Potassium-based muds are used in areas where inhibition is required to limit chemical alteration of shales. Potassium performance is based on cationic exchange of potassium for sodium or calcium ions on smectites and interlayered clays. The potassium ion compared to calcium ion or other inhibi-tive ions, fits more closely into the clay lattice structure, thereby greatly reducing hydration of clays. Potassium-based muds perform best on shales containing large quantities of smectite or interlayered clays in the total clay fraction. Shallow shales, containing large amounts of montmorillonite, however, still swell in a potassium-based system. The benefits may not justify the cost of using a potassium-based mud in this type of environment.
Potassium interactions with clay surfaces can be traced to two effects: ionic size and hydrational energy. Potassium ions are of the proper size to fit snugly into the spaces between the two silica tetra-hedral layers which contact each other in the formation of a three-layer clay packet. The ionic diameter of potassium is 2.66 A, very close to the available distance of 2.8 A in the lattice space of the clay structure. A cation slightly smaller than 2.8 A is desirable to allow for crystalline compaction.
Stabilization of problem shales by potassium ions appears to take place in the following manner. When montmorillonite is present, potassium exchanges for sodium and calcium and results in a more stable, less hydratable structure. When illites are present, the potassium replaces any exchangeable cation impurities in the structure. The potential for further base exchange is substantially reduced after K+ substitution, and the shale is more stable. In mixed-layered clays, potassium works both on the illite and the montmorillonite and reduces the amount of differential swelling that occurs. Therefore, potassium cations stabilize shales which have a larger percentage of illite or illite/smectite layer combinations.
Several general statements can be made regarding whether a potassium-based mud should be used on a particular shale. These generalizations are based on the results of laboratory and field tests. The potassium cation performs best on shales containing large quantities of illite or inter-layered clays in the total clay fraction. This is true as long as the shale is not an extremely hard, brittle variety with a matrix containing numerous microfractures. In these cases, a small percentage of the total swelling potential may be sufficient to jeopardize the wellbore. Intrusion of fluid along the microfractures helps to accelerate swelling. Even an 80% reduction in hydration may not be enough to stabilize the formation. However, these shales have been drilled successfully with potassium-based muds containing asphaltites and asphaltenes. Potassium-based muds have also been used to drill very hard illitic shales. Ideally, shales of this type should be tested before a definite recommendation is made.
Shales that contain large amounts of montmorillonite will still swell, to some extent, in a potassium-based mud system. The degree of inhibition required for these shales may not be sufficient to justify the cost of using a potassium-based system, particularly since most shales of this type appear at shallow depths (gumbo). Extremely large potassium treatments are required for the base exchange to occur, particularly in large holes with a high penetration rate. Again, testing of the particular shale should be done to decide whether the degree of inhibition justifies the costs. If cuttings dispersion, rather than hole erosion, is the main concern, then a potassium-based mud system may reduce the problem significantly.
The benefit of laboratory testing before using a potassium-based inhibitive fluid in a problem shale zone cannot be overemphasized. If core material is available from an offset well, an entire series of laboratory tests, including x-ray analysis, absorption isotherm, swelling, and dispersion studies should be performed. If cores are not available, then dispersion tests on cuttings from a previous well in the area can be used to obtain information. Without samples of any kind, an estimate as to the type of shale to be encountered must be based on the depth of burial, geologic correlation, and available logging data.
Base exchange reactions with cations in the shale, cuttings, and wellbore (also with cations on the surfaces of clay added to the drilling fluid), effectively reduce the potassium level in the mud as drilling proceeds. Since a sufficient concentration must be maintained at all times to guarantee inhibition, an excess of potassium should be maintained in the system.
The theory of ionic inhibition of each of the systems is essentially the same; however, selection of the particular system to use will depend on one or more of the following factors: operator's preference, mud densities required, types of formations to be drilled, temperatures expected, fluid loss required, rig equipment, and available solids control equipment. The importance of proper solids control also cannot be overemphasized.
The main concern when using any of these systems is that enough potassium be available for base exchange with an excess remaining in solution. If the potassium level ever falls below the required amount, the clays or shales will begin hydrating, leading to borehole instability and mud problems. If sufficient remedial treatment is not made quickly, the entire advantage of potassium-based mud systems could be lost.
KCl-Polymer (KCl-PHPA) Muds
KCl-Polymer Muds were developed to provide wellbore stability and minimize cuttings dispersion. When properly formulated, benefits such as low formation damage and high return permeability encourage their use for drilling water-sensitive formations. Potassium chloride (KCl) muds not only use a wide variety of potassium chloride concentrations from 3 to 15 wt%, but also a wide variety of types and concentrations of polymers. For KCl muds to be economical, drill solids concentrations should be low and efficient solids control practices must be used. For additional information on KCl-PHPA muds, see Appendix C.
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