**2.2 Drilling and completion**

Adverse economic impact posed by swelling clays during drilling and completion caused intensification in studies of clay-fluid interactions with the aim of understanding the problem and solving it in the shortest possible time. Research was thus aimed at understanding the mechanisms that drive clay swelling during interactions with engineered muds. The conditions of clay swelling and accompanying complications were studied thoroughly with abundant literature to that effect [10–16].

Van Oort et al. [14] undertook an overview of the mechanisms guiding clay-fluid interactions. Their work identified the mechanisms by which various engineered drilling fluids suppress adverse reactions of clay minerals with water based fluids during drilling and completions. In concluding, the authors simplified their work by categorising drilling fluids into five groups based on the mechanism by which they stabilise clays in shale formations during drilling.

Shukla et al. [17] conducted a review of earlier works on clay mineral swelling in unconventional reservoir systems. They concentrated on the various conditions under which clays swell and the types of clay swelling. They also identified various clay stabilisers and gave a brief on how these work.

#### **2.3 Enhanced hydrocarbon recovery**

At inception of shale gas, tight sands and other unconventional petroleum systems development, enhanced hydrocarbon recovery techniques were at advanced stage. The need to fine-tune these technologies to the needs of unconventional reservoir systems spurred another era of research focussed on clay-fluid interactions in

unconventional petroleum systems. One of the earliest works in this area was conducted by Zhou et al. [15] who premised their research on the fact that injected fluids caused formation damage due to clay swelling. They identified two types of swelling due to these interactions; crystalline swelling and osmotic swelling, the later posing significant adverse effects on reservoir quality. Alalli et al. [18] also noted that injected fluids caused disequilibrium in formation which leads to dissolution and precipitation of minerals in an attempt to return to equilibrium state. Dissolution and precipitation patterns were thus examined in order to identify their impact on reservoir quality. Dissolution of minerals, they noted, enhanced the porosity and permeability of formation by creating additional pore volume and linking previously unconnected pores; whereas precipitation of new minerals had adverse impact on reservoir quality due to the occlusion of flow paths, due to mineral growth within the existing pore space.

Buller et al. [19] analysed the Haynesville shale play in East Texas to understand what factors were responsible for efficiency of hydraulic fracturing in this formation. Their work concluded that, in high clay content zones, the efficiency of the fracturing was low due to massive proppant embedment and migration of fines. They postulated that post fracturing diagenetic events could also be initiated due to clay minerals interaction with fracturing fluids.

A similar effort was undertaken by Radonjic et al. [20] in their research focused on Caney shale. They sought to draw the link between mineralogical composition and microstructure of Caney shale to mechanical responses in order to delineate formations suitable for fracturing as well as predict mechanical responses of these formations.
