Clays permeability reduction

Prepared saltwater completion fluids are made of fresh surface water, with sufficient salts added to produce the proper salt concentration. Usually, the addition of 5 to 10% NaCl, 2% CaClj, or 2% KCl is considered satisfactory for clay inhibition in most formations. Sodium chloride solutions have been extensively used for many years as completion fluids these brines have densities up to 10 Ib/gal. Calcium chloride solutions may have densities up to 11.7 lb/ gal. The limitations of CaClj solutions are (1) flocculation of certain clays, causing permeability reduction, and (2) high pH (10 to 10.5) that may accelerate formation clays dispersion. In such cases, CaC12-based completion fluids should be replaced with potassium chloride solutions. Other clear brines can be formulated using various salts over wide range of densities, as shown in Figure 4-123 [28]. 

Veley, C.D. "How Hydrolyzable Metal Ions Stabilize Clays to Prevent Permeability Reduction" SPE paper 2188, 1968 SPE Annual Meeting of AIME, Houston, September 29-October 2. 

Alkaline solutions erode formation rocks and clays. In some cases, the permeability could be increased owing to the erosion. In most cases, permeability is reduced because eroded rocks and clays migrate and block pore throats. The permeability reduction in low permeabihty rocks is higher than that in high permeability rocks, as shown in Figure 13.36. The reason is that the pore throats... 

The permeability reduction may be due to the dislodging of fine particles and subsequent trapping in pore restrictions, or the swelling of clay particles, or both. 

The dispersion of nanoclays is known to have a strong role in the barrier properties. Thus, randomly exfoliated systems are expected to yield optimum barrier performance. Crystallinity does also play an important role in promoting barrier properties, however, since crystallinity was not seen to increase significantly in the systems study here, the observed permeability reduction is therefore directly ascribed to the observation of a highly dispersed morphology of the clay in the polymer. 

Soils properties are very sensitive to the type of exchangeable ions. Calcium imparts favorable physical properties to the soil, while adsorbed sodium causes clay dispersion and swelling. It is generally recognized that an exchangeable sodium percentage of 10 is sufficient to cause soil dispersion, reduction of soil permeability and impaired growth of some crop plants. On the other hand, excess salt concentration prevents the dispersive effect of adsorbed sodium. 

FIG. 13 Reduction of the relative permeability coefficient is dependent on the clay platelet aspect ratio in the system of polyimide-clay hybrid with water vapor as the permeate. Each hybrid contains 2 wt% clay. The aspect ratios for hectorite, saponite, montmorillonite, and synthetic mica are 46, 165, 218, and 1230, respectively. (From Ref. 71.)... 

Clay minerals, inevitably present in a petroleum formation, are sensitive to the type and concentration of ions contained in the well fluid filtrate lost to the reservoir. This sensitivity is demonstrated by a reduction in the permeability caused by the well fluid filtrate flowing through the core under investigation. 

Confirmatory soil sampling was subsequently performed. Three soil borings were drilled and samples retrieved from the impacted area. Soil from a depth of 30 ft and below were reported as nondetectable however, samples from a depth of 21 ft still contained significant gasoline components with TPH-gasoline ranging up to 3600 ppm. This zone of elevated hydrocarbons was anticipated due to the presence of clays and silt at this depth. Ventilation of these low-permeability soils was not deemed cost-effective, and significant reduction of the residual hydrocarbon concentrations unlikely. 

The general observation for all three rock types is decreasing bulk flow with increasing Oj lo ratio, and some effect of the fracture shear displacement on the bulk flow. For the clay containing YBS, bulk flow is actually reduced by 90% for o la = 1.3. The strongest rock (YBS) shows the most pronounced cross-flow reduction, whereas the weakest rock (LC) shows the least reduction. The explanation for the limited cross-flow reduction for the LC sample lies in the low matrix permeability in combination with its special mineralogical composition (>90% carbonate), which reduces the effect of a low permeability gouge layer on the bulk flow. 

To the contrary, if a layer of clay is present- in the sequence which is sheared, then continuation of the displacement is facilitated by the intrinsic weakness of the material in the shear zone. A distinction between shear in a homogeneous sandstone in which localisation is dependent on progressive reduction of stress within the shear zone, and shearing in heterogeneous sandstone localised by intrinsic material weakness, is theoretically predictable. The theory is found to be consistent with measurements of the permeability of shear bands. These measurements reveal a range of permeabilities which closely approach but rarely reach a value corresponding to the presence of a clay fraction. 

As a result, the permeabilities in these domains within the formation become more uniform. Reduction in permeability in the more permeable domains improves the mobility ratio of waterflood. Premature breakthrough is thus reduced, and the efficiency of the waterflood is improved (Boston et al., 1969). Poorly cemented clay particles, such as kaolinite and illite, can become detached during aqueous flow, especially when flowing brines become fresher. 

Well plugging. Reduction of the permeability of the formation or well screens can be caused by precipitation, clay swelling as a result of dilution, or mobilization of fine particles. 

Up to now two-phase two-component flow under non-isothermal conditions and coupled THM (one-phase flow) have been implemented in this code and validated against different experimental results. For the modelling of water penetration into unsaturated bentonite or clay, a swelling model is available in the code. If the test sample is confined within a constant volume, then a swelling pressure will build up which causes changes to the pore structure and reduces the porosity. A small change in porosity can, however, create a considerable reduction in permeability. 

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