Summary
Formation damage risks are well documented for standard mud formulations but they are poorly analyzed for new types of non-polluting muds. This paper describes extensive laboratory work aimed at evaluating the behavior of these new types of mud formulations. First, static and dynamic filtration experiments were conducted on paper filters and rock slices. Examination of mud cakes by means of cryo-SEM has permitted correlation of filtration behavior with the structural characteristics of both external and internal mud cakes. Then, the simulation of the full process of mud invasion in oil-bearing reservoirs was achieved by performing static and dynamic filtration experiments in a specially designed core-holder cell containing 40 cm long sandstone core samples. Cumulative filtrate losses and pressure drops across six sections of the core, while circulating the mud and back flushing the oil, were continuously monitored to evaluate the permeability damage. Damage arising from overbalanced conditions has been evaluated in terms of cake permeability, fluid loss characterization and reduction in oil permeability after mud exposure. For the three mud compositions tested here, the filtration process in high permeability sandstones is mainly controlled by external mud cakes. Damage is severe but an additional damage due to the trapped aqueous filtrate phase can strongly affect the oil relative permeabilities. This approach has given a complete behavior understanding of new water based drilling fluid formulations including evaluation of their performances, limits of use at high temperature and assessment of risks for different operational conditions.
