**3.3 Clay mineral properties**

Clay minerals are unique in a number of properties they exhibit; however, the following attributes of clay minerals have significant impacts on their interactions with fluids and are briefly captured below.

#### *Emerging Technologies in Hydraulic Fracturing and Gas Flow Modelling*


#### **Table 1.**

*Four major types of clay minerals relevant to hydraulic fracturing.*

#### *3.3.1 Cation exchange capacity (CEC)*

Cation exchange capacity (CEC) is defined as the amount of positive ion substitution that takes place per unit weight of dry rock [35] and is expressed in meq/100 g (milliequivalents per one hundred grams) of dry rock. Substitution of ions in minerals is the product of interfacial electrochemical interactions. Some of the most common cations exchanged are calcium (Ca2+), magnesium (Mg2+), potassium (K+ ), sodium (Na+ ) and ammonium (NH4 + ). CEC controls contribution of clay minerals and clay-bound water to electrical conductivity of rocks as well as the wettability characteristics of clay minerals during clay-fluid interactions.

Researchers developed various methods of measuring CEC over the years with more accurate methods still being developed. Some of the earlier methods have been exhaustively discussed in literature [36–40]. The most common methods currently used for CEC determination include: wet chemistry method; multiple salinity method and membrane potential method. These are however not without their limitations.

Bush and Jenkins [40] developed a method based on the use of the wet chemistry method in which several samples were investigated and a plot of best fit generated. The main challenge with their method is that, some minerals are capable of adsorbing water in humid environments though they have no CEC. Bush and Jenkins [40] proposed their method as a supplementary method for the wet chemistry method rather than a replacement.

Cheng and Heidari, [41, 42] introduced a new theoretical model of measuring CEC based on energy balance between chemical potential and electric potential energy. This involved the combined analysis of data collected from XRD (X-Ray Diffraction), NMR (Nuclear Magnetic Resonance) and nitrogen adsorption–desorption isotherm measurements with direct evaluation of CEC based on ammonium acetate method and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) measurements used in cross-validation of the results. They however alluded to the fact that their method was yet to be developed for complex rock composition.

#### *3.3.2 Clay swelling*

Clay swelling results mainly from fluid intake into the inter-layered structure of clay minerals. Electrochemical interactions between clay minerals and fluids are central to the swelling of clays. The type, quantity and charge of cations in the interlayer zones of clay are the main driving forces in the swelling process. Clay swelling and formation damage during enhanced oil recovery have also been discussed extensively [43, 44].

*Review of Geochemical and Geo-Mechanical Impact of Clay-Fluid Interactions Relevant… DOI: http://dx.doi.org/10.5772/intechopen.98881*

Two main types of swelling mechanisms have been identified in clay minerals which include crystalline swelling and osmotic swelling [45, 46]. During crystalline or surface hydration mechanism, the water molecules are adsorbed on the crystal surfaces with hydrogen bonding holding the water molecules to oxygen atoms exposed from the crystal surface. Subsequent layers of water molecules align to form a quasi-crystalline structure between unit layers, which results in an increased c-spacing. This type of swelling is common to all types of clay minerals, although to a different degree. In osmotic swelling mechanism, the concentration of cations between unit layers in clay minerals is higher than that in the surrounding water, water is therefore osmotically drawn between the unit layers and the c-spacing is increased. Osmotic swelling mechanism causes a larger swelling relative to the crystalline swelling but only a few clay minerals, such as sodium montmorillonite, swell in this manner [47].
