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Oil API Gravity
Knowledge of the oil API gravity is helpful in the interpretation of SCAL data and core photographs taken under ultra-violet light, and estimates of its value may be useful before the well is completed. Laboratory estimates of the in-place oil gravity can be made using one of several techniques. A common method requires the collection of oil samples, either from a drill stem test or a wireline formation tester fluid sample, from the zone of interest. The API gravity is determined on these oil samples, which are then retorted. The API gravity is then determined on the oil recovered in the retorting process, and correlations are made between the retorted oil gravity and the initial, non-retorted oil gravity. Retort oil from the core analysis can be used with the correlations in subsequent wells. Data included on the core analysis report represent a non-retorted oil gravity. Alternatively, oil samples may be obtained from wireline formation testers.
In cases involving the analysis of heavy oil and tar sands, where no produced oil is available or likely to be initially available, oil is actually recovered from the core. Sufficient oil can sometimes be recovered by centrifuging the core. In other cases, solvents such as methylene chloride have successfully removed the oil from the core. The solvent is subsequently vaporized and measurements are made on the remaining oil. Application of heat and agitation is necessary to remove the solvent from the oil, and some uncertainty exists as to how representative the gravity of the remaining fluid actually is. Nevertheless, this measurement has sometimes proved useful in those cases where no other source of oil gravity was available.
Methods for Determining Oil API Gravity
Three basic techniques exist for the determination of oil gravity. The first technique involves filling and weighing a capillary of small volume with water, and then filling and weighing the capillary with the sample of oil. Comparison of the two weights establishes the specific oil gravity, from which the oil’s API gravity is calculated.
The second technique involves the suspension of an oil droplet in a water-alcohol mixture. The mixture is adjusted by water or alcohol addition to cause the oil droplet to float. When this adjustment has been completed, a hydrometer is inserted into the mixture and the specific gravity of both the oil and the mixture is read.
The third technique, mentioned previously, is commonly used in sidewall core analysis. The refractive index of the retorted oil is measured, and through the use of correlations, the API gravity of the non-retorted oil is estimated. This is a relatively rapid measurement technique. It requires previously developed correlations between non-retorted oil API gravity and retorted oil refractive index. The following equations express the relationship between specific gravity and API gravity.
