Quantifying Separator Oil Shrinkage

Abstract

In tight unconventionals, oil and gas rates are often measured daily at separator conditions. Consequently, it is necessary to reliably convert these rates to volumes at standard conditions in cases where direct stock-tank measurements are not available. Due to changes in producing wellstream compositions and separator conditions, the separator-oil shrinkage factor (SF) can change significantly over time. This 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 are also proposed. To study how separator-oil shrinkage changes through time for tight unconventional wells, four reservoir fluids have been studied using a compositional reservoir simulator: (1) black oil, (2) volatile oil, (3) near-critical oil, and (4) gas condensate. All the simulations were conducted using a common equation of state (EOS) model, such that the only difference between each of the different runs were the in-situ reservoir fluid composition. The daily simulated wellstreams were thereafter run through a set of i) constant separator conditions and ii) changing separator conditions to analyse how daily separator oil shrinkage factors (SF) change with time. Separator oil shrinkage factor, and the associated liberated gas, change with time due to changes in producing wellstream compositions and separator conditions (zi, psep, Tsep). The variations of shrinkage factors through time are more severe for near-critical volatile oils and gas condensates, than for black- and volatile oils. This paper shows that not accounting for these changes can lead to errors in converted stock-tank oil rates of more than 20%. To convert separator rates to stock tank volumes, the common practice is to apply one constant shrinkage factor for a well and/or region. This paper shows that this practice can lead to significant errors, especially when producing from near-critical volatile oil and gas condensate fluid systems. This paper provides a novel approach for rigorous and consistent translation of daily separator-oil rate measurements into stock-tank rates, where we believe this procedure is seldom applied and can be important to allocation, ownership royalty determination, and consistency related to forecasting.