Permeability Modifications by In-Situ Cations Hydrolysis

Abstract

The precipitation of inorganic compounds by cation hydrolysis could be an alternative method to modify injection and production profiles. The precipitation in porous media can be caused by a pH increase as result of fluid-rock interactions, direct neutralization with base or thermal decomposition of a basic precursor, placed as additive. This paper ascertains the effects of precipitant solutions of zinc or aluminum salts, plus urea as basic precursor, on permeability, at reservoir conditions. Flow tests were conducted in 30 cm Berea cores, at 140°C, 800 psi. Different injection schemes were tested, varying the number of precipitant solution batches, i.e. 1 and 2, and the shut-in time, i.e. 24 and 48 h. After that, fresh water was injected to determine permeability reduction, as well as the effect stability, along the core and in 3 sections of 10 cm each. The results show 98% as maximum permeability reduction located mostly in the inlet section, while in the outlet section the maximal reduction achieved was 80%. In all cases, the effect was stable for at least 10 PV of water injected. Furthermore, permeability was not restored by 0.1 N HCl solutions. Precipitation occurs at T>70°C; it takes 2–4 hours at 80°C, and 40 min. at 140°C; enough time for the placement of the solution into porous media. Scanning electron microscopy images of the treated Berea cores demonstrated different shapes of particles precipitated, depending on the cation; all of them reduce permeability by filling the porous space. The aforementioned results show that in-situ cations hydrolysis induced by basic precursor have the potential to modify injection and production profiles in the field, with some possible placement advantages. Introduction Water injection profiles and production water are important factors that require optimization during the life of the enhanced oil recovery projects. They have incidence in oil producing rate and costs for treatment-disposal of produced water. Once water breakthrough, preferential flow channels form leaving oil-saturated areas not contacted by the displacing fluid. In such cases, high permeability areas must be isolated or sealed in order to divert injection fluids towards oil bearing zones. Commonly sealing chemical technologies include gels1,2,3 and cements4,5. Nowadays, research on alternative technologies is looking for lower formulation and operational costs and placement advantages. Among these technologies is cation in-situ precipitation, subject of this paper. The precipitation of NaCl and KCl by salting-out effect has already been evaluated in porous media at reservoir conditions. Although significant permeability reduction can be achieved, the final effect is not stable to water flow; in addition, the brines used can induce corrosion6. Other approach considers precipitation of hydroxides and oxyhydroxides by cation hydrolysis, either by addition of an alkali to metal salt solutions or by reaction of these solutions with carbonate minerals7,8. Lakatos et al.9 tested an iron hydroxide-based application in injector and producer wells in Algyö field, Hungary. Treatment scheme consisted in the neutralization, inside porous media, of acid solutions of FeCl3 by sequential injection of K2CO3. Preliminary evaluation of the results pointed 60% technical success and 40% of the treatment profitable, making the technology very attractive. However, the above procedure may drive to precipitation in a relative small mixing section, yielding low efficiency treatment. On the other hand, the method based on the reservoir rock carbonate content is of limited application, and it could have competition between channel formation due to dissolution and pore plugging due to precipitation. In order to achieve full precipitation and plugging of the whole flooded section, basic precursors may be appropriate to use. These compounds hydrolyze under certain conditions of temperature, changing pH of medium and inducing precipitation by cation hydrolysis.