Diffusion Model Coupled with the Flory–Huggins–Zuo Equation of State and Yen–Mullins Model Accounts for Large Viscosity and Asphaltene Variations in a Reservoir Undergoing Active Biodegradation

Abstract

A very large monotonic variation of asphaltenes and viscosity has been measured by downhole fluid analysis (DFA) in crude oils in five-stacked sandstone reservoirs in the northern part of the Barmer Basin, northwest India, undergoing active biodegradation; each of the five-layered sand bodies shows overlaying fluid gradients with depth providing replicate validation of the measurements. Fluid data from four wells across the field shows that the gradients are uniform across the formation. The crude oil in the upper half of the oil column exhibits an equilibrium distribution of asphaltenes matching predictions of the Flory-Huggins-Zuo Equation of state (FHZ EoS) with the gravity term only using asphaltene nanoaggregates of the Yen-Mullins model. However, the bottom half of the reservoir reveals a large asphaltene gradient approximately three times larger than the equilibrium predictions from the FHZ EoS. This increase in asphaltenes creates a very large (8x) viscosity gradient and is a major production concern. In addition, these shallow reservoirs are undergoing active biodegradation at temperatures of 55 °C to 61 °C.A simple diffusive model coupled with the FHZ EoS is shown to account for the entire observed asphaltene distribution in each of the five sand layers. Alkanes (and some aromatics) are rapidly consumed at the oil-water contact at the base of the oil column. The rate-limiting step is the diffusion of these compounds to the oil-water contact. The loss of these oil components decreases the oil volume yielding an increase in asphaltene concentration and a significant increase in viscosity. The limited geologic time of the oil in the reservoir limits the vertical extent of the diffusive process accounting for the observation of asphaltene equilibrium at the top of the column. Specifically, petroleum system modeling of this basin indicates that the oil commenced undergoing biodegradation approximately 50 million years ago, and this duration matches the analysis using the diffusion model plus the FHZEoS. Gas chromatography applied to the oils from the top to the bottom of the oil column provides detailed compositional confirmation of the diffusive mechanism proposed. In particular, all measured compositional properties of the oil column are shown to be consistent with this simple diffusive model. The ability to account for asphaltene and viscosity variations in the five stacked sand layers with a simple diffusive model coupled with the FHZ EoS and the Yen-Mullins model provides a robust model for improving efficiency of reservoir engineering and oil production.

Integrating comprehensive two-dimensional gas chromatography and downhole fluid analysis to validate a spill-fill sequence of reservoirs with variations of biodegradation, water washing and thermal maturity

“…[Mullins, 2008] Also shown in the gas chromatography (GC) that towards the base of the oil column, the alkanes are diminished, while towards the top of the oil column, the n-alkanes are present and clearly seen as a series of large spikes in the GC. [Zuo et al, 2015] Clearly, the alkane distribution is not equilibrated in this oil column; equilibration of n-alkanes would correspond to a much more homogeneous distribution than observed. Also, the drilling fluid filtrate contamination is evident towards the base of the oil column where the oils are enriched in asphaltene, more viscous and harder to sample without filtrate contamination.…”

“…An example of an integrated analysis is shown below for the Bhagyam reservoir in the Rajasthan basin, India. [Zuo et al, 2015] A large, disequilibrium gradient of asphaltenes exists in the oilfield as shown in Fig. 1.…”

“…Also, the drilling fluid filtrate contamination is evident towards the base of the oil column where the oils are enriched in asphaltene, more viscous and harder to sample without filtrate contamination. [Zuo et al, 2015] A) Asphaltenes versus depth (true vertical depth, subsea) Figure 1B shows the DFA-measured asphaltene gradients fit with two different theoretical expressions. The fit with the Flory-Huggins-Zuo equation of state (FHZ EoS) Zuo et al, 2013] using the 2nm asphaltene nanoaggregate from the Yen-Mullins model [Mullins, 2010] works only for the upper half of the oil column but does not fit the lower half of the oil column due to ongoing biodegradation.…”

“…1B. [Zuo et al, 2015] The microbes can consume at most 2/3 of the oil [Head et al, 2003] thus extensive biodegradation can yield a tripling of the asphaltene content beyond what it would have been in unperturbed crude oil. Fig.…”

“…1B is 50 million years matching the start of reservoir charging obtained from petroleum system modeling of the Bhagyam reservoir. [Zuo et al, 2015] In some reservoirs, more complex process can occur. In the Llanos basin, in Colombia in the eastern forelands of the Andes, reservoirs can undergo biodegradation, multiple charging and water washing.…”