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]. 

Starch is also used for fluid loss control. It does not provide carrying capacity therefore other polymers are required. Although starch is relatively cheap, it has two serious limitations (1) starch is subject to fermentation, and (2) it causes significant permeability reduction due to plugging. 

Polyacrylates are often added to drilling fluids to increase viscosity and limit formation damage. The filter-cake is critical in preventing reservoir invasion by mud filtrate. Polymer invasion of the reservoir has been shown to have a great impact on permeability reduction [98]. The invasion of filtrate and solids in drilling in fluid can cause serious reservoir damage. 

E. Souto, B. Bazin, and M. Sardin. Ion exchange between hydrogen and homoionic brines related to permeability reduction. In Proceedings Volume, pages 491-500. SPE Oilfield Chem Int Symp (New Orleans, LA, 3/2-3Z5), 1993. 

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. 

Ghazali, H.A. Willhite, G.P. "Permeability Modification Using Aluminum Citrate/Polymer Treatments Mechanisms of Permeability Reduction in Sandpacks," SPE Paper 13583, International Symposium on Oilfield and Geothermal Chemistry, Phoenix, April 9-11. 

The results, shown in Table II, quantify the adverse effect of common well fluid ions on permeability. Especially relevant are the data indicating that even calcium chloride/calcium bromide fluids cause some permeability reduction, although the cause of the adverse interaction is obscure. 

Even very small amounts of calcium provide a desirable decrease in the Na/Ca ratio. Prior studies indicating potassium chloride totally negates permeability reduction may have utilized water that contained some small amount of calcium ion to measure KC1 solution permeability. A second factor, which might explain the lack of KC1 damage reported in prior studies is a low ionic concentration, especially calcium, in the water used to equilibrate the cores prior to the KC1 tests. 

Hawkins, G.W. "Laboratory Study of Proppant-Pack Permeability Reduction Caused by Fracturing Fluids Concentrated During Closure," SPE paper 18261, 1988 Annual Technical Conference and Exhibition, Houston, Oct. 2-5. 

Mungan, N. "Permeability Reduction Through Changes in pH and Salinity," J. Pet. Technol.. December 1965, 1449-53. 

Although the permeability reduction, which is defined as the ratio of flow rates before and after adsorption, was strongly affected by the nature of porous media, its dependence on the shear rate was similar for both polymers. A clear and reversible change of LH... 

Figure 4.7 Correlation between O2, N2 and CH4 permeability reduction rates and their volumetric relaxation rates for thin films of various glassy polymers [49], Reproduced with permission of Elsevier.

Figure 2. Permeability reduction because of the presence of non-communicating shales. When the aspect ratio (Az/Ax) is small, as in the case of vertical permeability, the reduction is pronounced. When it is large, as for horizontal permeability, the reduction is small. Both extremes can be dealt with analytically. (Reproduced from Ref. 2.)...

At the surfactant concentrations employed in the core floodiil, there was no evidence of severe permeability reduction that would cause substantially reduced injectivity. 

Review of the literature resulted in several references relating to the use of emulsions as agents for causing permeability reduction. McAuliffe (2) demonstrated that injection of externally produced oil-in-water emulsions at 24 C effectively reduces the water permeabilities of sandstone cores. These laboratory findings were later substantiated by a field test of emulsion injection followed by waterflooding in the Midway-Sunset Field (3). 

Finally, permeability reductions attributed in the literature to the formation of water-in-oil emulsions are evidently caused by the high viscosity of those emulsions or to the formation of an oil film (lamella) across the pore throat (9). 

Figures 8, 9, and 10 show the permeability reductions that resulted from injecting 0.5% emulsions produced from the Wilmington crude oil and caustic. The temperatures are, respectively, 25°, 90°, and 110°C. The reductions in permeability were from 84 to 88%, with most of the reduction occurring within 1 PV of emulsion injection.

In Situ Emulsification. Coreflood experiments designed to cause permeability reductions by in situ creation of oil-in-water emulsions have been less successful than injection of externally produced emulsions, but still show significant reductions in permeability. The data are summarized in Table IX. 

The injection of emulsifier caustic into the Kern River sandpack (test 4) caused a significant permeability reduction, but not until injection of 1 PV of water. The stability of this "block" to steam was not tested. The photograph of the sandpack in Figure 11 reveals that brown, oil-in-water emulsions were formed in situ during the experiment. 

In-depth dispersion placement and permeability reduction was achieved in the immiscible-C02 diversion test(4). In the Siggins Field test, the severe channeling of gas to a nearby producer was blocked(2). 

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