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Intergranular Tar in a Deepwater Reservoir: Part I – Mechanisms of Asphaltene Deposition

Keith I. Mahon1, Harry Dembicki, Jr.1, Ahmed Chaouche1, John Meredith1, Howard J. White1, Nishanth Kalyanaraman1,2, David Kennedy3, and Dan Carruthers3
1Anadarko Petroleum Corporation, 1201 Lake Robbins Drive, The Woodlands, TX 77382
2Bureau of Economic Geology, Jackson School of Geosciences, Austin, TX 78758
3The Permedia Research Group, 577 Somerset St. W, Ottawa, Ontario, Canada

Deepwater exploration and development experiences in the Gulf of Mexico have brought to the forefront the detrimental impact that massive and intergranular tar can have on drilling and production. There are several challenges faced by petroleum system modelers attempting to predict subsurface tar occurrences. The first of these challenges is communicating with exploration and development engineers and geologists in a common language that improves our understanding of the characteristic form of the tar and how it impacts our understanding of the petroleum system. The second challenge is to categorize tar occurrences based on reasonable geological and geochemical mechanisms. The final challenge is to integrate our understanding of these mechanisms with knowledge of the basin and geochemical evolution into a predictive model of tar occurrence in the subsurface.

This paper presents a consistent vocabulary for subsurface tar, specifically asphaltene-rich solids, to better communicate in physical and chemical terms. Several asphaltene flocculation and precipitation mechanisms that result in intergranular tar are reviewed. In general, asphaltene precipitation within a reservoir or migration pathway may be the result of (1) biodegradation, (2) fluid mixing, and/or (3) pressure drop below the asphaltene onset pressure (AOP).

A predictive model of asphaltene precipitation within a deepwater Gulf of Mexico field is introduced. This model was developed by using the pressure, temperature, and hydrocarbon generation output from a PetroMod petroleum system model as input to an MPath migration model with a plug-in simulating asphaltene flocculation when the pressure and temperature drops below the AOP curve. The results show that asphaltene precipitation occurs along hydrocarbon migration pathways, within the reservoir, and in leaky seals. The model predicts that the highest concentrations of intergranular tar are in breeched paleo-traps (above and below salt) and near the top of the main reservoir interval.

Future applications of this technology include the development of a series of AOP and bubble point envelopes for a range of GORs to simulate fluid mixing. For example, the migration of early-charged oil into a horizon containing “fizz gas” may result in a higher GOR fluid mixture and a shift to higher AOP as compared to the migrated oil. Another example is charging an existing accumulation of unstable oil with a high maturity, high GOR fluid resulting in higher AOP for the fluid accumulation. These two end-members increase the probability of asphaltene flocculation within the oil and precipitation of an intergranular tar at the point of mixing. The development of these types of models improves our understanding of mechanisms that cause intergranular tar in deepwater reservoirs in the Gulf of Mexico, and may lead to a comprehensive model for intergranular and massive tar occurrences within this basin.


AAPG Search and Discovery Article #90091©2009 AAPG Hedberg Research Conference, May 3-7, 2009 - Napa, California, U.S.A.