Solution gas drive

Solution gas drive (or depletion drive) Gas cap drive Water drive with a large underlying aquifer Undersaturated oil (no gas cap) Saturated oil with a gas cap Saturated or undersaturated oil... 

Solution gas drive occurs in a reservoir which contains no initial gas cap or underlying active aquifer to support the pressure and therefore oil is produced by the driving force due to the expansion of oil and connate water, plus any compaction drive.. The contribution to drive energy from compaction and connate water is small, so the oil compressibility initially dominates the drive energy. Because the oil compressibility itself is low, pressure drops rapidly as production takes place, until the pressure reaches the bubble point. 

The characteristic production profile for a reservoir developed by solution gas drive is shown in Figure 8.3. 

Figure 8.3 Production profile for solution gas drive reservoir...

In the solution gas drive case, once production starts the reservoir pressure drops very quickly, especially above the bubble point, since the compressibility of the system is low. Consequently, the producing wells rapidly lose the potential to flow to surface, and not only is the plateau period short, but the decline is rapid. 

Commonly the wafer cuf remains small in solution gas drive reservoirs, assuming that there is little pressure support provided by the underlying aquifer. Water cut is also referred to as BS W(base sediment and water), and is defined as ... 

As solution gas drive reservoirs lose pressure, produced GORs increase and larger volumes of gas require processing. Oil production can become constrained by gas handling capacity, for example by the limited compression facilities. It may be possible to install additional equipment, but the added operating cost towards the end of field life is often unattractive, and may ultimately contribute to increased abandonment costs. 

The well is then opened, the CO2 provides a solution gas drive, and oil mobilized by the CO2 soak is produced. 

B. licheniformis JF-2 and Clostridium acetogutylicum were investigated under simulated reservoir conditions. Sandstone cores were equilibrated to the desired simulated reservoir conditions, saturated with oil and brine, and flooded to residual oil saturation. The waterflood brine was displaced with a nutrient solution. The MEOR efficiency was directly related to the dissolved gas/oil ratio. The principal MEOR mechanism observed in this work was solution gas drive [505]. 

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... 

In the reservoir imder consideration the energy available for expulsion of oil and gas comes entirely from the evolution of solution gas on pressure reduction. Consequently, this type of reservoir is designated as a solution gas drive reservoir to distinguish it from those whose recovery mechanisms involve energy from the expansion of a gas cap (gas expansion reservoirs) or from the encroachment of water (water drive reservoirs). The behavior of a solution gas drive reservoir may be predicted if the following data are available (1) the original reservoir pressure and temperature (2) values of r, and v as a fimction of pressure (3) values of the reservoir fluid viscosities r as a function of pressure at reservoir temperature (4) the constant water saturation 8w) (5) values of Kg/Ko as a fimction of saturation and (6) the number of barrels of stock tank oil originally in reservoir (iV). The computations are carried out stepwise as shown below. 

Shrinkage factor, 110 Shrinkage of oil, 110, 114 Single-component systems, 49 ff. Solution gas drive reservoir, 171 ff. Solutions, 79 ff. 

In this chapter the properties of nonaqueous hydrocarbon foams will be reviewed and the effects of foam formation on flow of oil—gas mixtures in porous media will be discussed A laboratory technique for investigating the role of foamy-oil behavior in solution gas drive is described, and experimental verification of the in situ formation of non-aqueous foams under solution gas drive conditions is presented The experimental results show that the in situ formation of nonaqueous foam retards the formation of a continuous gas phase and dramatically increases the apparent trapped-gas saturation. This condition provides a natural pressure maintenance mechanism and leads to recovery of a much higher fraction of the original oil in place under solution gas drive. 

Most likely, the in situ formation of an oil-continuous foam plays a role in the production of heavy oils under solution gas drive. Before discussing this role of oil-continuous foams, the similarities and differences between such nonpolar, nonaqueous foams and the aqueous foams will first be reviewed. 

As mentioned earlier, heavy oil produced by solution gas drive often displays marked foaminess in wellhead samples. This feature is not surprising because the two key factors needed for nonpolar foam stability are present in the heavy-oil system. The viscosity of the liquid phase (heavy oil) is high enough to retard drainage of liquid films by capillary... 

