Summary

A new clay stabilization system has been developed which prevents hydration of swellable clays without impairing permeability. The system uses a polymer that can be mixed in the HCl overflush of a typical mud acid treatment without loss of activity. Case histories proved the system to be very effective in controlling swelling clay problems.

Introduction

One major problem in producing wells is migration of fines which can reduce the productivity of a formation by the Jamin effect-migrating fines trapped in pore throats of the formation resulting in a drastic decrease in permeability. The source of fines that usually cause this can be divided into two classes:

  1. migrating clays and

  2. swelling clays.

Migrating clays such as kaolinite are not strongly cemented in the formation and are dispersed easily by treating fluids such as acids, brines, and gravel packing fluids. They then move toward the wellbore when the well is put on production, and accumulate in or near the perforations as the produced fluid converges into the wellbore. Hydratable clays such as montmorillonite cause problems by exchanging loosely bound cations (e.g., sodium and potassium) for water. This causes the clays to swell, which decreases the permeability of the formation. Another possibility is that the clays break loose and become migrating fines.Permanent methods of stabilizing clays generally involve use of multisited cationic chemicals. Polymers with cationic functional groups which are adsorbed on clay platelets have proved to be one of the most permanent means of stabilizing clays. Examples are partially hydrolyzed polyacrylamides, flaxmeal, and Halliburton Services' CLA-STA. Polymeric agents are advantageous in that they offer a permanent protection as a result of multiple sites of attachment. Release of the adsorbed polymer requires simultaneous release of numerous sites, and substitution of another cation at each site. Polymers are mixed easily with other treating fluids and are effective clay inhibitors over a wide range of conditions.The findings presented in this paper show the parameters that affect the clay stabilization ability of certain polymers. Maintaining permeability after treatment with polymers for clay stabilization has been found to be a function of concentration, temperature, and sequence of application. Production data has shown that proper use of a polymeric clay stabilizer affords permanent protection against clay swelling and subsequent migration which may result in drastic decreases in production.

Clay Chemistry

Sedimentary deposits of erosional products are the source of clays. They are apparently unchanged during the transport process and exhibit the weathering that produced them. During the weathering process, clays are degraded by the leaching of ionic potassium, magnesium, iron, aluminum, and silicon. As the weathered clays are deposited, the ions may re-enter the crystal lattice reconstituting the original minerals, sometimes with accompanied crystal growth. The same ion is not always replaced during the process.

JPT

P. 1211^

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