Recovery factors

The volumetries of a field, along with the anticipated recovery factors, control the reserves in the field those hydrocarbons which will be produced in the future. The value of an oil or gas company lies predominantly in its hydrocarbon reserves which are used by shareholders and investors as one indication of the strength of the company, both at present and in the future. A reliable estimate of the reserves of a company is therefore important to the current value as well as the longer term prospects of an oil or gas company. 

Ultimate recovery (UR) and reserves are linked to the volumes initially in place by the recovery factor, or fraction of the in place volume which will be produced. Before production starts reserves and ultimate recovery are the same. 

RF recovery factor the recoverable fraction of oil initially in place... 

The purpose of this exercise is to identify what parameters need to be further investigated if the current range of uncertainty in reserves is too great to commit to a development. In this example, the engineer may recommend more appraisal wells or better definition seismic to reduce the uncertainty in the reservoir area and the net-to-gross ratio, plus a more detailed study of the development mechanism to refine the understanding of the recovery factor. Afluid properties study to reduce uncertainty in (linked to the shrinkage... 

It should be noted that the recovery factor for a reservoir is highly dependenf upon the development plan, and that initial conditions alone cannot be used to determine this parameter. 

The same procedure may be used to rank the parameters themselves (GRV, N/G, ((>, S, Bg, recovery factor), to indicate which has the greatest influence on the HCIIP or ultimate recovery (UR). 

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. 

This section will consider the behaviour of the reservoir fluids in the bulk of the reservoir, away from the wells, to describe what controls the displacement of fluids towards the wells. Understanding this behaviour is important when estimating the recovery factor for hydrocarbons, and the production forecast for both hydrocarbons and water. In Section 9.0, the behaviour of fluid flow at the wellbore will be considered this will influence the number of wells required for development, and the positioning of the wells. 

This rather low recovery factor may be boosted by implementing secondary recovery techniques, particularly water Injection, or gas injection, with the aim of maintaining reservoir pressure and prolonging both plateau and decline periods. The decision to implement these techniques (only one of which would be selected) Is both technical and economic. Technical considerations would be the external supply of gas, and the... 

The recovery factor (RF) is in the range 30-70%, depending on the strength of the natural aquifer, or the efficiency with which the injected water sweeps the oil. The high RF is an incentive for water injection into reservoirs which lack natural water drive. 

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). 

The subscript i refers to the initial pressure, and the subscript ab refers to the abandonment pressure the pressure at which the reservoir can no longer produce gas to the surface. If the abandonment conditions can be predicted, then an estimate of the recovery factor can be made from the plot. Gp is the cumulative gas produced, and G is the gas initially In place (GIIP). This is an example of the use of PVT properties and reservoir pressure data being used in a material balance calculation as a predictive tool. 

From the above plot, it can be seen that the recovery factor for gas reservoirs depends upon how low an abandonment pressure can be achieved. To produce at a specified delivery pressure, the reservoir pressure has to overcome a series of pressure drops the drawdown pressure (refer to Figure 9.2), and the pressure drops in the tubing, processing facility and export pipeline (refer to Figure 9.12). To improve recovery of gas, compression facilities are often provided on surface to boost the pressure to overcome the pressure drops in the export line and meet the delivery pressure specified. 

Typical recovery factors for gas field development are in the range 50 to 80 percent, depending on the continuity and quality of the reservoir, and the amount of compression installed (i.e. how low an abandonment pressure can be achieved). 

The recovery factors for oil reservoirs mentioned in the previous section varied from 5 to 70 percent, depending on the drive meohanism. The explanation as to why the other 95 to 30 percent remains in the reservoir is not only due to the abandonment necessitated by lack of reservoir pressure or high water cuts, but also to the displacement of oil in the reservoir. 

This must be combined with the macroscopic sweep efficiency to determine the recovery factor (RF) for oil (in this example). 

Recall that the recovery factor (RF) defines the relationship between the hydrocarbons initially in place (HCIIP) and the ultimate recovery for the field. 

Section 8.2 indicated the ranges of recovery factors which can be anticipated for different drive mechanisms, but these were too broad to use when trying to establish a range of recovery factors for a specifio field. The main techniques for estimating the recovery factor are... 

