RESERVOIR DRIVE MECHANISMS

Transcription

1 RESERVOIR DRIVE MECHANISMS There are several ways in which oil can be naturally displaced and produced from a reservoir, and these may be termed mechanisms or "drives". Where one mechanism is dominant, the reservoir may be said to be operating under a particular "drive". Possible energy sources for expulsion of the reservoir fluids are: (i) (ii) (iii) (iv) (v) Expansion of the undersaturated oil above the bubble point. Compaction of the rock and expansion of connate water. Expansion of gas released from solution in the oil below the bubble point. Invasion of the original oil bearing reservoir by the expansion of the gas from a free gas cap. Invasion of the original oil bearing reservoir by the expansion of the water from an adjacent or underlying aquifer. All of the above displacement processes are related to some form of expansion or compaction: in other words, some form of compressibility is the driving force (energy) for expulsion of oil from the reservoir rock. The pressure drop in the reservoir, due to oil production, may be small if gas caps and aquifers are large and permeable; under favourable circumstances pressures may stabilize at constant or declining reservoir production rates. On the other hand, the compressibilities of undersaturated oil, rock, and connate water are so small that pressures in undersaturated reservoirs will fall rapidly to the bubble point if there is no aquifer to provide water drive. So these expansion mechanisms are not usually considered separately, and the three principal categories of reservoirs are: (a) (b) (c) Solution gas drive (or depletion drive) reservoirs. Gas cap expansion drive reservoirs. Water drive reservoirs 4.1 Solution Gas Drive Reservoirs Fig. 4.1a shows oil production, gas-oil pressure behaviour in a solution gas drive reservoir. If a reservoir at its bubble point is put on production, the pressure will fall below the bubble point pressure and gas will come out of solution (see Fig. 4.1b). Initially this gas may be a dispersed, discontinuous phase, any will be- essentially immobile until some minimum saturation - the critical gas saturation is attained. The actual order of values for critical gas but there is considerable evidence to saturation are in some doubt, support the view that values may be very low, about 1 % to 2 %. Once the critical gas saturation has been established gas will be mobile, and will flow to the producing wells if the pressure gradient is dominant. Initially the producing gas-oil ratio of a well producing from a closed reservoir will equal the solution GOR, R so. At early times, as pressure declines and gas comes out of solution, but cannot flow to 52

2 producing wells, the producing GOR will decline. When the critical gas saturation is established gas will flow towards producing wells. saturation is established gas will flow towards producing wells. Fig. 4.1a: Solution gas drive reservoir.. Fig. 4.1b: Dissolved gas drive reservoir. 53

3 The effective permeability to oil will be lower than at initial conditions, and there will be a finite effective permeability to gas so that the producing GDR, RP, at surface will rise. As more gas comes out of solution, and gas saturation increases, effective permeability to gas increases, that to oil diminishes, and this trend accelerates (see Fig. 4.2). Ultimately, as reservoir pressure declines towards abandonment pressure, the change in gas formation volume factor offsets the increasing mobility of gas over that of oil and the gas-oil ratio trend is reversed; i.e. while the ratio of the gas to oil volumes at reservoir conditions may continue to increase, in terms of standard volumes, the ratio of the produced gas to oil may decline. Fig. 4.2 Producing gas-oil ratio trends for reservoirs under various drives. In truly stratigraphic traps there is an absence of extensive lateral continuity, and relatively moderate structural dip. These factors imply reservoir communication with an aquifer of limited volumetric extent, if any, and little possibility for gravity segregation of reservoir fluids (i.e. no secondary gas cap formation). Therefore the most common drive mechanism in stratigraphic traps is depletion drive. This mechanism gives oil recoveries of 5-30% of the original oil in place. 54

4 4.2 Gas Cap Expansion Drive Reservoirs: Fig. 4.3a: Gas cap drive reservoir. On many occasion soil accumulations occurred in nature in which there were greater volumes of light components than would dissolve in the oil at reservoir temperature and pressure. When this occurred the light materials formed a free gas phase at the top of the reservoir (see Fig. 4.3b). As shown in Fig. 4.4, the presence of this free gas phase retards the decline in reservoir pressure. At the higher reservoir pressure, the rate of release of gas from solution is decreased and so too is the build up gas saturation in the oil zone. Provided that the free gas phase (cap) can be controlled and not produced directly from the producing wells, better well productivities and lower producing gas-oil ratios can be maintained. Eventually, however, the gas cap expands and soon reaches the wells upstructure, causing the producing GOR to increase. Refer to Fig Fig. 4.3b: Gas cap drive reservoir. 55

5 Fig. 4.4 Reservoir pressure trends for reservoirs under various drives. In as much as the segregation of reservoir fluids requires considerable dip, gas cap drives are found mostly in structural traps of sufficient relief. This type of drive produces % of the original oil in place. 4.3 Water Drive Reservoirs: Fig. 4.5a: Water drive reservoir. 56

6 Fig. 4.5a shows various strengths of water drive. An efficient water driven reservoir (Fig. 4.5b) requires a large aquifer body with a high degree of transmissivity to allow large volumes of water to move across the oil-water contact in response to small pressure drops. Owing to the low compressibility of water sufficient water influx can only be obtained if the aquifer is large volumetrically. Fig. 4.4 shows that in an active water drive reservoir, pressure decline is usually much less than with the other drives. Consequently, the producing GOR remains, more or less, at a constant level. Oil recoveries are typically %. Frequently two or all three mechanisms (together with rock and connate water expansion) occur simultaneously. See Fig Fig. 4.5b Water drive reservoir. Fig. 4.6 Combination drive reservoir.. 57

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