventing caving, controlling formation pressure, cooling and lubricating the bit, and carrying formation cuttings to the surface. Drill stein test - procedure for releasing reservoir gas and fluids into a perforated anchor pipe attached to the drill pipe to measure reservoir pressures and flow capacity. Generally run to determine the ability of a well to produce. Fault - a displacement of rock formations along a plane or zone of frac- ture. Frequently forms a reservoir boundary. Flow test - to determine the potential production of a well by opening surface or subsurface control valves. Conducted through production pipe as opposed to drill stem test which is conducted through drill pipe.
167 Gas deviation or compressibility factor - a factor introduced into the ideal-gas law to account for the departure of true gases from ideal behavior. Gas saturation - the fraction of the pore spaces in a rock which are occupied by gas. Induction-Electrical log - an electric log obtained without the use of electrodes, by lowering into the uncased borehole a generating coil (fed with alternating current) that induces in rocks surrounding the borehole currents that are concentric with the hole and that are detected by a receiver coil giving a continuous record of the conductivity with depth. One of several logs usually run in a hole to determine the char- acter of rock and its content. Interstitial water saturation - the fraction of the pore space in a reservoir rock which is occupied by water. j^sopachous map - a map showing lines connecting points of equal form- ation thickness. Used to determine the volume of a reservoir. Hydrocarbon - any organic compound, gaseous, liquid, or solid, con- sisting solely of carbon and hydrogen; the term is used principally to designate oil or gas. MMcf - million cubic feet. Net effective feet - that portion of a reservoir rock, measured in feet, which has sufficient porosity and permeability to permit the passage of fluids. Net pay - the net vertical thickness of the rocks in a reservoir which is potentially productive of hydrocarbons.
168 Nonassociated gas - gas which does not occur with liquid hydrocarbons; it often has a high methane content, in which case it is called "dry"- Permeability - the ability of a rock to transmit gas, oil, or water through interconnected pore or void spaces. Planimeter - a mechanical instrument for measuring the area of any plane figure by means of a pointer or moving arm that traces its boundary or perimeter. Porosity - the percentage of pores (usually occupied by gas, oil, or water) or voids in a rock. In sandstones, porosity occurs between the grains. In carbonates, porosity can occur between the grains (intergranular porosity), by breakage of the rock (fracture porosity), or by solution of the rock (solution porosity). Porosity log - a well logging device that measures the porosity of rock formations. Recovery factor - the ratio of recoverable fluid to the total fluid in-place in a reservoir. Reservoir - a rock body capable of containing free fluids and yielding them when penetrated by the drill. Reservoir volume factor - factor by which standard surface volumes of oil are multiplied to obtain reservoir volumes. Residual gas saturation - the fraction of the pore space in a rock which contains gas after depletion, usually applied to water drive. Resistivity - the relative resistance of a material to conducting electri- city; the reciprocal of conductivity; in electrical well logging resistivity and conductivity graphical plots reveal certain characteristics of the rock layers and their content.
169 Sandstone or sand - a rock type composed of very small grains, usually silica, which are cemented together. Sandstones and carbonates are the two major rock types of petroleum reservoirs. Seismic interpretations - the maps or cross-sections prepared by a geophysicist or geologist using seismic data. Sidewall core - a finger-sized cylinder of rock extracted by sidewall sampling from the walls of an uncased drillhole. Generally recovered in a hollow bullet fired into the formation with an explosive charge. Solution gas drive (gas-expansion drive) - the oil producing mechanism resulting from expanding gas that has been released from solution in the oil of a reservoir. Reservoir pressure causes gas to go into solution; as pressure is reduced, gas expands and comes out of solution in the gaseous phase. Solution gas-oil ratio - the volume of gas disolved in a unit-volume of oil, usually expressed in standard cubic feet of gas per barrel of oil. Specific gravity of separator condensate - the ratio of the mass of the body to the mass of an equal volume of water at standard conditions, measured by means of a hydrometer using a scale measured in degrees API (American Petroleum Institute). Specific gravity of separator gas - the ratio of the density of the separator gas at a given pressure and temperature to the density of air at the same temperature and pressure. Structure - any physical arrangement of rock layers that may lead to the accumulation of oil or gas.
170 Structural map - a map that portrays-subsurface configuration by means of contour lines drawn through points of equal subsurface depth. Total net feet - in a given sand body, the number of feet of formation that have porosity and permeability. Water drive - the expulsion or displacement of hydrocarbons in a reservoir by water invasion. Invasion may be artificially induced (water injection) or may be due to naturally occurring reservoir waters. Water saturation - the fraction of pore space in a rock which contains water.