Analytical models using classical reservoir engineering techniques such as material balance, aquifer modelling and displacement calculations can be used in combination with field and laboratory data to estimate recovery factors for specific situations. These methods are most applicable when there is limited data, time and resources, and would be sufficient for most exploration and early appraisal decisions. However, when the development planning stage is reached, it is becoming common practice to build a reservoir simulation model, which allows more sensitivities to be considered in a shorter time frame. The typical sorts of questions addressed by reservoir simulations are listed in Section 8.5. 

When estimating the recovery factor, it is important to remember that a range of estimates should be provided as input to the calculation of ultimate recovery, to reflect the uncertainty in the value. 

The type of development, type and number of development wells, recovery factor and production profile are all inter-linked. Their dependency may be estimated using the above approach, but lends itself to the techniques of reservoir simulation introduced in Section 8.4. There is never an obvious single development plan for a field, and the optimum plan also involves the cost of the surface facilities required. The decision as to which development plan is the best is usually based on the economic criterion of profitability. Figure 9.1 represents a series of calculations, aimed at determining the optimum development plan (the one with the highest net present value, as defined in Section 13). 

In gas field development, the recovery factor is largely determined by how low a reservoir pressure can be achieved before finally reaching the abandonment pressure. As the reservoir pressure declines, it is therefore common to install compression facilities at the surface to pump the gas from the wellhead through the surface facilities to the delivery point. This compression may be installed in stages through the field lifetime. 

Recovery factor Reduced column length Reduced plate height Reduced velocity Relative retention ratio Retardation factor d Retention time Retention volume Selectivity coefficient Separation factor... 

The resulting overall energy balance for the plant at nominal load conditions is shown in Table 3. The primary combustor operates at 760 kPa (7.5 atm) pressure the equivalence ratio is 0.9 the heat loss is about 3.5%. The channel operates in the subsonic mode, in a peak magnetic field of 6 T. AH critical electrical and gas dynamic operating parameters of the channel are within prescribed constraints the magnetic field and electrical loading are tailored to limit the maximum axial electrical field to 2 kV/m, the transverse current density to 0.9 A/cm , and the Hall parameter to 4. The diffuser pressure recovery factor is 0.6. 

The most common approach to fixed cost estimation iavolves the use of a capital recovery factor to give the annual depreciation and return on capital. This factor typically is between 15 and 20% of the total capital investment. Property taxes are taken as 1—5% of the fixed capital and iasurance is assumed to be 1—2% of the fixed capital. If annual depreciation is estimated separately, it is assumed to be about 10% of the fixed capital investment. The annual iaterest expense is sometimes neglected as an expense ia preliminary studies. Some economists even beHeve that iaterest should be treated as a return on capital and not as part of the manufactufing expense. 

The facdor K would be 1 in the case of full momentum recoveiy, or 0.5 in the case of negligible viscous losses in the portion of flow which remains in the pipe after the flow divides at a takeoff point (Denn, pp. 126-127). Experimental data (Van der Hegge Zijnen, Appl. Set. Re.s., A3,144-162 [1951-1953] and Bailey, ]. Mech. Eng. ScL, 17, 338-347 [1975]), while scattered, show that K is probably close to 0.5 for discharge manifolds. For inertiaUy dominated flows, Ap will be negative. For return manifolds the recovery factor K is close to 1.0, and the pressure drop between the first hole and the exit is given by... 

CRF Capital recovery factor RDF Refuse-derived fuel... 

This is then converted to annualized capital costs (ACC) with the use of the capital recovery factor (CRF), which can be calculated from the following equation ... 

For any control valve design be sure to use one of the modem methods, such as that given here, that takes into account such things as control valve pressure recovery factors and gas transition to incompressible flow at critical pressure drop. 

Care should also be taken in the use of recovery factors, because these can exert a significant effect. In general, recovery paths are appropriate where there is a specific mechanism to aid error recovery, that is an alarm, a supervising check, or a routine walk round inspection. 

The CCE spreads the investment over the lifetime of the measure into equal annual payments with the familiar capital recovery factor. The annual payment is then divided by the annual energy savings to yield a cost of saving a unit of energy. It is calculated using the following formula ... 

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