Reservoir efficiency

Zhang, J., Delshad, M., Sepehmoori, K., and Pope, G.A. 2005. An Efficient Reservoir-Simulation Approach to Design and Pptimize Improved Gil Recovery Processes With Distributed Computing. Paper SPE 94733 presented at the SPE Latin American and Caribbean Petroleum Engineering Conference, Rio de Janeiro, 20-23 June. DPI 10.2118/94733-MS. 

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 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. 

The macroscopic sweep efficiency s the fraction of the total reservoir which is swept by water (or by gas in the case of gas cap drive). This will depend upon the reservoir quality and continuity, and the rate at which the displacement takes place. At higher rates, displacement will take place even more preferentially in the high permeability layers, and the macroscopic displacement efficiency will be reduced. 

The microscopic displacement efficiency is the fraction of the oil which is recovered in the swept part of the reservoir. If the initial oil saturation is S i, then... 

Field analogues should be based on reservoir rock type (e.g. tight sandstone, fractured carbonate), fluid type, and environment of deposition. This technique should not be overlooked, especially where little information is available, such as at the exploration stage. Summary charts such as the one shown in Figure 8.19 may be used in conjunction with estimates of macroscopic sweep efficiency (which will depend upon well density and positioning, reservoir homogeneity, offtake rate and fluid type) and microscopic displacement efficiency (which may be estimated if core measurements of residual oil saturation are available). 

Miscible processes are aimed at recovering oil which would normally be left behind as residual oil, by using a displacing fluid which actually mixes with the oil. Because the miscible drive fluid is usually more mobile than oil, it tends to bypass the oil giving rise to a low macroscopic sweep efficiency. The method is therefore best suited to high dip reservoirs. Typical miscible drive fluids include hydrocarbon solvents, hydrocarbon gases, carbon dioxide and nitrogen. 

Drawdown and build-up surveys are typically performed once a production well has been completed, to establish the reservoir property of permeability (k), the well completion efficiency as denoted by its skin factor (S), and the well productivity index (PI). Unless the routine production tests indicate some unexpected change in the well s productivity, only SBHP surveys may be run, say once a year. A full drawdown and build-up survey would be run to establish the cause of unexplained changes in the well s productivity. 

As of this writing, it has not been possible to use the seismic data which defines the volume of the reservoir to also determine the joint stmcture. Extended flow testing is the most direct measure of the efficiency and sustainabiUty of energy recovery from the reservoir. The use of chemical tracers in the circulating fluid can also provide valuable supporting data with regard to the multiplicity of flow paths and the transit time of fluid within the reservoir (37). 

Primary production typically recovers 10—25% of the oil originally ia the reservoir. Efficiency of primary production is related to oil properties, reservoir properties, geometric placement of oil wells, and the drilling and completion technology used to drill the wells and prepare them for production. Pumping the well can maintain production at economic levels for years. 

There are two principal mechanisms of enhanced oil recovery increasing volumetric sweep efficiency of the injected fluid and increasing oil displacement efficiency by the injected fluid. In both, chemicals are used to modify the properties of an injected fluid whether water, steam, a miscible gas such as CO2 or natural gas, or an immiscible gas, usually nitrogen. Poor reservoir volumetric sweep efficiency is the greatest obstacle to increasing oil recovery (9). 

Wettabihty is defined as the tendency of one fluid to spread on or adhere to a soHd surface (rock) in the presence of other immiscible fluids (5). As many as 50% of all sandstone reservoirs and 80% of all carbonate reservoirs are oil-wet (10). Strongly water-wet reservoirs are quite rare (11). Rock wettabihty can affect fluid injection rates, flow patterns of fluids within the reservoir, and oil displacement efficiency (11). Rock wettabihty can strongly affect its relative permeabihty to water and oil (5,12). When rock is water-wet, water occupies most of the small flow channels and is in contact with most of the rock surfaces as a film. Cmde oil does the same in oil-wet rock. Alteration of rock wettabihty by adsorption of polar materials, such as surfactants and corrosion inhibitors, or by the deposition of polar cmde oil components (13), can strongly alter the behavior of the rock (12). 

The pressure/composition requirement for miscibility limits the oil reservoirs in which this technology has been appHed. However, the low iajected fluid viscosity often results in poor volumetic sweep efficiency. 

Polymer Flooding. Even in the absence of fractures and thief 2ones, the volumetric sweep efficiency of injected fluids can be quite low. The poor volumetric sweep efficiency exhibited in waterfloods is related to the mobiUty ratio, Af, the mobiUty of the injected water in the highly flooded (low oil saturation) rock, divided by the mobiUty of the oil in oil-bearing portions of the reservoir, (72,73). The mobiUty ratio is related to the rock permeabihty to oil, and injected water, and to the viscosity of these fluids by the following equation ... 

Microbial-enhanced oil recovery involves injection of carefully chosen microbes. Subsequent injection of a nutrient is sometimes employed to promote bacterial growth. Molasses is the nutrient of choice owing to its low (ca 100/t) cost. The main nutrient source for the microbes is often the cmde oil in the reservoir. A rapidly growing microbe population can reduce the permeabiHty of thief zones improving volumetric sweep efficiency. Microbes, particularly species of Clostridium and Bacillus, have also been used to produce surfactants, alcohols, solvents, and gases in situ (270). These chemicals improve waterflood oil displacement efficiency (see also Bioremediation (Supplement)). 

Thermally stable foam additives, such as alkylaryl sulfonates and C -C g alpha-olefin sulfonates, are being used in EOR steam flooding for heavy od production. The foam is used to increase reservoir sweep efficiency (178,179). Foaming agents are under evaluation in chemical CO2 EOR flooding to reduce CO2 channeling and thus increase sweep efficiency (180). 

It is possible to breed plants that have more efficient systems for utilization of water, and agricultural technology can help existing crop plants by spraying impervious coatings on them. Extremely small amounts of long-chain, fatty alcohols reduce evaporation losses from quiet lakes or reservoirs to less than 5% of the normal surface evaporation. 

Low-loss system. In a low-loss system the winding also resides in a reservoir of liquid helium but a very efficient insulation system enables the magnet to operate for long periods, typically I year or more, between liquid helium refills. An important feature of these systems is that they are relatively immune to short-term electrical power failures, which has enabled complete reliability even in extremely difficult environments. 

Although the [CHTJr cycle is internally reversible, external irreversibility is involved in the heat supply from the external reservoir at temperature Tr and the heat rejection to a reservoir at temperature 7. So a consideration of the internal thermal efficiency alone does not provide a full discussion of the thermodynamic performance of the plant. If the reservoirs for heat supply and rejection are of infinite capacity, then it may be shown that the irreversibilities in the heat supply ( r) f d the heat rejection respectively, both positive, are... 

There is a problem with Carnot s analysis, however, since at that time almost all physicists (including Carnot) thought heat consisted of a substance called caloric, which could not be created or destroyed. As a result, the amount of heat taken from the hot source at temperature T, would have to be the same as that delivered to the cold reservoir at temperature T(. Because no heat was converted into work, the efficiency of such an engine would be zero. 

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