Reservoir drive mechanisms

The previous section showed that the fluids present in the reservoir, their compressibilities, and the reservoir pressure all determine the amount of energy stored in the system. Three sets of initial conditions can be distinguished, and reservoir and production behaviour may be characterised in each case  

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 material balance equation relating produced volume of oil (Np stb) to the pressure drop in the reservoir (AP) is given by  

Once the bubble point is reached, solution gas starts to become liberated from the oil, and since the liberated gas has a high compressibility, the rate of decline of pressure per unit of production slows down. 

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In Section 5.2.8 we shall look at pressure-depth relationships, and will see that the relationship is a linear function of the density of the fluid. Since water is the one fluid which is always associated with a petroleum reservoir, an understanding of what controls formation water density is required. Additionally, reservoir engineers need to know the fluid properties of the formation water to predict its expansion and movement, which can contribute significantly to the drive mechanism in a reservoir, especially if the volume of water surrounding the hydrocarbon accumulation is large. 

Keywords compressibility, primary-, secondary- and enhanced oil-recovery, drive mechanisms (solution gas-, gas cap-, water-drive), secondary gas cap, first production date, build-up period, plateau period, production decline, water cut, Darcy s law, recovery factor, sweep efficiency, by-passing of oil, residual oil, relative permeability, production forecasts, offtake rate, coning, cusping, horizontal wells, reservoir simulation, material balance, rate dependent processes, pre-drilling. 

The expansion of the reservoir fluids, which is a function of their volume and compressibility, act as a source of drive energy which can act to support primary producf/on from the reservoir. Primary production means using the natural energy stored in the reservoir as a drive mechanism for production. Secondary recovery would imply adding some energy to the reservoir by injecting fluids such as water or gas, to help to support the reservoir pressure as production takes place. 

Gas reservoirs are produced by expansion of the gas contained in the reservoir. The high compressibility of the gas relative to the water in the reservoir (either connate water or underlying aquifer) make the gas expansion the dominant drive mechanism. Relative to oil reservoirs, the material balance calculation for gas reservoirs is rather simple. A major challenge in gas field development is to ensure a long sustainable plateau (typically 10 years) to attain a good sales price for the gas the customer usually requires a reliable supply of gas at an agreed rate over many years. The recovery factor for gas reservoirs depends upon how low the abandonment pressure can be reduced, which is why compression facilities are often provided on surface. Typical recovery factors are In the range 50 to 80 percent. 

The primary drive mechanism for gas field production is the expansion of the gas contained in the reservoir. Relative to oil reservoirs, the material balance calculations for gas reservoirs is rather simple the recovery factor is linked to the drop in reservoir pressure in an almost linear manner. The non-linearity is due to the changing z-factor (introduced in Section 5.2.4) as the pressure drops. A plot of (P/ z) against the recovery factor is linear if aquifer influx and pore compaction are negligible. The material balance may therefore be represented by the following plot (often called the P over z plot). 

A modern solvent delivery system consists of one or more pumps, solvent reservoirs, and a degassing system. HPLC pumps can be categorized in several ways by flow range, driving mechanism, or blending method. A typical analytical pump has a flow range of 0.001-10 mL/min, which handles comfortably the flow rates required for most analytical work (e.g., 0.5-3 mL/min). Preparative pumps can have a flow range from 30 mL/min up to L/m. 

Fig. 7. Diagram of a simple diaphragm reciprocating pump, a = motor, b = drive mechanism, c = plunger piston, d = piston seal, e = low-pressure hydraulic chamber, f = safety device, g = high-pressure hydraulic chamber, h = diaphragm, i = solvent chamber, j = column check valve, k = to column, 1 = reservoir check valve, m = reservoir.

West Sak reservoir located on the North Slope of Alaska is estimated to contain up to 25 billion barrels of heavy oil in place and represents the largest known heavy oil accumulation in the United States. The possibility of sharing the existing Kuparuk River Unit facilities makes the development and production of the West Sak reservoir, a near-term target. The absence of natural drive mechanism in this reservoir makes it a target for the application of enhanced oil recovery processes. Miscible flooding is considered as one of the candidates for recovery of West Sak crude. ... 

Development planning and production are usually based on the expected production profile which depends strongly on the mechanism providing the driving force in the reservoir. The production profile will determine the facilities required and the number and phasing of wells to be drilled. The production profile shown in Figure 1.1 is characterised by three phases ... 

As reservoir pressure is reduced by oil production, additional recovery mechanisms may operate. One such mechanism is natural water drive. Water from an adjacent more highly pressured formation is forced into the oil-bearing formation by the pressure differential between the formations. Another mechanism is gas drive. Expansion of a gas cap above the oil as oil pressure declines can also drive additional oil to the wellbore. Produced gas may be reinjected to maintain gas cap pressure as is done on the Alaskan North Slope. Additional oil may also be produced by compaction of the reservoir rock as oil production reduces reservoir pressure. 

Each resei voir generally has a dominant drive, an optimal pattern ofwell locations, and a maximum efficient rate of production (MER), which, if exceeded, would lead to an avoidable loss of ultimate oil recovery. Unfortunately, oil, gas, and water are not evenly distributed within the reservoir. With multiple leases above the reservoir, some lease owners will have more oil, gas, or water than will others, and coordination among competing firms in well placement and in controlling production rates is difficult. Efficient production of the reservoir suggests that some leases not be produced at all. Further, since each firm s production inflicts external costs on the other firms on the formation, some mechanism must be found to internalize those costs in production decisions. 

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]. 

The mechanism and sequence of events that control delivery of protons and electrons to the FeMo cofactor during substrate reduction is not well understood in its particulars.8 It is believed that conformational change in MoFe-protein is necessary for electron transfer from the P-cluster to the M center (FeMoco) and that ATP hydrolysis and P release occurring on the Fe-protein drive the process. Hypothetically, P-clusters provide a reservoir of reducing equivalents that are transferred to substrate bound at FeMoco. Electrons are transferred one at a time from Fe-protein but the P-cluster and M center have electron buffering capacity, allowing successive two-electron transfers to, and protonations of, bound substrates.8 Neither component protein will reduce any substrate in the absence of its catalytic partner. Also, apoprotein (with any or all metal-sulfur clusters removed) will not reduce dinitrogen. 

In ER reservoir systems, a membrane surrounds a reservoir of the drug, also called the core of the system. The membrane controls the drug release and the driving force is the difference in chemical potential over the membrane, which can be correlated with a concentration gradient over the membrane. The transport of the drug through the ER membranes can be divided into three different mechanisms [45-47] ... 

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. 

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