Introduction There are multiple links between ...

Introduction There are multiple links between geomechanical, rock physics and petrophysical properties of shales and most of those are related to specific surface area (SSA) and cation exchange capacity (CEC) of these materials, which in turn is related to the fine grain size and atomic structure of their clay mineral constituents. Variations in clay mineralogy are linked to changes in permeability and strength and such effects can be related to the development of wellbore instability or high pore fluid pressures in the subsurface (or both). The dielectric permittivity of a shale relates to the amount of electrical charge stored over a cycle of alternating electric field, and as it increases with decreasing frequency from as more polarization mechanisms become involved in the composite material (von Hipel, 1954). Clay minerals have electrically active surfaces when hydrated and the surface charges on and around clay particles become increasingly polarized by space-charge mechanisms as frequency decreases, such that the dielectric constant increases with decreasing frequency, related directly to CEC. The electrical properties of clays and shales therefore exhibit a complex frequency dependence controlled by water content, surface area and charge density, pore fluid salinity and microstructural characteristics. The chemohydromechanical interaction between shale formations and water-based drilling fluids can lead to a variety of drilling and wellbore stability problems. The most common control is to adjust the salinity of the drilling mud to reverse the osmotically-driven flow so that the hydraulic pore pressure surrounding the wellbore does not increase during operations (e.g. Chenevert 1970). An informed approach to engineering of the drilling fluids is one that makes use of characterisation of the chemoporomechanical properties of the shale that determine how it will respond to changes in the drilling fluid salinity. Two of the critical parameters governing the chemomechanical stability of shales are the reflection coefficient, R, which represents an enrichment of the membrane efficiency concept (e.g. Sherwood 1994; Sarout and Detournay, 2011), and the chemomechanical coupling coefficient, α, which embodies the constitutive coupling between ionic content and swelling/shrinking of the shale. Shale swelling or shrinkage is directly related to the CEC of a given shale, those with large amounts of smectite (highest CEC) are prone to be particularly troublesome in this regard. This paper details recent experiments investigating the links between CEC, SSA, chemomechanical properties, dielectric response and rock strength. Methodology For the purpose of measuring the dielectric properties of drill cuttings and small quantities of powdered samples, a 16 mm coaxial transmission line dielectric cell (e.g. Weir, 1974) was developed. The cell is connected to an Agilent E5070B network analyzer to measure the reflection and transmission scattering (S) parameters from 300 KHz to 3 GHz. The cell was used to measure standard clay mineral powders and pulverised shale samples under room-dry conditions. We also prepared pastes from the ground powders. The pastes are best measured with an end-terminated coaxial probe (Figure 1, left) using the same network analyser, calibrated against a deionised water standard. Analysis is based on an inversion algorithm for the scattering parameters measured for a section of coaxial transmission line terminated against the surface of the sample. Probes of this nature are described by Burdette, Cain and Seals (1980). While not particularly accurate and with limited bandwidth compared with a coaxial cell, the end-loaded probe is a rapid measurement system. Generally its accuracy is much better for liquids and compliant solids, which makes it perfect for large batches of drill cuttings (cf. Leung and Steiger, 1992). For chemomechanical characterisation, specimens are prepared by lapping a small piece of shale in order to obtain parallel ends, and then grinding the specimen into a cylinder shape. Specimens were generally prepared under oil for preservation purposes. The specimen is placed in the cell in an initial bath of 3.5% by weight NaCl solution and the 19 g hanger plus one 97 g weight is applied as an initial load (Figure 1, right). Note that here and in the incremental loading step, the weights are attached to a Fourth EAGE Shale Workshop Shales: What do they have in common? 6-9 April 2014, Porto, Portugal

lever arm with such that the load applied to the specimen is twice the loading applied to the hanger by the weights. The specimen is allowed to settle for 1-2 days. After this settlement time, mechanical loading occurs through the addition of six, 97 g weights to the hanger at 2-3 minute intervals. This loading gives an axial strain versus axial stress curve. The specimen is then allowed to equilibrate, which in most cases takes approximately one day. Finally, the chemical loading stage is applied through addition of 35 g NaCl to the 500 ml of fluid in the cell by externally switching the auger on. In this way the osmotic pressure of the bath is increased by a nominal 3 MPa without opening the temperature chamber or otherwise disturbing the apparatus. The transient response is monitored for 20 hours or more. Further details are in Dewhurst et al., (2013).

