The Mud Damage Problem
Keelan and Koepf (1977) explain that drilling muds contain solid particles that form a filter cake over the wellbore wall, the filter cake restricts the mud flow into the near well bore formation, but some filtrate and fine particle invasion are unavoidable and usually occurs. The filtrate may react with the resident formation clays causing clay swelling, mobilization, and migration. The released particles and the fine particles carried into the formation by the filtrate can plug the pores and reduce permeability of the formation. The water-based filtrates increase the irreducible water saturation and create water block and hydrocarbon permeability reduction.
Evaluation of Drilling Muds—Damage Potential and Removal
Keelan and Koepf (1977) explain that "This test indicates impairment of productivity by clay hydration and movement of fines into the formation during the drilling operation, and any benefit of the fines' removal when the well flow in a reverse direction into the wellbore." The core plug is saturated with the brine to be tested and may or may not contain irreducible, immobile oil. Hence, the water-block effect is eliminated because the water saturation is constant. During these tests, the filtrate volume or rate versus the filtration time is measured until mudoff. If the experimental design permits, the filter cake properties, such as porosity, permeability, and thickness, and the effluent fines and liquid volumes should also be measured. The pressure difference applied to the core plug should be determined by scaling from the planned drilling over balance pressure (Keelan and Koepf, 1977).
Keelan and Koepf (1977) delineates the test sequence with the presence of mobile hydrocarbons and shows the equations used to determine the magnitude of formation damage or remediation. They explain that this test indicates "the water-block potential of a formation." In this test, the water saturation varies and is calculated by measuring the effluent filtrate volume. Any permeability reduction remaining, after the production of all the injected, extraneous filtrate water, is attributed to clay hydration and/or mud-solids invasion (Keelan and Koepf, 1977).
Keelan and Koepf (1977) depicts the results of the evaluation tests of two muds, referred to as Muds A and B. Mud A causes more damage than Mud B. In the case of Mud A, the return permeability is only 6% of the initial permeability, while it is 54% for Mud B. Keelan and Koepf (1977) conducted evaluation tests for two different drilling mud fluids, specially prepared for stabilizing the formation to
avoid formation damage during water flooding. Keelan and Koepf (1977) used fresh cores containing irreducible oil. They recommend running tests with the presence of irreducible oil because they explain that "The presence of residual oil, or associated organic compounds, sometimes protects clay surfaces, making them less sensitive to alteration when contacted by incompatible brines." They injected coarsely filtered mud
filtrates (thus containing fine particles) into core plugs and measured the permeability impairment. Keelan and Koepf (1977) presents the results of injecting formation brine, filtrate, and injection brine samples into the core plugs. As can be seen, the effective permeability is 30% higher for KC1 mud filtrate compared to that of lignosulfonate mud filtrate.
Keelan and Koepf (1977) summarized their interpretation of the acid treatment results as following:
1. Similar permeability reduction to each filtrate was noted in these test cores.
2. In the cores contacted with KC1 filtrate, HF acid yielded 136% higher permeability to injection brine than did the regular mud acid, and resulted in a net permeability enhancement above initial. The regular mud acid was not effective, and final permeability to injection brine was no higher than when the acid wash was not used.
3. In the lignosulfonate-contacted cores, the regular mud acid and the HF acid were equally effective, and each yielded a permeability greater than the original.
4. In summary, either mud was suitable if HF acid was used for remedial treatment. If the regular mud acid was to be used, the most suitable drilling mud would be lignosulfonate.
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