Water-swelling clay stabilization

Results indicated that swelling clay stabilizers such as poly (DMA-co-EPl) which do not possess a quaternary nitrogen atom in a pendant chain may not be very effective at preventing permeability damage due to fines migration in the absence of water-swelling clays. 

The problems caused by shales in petroleum activities are not new. At the beginning of the 1950s, many soil mechanics experts were interested in the swelling of clays. It is important to maintain wellbore stability dining drilling, especially in water-sensitive shale and clay formations. The rocks within these types of formations absorb the fluid used in drilling this absorption causes the rock to swell and may lead to a wellbore collapse. The swelling of clays and the problems that may arise from these phenomena are reviewed in the literature [528,529,1788,1900]. Various additives for clay stabilization are shown in Table 3-1. 

The major class of plate-like colloids is tliat of clay suspensions [21]. Many of tliese swell in water to give a stack of parallel, tliin sheets, stabilized by electrical charges. Natural clays tend to be quite polydisperse. The syntlietic clay laponite is comparatively well defined, consisting of discs of about 1 nm in tliickness and 25 nm in diameter. It has been used in a number of studies (e.g. [22]). 

Clays or shales have the ability to absorb water, thus causing the instability of wells either because of the swelling of some mineral species or because the supporting pressure is suppressed by modification of the pore pressure. The response of a shale to a water-based fluid depends on its initial water activity and on the composition of the fluid. The behavior of shales can be classified into either deformation mechanisms or transport mechanisms [1765]. Optimization of mud salinity, density, and filter-cake properties is important in achieving optimal shale stability and drilling efficiency with water-based mud. 

Stresses caused by chemical forces, such as hydration stress, can have a considerable influence on the stability of a wellbore [364]. When the total pressure and the chemical potential of water increase, water is absorbed into the clay platelets, which results either in the platelets moving farther apart (swelling) if they are free to move or in generation of hydrational stress if swelling is constrained [1715]. Hydrational stress results in an increase in pore pressure and a subsequent reduction in effective mud support, which leads to a less stable wellbore condition. 

Polyelectrolytes provide excellent stabilisation of colloidal dispersions when attached to particle surfaces as there is both a steric and electrostatic contribution, i.e. the particles are electrosterically stabilised. In addition the origin of the electrostatic interactions is displaced away from the particle surface and the origin of the van der Waals attraction, reinforcing the stability. Kaolinite stabilised by poly(acrylic acid) is a combination that would be typical of a paper-coating clay system. Acrylic acid or methacrylic acid is often copolymerised into the latex particles used in cement sytems giving particles which swell considerably in water. Figure 3.23 illustrates a viscosity curve for a copoly(styrene-... 

Platelets are held together by cations. They impart a positive charge to the edge of the particles. These interlayer cations play a key role in the physicochemical properties of bentonite and in the stability of aqueous dispersions. Normally calcium is predominant and the clay swells to a moderate extent when dispersed in water. When Ca ions are replaced by Na, e.g., by reacting with Na2CC>3, the bentonite is said to be activated. This activation makes the clay much more swellable. 

Clays of the montmorillonite family are lamellar aluminosificates [46] used in many industrial processes and in products such as paints, softeners, and composite materials [47]. They swell when brought into contact with water, which is due to the insertion of water molecules between the sheets. Complete exfoliation can be induced leading to dispersions of disk-like particles of 10 A thickness and 300-3000 A in diameter, depending on the variety of clay used. These clay platelets bear a rather large surface electrical charge so that electrostatic interactions between them must be considered and are actually responsible for the colloidal stability of these dispersions. These suspensions have been widely studied as model colloids and also because they form physical thixotropic gels. 

A more pervasive problem is the maintenance of wellbore stability in shale formations [i.e., formations that have a high clay content, typically in excess of 50 wt% (10)]. In the presence of water, shales can take up water and swell and disperse or they can fracture. Problems associated with wellbore stability in shale sections are sticking of the drill pipe (usually termed stuck pipe), hole enlargement, and excessive generation of drilled solids. 

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