Abstract
Abstract
This paper introduces a new strategy for improving the high temperature stability of acrylamide polymer solutions and gels in aqueous solutions containing the divalent cations Ca2+ and Mg2+. Specifically, certain low molecular weight compounds capable of complexing with the divalent cations are shown to reduce significantly their negative impact on the solubility of acrylamide polymers and on the stability of crosslinked acrylamide polymer gels with regard to syneresis. Where the inhibitor-divalent ion complexes formed are soluble, it should be possible to propagate their polymer or gelant solutions through matrix rock; where insoluble divalent ion-inhibitor complexes are formed, these inhibitors may still be compatible with gel placement in fractured reservoirs or in the immediate vicinity of the wellbore. The results obtained offer the possibility to extend the upper temperature limit for the use of polyacrylamides and acrylamide copolymers in brines in both polymer flooding and polymer gel treatments.
Related studies show that pH is a key factor influencing the solubility of acrylamide polymers in the presence of divalent cations. This fundamental variable has been overlooked in earlier studies correlating PAAm solubility with the degree of polymer hydrolysis, divalent cation concentration, and temperature.
Introduction
Aqueous polymer gels command great interest at present because of their potential impact on petroleum production. Gel treatments have been widely employed on both producer wells, in order to reduce water production, and on injector wells, in order to direct the flow of injected fluids to the mobile oil-containing rock bypassed in previous water or miscible (vapor, methane or CO2) flooding operations. In the case of injector treatments, reservoir simulations have identified the importance of placing the gel a considerable distance within the reservoir in order to realize a significant increase in the ultimate oil recovery from water-flooded, unfractured reservoirs with large well-spacing.
Our program has focused on the problem of developing the chemistry necessary for carrying out polymer gel treatments on high temperature reservoirs and in particular for realizing the ambitious, in-depth placement of polymer gels in such reservoirs. One of the technical requirements that must be satisfied in order to arrive at a workable and general in-depth profile modification technology is the definition of delayed crosslinking chemistry capable of providing gelation times of the order of several weeks or more under reservoir conditions. In previous reports we have described the discovery and development of a powerful new Cr+3-based delayed gelation chemistry that largely satisfies the delayed gelation requirement to temperatures of 100C or more, while avoiding the negative environmental aspects associated with earlier technologies.
The large volumes of gelant solution required for in-depth profile modification treatments and the long pay-out times also place strict requirements on the stability of the gel under reservoir conditions. Harsh reservoir conditions (high temperature and salinity) in particular pose a demanding technical challenge. The growing interest in extending gel technology to deeper, hotter reservoirs has placed continuous demands on the existing polymer gel technology over the past decade. The chemical industry has responded to this requirement by pursuing the development of (crosslinkable) polymers possessing greater resistance to harsh reservoir conditions. Several aspects of the stability of the currently available polymers are reviewed briefly in the second part of the Introduction.
In this report we describe a new, simple, and potentially cost-effective approach, based on the use of divalent cation inhibitors, for extending the stability of polymer solutions and gels to high temperature and high salinity. The inhibitors may be used advantageously with both polyacrylamide (PAAm) and acrylamide copolymers, and hence their use is complementary with respect to earlier technological developments. The results have implications not only for in-depth gel treatments, but for any field application of polymer solutions or gels where long-term stability is required. Finally, some new experimental results on the solubility of hydrolyzed PAAm in synthetic seawater (SSW) are also described.
Stability of polymer solutions and gels: Background. Polyacrylamide and Xanthan gum have been widely employed in laboratory studies and field applications of polymer gels. While their aqueous gels are stable at low-to-moderate temperatures, neither of these polymers is suitable for use under harsh reservoir conditions.
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