The alteration of rock properties due to interactions with drilling-fluid components

Abstract

The effects of water-based drilling-fluid components on wettability and permeability have been evaluated. Most of the components tested are generally considered to be acceptable for a bland drilling mud, that is, a mud which will not alter rock properties. However, we have found that almost all of the components do alter wettability, and that some components reduce permeability significantly. Ten whole drilling fluids were also evaluated. Alteration was observed for several of these drilling fluids, although the extent of alteration was lower than observed for the individual components. The drilling fluids which led to the most alteration either lacked components needed to form a good filter cake, or contained particular especially damaging components. Both capillary pressure behavior and contact angle measurements were used to evaluate the effects of drilling fluid components on wettability. The effects of the whole drilling fluids were evaluated by free imbibition tests. The capillary pressure tests were performed on strongly water-wet Berea sandstone samples, and on Berea samples which had been treated to be oil-wet. Contact angles were measured on quartz and calcite crystals, and on asphaltene films. The bland components tested included bentonite, carboxymethylcellulose (CMC), dextrid, drispac (a polyanionic cellulose polymer), hydroxyethylcelulose (HEC), pregelatinized starch, and xanthan gum. Two components excluded from bland muds also were tested: Coat 415 (a filming amine) and lignosulfonate. All of the components considered to be bland, with the exception of bentonite filtrate, made oil-wet samples significantly less oil-wet. None of the bland components apparently affected the wettability of the water-wet samples. Coat 415 reversed the wettability of the water-wet samples, and lignosulfonate made the oil-wet samples less strongly oil-wet. CMC, starch, and dextrid were found to reduce permeability significantly. This work demonstrates that additives generally considered to be bland can, in fact, alter reservoir rock properties.