A core study was conducted to investigate the return-permeability and changes in the irreducible water saturation in Sandstone and Carbonate cores, when exposed to filtrate from actual oil-based and water-based drilling mud. Results indicate that the oil-based filtrate contains sufficient oil wetting surfactants and emulsifiers to reverse the wettability of rock surfaces from water wet to oil wet. The oil wet rock surface in the low permeability matrix, reduced the effective oil permeability drastically. In the high permeability matrix, the change in wettability to an oil wet system reduced the irreducible water saturation, resulting in an increase in the effective oil permeability. This effect was observed for Sandstone as well as the carbonate cores (1).
Clay swelling, dispersion and ion exchange combined with water blockage are believed to control the changes in flow properties for the Sandstone core samples. The introduction of colloidal particles and the dissolving of calcite are believed to be the mechanisms controlling the changes observed in the flow properties for the carbonate cores (1).
When drilling the producing interval, drilling mud filtrate and colloidal particles in the drilling mud will inevitably invade the formation adjacent to the wellbore. Any alteration to the virgin reservoir rock wettability, intrinsic permeability, water saturation and oil viscosity can be detrimental to the reservoir productivity. Further more, altered reservoir rock wettability can give misleading results in core analyses to be used in a reservoir flow simulator.
The most obvious permeability reduction from the drilling operation is the invasion of colloidal particles from the drilling mud. If these particles are not flushed out completely when the well is put on production, they will block constrictions in the flow channels.