During drilling, the invasion of drilling fluid components into the formation can be a major source of operational challenges such as borehole instability and formation damage. Filter cake, which is formed by particles that are captured near the wellbore surface, allows the movement of some molecules while it restricts the others. The properties of the filter cake as well the composition of the filtrate which permeate through it are very important parameters that can be tuned to prevent possible operational challenges. In this paper, the transport properties of certain ions (sodium, potassium, magnesium and calcium) through bentonite- based drilling fluid filter cake were investigated. The effect of in-house activated clay particles on the transport properties was also investigated. The capability of the cake to control the transport properties of the permeating ions were measured in terms of ion rejection. The results obtained for the pure bentonite follows the sequence: Ca2+ > Mg2+ > K+ > Na+. The trend was however, reversed by the addition of the activated clay to Mg2+ >Ca2+ >K+> Na+. The avearge rejection for the pure bentonite cake for Na+, K+, Mg2+ and Ca2+ are 4.45, 49.64, 53.33 and 94.43%, respectively. The rejection results for the mixed-matrix cake containing the activated clay were 6.38, 51.34, 86.19 and 78.09 for Na+, K+, Mg2+ and Ca2+, respectively. It was observed that the selectivity of the filter cake for Mg2+ and Ca2+ was reversed due to the addition of the activated clay.

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