Method for Accurately Measuring Produced Oil Volumes During Laboratory Waterflood Tests at Reservoir Conditions

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Introduction Much usable information can be obtained from laboratory waterflood tests conducted at reservoir temperatures and pressures using reservoir fluids. Consequently, reservoir-conditions waterflood tests have been conducted by several laboratories even though such tests are much more difficult and expensive than routine waterflood tests. One of the most difficult problems with these reservoir-conditions tests is measuring incremental and total displaced oil volumes accurately. This study describes a method that produces a continuous record of oil production during a high-pressure, high-temperature waterflood test. With this method, oil is accumulated in a high-pressure separator, where its volume is monitored with a very sensitive differential pressure transducer. If the dimensions of the separator are selected properly, very small volumes (less than 1% of the pore volumes of typical test cores) can be measured reliably using commercially available pressure transducers. The basic method also can be used for tests involving gas displacing oil or water and water displacing gas. With minor modifications of equipment and operating procedures, produced volumes of gas, oil, and water can be measured during three-phase flow tests. The waterflood technique discussed here now is being used routinely and has proved satisfactory for the past 15 years. Using this method experienced operators easily can record the production history from several core floods running concurrently. EXPERIMENTAL METHOD Fig. 1 schematically shows the main items of required equipment. All components are off-the-shelf items except the core holder and high pressure separator, which must be shop built. During a waterflood test, fluid leaving the core enters a high-pressure separator, where produced oil accumulates in a collecting tube. Both the core and separator are in a constant temperature oven. The separator is a blind cell, thus the inconvenience and hazards associated with viewing windows are avoided. The system is designed so only one phase (gas or water is displaced through a backpressure regulator. This design has two advantages over methods that require all production to flow through a regulator:conventional commercial backpressure regulators can be used without modification to reduce holdup volumes andpressure fluctuations at the upstream side of the regulator are reduced significantly so the pressures in the separator and at the outlet end of the core are kept much more uniform. The difference between pressures at the top of the collecting cube and the top of the annulus is measured with a sensitive differential pressure transducer. The portion of the annulus containing oil and the lines from the separator to the transducer normally are filled with live crude oil. At the start of the test water in the separator should have the same density as the flood water and water initially in the core. SPEJ P. 239