Clay swelling is one of the major causes of formation damage in hydrocarbon reservoirs and also can cause many drilling operations problems in shaly formations. Clay swelling is controlled primarily by the compositions of aqueous solutions with which the clay comes into contact. In this paper, we introduce a method to construct clay-swelling diagrams that can be used to determine the compatibility between swelling clays and aqueous fluids.
Smectite minerals can swell in two very different ways. In concentrated brine or in a solution dominated by divalent or multivalent cations, smectites experience only a small volume increase or swelling. In these solutions, water molecules are structured in layers on the clay surface; this type is called crystalline swelling. In dilute solutions where Na+ is the dominant cation, smectites experience a large volume increase or swelling. In these solutions, an electric double layer will develop on the clay surface and cause a strong repulsion between the clay platelets; this is called osmotic swelling. Osmotic swelling is much more damaging to hydrocarbon reservoirs than crystalline swelling. If the solution compositions leading to osmotic swelling conditions can be well defined, formation damage caused by clay swelling may be avoided.
Swelling characteristics of clay minerals can be quantified by the X-ray diffraction (XRD) method. To many petroleum geologists and engineers, XRD is a method useful only for mineral identification. However, with a proper sample holder, the method can also be very useful for clay-swelling studies because clay swelling is a result of the increase in interlayer spacing in clay particles; this increase in interlayer spacing [called (001) d spacing] can be quantified by an XRD method.
As a demonstration, we used the montmorillonite/NaCl/CaCl2 system. Sodium and calcium ions are the dominant cations in most reservoir and injection fluids. Anions have little effect on clay swelling. Therefore, the system chosen here has broad application. When montmorillonite is fully saturated and experiences crystalline swelling in NaCl/CaCl2 solutions, its (001) diffraction peak is sharp and very well defined, and its d spacing is 19 A. When montmorillonite experiences osmotic swelling in the solution, its (001) diffraction peak becomes very broad and its d spacing is usually >20 A. For example, the (001) d spacing of montmorillonite is 19.2 A in 0.14 N NaCl/0.06 N CaCl2 solution, but 51 A in 0.19 N NaCl/0.01 N CaCl2 solution. Therefore, use of XRD method to quantify the effect of solution composition on the swelling behavior of montmorillonite is very convenient. On the basis of this type of data, a swelling diagram like that in Fig. 1 can be constructed. The position of each point in NaCl/CaCl2 composition space defines the solution composition of this run. The solid dots indicate that crystalline swelling is observed under the solution composition; open circles indicate that osmotic swelling is observed under the solution composition. The solid line is a visual-fit boundary between crystalline and osmotic swelling. As pointed out earlier, formation damage is most likely to occur when clay minerals experience osmotic swelling. Because it delineates a composition field in which osmotic swelling would take place, Fig. 1 can be used as a simple tool to determine the compatibility between a montmorillonite-rich formation and foreign (drilling and injection) fluids. To minimize swelling-related formation damage, fluid composition falling in the osmotic-swelling field should be avoided as much as possible.
It is important to note that the ability of Ca2+ to inhibit montmorillonite swelling depends on Na concentration in solution. At low Na concentrations, a small amount of Ca in solution would effectively limit the montmorillonite swelling. At higher Na concentration, a much larger Ca concentration is needed to control montmorillonite swelling. This is because Ca2+ is preferred on clay surface in low-ionic-strength solutions and it is the surface Ca2+ coverage that controls the smectite swelling in reservoirs.
Fig. 1 is applicable for the systems in which major cations are Na+ and Ca2+. However, a similar approach can be used for other systems. For example, swelling diagrams can be constructed for montmorillonite in NaCl/KCl solutions or in NaCl/NH4Cl solutions. These swelling diagrams are similar to that of Fig. 1 and can be used the same way. Other swelling clays may behave differently from montmorillonite, and the crystalline-/osmotic-swelling boundary may shift as a result. If a reservoir clay is expected to behave very differently from montmorillonite, the reservoir clay can be used directly for swelling tests with XRD.
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*Conversion factor is exact.