Profiling oil production under WAG recovery
A suite of production logs can provide important information for fine-tuning tertiary recovery operations. In the example below, production logs were run with the objective of identifying the following:
- Intervals in which water enters the wellbore.
- Whether CO2 is dissolved in the water.
- Intervals in which oil (with CO2 in solution) enters the wellbore.
In this well, 5 1/2-in. casing is set to 4,463 ft. Below the casing, oil is produced in the open hole under WAG (water-alternating-gas) recovery.
The well produces 1381 RB/D of water, 119 RB/D of oil, and 245 RB/D of CO2. Carbon dioxide, CO2, dissolves primarily in the oil and secondarily in the water. (See Designing a miscible flood) Water is the continuous phase in the wellbore. The produced oil, with CO2 in solution, bubbles (or "percolates") up through the flowing water.
Production logs used
The well was logged flowing, then shut in overnight. Shut-in logs were recorded the next morning. The following logs were run:
A comparison of flowing and shut-in temperature profiles (Fig. 1a) shows that the major production originates from a thin interval just above 4,575 ft (Depth G). Because the thermal content of the stream is essentially water, a thin interval at Depth G is therefore the source of water production. Injected water travels along a permeable streak known to exist at the based of the porous interval. This is not a good profile for oil recovery. The water is warm because warmer brine is being injected into a formation cooled by years of waterflooding.
The fluid-capacitance log (Fig. 1b) (well flowing) responds to the deepest oil entry at Depth C on an up run. However, fouling of the capacitance probe by the heavy oil renders the remainder of the up run useless for detecting additional oil entries. The probe was cleaned by stationing it in the tubing, where the elevated flow velocity removed the heavy oil film; however, it again fouled upon exit from the tubing. The shut-in capacitance profile, recorded later, reveals an additional oil entry at Depth E.
One usually depends on the response of the flowing fluid-capacitance log to determine whether an entering fluid is water or oil. In this example, the failure of the flowing capacitance log to respond to all oil entries with the exception of the deepest one (C) was anticipated in advance of logging and is not a problem because the water production is localized, and a comparison of flowing and shut-in neutron logs, which respond to a change of CO2 concentration in the wellbore, can detect oil entries above Depth C.
The comparison of the separations between flowing and shut-in neutron logs (Fig. 1c) reveals the following:
- CO2 is dissolved in the water entering at (A).
- CO2 is dissolved in the water entering below (B), but at higher concentration than in the entry below (A).
- CO2 is dissolved in the oil entry at (C), which is the deepest oil entry according to the flowing capacitance log.
- Oil with dissolved CO2 enters just below (D).
- CO2 is dissolved in the fluid entering at (E), which is oil according to the shut-in capacitance log.
- There is a fluid entry at (F), but with no CO2 dissolved in the entering fluid. Because the water is produced below (C), the entering fluid is probably oil. The injected CO2 is not reaching as far up the formation as Depth F. Still, the gravity migration of carbon dioxide upward in the formation above the bottom permeability streak is much better than one might suspect from the water production profile alone.
In this example, none of the three log types (temperature, fluid capacitance, and neutron) is capable of accomplishing the logging objective by itself. Moreover, no combination of two of the logs is capable of fulfilling the objective. Only a thorough analysis of the three logs taken together can accomplish the objective, showing the importance of a carefully selected, comprehensive suite of logs.
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