Black Oil, Heavy Oil and Tar in One Oil Column Understood by Simple Asphaltene Nanoscience

Abstract

A Jurrasic oilfield in Saudi Arabia is characterized by black oil in the crest and with mobile heavy oil underneath and all underlain by a tar mat at the oil-water contact. The viscosities in the black oil section of the column are fairly similar and are quite manageable from a production standpoint. In contrast, the mobile heavy oil section of the column contains a large continuous increase in asphaltene content with increasing depth extending to the tar mat. The tar shows very high asphaltene content but not monotonically increasing with depth. Because viscosity depends exponentially on asphaltene content in these oils, the observed viscosity varies from several to ~ 1000 centipoise in the mobile heavy oil and increases to far greater viscosities in the tar mat. Both the excessive viscosity of the heavy oil and the existence of the tar mat represent major, distinct challenges in oil production. Conventional PVT modeling of this oil column grossly fails to account for these observations. Indeed, the very large height in this oil column represents a stringent challenge for any corresponding fluid model. A simple new formalism to characterize the asphaltene nanoscience in crude oils, the Yen-Mullins model, has enabled the industry's first predictive equation of state (EoS) for asphaltene gradients, the Flory-Huggins-Zuo (FHZ) EoS. For low GOR oils such as those in this field, the FHZ EoS reduces to the simple gravity term. Robust application of the FHZ EoS employing the Yen-Mullins model accounts for the major property variations in the oil column and by extension the tar mat as well. Moreover, as these crude oils are largely equilibrated throughout the field, reservoir connectivity is indicated in this field. This novel asphaltene science is dramatically improving understanding of important constraints on oil production in oil reservoirs.

Yen–Mullins Model Applies to Oilfield Reservoirs

“…by the existence of a tar mat with phase-separated asphaltenes below the heavy oil column. 143,144 The asphaltenes were added by convective flows of asphaltene-rich currents from higher in the column. 6,143,144 As the asphaltene content exceeds several percent in this oil, the asphaltenes form clusters.…”

“…143,144 The asphaltenes were added by convective flows of asphaltene-rich currents from higher in the column. 6,143,144 As the asphaltene content exceeds several percent in this oil, the asphaltenes form clusters. Over geologic time an equilibrated gravity gradient of clusters is established at the base of the oil column.…”

Critical review of asphaltene properties and factors impacting its stability in crude oil

“…This model describes the asphaltene structure based on size and behavior as a function of the crude oil that contains the asphaltene. In light oils, with high API gravity, the asphaltene will be present as small polyaromatic hydrocarbon molecules with an average diameter of 1.5 nm (Forte and Taylor 2014;Mishra et al 2012;Seifert et al 2012;Mullins et al 2013). In this case, the asphaltene concentration will be relatively low, and thus, the asphaltene size will not grow.…”

Determining Reservoir Connectivity and Compositional Grading by Mapping Asphaltene Gradients (Russian)

“…Однако, уравнения с переносом тепла и, таким образом, зависящие от времени, могут быть названы уравнениями состояния и фактически лишь незначительно отличаются от истинного состояния равновесия (Dong, 2012). Под этим подразумевается, что переменные, входящие в уравнение состояния, не зависят от времени: давление, температура и композиционный состав не меняются.…”

“…Рис.3 показывает градиенты содержания асфальтенов, для расчета которых была использована только гравитационная составляющая (Seifert, 2012). 2-Фактические данные и аппроксимация при помощи FHZ уравнения Следует отметить, что растворимость (и, соответственно, ГНФ) в данном случае является переменной, которая оказывает решающее влияние.…”