The Role Of Membrane Potential Measurements In Shaly Sand Evaluation'
- K.M. Diederix (Koninklijke Shell Exploratie En Produktie Laboratorium) | H.H. Yuan (Shell Development Bellaire Research Center,)
- Document ID
- Society of Petrophysicists and Well-Log Analysts
- The Log Analyst
- Publication Date
- November 1989
- Document Type
- Journal Paper
- 1989. Society of Petrophysicists & Well Log Analysts
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- 141 since 2007
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The accurate determination of the conductivity due to clay counterions, otherwise known as the clay conductivity, is important in evaluating water and oil saturations in shaly sands. In this paper we demonstrate, by reexamining the original data of Waxman and Smits, that the clay conductivities derived from membrane potential measurements provide more accurate estimates of the clay conductivity than those obtained from conductometric titration. The reasons for this are that the membrane potential measurements are performed where the clays are maintained in the environment in which they contribute to the clay conductivity, and that the clay conductivity derived from membrane potential measurements does not depend upon empirical shaly sand parameters such as the shaly sand formation resistivity factor, F*. A new equation that directly relates the clay conductivity to the membrane potential is developed from an approximate solution to the membrane potential integral equation and includes only the brine conductivity and the membrane potential of a perfect membrane, which can be calculated from sodium transport numbers. Shaly sand equations that incorporate membrane potential measurements are then derived by combining the approximate solution with the Waxman-Smits equations. For water-bearing shaly sands, the membrane potential equation has only one parameter, namely the shaly sand formation resistivity factor, F*, whereas for partially saturated shaly sands, the only dependent parameter is the shaly sand saturation exponent, n*. Consequently, the measurement of membrane potentials on core samples currently offers the most accurate, reliable, and direct methods for determining the clay conductivity.
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