Formation damage from swelling clays

Swelling clays, although relatively abundant in shales, do not occur as commonly in producing intervals. Thus, formation damage problems with swelling clays are not nearly as common as those associated with fines migration.

Impact of swelling clays

The most common swelling clays found in reservoir rock are smectites and mixed-layer illites. It was earlier thought that much of the water and rate sensitivity observed in sandstone permeability was caused by swelling clays. However, it is now well accepted that the water-sensitive and rate-sensitive behavior in sandstones is more commonly the result of fines migration and only rarely of swelling clays. Swelling clays reduce formation permeability by peeling off the pore surfaces and plugging pore throats, not by reducing porosity alone. Should this happen to any extent, large reductions in permeability are observed.

The presence of swelling clays is generally associated with drilling problems (i.e., hole quality and stuck pipe). This can result in poor cement jobs and sensitivity to completion fluids. Poor hole quality in the producing interval can result in significant migration of fluids behind pipe, resulting in reduced fluids control in the wellbore. These problems are encountered if either the producing formation or the intervening shales contain substantial quantities of swelling clays. When swelling clays are present in the producing interval, formation damage problems can occur because of rate sensitivity or water sensitivity. Care must be exercised to ensure that production rates and drawdowns in such wells are maintained so that the critical velocity is not exceeded in the near-wellbore region.

Swelling process

Clay minerals, such as smectites and mixed-layer illites, can expand in volume up to 20 times their original volume through adsorption of layers of water between their unit cells. Such 2:1 clay minerals are particularly prone to swelling because there is no hydrogen bonding between the octahedral layers of the unit cells.

Swelling is known to occur in three steps. In the first step, referred to as crystalline swelling, layers of water enter the interlayer space in the clay mineral, resulting in an increase in the C spacing of the clay mineral in steps. The size of these steps is observed to be approximately equal to the diameter of the water molecule. Extremely large swelling pressures can be generated through such an expansion of the clay lattice. The next stage in swelling is referred to as hydration swelling. This is thought to occur through the hydration and dehydration of ions entering the interlayer region. Several theories have been proposed to explain the observed repulsive hydration force observed in the presence of different cations. Finally, when the interlayer spacing is ≈ 50 Å or so, free swelling occurs. This is driven primarily by the balance between electrostatic and van der Waals forces between the layers of clay. In this stage of swelling, the clay layers are sufficiently far apart that very little mechanical integrity exists in the clay. Such clay minerals are liable to be dispersed in the flowing fluid and to plug pore throats.

Prevention

To prevent fines migration and clay swelling, various chemical treatments have been designed. These include polymers containing quaternary ammonium salts, hydrolyzable metal ions such as zirconium oxychloride, hydroxy-aluminum, and polymerizable ultrathin films.

Each of these methods relies on coating the fines (which are usually negatively charged) with large polyvalent cations that can attach irreversibly to the mineral surfaces. When the electrostatic charges on the fines are neutralized, the likelihood of fines migration is reduced significantly. Fines-stabilizing chemicals have been used in treatments such as acidizing, gravel packing, and fracturing. The effectiveness of such treatments is discussed extensively in Borchardt.

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See also

Formation damage

PEH:Formation_Damage