Electrical Properties Effect of Clays Effect of Wettability

Clays provide an alternate path for flow of electrical current, violating the assumption in Archie’s Equations that all current flows due to conductivity of formation water –Archie’s Equations are too pessimistic for shaly formations missed opportunities for development of potentially productive reservoirs wrong amounts and type of investment in reservoir development due to skewed economic projections Effect of Clays on Electrical Properties

The formation factor (F=R o /R w ) is constant for a clean sand; F decreases for shaly sand as value of R w increases –due to conductivity of clays becoming more significant compared to conductivity of water –same core sample with clay present varying only R w Effect of Clays on Formation Factor

Clays provide an alternate path for flow of electrical current The “best” saturation equation for shaly formations seems to vary –Jordan and Campbell, Table 6.9 shows 10 different models Depends on type of clays present (advanced topic not discussed here) –Waxman and Smits recommended as “definitive model” for resistivity behavior of R o in shaly rocks Waxman and Smits will be discussed as an important example of a saturation equation for shaly formations Effect of Clays Saturation Equations

Power Law Model I R =R t /R 0 =S w -n –No conductive materials (clay) present Review: Archie’s Saturation Equation Rock type 1 Rock type SwSw I R = RtR0RtR0 From NExT, 1999

For a clean formation R t  as S w  0 When conductive solids are present (e.g. clays) R t  R rock as S w  0 R rock is resistivity of rock with formation water not providing any conductivity Effect of Clays Resistivity Index / Saturation Relationship Note: Resistivity Index, I R =R t /R o

Waxman and Smits modified Archie’s model to account for the presence of clays I R = R t /R o = S w -n [(1+a)/(1+(a/S w ))] Effect of Clays Waxman and Smits Saturation Relationship The parameter, a, is semi-empirical and depends on –R w –B, ionic conductance electrolyte conductivity temperature –Q v, concentration of exchange cations

Application of the Waxman and Smits saturation equation is often done by matching observed data to curves with known values of n and a –no sliding allowed, simply determine which known curve the data follows –data points at higher S w values used to determine n As S w  1, results are the same as Archie’s model –data points at lower S w values used to determine a Asymptotic value of R t /R o as S w  0 Homework #7 will include exercises on this matching process Effect of Clays Waxman and Smits Saturation Relationship

Wettability determines which pores are occupied by phases and at what saturations, phases become discontinuous –wetting phase in small pores –when water starts to become discontinuous, R t tends to increase sharply Effect of Wettability on Electrical Properties

Sandstone formations –Wettability has a significant effect on: saturation exponent for Archie’s Law (n) model to be used (Archie’s model is a power law, i.e. log-log straight line) Effect of Wettability Resistivity Index / Saturation Relationship

Carbonate formations –Wettability has a significant effect on saturation exponent for Archie’s Law (n) model to be used (Archie’s model is a power law, i.e. log-log straight line) Effect of Wettability Resistivity Index / Saturation Relationship