The objective of the experimental study described next was to examine whether the in situ formation of a foam occurs in primary production of heavy oils by solution gas drive. A simple apparatus was designed to conduct primary depletion experiments in the laboratory. The presence of foam within the porous matrix was inferred from the observed production and pressure-drop behavior of the system. 

Thus the production versus drawdown behavior was consistent with the conventional picture of solution gas drive and revealed no surprises. 

Results of the experimental study suggest that the formation of an oil-continuous foam may be involved in flow of heavy oil under solution gas drive. Such foam formation can be very beneficial for increasing the oil recovery. It delays the formation of a continuous gas phase, and thereby acts as a natural pressure-maintenance mechanism. In terms of the conventional solution gas drive theory, it serves to greatly increase the apparent trapped-gas saturation. 

Relating this to the anomalous production behavior of Lloydminster type heavy-oil reservoirs under solution gas drive, the foam-flow hypothesis can explain the high primary recovery factors. However, it does not explain the high rate of production. Evidently, a different mechanism is responsible for the high rates. Most probably, an increase in the effective permeability caused by sand dilation is involved. The foam-flow mechanism also explains why some of these prolific producers under primary production may show poor response to steam stimulation. At elevated temperatures, the foam is likely to be less stable. Therefore, the gas phase would become continuous at much lower gas saturations. Consequently, the benefit of foamy-oil may not be available at steam temperatures. 

Vaziri (54) extended the Risnes model (53) by incorporating several features important to solution gas drive processes. Dissolved gas can come out of solution as the reservoir pressure is depleted below the oil bubble point/ Solution gas drive refers to oil production resulting from expansion of the gas phase. Vaziri assumed that liquid and gas form a single phase completely filling the pore space. Mechanical properties of the fluid (e.g., compressibility) vary with proportion of the gas phase and can be determined by application of Boyle s and Henry s laws. An expression for a fluid compressibility capacity, termed fluid flexibility, of the following form is used ... 

Even though the oils have a low gas-to-oil ratio, solution gas drive appears to be major factor (97, 100). Gas that comes out of solution is slow to coalesce because of the viscosity of the oil, and a foamy oil flow results. The nature of the foamy oil has been discussed (101), and it has been postulated that it is one of several factors contributing to the production of solids from these types of reservoirs. 

Mechanisms of oil recovery due to this process include (1) reduction of flow resistance near the well bore by reducing the crude oil viscosity and (2) enhancement of the solution gas drive mechanism by decreasing the gas solubility in an oil as temperature increases. 

Steam injection might not be necessary because of the existence of an unusual cold production mechanism called foamy solution gas drive, which results in a porous medium flow rate up to 10 times higher than predicted by Darcy s law, and an anomalously high final recovery, e.g., 15%. These features were found in heavy crude oilfields both in Canada and Venezuela, and appear to be highly favorable behavior of those deposits, though the main reasons for them are still unclear (12—18). As depletion takes place, very tiny gas bubbles start forming and do not coalesce, contrary to what happens usually in most oil production situations. Indeed, most apolar liquid foams are unstable be-Table ICharacteristics of Crudes from Orinoco OU Belt... 

Whatever the reason for this behavior, field and laboratory data indicate that the foamy solution gas drive regime is attained only at high drawdown pressure. The current state of the art indicates that a high depletion rate is required to trigger and maintain this mechanism, and that it should be applied early in the production history. The confirmation of these trends would probably compel producers to apply shorter well-spacing patterns. 

GE Smith, Fluid flow and sand production in heavy oil reservoir under solution gas drive. SPE Prod Eng (May) 169—177, 1988. 

The above fundamental equation is of importance in nucleation theory. Nuclea-tion is a phenomenon of interest in many engineering applications, including metallurgical processes and solution-gas drive in porous media for oil production (Firoozabadi and Kashchiev, 1996). 

Firoozabadi, A., and D. Kashchiev Pressure and Volume Evolution During Gas Phase Formation in Solution Gas Drive, Soc.Pet.Eng.J.p. 219, Sept. 1996. 

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