Figure 1 (left) end loaded probe for performing dielectric measurements on powders and pastes; (right) cell for performing chemoporomechanical measurements on small samples of intact shales (based on Dewhurst et al., 2013).

Results Figure 2 shows the complete dielectric response for five well paste samples from a single borehole selected to show how typical dielectric responses of shales are related to their SSA. The real dielectric response reduces with frequency to a plateau value of approximately 30-40 but the real dielectric permittivity at 1 MHz is approximately 1 order of magnitude higher than the high frequency plateau and more spread out. The real dielectric permittivity at 10-30 MHz is well ordered according to SSA, with an increased SSA leading to an increased dielectric permittivity. In the frequency range between 100 MHz and 1 GHz the separation according to SSA is lost and it is apparent that the high frequency real dielectric permittivity is governed by other factors, especially paste water content as we know from the previous section. Likewise the equivalent imaginary dielectric permittivity exhibits a simple downward 1-to-1 trend in frequency space which is consistent with conduction dominated loss, and is supported by the inset conductivity response. Above 100 MHz the polarisation loss mechanisms begin to appear, but don’t dominate equivalent imaginary dielectric permittivity until beyond 1 GHz. The correlation between SSA and dielectric loss is well ordered at high frequency. The chemoporomechanical measurements show that there is a general correlation between R and α and the values cluster around the α = R line (Dewhurst et al., 2013). Closer examination reveals that α is slightly larger than R in a majority of cases. The few instances where α is less than R are associated with kaolinitic shales. In addition to being empirically correlated with one another, the results show that R and α are moderately correlated with CEC. CEC and SSA are strongly correlated with one another (Figure 3, left), and so it is not surprising that direct correlations of R or α with SSA yield similar results to the correlations with CEC. However, in spite of the moderate individual correlations of α and R with SSA or CEC, it turns out that stronger correlations exist between R (not shown) or α and the product CEC and SSA (Figure 3, right). Fourth EAGE Shale Workshop Shales: What do they have in common? 6-9 April 2014, Porto, Portugal

Real Dielectric Response of Nagra Paste Samples Relating to SSA

Conductivity of

Nagra Pastes Relating to SSA 1

NagraE ε'r NagraJ ε'r

Equivalent Conductivity σ (S/m)

Real RelativeDielectric Permittivity ε'r

1000

NagraD ε'r NagraB ε'r NagraF ε'r

100

Increasing SSA

Upper Frequency Asymptote

10 1E+06

1E+07

1E+08

1E+09

1E+10

0.1

SSA=121 σ SSA=110.1 σ SSA=58.3 σ SSA=41.5 σ SSA=17.5 σ 0.01 1E+06

1E+07

1E+08

1E+09

1E+10

Frequency (Hz)

Frequency (Hz)

Figure 2 Dielectric responses of a shale paste samples from a single borehole in relation to SSA. (left) Real dielectric permittivity, (right) the equivalent effective conductivities.

