Completion Fluid Invasion Simulation and Permeability Restoration by Sodium- and Potassium-Based Brines

Abstract

A laboratory study was performed to simulate completion fluid invasions into large Berea and Casper sandstone cores. Permeability was initially stabilized with a 4% NaCl reference brine, then flow direction was reversed through each core with sodium and potassium completion brines. After a stabilized permeability was obtained under invasion permeability was obtained under invasion conditions, the reference brine was flowed again through the cores to simulate production after completion. In Casper sandstone the permeability stabilized 5% higher than the originally established level for the KCl completion fluid, while it only increased 2% with NaCl brine. No increase in permeability was obtained in Berea cores with either KCl or NaCl completion fluids. Experiments were also performed to determine the effects of completion fluid and flow reversal on stabilizing the permeability of a formation with clay dispersion damage. Results of these studies indicate that for each sandstone a threshold KCl brine saturation exists (10% in Berea and 5% in Casper) above which the mite clay lattices collapse around the exchanged K+. Once a formation is exposed to a KCl brine saturation higher than the threshold saturation excessive clay dispersion and swelling damage of the rock is inhibited even after the formation is exposed to much lower salinity levels. Introduction Absolute permeability reduction of reservoir sand in the vicinity of a wellbore can be a result of pore blockage by formation fines or clay swelling and dispersion caused by the introduction of an incompatible non-native fluid. Because the composition duplication of the formation water is difficult and costly, it is rare that the fluids introduced during all stages of well development and workover operations match the composition of formation water. Consequently, an incompatible fluid can choke the formation permeability and impair productivity or injectivity. Previous core flooding experiments were Previous core flooding experiments were focused on the effect of singular brine concentrations a series of low brine concentrations or high density calcium and zinc bromide brines. Earlier, the authors presented permeability changes with brine presented permeability changes with brine concentrations ranging from zero to 22% by weight for NaCl and KCl. The effects of isomorphic substitution of cations on the exchange sites of clays and the tendency of clay lattices to attract potassium ions rather than sodium ions were discussed. In both Berea and Casper sandstones, after the initial Ca+ and Mg+ ions are substituted on the exchange sites of clays by Na, the permeability to NaCl brine was controlled by permeability to NaCl brine was controlled by osmotic forces and reversible clay swelling. As the NaCl brine concentration was reduced below the critical salt saturation clay dispersion and migration caused total loss of permeability in all the cores. This paper permeability in all the cores. This paper presents further laboratory experiments to presents further laboratory experiments to establish the advantages of KCl instead of NaCl as a completion fluid. EXPERIMENTAL APPARATUS Berea and Casper sandstones were selected for testing. Berea was used as a reference sandstone because of its homogeneity and low clay and carbonate contents. Casper sandstone in contrast has secondary dolomite grains intermixed with authigenic clays. P. 237