Petroleum Reservoir Rocks

For displacement of the oil in the pores and capillaries of petroleum reservoir rock, an aqueous solution-oil interfacial tension of 10 3 mN/m (dyn/cm) is generally required. To reach so low a value, the value of yah (equation 5.1), the interaction energy across the interface (Figure 5-1) must be large. This means that the nature of the material on both sides of the interface must be very similar. Since oil and water have very different natures, a situation where both sides of an oil-water interface can have similar natures can occur only when both sides of the interface have similar concentrations of surfactant, oil, and water. There are a number of ways in which such a situation can be created. 

CBM is adsorbed to the surface of the coal and the adsorption sites can store commercial quantities of gas as part of the coal matrix. This must not be confused with conventional pore-volume storage. Gas within petroleum reservoir rock as a gas and the traditional pressure/temperature/volume relationships hold. Adsorbed gas molecules do not behave as a gas (1) they do not conform to the shape of the container, (2) they do not conform to the modified ideal gas laws (i.e., PV ZnRT), and (3) they take up substantially less volume than the same mass of gas would require within a pore volume. 

Burdine, N. T., Gournay, L. S. and Reichertz, P. P. (1950) Pore size distribution of petroleum reservoir rocks. Trans. AIME 189, 195. 

Low interfacial tensions have been intensively studied in the eighties, owing to its application in enhanced tertiary recovery of petroleum oil. For displacement of the oil in the pores and capillaries of petroleum reservoir rock, aqueous solution—oil interfacial tensions below 10 mN/m are generally required. It is the field of microemulsions [26-30], where such low values are achieved. Zhou et al. demonstrated there is a clear correlation between a compact packing in the interface and a low interfacial tension [31]. 

Dandekar, A.Y. (2006) Petroleum Reservoir Rock and Fluid Properties, Taylor Frands, Boca Raton. 

Sawyer, W.K., Pierce, C.L, Lowe, R.B., 2001. Electrical and hydraulic flow properties of Appalachian petroleum reservoir rocks. Petrophysics 42 (2), 71-82. 

By 1980, research and development shifted from relatively inexpensive surfactants such as petroleum sulfonates to more cosdy but more effective surfactants tailored to reservoir and cmde oil properties. Critical surfactant issues are performance in saline injection waters, adsorption on reservoir rock, partitioning into reservoir cmde oil, chemical stabiUty in the reservoir, interactions with the mobiUty control polymer, and production problems caused by resultant emulsions. Reservoir heterogeneity can also greatly reduce process effectiveness. The decline in oil prices in the early 1980s halted much of the work because of the relatively high cost of micellar processes. 

Natural Gas Natural gas is a combustible gas that occurs in porous rock of the earth s crust and is found with or near accumulations of crude oil. It may occur alone in separate reservoirs, but more commonly it forms a gas cap entrapped between petroleum and an impervious, capping rock layer in a petroleum reservoir. Under high-pressure conditions, it is mixed with or dissolved in crude oil. Natural gas termed dry has less than 0.013 dmVm (0.1 gaLlOOO fF) of gasoline. Above this amount, it is termed wet. 

Increasing the water-wet surface area of a petroleum reservoir is one mechanism by which alkaline floods recover incremental oil(19). Under basic pH conditions, organic acids in acidic crudes produce natural surfactants which can alter the wettability of pore surfaces. Recovery of incremental oil by alkaline flooding is dependent on the pH and salinity of the brine (20), the acidity of the crude and the wettability of the porous medium(1,19,21,22). Thus, alkaline flooding is an oil and reservoir specific recovery process which can not be used in all reservoirs. The usefulness of alkaline flooding is also limited by the large volumes of caustic required to satisfy rock reactions(23). 

Fig. 30.4. Changes in the volumes of minerals in the reservoir rock during the simulated alkali floods (Fig. 30.3) of a clastic petroleum reservoir using NaOH, Na2CC>3, and Na2SiC>3 solutions. Minerals that react in small volumes are omitted from the plots. Abbreviations Anal = analcime, Cc = calcite, Daw = dawsonite, Dol = dolomite, Kaol = kaolinite, Muse = muscovite, Parag = paragonite, Phlog = phlogopite, Qtz= quartz, Trid = tridymite.

In primary recovery the natural energy comes mainly from gas and water in reservoir rocks. The gas may be dissolved in the oil or separated at the top of it in the form of a gas cap. Water, which is heavier than oil, collects below the petroleum. Depending on the source, the energy in the reservoir is called solution-gas drive, gas-cap drive, or water drive. In solution-gas drive, the gas expands and moves toward the opening, carrying some of the liquid with it. In gas-cap drive, gas is trapped in a cap above the oil as well as dissolved in it. As oil is produced from the reservoir, the gas cap expands and drives the oil toward the well. In water drive, water in a reservoir is held in place mainly by underground pressure. If the volume of water is sufficiently... 

Thus gas, if any is present, is found in the highest parts of the trap, followed by oil (and oil with gas) below the gas, and finally salt water below the oil. Experience has indicated that the salt water seldom was completely displaced by oil or gas from the pore spaces, even w ithin the trap. Even in the midst of oil and gas accumulation, pore spaces within the trap may contain from 10 to 50% or more of salt water, It appears that the remaining water (termed connate water) fills the smaller pores ancl also exists as a coating or film, covering the rock surfaces of the larger pore spaces thus oil and/or gas are apparently contained in water-jacketed pore spaces. The geological structures called traps are petroleum reservoirs, i.e.. they are the oil and gas fields that me explored and produced. All oil fields contain some gas, but the quantity may range widely. See also Natural Gas. 

The reservoir rocks that yield crude oil range in age from Precambrian to Recent geologic time but rocks deposited during the Tertiary, Cretaceous, Permian, Pennsylvanian, Mississippian, Devonian, and Ordovician periods are particularly productive. In contrast, rocks of Jurassic, Triassic, Silurian, and Cambrian age are less productive and rocks of Precambrian age yield petroleum only under exceptional circumstances. 

Aquifer a subsurface rock interval that will produce water often the underlay of a petroleum reservoir. 

Combustion zone the volume of reservoir rock wherein petroleum is undergoing combustion during enhanced oil recovery. 

Petroleum (crude oil) a naturally occurring mixture of gaseous, liquid, and solid hydrocarbon compounds usually found trapped deep underground beneath impermeable cap rock and above a lower dome of sedimentary rock such as shale most petroleum reservoirs occur in sedimentary rocks of marine, deltaic, or estuarine origin. 

This book evolved from notes prepared and developed by me over the past six years. This material forms the subj ect matter for a one-semester petroleum engineering course at The Pennsylvania State University. This course along with one dealing with the fundamental properties of reservoir rocks and rook-fluid systems are prerequisites for more advanced courses in reservoir engineering. These basic courses are an introduction to the fundamentals which must be mastered by the student before the treatment of more complex systems can be attempted. 

Several attempts have been made to integrate geochemical parameters into this framework of system tracts to assist in differentiating one facies from another. Potential source rocks can be found in all three system tracts, with the richest source rocks generally found in the condensed section within a TST. The best petroleum reservoirs are found in LST. With the use of geochemical data, the various system tracts in a sequence can be identified and therefore used to distinguish the location of both potential petroleum source rocks and reservoirs. 

---