In oil and gas drilling and hydraulic fracturing, shale and/or clay may swell in the presence of water, which may cause a majority of problems including: wellbore instability, cuttings disintegration, bit balling, and fines migration. All of these issues can result in higher drilling costs, slower well productivity and can even destroy the wellbore. Clay stabilization products (also called anti-swelling agents) could reduce and prevent clay swelling during different operations including drilling mud, water injection, acidizing, fracturing, and enhanced oil recovery (EOR). It is widely used and plays an important role in oil and natural gas exploration.
In recent years, clay stabilizers and shale inhibitors have been studied and developed around the world. Generally they have the same structures of quaternized amines besides KCl (potassium chloride), such as NH4Cl (ammonium chloride, AC), [(CH3)4N]Cl (Tetramethylammonium chloride, TMAC), and some polymeric amines, etc. “Small cations”(e.g. AC, TMAC) and KCl usually function through ion exchange. They enter into the crystal lattice of clay, where the cations (NH4+, (CH3)4N+, K+) of clay stabilizer replace the Na+ on the surface of clay to prevent swelling. However, the polymeric amines as stabilizers just adsorb on the surface of clay or shale, and they have a distinct mechanism of stabilizer from the “small cations” and KCl.
Some examples of commercial clay and shale stabilization products are introduced as follows:
KCl has been the primary choice for a temporary clay stabilizer, but it can be incompatible with other materials. It is only effective in the salt-containing drilling fluid/mud. Once fresh water is added, this balance will break and the clay will swell once again. In addition, the use of KCl in drilling operations carries some environmental concerns due to its high content of Cl-.
TMAC can replace KCl as a clay and shale stabilizer, especially in the fracturing fluid. The quantity of TMAC is just 1/10 of KCl, TMAC provides temporary protection because of the NH + single cation. TMAC is very cheap and suitable for short-time operations, however, the main drawback is its toxicity and odor. Although it can be used in oilfield operations because of its strong adsorption on the clay, which will reduce the content of that in the drilling fluid. Traditional clay and shale stabilization products have a limited application due to temperature and pH stability, strong odors, bad compatibility or lower level of inhibition, especially since they are more toxic than HETMAC in both onshore and offshore as drilling fluid additives.
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