Clay minerals

2. Literature review

2.1 Sandstone

Sedimentary rocks, as sandstone, are molded as an effect of deposition of clastic material or detritus.

These depositions are usually attributed to high energy sedimentary environments, and are

accumulated in deserts, beaches, flood plains and/or deltas. Sandstone originate from older igneous, metamorphic or sedimentary rock, where fragmentation, erosion and weathering produce the building blocks (Nichols, 2009; Zolotuchin et. al., 2000)

The size range of the grains varies from 63 μm to 2 mm (Nichols, 2009). Diagenesis is any chemical, physical, or biological change undertaken by sediment or sedimentary rock during and after lithification / formation. Lithification is the process in which the sediments compact as the

overburden pressure increase as the grains are buried and the chemically dissolved minerals cause compaction and cementation. Quartz (SiO2) is the most common mineral species in sandstone reservoirs, there are however a range of different minerals that may occur, such as mica feldspar, heavy minerals, lithic fragments, biogenic particles and many other mineral species which have all been observed in sandstones. Sandstones are often denoted as silici-clastic rocks due to their high silica content. The sandstone has some common cementing material that is attached as a coating to the grains, such as silica, calcium carbonate, iron oxide and clay minerals. After diagenesis the resulting rock has a density of about 2.65 g/cm³ (Zolotuchin et. Al., 2000)

2.2 Clay minerals

Clay is basically described chemically as aluminum silicates, and consists of a range of different materials, such as silica, alumina, water, and frequently with large quantities of iron and magnesium and lesser amount of sodium and potassium. Clays usually found in sandstone reservoirs is made up by a crystal structure with two simple fundamental building units, sheets of tetrahedral silica and octahedral aluminum layers. These layers are linked to each other into planar layers by sharing oxygen ions between Si⁴⁺ or Al³⁺ ions of the adjacent tetrahedral or octahedral. The space between the oxygen octahedral and tetrahedral are mostly taken by the Si⁴⁺ and Al³⁺ ions, but to ensure charge balance other cations such as potassium, calcium, magnesium and iron are necessary in the clay structure (Morad et. Al.,2003). The tetrahedral silica and octahedral aluminum layers join together to form the structure of the clay, which defines the units the clay is made up with.

Kaolinite is clay minerals that consist of one tetrahedral layer linked through oxygen atoms to one octahedral layer with no interlayer cations, and is connected by O-H-O bonds in a 1:1 layer structure.

The chemical composition is Al2Si2O5(OH)4 (Morad et. Al., 2003, Wikipedia). Kaolinite is typically

3 described as booklet pages, it can cause pore blockage if mobilized by liquid flow, however, it does not break up under chemical treatment.

Illite is a non-expanding clay mineral and a phyllosilicate or layered alumino-silicate. Its structure is constituted by the repetition of tetrahedral – octahedral – tetrahedral (TOT) layers, termed 2:1 structure. Two opposing tetrahedral layers are connected by O-K-O bonds, and poorly hydrated K⁺ mainly occupies the interlayer space, responsible for the absence of swelling. Al³⁺ partially substitutes Si⁴⁺ in the tetrahedral layer, and a substitution of divalent cations for Al³⁺ in the octahedral layer occurs, the K⁺ is required for charge balance. The chemical formula is given as (K,H3O)(Al,Mg,Fe)2(Si,Al)4O10[(OH)2,(H2O)], but there is considerable ion substitution (Morad et. Al., 2003).

Chlorite has a 2:1 sandwich structure, consisting of negatively charged tetrahedral – octahedral – tetrahedral layers. Unlike other 2:1 clay minerals, a chlorite's interlayer space consist of an additional octahedral layer that is positively charged and comprised of cations and hydroxyl ions, (Mg²⁺,

Fe³⁺)(OH)6, commonly described as the brucite -like layer. Chlorite´s structure will then have the following build up; T – O – T – Brucite – T – O – T. (Morad et. Al., 2003)

Montmorillonite, as chlorite, has a 2:1 sandwich structure, two tetrahedral layers sandwiching a central octahedral layer. The particles have an average diameter of about 1 μm, and are plate shaped. Montmorillonite is a member of the smectite family, and is the main component of

the volcanic ash weathering product, bentonite. It increases greatly in volume when it absorbs water, and the original water content is variable. The chemical formula is given as

(^Na,Ca)0.33(Al,Mg)2(Si4O10)(OH)2·nH2O, named hydrated sodium calcium aluminium magnesium silicate hydroxide. Usual substitutes are Iron, potassium and other cations, but the exact ratio of cations varies with source. It often occurs blended with chlorite, muscovite, illite, and kaolinite.

Montmorillonite, Illite, and Chlorite are all formed as a volcanic rock weathering product, especially volcanic glass.

Clay minerals has unbalanced negative charges on the edges of the unit cells, this is a characteristic that separates them from the other silicates. These negative charges originate from the broken bonds at the edges and surface of the clay structure, and from the dissociation of accessible hydroxyl groups, where the isomorphous substitution of Al³⁺ for Si⁴⁺ occurs. To attain neutrality, these

negatively charged sites attract positively charged ions from the surrounding pore fluid. Some materials have the ability to exchange cations, either by absorption to the external surface or between the layers of the structure, and are described as cation exchange materials (Hamilton,

4 2009). A clays capacity to attract and hold cations from a solution is measured in CEC (Cation

exchange capacity). The Cation exchange capacity is defined as the maximum quantity of total exchangeable cations that the clay is capable of holding at a given pH, usually at a pH of 7. CEC is commonly measured in milliequivalent of hydrogen per 100 gram of clay (meq+/100g) (Bergaya et.

Al., 2006, Wikipedia). Cations in the solution are attracted and held by weak quasi-bonding forces, including electrostatic and van der Waals forces, and depending on the conditions they are

exchanged and not held permanently. Various cations have different relative strengths and replacing power. Weakly adsorbed cations may easily be exchanged, and therefore the relative replacing power of a particular cationic species depends on its strength of binding.

It is believed that the relative replacing power of cations in room temperature is as follows (IDF, 1982, Beaton et. Al., 2011):

Li⁺<Na⁺<K⁺<Mg²⁺<Ca²⁺<Sr²⁺<Ba²⁺<H⁺<Al³⁺

As a result, at equal concentrations, H⁺ will be more successful to displace Li⁺ from the clay surface, then Li⁺ to displace H⁺. However, if the relative concentration of the weaker ion is high enough it may be able to replace ions with a relatively higher replacing power. Characteristics from the four most common clays found in sandstone oil reservoir are listed below.

Property Kaolinite Illite / Mica Montmorillonite Chlorite

Structure 1:1 2:1 2:1 2:1:1

Particle sice (micron)

5-0.5 Large sheets to

0.5

2-0.1 5-0.1

CEC (meq/100g) 3-15 10-40 80-150 10-40

Surface area BET (m²/g)

15-25 50-110 30-80 140

Table 2.1: Clay characteristics and properties (IDF, 1982)

5