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Waterdrive reservoirs

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Waterdrive (or water drive) petroleum reservoirs are characteristically bounded by and in communication with aquifers. As pressure decreases during pressure depletion, the compressed waters within the aquifers expand and overflow into the petroleum reservoir. The invading water helps drive the oil to the producing wells, leading to improved oil recoveries. Like gas reinjection and gas cap expansion, water influx also acts to mitigate the pressure decline. The degree to which water influx improves oil recovery depends on the size of the adjoining aquifer, the degree of communication between the aquifer and petroleum reservoir, and ultimately the amount of water that encroaches into the reservoir.

Overview

Some of the most prolific oil fields in the world are waterdrive reservoirs. Perhaps the most celebrated example is the East Texas field. The final oil recovery in the East Texas field is projected to be approximately 79%.[1] As this example shows, water influx has the potential to improve oil recovery considerably.

Once a water influx mechanism has been identified, it is important to monitor the producing wells closely and to minimize water production. Minimizing water production in “edgewater drives” may require systematically shutting in flank wells once the advancing water reaches them. Minimizing water production in “bottomwater drives” may require systematically cementing in lower perforations as the bottom water slowly rises.

An integral part of reservoir surveillance for waterdrives is an active assessment program. The first phase of assessment includes diagnosis, classification, and characterization. The second phase identifies mathematical models that effectively simulate the aquifer, especially its deliverability. This phase includes reliably estimating aquifer model parameters. The third and final phase includes combining aquifer and reservoir models into a common model that can be used to forecast future recovery effectively and to identify optimal depletion strategies. The success of the third phase depends heavily on the success of the preceding two phases.

Waterdrive and aquifer classification

Waterdrives are classified in several ways. First, they are classified according to the location of the aquifer relative to the reservoir:

  • Peripheral waterdrive -- the aquifer areally encircles the reservoir, either partially or wholly,
  • Edgewater drive -- the aquifer exclusively feeds one side or flank of the reservoir
  • Bottomwater drive -- the aquifer underlays the reservoir and feeds it from beneath

Waterdrives also are classified according to the aquifer’s strength and to how well the aquifer delivers recharge water to the reservoir. The aquifer strength also refers to how well the aquifer mitigates the reservoir’s normal pressure decline. A strong aquifer refers to one in which the water-influx rate approaches the reservoir’s fluid withdrawal rate at reservoir conditions. These reservoirs also are called complete waterdrives and are characterized by minimal pressure decline. Strong aquifers are generally very large in size and highly conductive. A moderate or weak aquifer is one in which the water recharge rate is appreciably less than the reservoir’s fluid withdrawal rate. These reservoirs are called partial waterdrives and they are characterized by pressure declines greater than a complete waterdrive but less than a volumetric reservoir. An aquifer’s weakness is related directly to its lack in size or conductivity.

Waterdrive diagnosis

There are several diagnostic indicators to help identify or discount a possible active aquifer.

First, an understanding of the reservoir’s geology is important. The entire outer surface of the reservoir must be scrutinized carefully to identify communicating and noncommunicating pathways; communicating pathways represent possible water entry points. Geological maps should be consulted to identify the type of reservoir trap and the trapping surfaces. Trapping surfaces represent impenetrable surfaces and are discounted automatically as possible water-entry points. The remaining outer surfaces need to be evaluated and classified. If no communicating pathways exist, then the reservoir can be confidently discounted as a possible waterdrive; however, if communicating pathways exist, then the reservoir remains a candidate waterdrive.

Second, and perhaps most importantly, the water cut history of all producing wells should be recorded and regularly monitored. A steady rise in a well’s water cut is a good indicator of an active aquifer. Although this is among the best indicators, it is not foolproof. For instance, an increasing water cut might be caused by water coning instead of an active waterdrive. Special precautions need to be exercised to avoid water coning. A rising water/oil contact (WOC) is a good indicator of a bottomwater drive. Special attention should be paid to the location of high-water-cut wells. Their location will help define the position of the reservoir/aquifer boundary in peripheral and edgewater drives.

Third, the change in reservoir pressure also can be a helpful indicator. Strong waterdrive reservoirs are characterized by a slow or negligible pressure decline. Thus, a slower-than-expected pressure decline can help indicate a waterdrive. Material-balance calculations are important to help identify and confirm a slower-than-expected pressure decline.

The reservoir pressure distribution also can help diagnose an active aquifer. For peripheral-water and edgewater drives, higher pressures tend to exist along the reservoir/aquifer boundary while lower pressures tend to exist at locations that are more distant. A pressure contour map is sometimes helpful to identify pressure disparities.

Fourth, the producing gas/oil ratio (GOR) can be a helpful indicator. Strong waterdrives are characterized by small changes in the producing GOR. The small GOR change is directly related to the small pressure decline.

Fifth, a material balance analysis can help diagnose water influx. Several different types of material-balance analyses such as the McEwen[2] analysis can identify water influx.

