1. Notes on CEC and Bound Water Andy May September 4, 2013

2. Considerations Clay-bound water, due to the high CEC of clays is only part of the total bound water. CEC is dependant upon mineralogy, but also the surrounding water, its salinity, pH, temperature, pressure, and the type of cations in the water. Using XRD minerology, only, and textbook CEC values to determine bound water volume can result in errors and checking the volume with capillary pressure measurements is critical. Slide - 2

3. Mineral versus Rock Model This model allows the CEC measured in core plugs to be directly compared to log derived values. Shale Model XRD clay Model

4. CEC and Bound Water Both Schlumberger and Shell research have vastly over-simplified CEC and its relationship with bound water. Reality, from Grim, 1968: CEC is a very complex quantity and it is dependant not just on salinity and mineral type (Juhasz, 1979 and 1981), but also on temperature (Best, 1978), pH, particle size (or surface area), and cation type. These latter three factors are important and not addressed by either Juhasz or Schlumberger.

5. Key publications Grim, Ralph, “Clay Mineralogy,” McGraw-Hill, 1968 Best, Gardner, and Dumanoir, “A Computer-Processed Wellsite Log Computation,” SPWLA Trans., 1978, Paper Z. Juhasz, “Normalized Qv…,” SPWLA Trans., 1981 Waxman and Smits, “Electrical Conductivities in Oil-Bearing Shaly Sands,” 1968, SPE 1863 Waxman and Thomas, “Electrical Conductivities in Shaly Sands…,” 1974, SPE 4094 Juhasz, “The Central Role of Qv…,” SPWLA Trans. 1979, Paper AA Clavier, Coates and Dumanoir, “Theoretical and Experimental Bases for the Dual-Water Model…,” SPE, 1984 Hill, Shirley and Klein, “ Bound Water in Shaly Sands…,” Log Analyst, 1979. Mollison, Rick, “Implementation of Hill, Shirley and Klein…,” Baker-Hughes, 1999. Dacy and Martin, “Practical Advances in Core-Based Sw…,” Petrophysics, Feb. 2008. Slide - 5

6. Juhasz, 1979, page 9 & 10 “…Dual water model predicts less CBW than we arrive at on the basis of the HSK equation combined with the assumption that the amount of CBW is not temperature dependant but is solely a function of CEC and salinity.” This is true as written, but Grim says CEC is dependant upon temperature, pH, particle size, and other factors. “…the maximum amount of hydrocarbons [from capillary pressure measurements] should not exceed the available effective pore space, i.e. Φt(1-Swt) < Φe.” Juhasz (1979) acknowledges that capillary pressure is the standard definition of bound water and Qv must be changed to match capillary pressure measurements (page 10 & 11).

7. Swb > Sw? Slide - 7 NoYes, bad parameters Swb = Vsh*Φ sh /Φ t or a function of clay volume and CEC Either way, check Swb versus Sw! Make sure Swb is not truncated at Sw or greater than Sw. Also check Φ t (1-Sw t ), make sure it is less than Φ e Check Swb against capillary pressure measurements.

8. Best, 1978 “Na+ ions are kept some distance from clay surface.” Shell and Grim disagree with this, Grim (1968) has X- Ray data that shows adsorbed cations in contact with the clay surface. Grim, 1968, page 196: “…adsorbed cations around the edges of flakes are held directly in contact or at least very close to the clay- mineral surface…” The type of cation is very important, “negative charges on the clay mineral can be neutralized locally only by monovalent cations.” Thus sodium can do it, but not calcium. Sodium is held more closely than calcium.

9. Clavier, Coates and Dumanoir “The conductivity of the clay-water is quasiuniversal and depends mainly on temperature” Shell and Grim disagree with this, Grim (1968) presents abundant contradictory data. Slide - 9

10. Bound Water Clay-bound water, bound by CEC Capillary-bound water, bound by wettability, interfacial tension with other fluids and capillary pressure Normally Rwb<Rw, that is the bound water salinity is normally higher than the free water salinity (Brown, 1986; Juhasz, 1981, page 4: “The excess bulk water conductivity is due to the presence of excess cations in the water surrounding the clay particles…”) Schlumberger claims that Rwb can be greater than Rw, this is rare, in my experience.

11. Causes of CEC Grim, 1968, page 192: “All inorganic minerals have a small CEC as a result of broken bonds around their edges. CEC increases as particle size decreases…” In smectites and vermiculites, broken bonds are a small portion (20%+-) of CEC, the remainder are lattice substitutions.

12. CEC, Temperature and Cation Type CEC is, in part, a function of temperature and cation type, data from Grim, 1968, page 206. Also noted in Clavier, Coates, and Dumanoir, 1984. Note sodium is held longer than calcium.

13. CEC is strongly dependant upon pH From Grim, 1968, Clay Minerology Standard pH = 7 Normal drilling mud pH=9 to 9.5

14. Effects on CEC Particle size or surface area, Grim, 1968: “…CEC capacities of kaolinite and illite increase as the particle size decreases…Ormsby (1962) found a linear relation between surface area and CEC for kaolinites of Georgia.” Perfection of crystallinity is also a factor, but Ormsby found that surface area was more important. CEC of smectite is only slightly affected by grain size because the CEC is dominated by lattice substitutions

15. CEC pH=7 From Grim, 1968, Clay Minerology Grimm, 1968, page 190. The difference between the CEC measured at a pH of 3.5 and one at a pH at 7 can be 100 meq/100 g. As pH goes up CEC goes up. In the table above, all measurements are at a pH of 7.

16. Shale Φe Clay Model Φe Silt Sand From Corelab

17. Silty Sandstone Shale=53.8% Clay = 23%

18. High Feldspar! If very fine grained and slightly altered to clay can have a high CEC, but be classified as feldspar by XRD.

19. CEC from core Wet Chemistry, titration on crushed sample. Grim, 1968, page 224: “The determination of CEC is at best a more or less arbitrary matter, and no high degree of accuracy can be claimed.” CoCw core measurements, non-destructive, This the preferred method, but is on a rock sample, not just the clays and the sample must have permeability, thus it is not of a shale, but the sample has a Vshale. This is the only measurement of effective CEC. (Waxman-Smits, 1968, Waxman, Thomas, 1974 and Dacy and Martin, 2008) Derive from XRD, Requires too many assumptions about clay mineral CEC and clay distribution

20. CEC from logs Where: C 0 is the NaCl concentration in kppm CEC = Qv*Φ t From Mollison, 1999 and Juhasz, 1986

21. Clay-Bound water summary Clay-bound water can be defined as the water bound to the clays due to cation exchange capacity. The value is highly dependant upon the surrounding water pH, particle size, cation (divalent vs. monovalent) type and salinity. Clay-bound water is only a portion of the total bound water and arbitrarily defined. The remaining bound water can be called capillary-bound water. Total bound water is determined with a capillary pressure model.

22. Capillary-bound Water Three forces combine to create a “capillary force” 1.Wettability ( Ɵ, contact angle between water and solid) 2.Interfacial tension (σ) 3.Capillary pressure (Pc) λ is the pore size distribution index Pc = ρ w - ρ g σ – curvature of the contact between the two fluids

23. Capillary-bound water summary Capillary forces are very strong and, like cation exchange forces, dependant upon grain size and surface area In rocks with little clay, capillary-bound water will be the dominant bound water Capillary-bound water volume is estimated with core measurements and a model

24. Conclusions CBW does not equal BW, but is a smaller volume BW volumes should be checked against capillary pressure measurements and adjusted to match CEC is difficult to measure and is not a mineral property, but a property of the mineral and the environment the mineral is in. Slide - 24