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Colloidally-Induced Fines Release in Porous Media

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Colloidally-Induced Fines Release in Porous Media Journal of Petroleum Science and Engineering, 4 (1990) 213-221 213 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands Colloidally-induced fines release in porous media Kartic C. Khilar% R.N. Vaidya2 and H.S. Fogler2'3 1Department of Chemical Engineering, Indian Institute of Technology, Bombay Powai, Bombay, 400 076 (India) 2Department of Chemical Engineering, The University of Michigan, Ann Arbor, Michigan 48109-2136 (U.S.A.) (Accepted after revision June 9, 1989) ABSTRACT Khilar, K.C., Vaidya, R.N. and Fogler, H.S., 1990. Colloidally-induced fines release in porous media. J. Pet. Sci. Eng., 4: 213-221. A critical value of the total ionic strength (CTIS) has been found to exist for mixed salt solutions flowing in porous media. If the ionic strength drops below this value, significant amounts of fines are released in-situ due to colloidal forces, causing drastic formation damage. For a NaCI/CaC12 system, the CTIS is strongly dependent on the relative amount of CaC12 present in the solution. The concept of a critical salt concentration (CSC) and the analysis based on DLVO theory has been extended to mixed salt systems to estimate the CTIS. The difference between critical flocculation concentration (CFC), and the present definition of CTIS has been pointed out. Predictions of this analysis are consistent with experi- mental observations. Introduction Schofield, 1955; Rowell et al., 1969; Hardcas- tle and Mitchell, 1974; Kolakowski and Mati- Colloidally-induced release and migration in jevic, 1979; Khilar and Fogler, 1984). The lat- porous media is important in numerous pro- est of these studies has addressed the concept cesses such as enhanced oil recovery, migra- of critical salt concentration in detail and an tion of organic waste from landfills, regenera- experimental method has also been proposed tion of filter beds, and erosion of earthen to determine CSC for fines migration in Berea embankments (Khilar and Fogler, 1984, 1987; sandstone (Khilar and Fogler, 1984). The mi- Khilar et al., 1985). The phenomenon of col- grating fines in Berea sandstone were found loidally-induced fines migration is of signifi- predominantly to be kaolinite clay. In addi- cant importance in the petroleum industry as tion, an analysis has been formulated deline- these released fines can migrate and plug re- ating the effects of flowrate, pH, temperature gions in the formation causing damage and and, charge and size of cation on CSC in a sin- hence reduction in production. A number of gle salt system. This analysis is based on DLVO studies using single salt systems (NaC1, KC1, theory of stability of lyphobic colloids and the etc. ) have conclusively shown that colloidally- CSC is analogous to the critical flocculation induced fines release is a threshold type of pro- concentration (CFC) in colloid sol stability cess that can occur when the salt concentra- (Hiemenz, 1986). tion of the flowing solution decreases below a Studies have been reported on colloidally- threshold (or a critical) value known as criti- induced fines migration with a mixed salt sys- cal salt concentration (CSC) (Quirk and tem of NaC1/CaC12 (Quirk and Schofield, 1955; Jones, 1964; Kia et al., 1987b). These 3Author to whom all correspondence should be addressed. studies primarily emphasize the importance of 0920-4105/90/$03.50 © 1990 Elsevier Science Publishers B.V. 214 K.C. KH1LAR, R.N. VAIDYA AND H.S. FOGDzR Nomenclature a radius of sphere Subscripts A Hamaker constant i,j species, (1,2..) e electronic charge LVA London-Van der Waals F force BR Born repulsion h distance of separation DLR Double layer I total ionic strength HR hydrodynamic k Boltzman constant T total n bulk concentration of ions 7" temperature Acronyms I potential energy CFC Critical Flocculation Concentration y dimensionless potential (Eq. 5c) CSC Critical Salt Concentration _- valency of the ion CTIS Critical Total Ionic Strength DLVO Deryaguin-Landau-Verwey-Overbeek Greek symbols permitivity of medium x Debye Huckel parameter (Eq. 5b) cr Born repulsion parameter zeta potential T surface potential (all in consistent C.G.S. units) the presence of a minimum amount of Ca 2+ cium ions is greater than a critical value of ions in solution or on the surface of the fines 75%, then the zeta potential is sufficiently re- (clay) to prevent their release and migration. duced for the prevention of colloidally-in- Quirk and Schofield (1955) have observed duced fines release. The surface coverage of that soil permeability decreases due to the clay calcium ions depends strongly on the total swelling* and dispersion when the concentra- ionic strength as well as on the calcium ionic tion of Ca 2+ ions in the mixed salt solution