Seminar: Tar Mats and Bitumen Deposition; Origins and Properties
29 January 2019
Instructor: Oliver C. Mullins, Scientific Advisor, Schlumberger
Tar and bitumen deposition can be upstructure, or at the oil-water contact or various locations in-between. Tar can be impermeable and might or might not be a regional seal. In other cases, tar zones can be highly permeable flowing light oil. Tar can have the rheology of coal, or can flow (or ooze). The origins of this bewildering array of tar properties have been grossly misunderstood in part due to the lack of any thermodynamic treatment of asphaltenes. The recent development of the Flory-Huggins-Zuo equation of state (FHZ EoS) for asphaltenes, with its reliance on the Yen-Mullins model of asphaltenes, has led to a simple, concise form of asphaltene thermodynamics, thereby enabling a 1st-principles approach addressing tar deposition and tar mats. Another prior shortcoming was the lack of reservoir fluid data to enable a robust evaluation of asphaltene thermodynamics. Downhole Fluid Analysis (DFA) is now ubiquitously acquired and provides asphaltene gradients in crude oil with exquisite accuracy. A decade of reservoir studies employing these advances has elucidated a broad array of reservoir fluid geodynamic processes that give rise to asphaltene accumulation and destabilization yielding bitumen deposition and tar mat formation. Wide-ranging data streams support conclusions of the reservoir studies including production data, pressure transient analysis, analytical chemistry, geochemistry, flow assurance analysis, core extracts, mud gas analysis and petrophysics. This new vantage provides an excellent foundation for treating tar and bitumen deposition in virtually all reservoirs.