Langmuir 1994,10, 1544-1549
1544
Effect of Clay/Polymer Interactions on Shale Stabilization during Drilling L. Bailey* and M. Keallt Schlumberger Cambridge Research, High Cross, Madingley Road, Cambridge, U.K.
A. Audibert and J. Lecourtier Institut FranCais du Pgtrole, 1-4, avenue de Bois-PrCau, Rueil-Malmaison, France Received June 15, 1993. I n Final Form: January 24, 1994@
Adsorption measurements were used to investigate the mechanisms of shale stabilization by watersoluble polymers, which are commonly added to drilling fluids to control reactive shale formations. High molecular weight (107)polyacrylamidesof varying ionicitywere adsorbed on well-characterizedK-smectite, from solutionsof varying electrolyteconcentration. Polymer adsorption was found to be stronglydependent on the ionicity of the polymer and on the ionic strength of the medium, due to charge screening effects and the salt-dependent energy of tactoid assembly. The interaction of these polymers with a standard shale rock was investigated by conventional oilfield cuttings rolling tests and in an apparatus designed to simulatewellbore conditionsof pressureand flow rates. The results show that under conditionspromoting adsorption of polymers, disintegration and dispersion are inhibited, although the near wellbore material still imbibes water and ions. The stability of the wellbore is also dependent on the nature of the polymerclay interaction; the strongly bound cationic polymer is able to limit clay swelling. 1. Introduction Drilling through reactive shale formations requires the use of a stabilizing drilling fluid to prevent degradation of the wellbore wall and to minimize disintegration and dispersion of drilled cuttings during their removal from the borehole. Inhibitive polymer-electrolyte systems are commonly used;' their stabilizing action depends on polymer interaction with the clay minerals, although the mechanisms are not well understood. In the present work we attempt to use the understanding of polymer-clay interactions at the colloidal level to explain the inhibitive effects exploited in drilling fluids. In this study we measured the adsorption of acrylamide copolymers on a well-characterized model clay, over a range of electrolyte concentrations, and applied the findings to behavior observed with a shale under simulated wellbore conditions. Two types of experiment were carried out on polymer interaction with shale, which were designed to probe the different functions of the polymer. Firstly, a cuttings rolling test was used to investigate the effects of the polymers on drilled solids. Secondly the effect on the shale wellbore wall was investigated using a specially built apparatus which simulates wellbore conditions of geometry, pressure, and flow rates. 2. Adsorption on Homoionic Clays Polymers. We have chosen to look a t a family of three acrylamide copolymers which show a range of interactions with mineral oxides;2-12polymer A, a partially hydrolyzed
* Author to whom correspondence should be addressed.
+ Now a t Stanger Consultants, Ltd., Salford, U.K.
*Abstract published in Advance A C S Abstracts, April 1, 1994. (1)Clark, R. K.; Scheuerman, R. F.; Rath, H.; Van Laar, H. G. J. Pet. Technol. 1976, June, 719. (2) Lee, L. T.; Rahbari, R.; Lecourtier, J.; Chauveteau, G. J . Colloid Interface Sci. 1991, 147, 351. (3) Audibert, A.; Bailey, L.; Hall, P. L.; Keall, M.; Lecourtier, J. In Physical Chemistry of Colloids andlnterfaces in OilProduction, Touloat, H., Lecourtier, J., Eds.; Editions Technip: Paris, 1992. (4)Bottero, J. Y.; Bruant, M.; Cases, J. M.; Canet, D.; Fiessinger, F. J. Colloid Interface Sci. 1988, 124, 515. (5) Lee, L. T.; Somasundaran, P. J. Colloid Interface Sci. 1991, 142, 470.
0743-7463/94/2410-1544$04.50/0
polyacrylamide, with a degree of hydrolysis of 27 % ,which is typical of the inhibitive polymers used in current drilling fluids, polymer N, a near neutral polyacrylamide (IwmrV
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Figure 8. Water invasion profiles of shale samples exposed to
polyacrylamide copolymer solutions (with no added salt). The dash dotted line parallel to the y ordinate indicates the original position of the wellbore.
Paiymr A
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Figure 7. Wellbore evolution for shale samples exposed to polyacrylamide copolymer solutions in 5% KC1. Each pair of matched lines represents the mean diameter *1 standard
deviation.
