In order to confirm the expected high recoveries obtained in gravity drainage we performed lab tests, oil relative permeability calculation and reservoir simulation for a Brazilian offshore oil field.
The lab tests, at ambient temperature and low pressures, were performed in long cores(90-252cm long) with dead oil and conventional drainage where the oil was produced at the lower end of the vertical core and air was entering the upper end. For one test we performed a counter-flow drainage test using live oil where the production was done at the lower end of the core and the liberated dissolved gas was segregated counter-current. The saturation evolution inside the core was monitored by an X-ray system. Using the same rock and fluids centrifuge tests were performed using 3- 4cm.long plugs to compare results. When the results did not agree an explanation using a model of bundles of capillary tubes was obtained. Verification using minimum energy principle was performed.
A computer program was implemented to calculate the oil relative permeability using the lab data, whose results were compared with published literature.
The calculated relative permeability was used to simulate a Brazilian offshore oil field behaviour which was exploited with water injection and due to appearance of a secondary gas cap was started a gas injection into the crest of the structure.
Numerous papers described lab tests results concerning the gravity drainage (2,3,4,7,8,10,12,14,16,19,23,24,26,27).
In our analyses we present the results of long core tests and centrifuge tests for the same rock and fluid properties. In those situations where they did not agree we calculate the results considering a bundle of capillaries tubes to understand the difference. The long core tests showed accordingly to the literature a very low residual oil saturation at the top of the core.
The results of these tests were used to calculate the oil relative permeability to be used in next studies.
An offshore Brazilian oil field with an 110.6MMSTm3 OOIP has been under production for 13 years under peripheral water injection and due to the appearance of an secondary gas cap has been supplemented with a crest gas injection. After an history match we performed the production forecast for both water and combined water and gas injection.
After the traditional encapsulating process, the sequence of the tests can be summarized in the following steps:
Measurement of rock and fluids properties;
Water saturation of the sample under vacuum;
Displacement of the water by oil and establishment of the initial water saturation;
Drainage of oil in the presence of air.
The tests were performed with several core lengths and fluids at laboratory conditions of temperature and pressure. We selected three tests to be detailed. Production measurements and periodic registration of the saturation profiles were obtained during the vertical drainage process in the long core samples. Fluid production was obtained by weighting the core. An X-ray device periodically registered the evolution of the saturation profiles during several weeks.