Figure 3 (left) Strong correlation between SSA and CEC on a global population of shales tested for chemoporomechanical response; (right) moderately strong correlation between α and the product of CEC and SSA for the same global population of shales. Discussion The measurements taken clearly show that shale dielectric permittivity depends on the volumetric water content, specific surface area, and the surface charge density, and therefore CEC. These properties correlate with each other in a generally linear way, while to predict surface area from permittivity, we need to introduce a weakly non-linear relationship. In preserved shales, the high frequency permittivity (100 MHz was possible in our parallel plate apparatus) ranges from less than 10 in the most compact and inert shales to over 40 for shales with appreciable smectite that also have high water contents; many mudrocks have permittivity close to the average value range of 15-20. For different shales, the 10 MHz brine permittivity ranges from around 15 to around 130 (average ε’r is 40). High values of dielectric enhancement, from around ε’r = 30 at 1 GHz typically to ε’r > 50 to 80 at 10 MHz are the rule in pastes made from pulverized shales and there is a strong correlation between the 10-30 MHz dielectric properties and both CEC and SSA. The weaker correlation between these surface dependent properties and preserved shale permittivity compared with paste permittivity is notable, and shows the importance of microstructural features in controlling both transport and polarization of electrical charges. Chemical reactivity/sensitivity or swelling capacity of shales, and clay-rich rocks in general, is often attributed to the magnitude of their CEC. Typically, negative charges on clays hinder water molecules and ionic mobility in the shale pore network. An additional effect is expected from the natural Fourth EAGE Shale Workshop Shales: What do they have in common? 6-9 April 2014, Porto, Portugal

coupling between hydraulic and osmotic pressure gradients, that is (i) the motion of ions indirectly induced by a hydraulic pressure gradient; and (ii) the motion of water molecules indirectly induced by a gradient in ions concentration (so-called osmotic pressure). To account for this additional coupling effect, the reflection coefficient R has been in introduced in the chemoporoelastic theory. This is a macroscopic parameter that reflects the underlying microscopic mechanisms at work, and as such is expected to correlate with the CEC (Dewhurst et al., 2013). In addition to these transport-related phenomena, CEC is known to correlate with the ability of clays to swell/shrink (deform). For instance, smectite swells much more than other clays according to their respective CEC. This constitutive effect is quantified through the chemo-mechanical parameter α in the chemoporoelastic theory. Both transport and constitutive phenomena at the macro-scale are related to the same physical mechanism at the micro-scale, which in turn is related to the negatively charged surface of clays. Therefore, one would intuitively expect the two parameters α and R to correlate with the CEC. Conclusions CEC and SSA in shales have been shown to relate to many other mechanical and physical properties of these rocks. Knowledge of the CEC and SSA, whether measured or predicted from a downhole dielectric tool for example, can inform us of potential mechanical behaviour. Frequency dependence of dielectric response is shown to be related to the SSA (and by inference CEC) and also chemomechanical behaviour in a global suite of shales. CEC is also related to rock strength and given enough testing, improved strength prediction would be a future goal. Acknowledgements This work was partly performed as part of the SHARC consortium funded by ExxonMobil, Chevron, Sinopec, ConocoPhillips, Total, BG-Group and Statoil. The single borehole study was funded by Nagra. References

Chenevert. M.E., 1970, Shale Control with Balanced-Activity Oil-Continuous Muds. J. Petroleum Tech. 22:1309-1316. Dewhurst, D., Bunger, A., Josh, M., Sarout, J., Delle Piane, C., Esteban, L., Clennell, B., 2013, Mechanics, Physics, Chemistry and Shale Rock Properties. Paper 13-0151, Proceedings of the 47th American Rock Mechanics Association Annual Conference, San Francisco, 11pp. Sarout, J. and Detournay, E., 2011, Pore pressure transmission test for reactive shales: Chemoporoelastic analysis and experimental validation. Int. J. Rock Mech. Min. Sci. 48:759-772. Sherwood, J.D. 1994. A model of hindered solute transport in a poroelastic shale. Proc. Royal Soc. London A 445:365-377. Von Hipel, A.R., 1954. Dielectric Materials and their Applications, The Technology Press of M.I.T., United States of America. Weir, W. B., 1974, Automatic measurement of complex dielectric constant and permeability at microwave frequencies, IEEE Trans., vol. 62, 33-36.

Fourth EAGE Shale Workshop Shales: What do they have in common? 6-9 April 2014, Porto, Portugal