(694e) Modeling of Reservoir Connectivity and Tar-Mat Using Gravity-Induced Asphaltene Compositional Grading

Reservoir compartmentalization is one of the major issues in both onshore and offshore reservoirs. High capital costs are involved especially in deepwater exploration and production, making it essential to assess prior to production the extent of compartmentalization within a reservoir. Seismic data and discontinuous variation of fluid properties (e.g. chemistry, density, and viscosity) are commonly used in an attempt to assess the level of compartmentalization. Within a single oil column, fluid properties vary along the depth of a reservoir due to compositional grading. Considerable fluid flow is required to yield such compositional gradients, suggesting connectivity. In particular, reservoir connectivity can be best understood based on continuous characteristics of asphaltene compositional gradient, because to equilibrate asphaltene, the heaviest and least mobile crude oil component, necessitates substantial permeability. 

Compositional grading studies available in the literature account for gas-to-oil ratio and composition variations, but do not analyze compositional grading related to asphaltenes.  In this work, we present an algorithm based, in part, on  Whitson’s analysis [1] and use the PC-SAFT equation of state to address asphaltene compositional grading due to the attainment of thermodynamic equilibrium in a uniform gravitational field. The results are compared with field data to evaluate the compartmentalization of a reservoir. Based on the solution thermodynamics, an approximate analytical solution for calculating the asphaltene gradient is also presented. The analytical solution will be helpful for sensitivity analysis and approximate estimate of the asphaltene compositional gradient.

Asphaltene compositional gradient leads to significant variations in crude oil viscosity [2], and extreme cases of asphaltene compositional grading can lead to tar-mat formation [3]. The PC-SAFT asphaltene compositional grading introduced in this study is extended to model the possibility of a tar-mat formation due to gravitational segregation of asphaltene.

References

1.         Whitson C H, Trondheim U, Belery P. Centennial Petroleum Symposium, 1994. Tulsa, USA. SPE 28000:443-459.

2.         Hirschberg A. Journal of Petroleum Technology, 1988. 40(1):89-94.

3.         Carpentiera B, Arab H, Pluchery E, Chautru J M. Journal of Petroleum Science and Engineering, 2007. 58(3-4):472-490.