Method Quantifies Separator-Oil Shrinkage
Authors: Carpenter Chris 
Published: January 2021

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper IPTC 19775, “Quantifying Separator-Oil Shrinkage,” by Mathias Lia Carlsen, SPE, and Curtis Hays Whitson, SPE, Whitson, prepared for the 2020 International Petroleum Technology Conference, Dhahran, Saudi Arabia, 13-15 January. The paper has not been peer reviewed. Copyright 2020 International Petroleum Technology Conference. Reproduced by permission. In tight unconventionals, oil and gas rates often are measured daily at separator conditions. Consequently, converting these rates reliably to volumes at standard conditions is necessary in cases where direct stock-tank measurements are not available. Because of changes in producing-wellstream compositions and separator conditions, the separator-oil shrinkage factor (SF) can change significantly over time. The complete paper presents a rigorous and consistent method to convert daily separator rates into stock-tank volumes. Recommendations for developing field-specific shrinkage correlations using field test data also are proposed.

SF and Flash Factor (FF)

Separator-Oil SF. Separator-oil SF is the fraction of metered separator oil rate that remains (or transforms into) stock-tank oil after further processing to standard conditions of 1 atm and 60°F. Put simply, the SF quantifies the decrease in oil volume from separator conditions to stock tank. The magnitude can range from less than 0.65 to 0.99.

Separator-Oil FF. Separator-oil FF is the ratio of liberated gas from metered separator oil after further processing to standard conditions of 1 atm and 60°F. The FF accounts for the increase in gas volume from separator conditions to stock tank and explains why oil is shrinking (i.e., gas is coming out of the solution). The magnitude of the FF can range from 5 to 1,000 scf/STB. Total producing gas/oil ratio (GOR) can be calculated easily when SF and FF are known. An SF always is associated with an FF and is literally the solution GOR of the separator oil. Both SF and FF are a function of the top-side surface process and an associated wellstream composition.

Surface Process. The surface process represents the number of topside separation stages and the associated separator pressure and temperature of each stage. In shale basins, two- and three-stage separation trains are common. The number of separation stages typically is fixed throughout the lifetime of a well. However, the separator temperature and pressure may vary significantly.

Wellstream Composition. The well-stream composition quantifies the relative amounts of different components flowing out of a well at a given day. This measurement is typically expressed in mol%. Tight unconventional basins contain many kinds of in-situ reservoir fluid compositions from dry gas to black oils. The produced-wellstream compositions from these systems tend to change considerably with time because of producing flowing bottomhole pressures below the saturation pressure, as seen in the field example presented in Fig. 1. In the figure, the shut-in period after approximately 330 days results in a transient period with large compositional changes.