Fluid flow in mudstones: a function of pore size distribution or grain size distribution

Abstract

Mudstones play vital roles in the entrapment and accumulation of hydrocarbon in reservoirs. They are of major consideration during prospect analysis and evaluating charging risk. Their integrity could be the basis for frontier exploration. Closer examination of mudstones based on Mercury Injection Capillary Pressure Analysis (MICPA) and Sedigraph data for the basis of fluid flow proves that mudstones lithology offers a limited explanation. However, pore size distribution and/or grain size distribution in corroboration with the pore pressure and structural history seem to offer better explanation for fluid flow in mudstones; they also provide better understanding of the leakage mechanisms and pathways of leaked hydrocarbons in mudstone caprocks. In this case study on the caprocks of Valhall oil field in the North Sea, the pore size distribution has been suggested to reflect the grain to grain contact in the matrix of the caprock due to the effective stress by the overburden. Hence, compaction processes better explains the phenomena of similar permeability in different lithologies reflecting distribution profiles for finer and coarser grained sizes of different formations.

Keywords: Caprock, Leakage, Fluid Flow, Permeability, Porosity