The Impact of Oil Viscosity Heterogeneity on Production Characteristics of Heavy Oil and Tar Sand (HOTS) Reservoirs

  • Date: 02/20/2007
Speaker(s):

Steve Larter (University of Calgary)

Location: 

Calgary Place Tower (Shell)

Topic: 

The world oil inventory is dominated by heavy oils and tar sand (HOTS) bitumens generated almost entirely by the process of biodegradation. This process is a biologically-driven, complex reactive diffusion-dominated in-reservoir oil alteration process that occurs under anaerobic conditions driven by oil-water reactions, usually at the oil column base producing methane and heavy oil as by-products. Because large volumes of lighter hydrocarbon components are consumed by the biological process at the oil-water contact or transition zone, significant vertical and lateral gradients in oil composition and thus oil viscosity result . The ultimate controls on the increase of oil alteration and thus viscosity rise are related to the oil charge composition and charge rate history, mixing of fresh and biodegraded oils and diffusion of oil components , the extent of the water leg in the reservoir and nutrient supply, and the reservoir temperature history . Temperature ultimately controls the survival of microrganisms in the subsurface with reservoir pasteurisation at 80 C and above.
The defining characteristic of heavy and super heavy oilfields is large spatial variation in fluid properties, such as oil viscosity, commonly seen within the reservoirs. Viscosity can increase by up to one hundred times across a 40 m thick reservoir. Traditional heavy oil and tar sand exploration and production strategies rely significantly on characterization of key reservoir heterogeneities and assessments of fluid saturations. It is important to understand how these properties vary as the spatial distribution of fluid properties can often dominate the oil phase mobility ratio (oil effective permeability/oil viscosity) distribution which in turn controls production behavior under cold and thermal recovery but surprisingly, in most reservoir simulations, fluid variations are often ignored! Petroleum biodegradation proceeds at the oil-water contact (OWC), or basal reaction zone under anaerobic conditions in any reservoir that has a water leg and has not been heated to temperatures over 80?C. The process of biodegradation is driven from the basal reaction zone near the oil-water contact with reactive hydrocarbons brought to the reaction zone by diffusion and during field charging advection. This results in the most altered and viscous oils being located at the base of the oil column in most instances which is where wells are often placed for thermal gravity drainage processes such as SAGD or CSS.
We have developed several full physics numerical models of the biodegradation process (e.g. 1DRS) which couple charging and biodegradation in a reservoir simulation environment. These allow us to calculate changing compositional profiles through geological time and also predict viscosity gradients distant from well control in HOTS reservoirs. We describe why we find curved, parabolic or even exponential viscosity depth profiles in western Canadian reservoirs where oil charge has terminated but in actively charging traps elsewhere in the world, we typically see more sub-linear viscosity versus depth profiles. We also describe the dynamics of the biodegradation basal reaction zone which can be several meters thick and its impact on the production character of HOTS reservoirs. The ability to perform reactive reservoir simulations with full physical models of biodegradation allows us to populate full reservoir simulator models with mixtures of oils that fully reflect the viscosity heterogeneity of a typical western Canada HOTS reservoir. This together with advances in optimisation technology and parallel computing developments applied to reservoir simulations allow automated development of advanced recovery processes that are tailored to the heterogeneities found in HOTS reservoirs.

Other Information: 

The world oil inventory is dominated by heavy oils and tar sand (HOTS) bitumens generated almost entirely by the process of biodegradation. This process is a biologically-driven, complex reactive diffusion-dominated in-reservoir oil alteration process that occurs under anaerobic conditions driven by oil-water reactions, usually at the oil column base producing methane and heavy oil as by-products. Because large volumes of lighter hydrocarbon components are consumed by the biological process at the oil-water contact or transition zone, significant vertical and lateral gradients in oil composition and thus oil viscosity result . The ultimate controls on the increase of oil alteration and thus viscosity rise are related to the oil charge composition and charge rate history, mixing of fresh and biodegraded oils and diffusion of oil components , the extent of the water leg in the reservoir and nutrient supply, and the reservoir temperature history . Temperature ultimately controls the survival of microrganisms in the subsurface with reservoir pasteurisation at 80 C and above.
The defining characteristic of heavy and super heavy oilfields is large spatial variation in fluid properties, such as oil viscosity, commonly seen within the reservoirs. Viscosity can increase by up to one hundred times across a 40 m thick reservoir. Traditional heavy oil and tar sand exploration and production strategies rely significantly on characterization of key reservoir heterogeneities and assessments of fluid saturations. It is important to understand how these properties vary as the spatial distribution of fluid properties can often dominate the oil phase mobility ratio (oil effective permeability/oil viscosity) distribution which in turn controls production behavior under cold and thermal recovery but surprisingly, in most reservoir simulations, fluid variations are often ignored! Petroleum biodegradation proceeds at the oil-water contact (OWC), or basal reaction zone under anaerobic conditions in any reservoir that has a water leg and has not been heated to temperatures over 80?C. The process of biodegradation is driven from the basal reaction zone near the oil-water contact with reactive hydrocarbons brought to the reaction zone by diffusion and during field charging advection. This results in the most altered and viscous oils being located at the base of the oil column in most instances which is where wells are often placed for thermal gravity drainage processes such as SAGD or CSS.
We have developed several full physics numerical models of the biodegradation process (e.g. 1DRS) which couple charging and biodegradation in a reservoir simulation environment. These allow us to calculate changing compositional profiles through geological time and also predict viscosity gradients distant from well control in HOTS reservoirs. We describe why we find curved, parabolic or even exponential viscosity depth profiles in western Canadian reservoirs where oil charge has terminated but in actively charging traps elsewhere in the world, we typically see more sub-linear viscosity versus depth profiles. We also describe the dynamics of the biodegradation basal reaction zone which can be several meters thick and its impact on the production character of HOTS reservoirs. The ability to perform reactive reservoir simulations with full physical models of biodegradation allows us to populate full reservoir simulator models with mixtures of oils that fully reflect the viscosity heterogeneity of a typical western Canada HOTS reservoir. This together with advances in optimisation technology and parallel computing developments applied to reservoir simulations allow automated development of advanced recovery processes that are tailored to the heterogeneities found in HOTS reservoirs.

Sponsor: 

pimsshellaisucal