tests), we have plotted the mean diameter fl standard deviation, averaged for both perpendicular diameters, over the full length of the sample. Thus the width of the band indicates the roughness of the wellbore, and movement of the mean from the initial position either swellingor erosion. As Figure 5 shows, the shale reacts strongly to the freshwater fluid, first swelling then as the swollen material becomes progressively weaker, it is eroded by the flowing mud. The light pressure of the caliper also removes weakened material. Addition of 5% KC1 to the test fluid only reduces the erosion to a limited extent; considerably higher KC1 concentrations are required to stabilize the shale completely.'g Figures 6 and 7 show the wellbore evolution during exposure to the polymers in low and high ionic strength electrolyte. We will consider the low ionic strength case first. The results for polymer A show a great deal of roughening and erosion of weakened material as the test proceeds, the wellbore is only slightly better than that in the freshwater control. By contrast, with polymer N a
strong swelling response is seen, but little erosion. The main difference between the polymers under these conditions is the amount adsorbed; only a small amount of polymer A adsorbs because of the divalent ions on the shale, but polymer N can adsorb strongly. The adsorbed polymer is able to give some mechanical integrity to the altered shale and stabilize it against erosion but is unable to prevent the osmotically driven swelling. There is still some erosion and roughening of the wellbore, as we saw with the cuttings tests; the weakened, but stabilized,shale can be mechanically degraded. The polymer C test shows some wellbore evolution, but not as much erosion as with polymer A nor the strong swelling observed with the polymer N. If we continue with the argument that the polymers do not influence the swelling process, but the amount of polymer adsorbed regulates the resistance of the swollen shale to erosion, we would expect polymer C to give as swollen a wellbore as polymer N, if not even more so, as polymer C adsorbs to much greater levels in low salinity. Clearly this is not the case, and we may deduce that polymer C is constraining the swelling process in some way, possibly by charge neutralization. After the tests, the cores were cut in half, axially, and water invasion profiles measured. Water contents are determined by weight loss on drying at 105 "C. The water invasion profiles, shown in Figure 8, are consistent with the wellbore diameter evolution. Near to the wellbore there is a zone of elevated water content for all the polymers, with both the uneroded, highly swollen zone for polymer N and the lower alteration with polymer C apparent. When we move to high ionic strength, differences in the adsorption of the polymers disappear. Looking at the wellbore evolution, Figure 7, there is very little alteration or difference between the polymers. However, the water invasion, Figure 9, still shows an altered zone, similar to that in the low ionic strength conditions. Increasing the ionic strength has reduced the electrostatic repulsion restricting the adsorption of polymer A and, as we can see, there is now no difference between polymer A and polymer N. Polymer C, by contrast, is still markedly different, with a much reduced altered zone; the adsorption interaction of the polymer C is very different to the other polymers despite the similar levels achieved. We believe that this difference in altered zone can be explained by the relative mobilities of the adsorbed polymers. Many studies point to the weakness of the interaction between the acrylamide group and the mineral surface, in particular the calorimetric study of Denoyel et al.,' which contrasts it with the much stronger interaction between the cationic group and the negatively charged
Shale Stabilization by Polymers
Langmuir, Vol. 10, No. 5, 1994 1549
0.3
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Figure 9. Water invasion profiles of shale samples exposed to polyacrylamide copolymersolutionsin 5 % KCl. The dash dotted line parallel to the y ordinate indicates the original position of the wellbore.
clay. These strong interactions are believed to give a relatively immobile polymer chain, although adsorption levels can be high for polymers of low cationicity due to the number of cationic groups required for charge neutralization and the inherent flexibility of the polymer molecule. Radiolabeling studiesa*gon the reversibility of the adsorption process with the more weakly bound polymer N confirm the mobility of the adsorbed polymer. Thus, under the test conditions, adsorbed chains of polymers A and N are able to rearrange because of their weak, and dynamic, links to the clay surface through the acrylamide groups, and can accommodate the expansion of the shale required by the imbalance in water chemical potential. Polymer C, however, has a much less mobile adsorbed layer due to the "frozen-in" configuration imposed by the stronger interaction of cationic groups with the charge sites and is unable to rearrange to the same degree, imposing a constraint on the swelling/failure process. Some swelling occurs because only a fraction of the bound segments are the cationic groups, charge neutralization is not complete, and rearrangement through the acrylamide/clay interactions is possible. Thus in contrast to the situation with low electrolyte levels where
the amount adsorbed dictated the stability of the shale, details of the interaction chemistry are the controlling factor. 4. Conclusions We find that the adsorption of high molecular weight acrylamide copolymers on K montmorillonite is strongly dependent on electrolyte concentration. A t high ionic strength, where the tactoids are strongly associated and differences in polymer charge are screened out, there is very little difference between high molecular weight polymer C, N, or A. At low ionic strength, polymer A does not adsorb, due to electrostatic repulsion between polymer and clay surface, but both polymers C and N adsorb strongly and are able to overcome the forces holding the tactoids together, increasing the available surface area by a factor of 2-3. In addition, the unscreened electrostatic attractions between polymer C and clay increase the adsorption still further in the low ionic strength environment. The adsorption behavior of the acrylamide copolymers on montmorillonite clay is seen to have a direct bearing on the inhibitive effects of these polymers on reactive shale in a wellbore environment. High adsorption levels help stabilize swollen and weakened material against erosion, as demonstrated by all the polymers in high salinity environments, where electrostatic interactions between polymer and clay are screened out. However, a strong interaction, which immobilizes the adsorbed chains, is required to minimize the swelling response. Of the polymers studied, only the cationic polymer C is able to Mill both these requirements. The high molecularweight of these polymers is probably a key factor in their stabilization of shale, and work is in progress to determine its influence. Acknowledgment. The authors thank Garry Goldsmith at SCR for his efforts in running the SWBS,Lionel Rousseau at IFP for his help with the adsorption measurements, and also various colleagues at SCR and IFP for useful discussions during the course of this work.
