Performance evaluation and formation damage potential of new water-based drilling formulas

Formation damage risks are well documented for standard mud formulations bu t they are poorly analyzed for new types of nonpolluting muds. This paper d escribes extensive laboratory work aimed at evaluating the behavior of thes e new types of mud formulations. First, static and dynamic filtration exper iments 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 cake s. Then, the simulation of the full process of mud invasion in oil-bearing reservoirs was achieved by performing static and dynamic filtration experim ents in a specially designed core-holder cell containing 40 cm long sandsto ne core samples. Cumulative filtrate losses and pressure drops across six s ections of-the core, while circulating the mud and back flushing the oil, w ere continuously monitored to evaluate the permeability damage. Damage aris ing from overbalanced conditions has been evaluated in terms of cake permea bility, fluid loss characterization and reduction in oil permeability after mud exposure. For the three mud compositions tested here, the filtration p rocess in high permeability sandstones is mainly controlled by external mud cakes. Damage is severe but an additional damage due to the trapped aqueou s filtrate phase can strongly affect the oil relative permeabilities. This approach has given a complete behavior understanding of new water based dri lling fluid formulations including evaluation of their performances, limits of use at high temperature and assessment of risks for different operation al conditions.