Biodegradation of Petroleum in Subsurface Reservoirs: A Numerical Analysis of the Impact of Produced Biogenic Gas on Oil Column Systematics

Jennifer Adams¹, Dennis Coombe², Steve Larter¹, and Haiping Huang^1
1 University of Calgary, Calgary, AB
² Computer Modelling Group, Calgary, AB

In the deep subsurface, biodegradation has altered most of the world's remaining petroleum resources, making the oil more difficult to produce and more costly to refine. To optimize production activities in existing accumulations, distributions of fluid properties and composition must be estimated. The complex spatial and temporal interplay of reservoir charging, oil degradation and oil mixing inherent to biodegraded oil accumulations require numerical simulation for good predictions. To date, 1D simulations of petroleum charging and biodegradation have inadequately addressed the complex physics and chemistry of a complex petroleum reservoir. Here, we evaluate the movement and impact of biogenic carbon dioxide and methane, generated near oil-water contacts, on the composition of oil columns. Two-dimensional, three-phase fluid flow coupled with biodegradation is simulated using the reservoir model, STARS, to fully examine in-reservoir biodegradation for an oilfield in the Liaohe basin, NE China. As simulated, the numerical microorganisms generate biogenic gas by metabolism of the multicomponent petroleum substrate. As modeled, oil charges at the top of a simple reservoir and degrades at the oil-water contact. Various metabolic schemes and oil charge scenarios show that biogenic gas generation during oil biodegradation aids convection and mixing of the oil column. A free gas phase can develop in the reservoir depending on reservoir conditions and gas saturation. In a homogenous reservoir, the oil column turnover is episodic, but more complex flow behaviour is predicted for more complex reservoir geometries and properties.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005