de- fraction or percentage in the solution. Conse- creases below a threshold value of 2.5 × 10 -4 quently, the threshold value of the concentra- M. Their data showed that this threshold value tion of Ca 2+ to prevent fines release must de- is independent of the concentration of Na + in pend on the mole percentage of calcium in the the solution; which is rather unconvincing. solution. Experimental work concerning a crit- Recent work of Kia et al. (1987b) focuses on ical total ionic strength (CTIS) of a mixed salt the amount of Ca 2+ ion on the surface rather system, the percentage of calcium in solution than that in the bulk solution. This is a more and its relationship with the phenomenon of reasonable approach, because the amount of colloidally-induced fines migration has not Ca 2+ ions on the surface of the fines deter- been reported, nor has an analysis been put mines the zeta potential of the fines which in forth that can be used to estimate the CTIS. turn controls the release process. Kia et al. have This information is useful in many chemical, shown that if the solution composition is ad- environmental and petroleum engineering op- justed so that coverage on the surface by cal- erations involving flow of mixed salt solutions through a porous medium. *Reduction in permeability due to clay swelling occurs Continuing our earlier work on the CSC because of reduction in flow cross-sectional area due to (Khilar and Fogler, 1984) we show here that volume changes of the clay. In fines migration, the clay particles detach from the pore walls and migrate with the there exists a critical total ionic strength fluid until they are captured at pore constrictions, thereby (CTIS), below which colloidally-induced fines reducing the flow cross-sectional area. release may occur in Berea sandstone. We have COLLOIDALLY-INDUCED FINES RELEASE IN POROUS MEDIA 215 Berea Sandstone, temperature = 300 K expanded our earlier analysis of the CSC to Ko = I 0 roD, Calcium percentage = 0 254 cm Diameter, 254 cm length, velocity = 19 7 crn/hr mixed salt systems consisting of symmetric and 1.6 unsymmetric electrolytes. In a NaC1/CaCI2 system, the CTIS strongly depends on the rel- O,e ative amount of CaC12 present in the solution o0.e (calcium molar percentage, %Ca). For exam- ple in a 5% calcium and 95% sodium solution, 0.4 Q- tx I~ a tx the CTIS is 0.025 M while it decreases signifi- °Q tOIo-lg- o. I° ° 8- ot: Io.°o o. o.O., cantly to 0.005 M for a 10% calcium and 90% ~)Z~ Z I i ~rz [ I I I I I I sodium solution. As we will soon see the pre- 0 40 80 120 160 200 240 280 320 360 400 dictions using this analysis agree well with the Pore Volumes experimental data. Fig. 2. Measurementof critical ionic strength from rela- tive permeability (Khilar, 1981 ). Measurement of the critical total ionic strength (CTIS) permeability (K/Ko). A typical measurement is shown in Fig. 2 (Khilar, 1981). The value A schematic of the experimental system is of the CSC for different cations, such as Li+, shown in Fig. 1. It consists of a positive dis- K +, NH~-, Cs ÷, have been reported by Khilar placement pump for fluid injection, a Hassler (1981). In our present study, the concentra- cell in which the core is contained and, pres- tions of the ions (Ca :÷ and Na ÷ ) were mea- sure and concentration measuring devices. For sured using specific ion electrodes. The partic- a constant injection rate, the pressure drop ulate content of the effluent was analyzed using across the core was monitored continuously as an Atomic Absorption Spectrometer (AAS)* a function of the ionic strength of the aqueous and a Scanning Electron Microscope (SEM). solution flowing through the core. All the Be- The ionic strength at which the pressure drop rea sandstone samples used for this study were across the core begins to increase and the clay from the same block, having a initial permea- particles appear in the effluent determines the bility of approximately 10 roD. The cores were critical concentration. A similar experimental not fired nor acidized prior to the experi- procedure was used in this work. ments. The Berea samples had a clay content In a typical experiment, a Berea sandstone of approximately 8 wt%, and 75% of which is core (2.54 cm both in diameter and length) kaolinite clay. was vacuum saturated with a NaC1/CaC12 salt The ionic strength of the solution is de- solution of 0.10 M ionic strength (I) and at a creased in a stepwise manner, and this tech- particular mole percent of calcium. The satu- nique has been used successfully (Khilar, 1981; rated Berea sandstone was placed in a core Khilar and Fogler, 1984) to determine the CSC holder and a NaCI/CaC12 salt solution was of various single salt systems. The critical ionic flown through the core at a superficial velocity strength is defined as the ionic strength at of 19.2 cm h- ] ( 15.1 ft day- 1).
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