--> A Case Study of Mobile Tar in Deepwater Gulf of Mexico, by Leon Dzou, Carl Sheliga, Ed Lisle, John Shaughnessy, and Harry Prewett; #90062 (2007)

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A Case Study of Mobile Tar in Deepwater Gulf of Mexico

Leon Dzou, Carl Sheliga, Ed Lisle, John Shaughnessy, and Harry Prewett
BP America, Houston, Texas

Anomalously high asphaltenes contents (to >50%) have been reported in Gulf of Mexico wells/fields. Asphaltenes are a group of high molecular weight, polar molecules indigenous to crude oils. Asphaltenes are held in a colloidal dispersion in crude oil by surrounding layers of aromatic compounds called maltenes and resins. These layers, composed of aromatic asphaltic compounds are of a lighter molecular weight and function by stabilizing the colloidal asphaltenes particles dispersed in the crude.

Tar occurrences have important implications for crude oil reservoir exploration, exploitation and production. Drilling problems associated with mobile tar in Gulf of Mexico range from minor to severe non-productive time, or loss of the borehole depending on the volume, mobility, and overburden. Torque and sticking issues caused by mobile tar can result in stuck bottom hole assemblies, drill pipe, and casing, along with riser and wellhead issues as mobile tar flows into the borehole and riser. From the exploitation side, a predictive model of tar mat occurrence and areal distribution would be beneficial for field development and enhanced oil recovery procedures, since tar mats may create flow barriers and the tars themselves are dominantly non-producible in-place oil. Asphaltene deposits can seriously affect crude oil production operations. During production subsurface deposition of asphaltenes around the wellbore, in gas lift valves, mandrels, sliding sleeves and tubing, as well as accumulations in wellhead valves, separators and tanks have been encountered, increasing the cost and frequency of well interventions.

Several processes have been considered to enhance asphaltenes concentrations from the oil: P-T phase changes, gas deasphalting, aerobic and anaerobic biodegradation, and others. In this presentation, we will discuss a model for tar formation that explains observations in GoM fields.

AAPG Search and Discovery Article #90062©2006 AAPG Hedberg Research Conference, Veracruz, Mexico