Publications Details
Modeling the electrical response of oilfield infrastructure
Weiss, Chester J.; Wilson, Glenn A.
The recurring problem in electrical and electromagnetic modeling of anthropogenically impacted geologic settings is the need for efficient representation of strong, thin, arbitrarily oriented electrical conductors, such as metal pipes or conductive fractures. The difficulty arises from discretization with roughly equidimensional elements of the governing Maxwell equations over these volumetrically insignificant regions; which by virtue of conductors' thinness, can easily number in the 100's of millions for even simple models. To address this problem, a novel hierarchical electrical model is proposed for unstructured tetrahedral finite element meshes, where the usual volume-based conductivity in tetrahedra is augmented by facet- and edge-based conductivity on the infinitesimally thin regions between elements. This allows a slender borehole casing of arbitrary shape to be approximated by a set of connected edges within the mesh, and on which a conductivity-area product is explicitly defined. Benchmark testing of the direct current (DC) resistivity problem shows excellent agreement between the facet/edge representations and independent analytic solutions. As a practical case, the metallic infrastructure of a mature oilfield in the Kern River Formation is modeled. The oilfield comprises roughly 2 km of surface pipeline and 122 vertical, steel-cased wells, each extending to a depth of 300 m. Numerical results demonstrate strong coupling between surface and downhole conductors and reveal a complex circuit of current flow within the (finite conductivity) steel. This would be difficult to quantify using alternative, approximate methods for accommodating the approximately 30 km of steel casing and surface pipe combined.