Performance

To illustrate the performance of waterdrives, simulation results of an 80-acre segment of a west Texas black oil reservoir are presented. The segment is assumed to be surrounded by an infinite radial-flow aquifer. The reservoir properties in Table 1 apply. The aquifer permeability and porosity are 37 md and 27%, respectively.

Fig. 1 shows the effect of water influx on a plot of pressure vs. fractional oil recovery. The initial reservoir pressure is 2,000 psia. Waterdrive and solution-gas-drive performances are compared. This figure shows that water influx consistently improves the fractional oil recovery at a given pressure. Alternatively, the waterdrive maintains a higher pressure at a given recovery.

Fig. 2 shows the reservoir performance with time. This figure contains four separate plots: GOR, saturation, oil-recovery, and pressure histories. Waterdrives typically yield a characteristic GOR history. After a brief increase, the GOR typically levels off. This behavior is explained by the gas-saturation history. The gas saturation increases as soon as the pressure falls below the bubblepoint. After a brief increase, the gas saturation also levels off. The gas saturation levels off because the invading water drives the oil toward the producers. The oil is concentrated in a nonswept region or shrinking oil zone. Without oil displacement, the gas saturation and GOR would rise unabated. The solution-gas drive yields a final GOR of 4,506 scf/STB and gas saturation of 0.287 PV, while the waterdrive yields 1,323 scf/STB and 0.19 PV, respectively. The GOR for the water drive in Fig. 2 actually peaks after approximately 10 years and then decreases slightly. A decrease in the GOR reflects good displacement efficiency of oil by water.

As expected, the waterdrive yields a substantially higher recovery. The waterdrive also lengthens the productive life of the reservoir considerably. In this example, the waterdrive recovers 53.2% of the OOIP after 32.6 years, while the solution-gas drive recovers 24.2% of the OOIP after 13.5 years. Both cases assume a terminal oil rate of 20 STB/D. This recovery level indicates a relatively moderate to strong waterdrive. The waterdrive also yields a higher gas recovery (80.5 vs. 53.1%). The water-influx history basically mimics the incremental oil-recovery history. The cumulative encroached water is 58% hydrocarbon pore volume (HCPV) or 0.46 PV. This translates to approximately 1% OOIP incremental recovery for each 0.16 PV (or 2.0% HCPV) of encroached water. The waterdrive in Fig. 2 consistently yields higher producing rates than solution-gas drive.

Also as expected, the waterdrive consistently yields a higher pressure at a given time. The waterdrive yields a lower terminal pressure because lower gas saturations are realized at a given pressure. The example water and solution-gas drives yield final pressures of 471 and 613 psia, respectively.

The performance trends noted in Figs. 1 and 2 are not without exception. Waterdrive performance is strongly influenced by the displacement efficiency of oil by water. Figs. 1 and 2 are representative of moderate-to-good displacement efficiency. If displacement efficiency is poor, lower oil recovery will occur. A less obvious result, however, is that the GOR history will exhibit much different character than already discussed. Instead of rising slightly and leveling off, the GOR acts much like solution-gas drive; namely, the GOR steadily and monotonically increases. This difference occurs because the invading water bypasses substantial oil and fails to drive enough oil toward the producers to arrest the natural increase in the gas saturation. The GOR of a waterdrive can even exceed the GOR of the solution-gas drive if the displacement efficiency is poor enough. Peripheral waterdrives do not tend to be as efficient as bottomwater drives.

In summary, water influx can markedly improve oil recovery in oil reservoirs. The final oil recovery in a waterdrive reservoir depends largely on the net volume of influxed water. The net volume of influxed water is defined as the volume of influxed water less the volume of produced water. As the net volume of influxed water increases, the oil recovery increases. The volume of the influxed water depends mainly on the size of the aquifer and the communication between the aquifer and the reservoir. The maximum possible net volume of influxed water expressed as a fraction of the reservoir PV is

 .......................(1)

where Sorw is the residual oil saturation to water, Sgrw is the residual gas saturation to water, and Swi is the initial water saturation. As this equation shows, the residual saturations directly affect oil recovery by limiting the net volume of water that can influx into the reservoir. The residual saturations are a direct measure of the displacement efficiency of water. Lower residual oil saturations are preferred over lower residual gas saturations to promote oil production over gas production. The maximum fractional oil recovery for an initially undersaturated black oil reservoir is

 ...........................(2)

where Ev is the volumetric sweep efficiency of the invading water. This equation assumes a complete waterdrive (i.e., no pressure depletion).

Nomenclature

Ev = volumetric sweep efficiency, fraction
Sgrw = residual gas saturation to water flow, fraction
Sorw = residual oil saturation to water flow, fraction
Swi = initial water saturation, fraction

References

  1. Roadifer, R.E. 1986. Size Distributions of World’s Largest Known Oil, Tar Accumulations. Oil & Gas J. (24 February): 93.
  2. McEwen, C.R. 1962. Material Balance Calculations With Water Influx in the Presence of Uncertainty in Pressures. SPE J. 2 (2): 120–128. SPE-225-PA. http://dx.doi.org/10.2118/225-PA

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