Purpose of this paper is to present laboratory data to aid in the evaluation of a damage of drilling fluids to oil sands. In this study were used onsolidated synthetic cores mixing river sand with cement. Permeabilities of these samples were between 7 and 3000 md. Firstly, the effective oil permeability of a core was measured. Secondly, the core was mounted on the assembly shown in Fig. 1 and mud fluid was invaded into the core under pressure of 5 kg/cm
2 for three hours. Thirdly, oil was forced to flow upward under certain pressure, and effective oil permeability was measured again. Thus, the recovery in effective oil permeability was obtained and the damage of drilling fluids to oil sands was evaluated. The results of these tests are shown in Figs. 2_??_5.
Fig. 2 indicates that in bentonite mud the permeability recovery was very low, especially in low oil pressure. If mud cake was removed, the recovery increased to some degree. Fig. 3 indicates that in oil emulsion mud the permeability recovery was also low. Figs. 4 and 5 indicate that the permeability recovery in oil-base mud was higher than that of water-base mud. The recovery, however, was not sufficiently high except for samples without mud cake.
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