Gas-oil-contact

An oil reservoir which exists at initial conditions with an overlying gas cap must by definition be at the bubble point pressure at the interface between the gas and the oil, the gas-oil-contact (GOC). Gas existing in an initial gas cap is called free gas, while the gas in solution in the oil is called dissolved or solution gas. 

This property is useful in helping to define the interface between fluids. The intercept between the gas and oil gradients indicates the gas-oil contact (GOG), while the intercept between the oil and water gradients indicates the free water level (FWL) which is related to the oil water contact (OWC) via the transition zone, as described in Section 5.9. 

For example, In the following situation, two wells have penetrated the same reservoir sand. The updip well finds the sand gas bearing, with gas down to (GOT) at the base of the sands, while the downdip well finds the same sand to be fully oil bearing, with an oil up to (OUT) at the top of the sand. Pressures taken at intervals in each well may be used to predict where the possible gas-oil contact (PGOC) lies. This method is known as the gradient intercept technique. 

The difference between the OWC and the FWL is greater in tight reservoirs, and may be up to 30m difference. A difference between gas-oil contact and free oil level exists for the same reasons, but is much smaller, and is often neglected. 

Give the probable hydrocarbon/water contact. Give the probable nature of the hydrocarbons and the gas/oil contact. 

The area of a gas reservoir enclosed by reservoir boundaries (gas-water contact, gas-oil contact, faults, zones of impermeability, or other reservoir boundaries). 

CO2 works very well in steeply dipping reservoirs where it can be injected at the top of the reservoir near the gas-oil contact. Gravity can then stabilize the sweep-out of the pattern as the fluids move down to the production wells. 

Unfortunately, detailed PVT data often depart radically from such simple assumptions. Usually, the saturation pressure (dew and bubble point) trends are not constant or linear with respect to depth, but curved (e.g. Grant 1959 Havlena 1968 Nutakki et al. 1996 Araque Auxiette 2002), and only converge on the reservoir pressures near the gas-oil contact (GOC) separating the two phases (Fig. 4c,d). This suggests that the two phases may only be in local thermodynamic equilibrium near the GOC. Often, the degree of under-saturation of the liquid phase decreases so rapidly with depth that the bubble points actually decrease with depth in an absolute sense (Fig. 4d). 

Fig. 21. Tilted rotated fault blocks with repetitive clastic lithologies of sandstones, shales and coals causes a multitude of down-to contacts, e.g. oil down-to contacts (ODC) and true oil-water contacts are seldom intersected by the well path. A slight tilting of the block results in significant mobilization of OWCs, gas-oil contacts and gas-water contacts, i.e. hydrocarbons get mixed within the individual compartments.

Gas is lighter than oil and oil lighter than water. There is thus a gravity segregation of the reservoir fluids. The top may consist of a gas zone, or cap, separated by the gas oil contact from the oil zone. The oil zone is separated from the water zone by the oil-water contact. The reservoir beneath the oil zone is termed the bottom water, and that adjacent to the field area is termed the edge water. 

Figure 2.4 shows another schematic of the variation of pressure and saturation pressure with depth. Note that there is no gas-oil contact. At the top, there is the gas phase, and at the bottom, there is the liquid phase. In such a case, the critical temperature is less than the reservoir temperature at the top and more than the reservoir temperature at the bottom the reservoir pressure is higher than dewpoint pressure at the top it is also higher than the bubblepoint pressure at the bottom. Oilfield examples of such a behavior are given by Neveux and Sathikumar (1988) and by Espach and Fry (1951). 

Figure 2.3 Schomatic of depth vs. pressure and saturation pressure in the gas cap and oil column with a distinct gas-oil contact (GOC).

The resistivity log can also be used to define oil / water or gas / water contacts. Figure 5.53 shows that the fluid contact can be defined as the point at which the resistivity begins to increase in the reservoir interval, inferring the presence of hydrocarbons above that point. 

Permeability (k) is a rock property, while viscosity (fi) is a fluid property. A typical oil viscosity is 0.5 cP, while a typical gas viscosity is 0.01 cP, water being around 0.3 cP. For a given reservoir, gas is therefore around two orders of magnitude more mobile than oil or water. In a gas reservoir underlain by an aquifer, the gas is highly mobile compared to the water and flows readily to the producers, provided that the permeability in the reservoir is continuous. For this reason, production of gas with zero water cut is common, at least in the early stages of development when the perforations are distant from the gas-water contact. 

Heavy fuel oil usually contains residuum that is mixed (cut back) to a specified viscosity with gas oils and fractionator bottoms. For some industrial purposes in which flames or flue gases contact the product (eg, ceramics, glass, heat treating, and open hearth furnaces), fuel oils must be blended to low sulfur specifications low sulfur residues are preferable for these fuels. 

In fluid catalytic cracking, a partially vaporized gas oil is contacted with zeoflte catalyst (see Fluidization). Contact time varies from 5 s—2 min pressure usually is in the range of 250—400 kPa (2.5—4 atm), depending on the design of the unit reaction temperatures are 720—850 K (see BuTYLENEs). 

The most dominant catalytic process in the United States is the fluid catalytic cracking process. In this process, partially vaporized medium-cut petroleum fractions called gas oils are brought in contact with a hot, moving, freshly regenerated catalyst stream for a short period of time at process conditions noted above. Spent catalyst moves continuously into a regenerator where deposited coke on the catalyst is burnt off. The hot, freshly regenerated catalyst moves back to the reactor to contact the hot gas oil (see Catalysts, regeneration). 

A sample of cracking catalyst in a fixed bed reactor is contacted with gas oil, an ASTM standard feed. Cracked liquid products are analyzed for unconverted material. Conversion is the difference between weight of feed and unconverted product. 

In two stage units, it is often economical to distill more gas oil in the vacuum stage and less in the atmospheric stage than the maximum attainable. Gas formed in the atmospheric tower bottoms piping at high temperatures tends to overload the vacuum system and thereby to reduce the capacity of the vacuum tower. The volume of crude vaporized at the flash zone is approximately equal to the total volume of distillate products. Of course, the vapor at this point contains some undesirable heavy material and the liquid still contains some valuable distillate products. The concentration of heavy ends in the vapor is reduced by contact with liquid on the trays as the vapor passes up the tower. This liquid reflux is induced by removing heat farther up in the tower. 

Fluid coking is very insensitive to poor gas-solids contacting, but has one problem not faced by cat cracking or hydroforming. If the heavy residual oil is fed too fast to the reactor, the coke particles will become wetted and stick together in large unfluidizable lumps. Correct control of feed rate is necessary to prevent this bogging. 

Oil is supplied to this system from the frame lube oil system or from an overhead tank. This oil comes in contact with and thus contaminates the gas being compressed. Gas/oil compatibility should be checked. 

Industries that burn wood, gas, oil or coal contribute most of the rest of airborne B(a)P. Studies on animals have shown that contact with BaP and PAH can cause skin cancer, but the effects of breathing or ingesting them are not yet well enough studied to draw a conclusion as to other cancers. Animal tests have shown that exposure to BaP may cause reproduction difficulty. The U.S. government considers BaP a human carcinogen. 

In an earlier study calorimetry achieved this objective for the compositional boundaries between two and three phases (2). Such boundaries are encountered both in "middle-phase microemulsion systems" of low tension flooding, and as the "gas, oil, and water" of multi-contact miscible EOR systems (LZ). The three-phase problem presents by far the most severe experimental and interpretational difficulties. Hence, the earlier results have encouraged us to continue the development of calorimetry for the measurement of phase compositions and excess enthalpies of conjugate phases in amphiphilic EOR systems. 

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