Fluid Flow Effects of Faults in Carbonate Reservoirs, an Example from the Kharyaga Field, Russia

Faults in carbonate reservoirs may have a dramatic effect on fluid flow and represent a major uncertainty in hydrocarbon exploration and reservoir characterization since they are often associated with high permeability conduits as well as potential barriers. The fluid flow properties result from a combination of mechanical and diagenetic processes controlled by the brittle and reactive characteristics of carbonate rocks. The Late Devonian reservoir in the Kharyaga Field of the Pechora Basin is extremely heterogeneous due to fractures and karstification superimposed on the depositional heterogeneity. Faults and open fractures strike dominantly E-W, and the data indicate that fracture density tends to increase towards faults and is associated with decreasing porosity in the matrix rocks. Dissolution and karstification is commonly associated with the faults and fractures. The best and most realistic match of historical production data was achieved when the faults in the simulation models were represented as conduits for flow parallel to the fault and barriers for flow perpendicular to the fault. Geologically this is explained by: a cemented cataclastic fault core representing the barrier, and by a fault damage zone consisting of a connected open fracture network enhanced by dissolution producing high permeability parallel to the fault.