Propagation of Chromium(III) Acetate Solutions Through Dolomite Rock
- H. Jin (U. of Kansas) | C.S. McCool (U. of Kansas) | G.P. Willhite (U. of Kansas) | D.W. Green (U. of Kansas) | M.J. Michnick (U. of Kansas)
- Document ID
- Society of Petroleum Engineers
- SPE Journal
- Publication Date
- June 2003
- Document Type
- Journal Paper
- 107 - 113
- 2003. Society of Petroleum Engineers
- 5.6.5 Tracers, 5.4.10 Microbial Methods, 5.7.2 Recovery Factors, 1.6.9 Coring, Fishing, 4.1.2 Separation and Treating, 4.2.3 Materials and Corrosion, 4.3.4 Scale
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Chromium(III)-polymer gel systems are often used in permeability modification treatments for flow control. In-depth treatment of the carbonate rock matrix by these systems is of concern because rock-fluid interactions can lead to loss of chromium and may limit the penetration of a gel treatment. This paper describes a study of chromium retention in dolomite cores in the absence of polymer and demonstrates that precipitation of chromium is the principal retention mechanism. Displacement experiments were conducted in which chromium(III) acetate solutions were continuously displaced at different flow rates through Baker dolomite cores with permeabilities from 19 to 25 md. In some experiments, the cores were shut in and the resident fluid displaced and analyzed to determine chromium retention during the shut-in period.
There was significant retention of chromium in the core in both continuous and shut-in experiments. In the continuous displacements, effluent concentration profiles were a function of flow rate, indicating that the retention was a rate-dependent process. In the range of concentrations studied, 0 to 1% by weight, the presence of KCl increased the retention rate. At the lowest flow rate used, effluent chromium concentration approached a steady-state value that was much smaller than the injected concentration. The retention capacity of the rock appeared to be infinite.
Chromium concentrations at various residence times were compared to values predicted from a kinetic model developed for precipitation from bulk solutions. The kinetic model fit the precipitation rate data well and is consistent with the conclusion that precipitation is the principal retention mechanism. However, the induction period observed in bulk solutions was not present in the data taken in the cores. Precipitation occurred faster in the cores than in the batch experiments. The experimental results indicate that chromium is precipitated at a relatively high rate when chromium acetate solution is in contact with dolomite core material. This loss of chromium may inhibit gelation and limit the depth of penetration of a gel system in dolomite matrix rock.
Poor sweep efficiency is a problem in oil reservoirs because of the heterogeneous flow properties of reservoir rocks. Preferential fluid movements during oil-recovery processes result in bypassed oil. Application of gelled polymer treatments is one method to modify and/or reduce the permeability in zones of the reservoir in order to redirect injected fluids to flow through previously unswept portions of the reservoir.
Aqueous polymer systems composed of high molecular weight, partially hydrolyzed polyacrylamide (HPAM), and xanthan polymers that are crosslinked by chromium(III) have been used for treatments. However, in-depth treatment of matrix rock using these systems may be limited by the poor propagation of the chromium crosslinker, particularly in reservoirs containing carbonate.
When a solution containing chromium acetate and partially hydrolyzed polyacrylamide is injected into a porous matrix, two competitive reactions occur. These are: (1) the reaction between chromium acetate and polymer in which an acetate ligand is displaced by the carboxyl group to form a chromium acetate-polymer bond, and (2) precipitation of chromium from solution caused by the hydrolysis of chromium acetate when acetate ligands are displaced by hydroxide groups, decreasing the solubility of the chromium acetate complex. The hydrolysis of chromium acetate is affected by the pH of the solution. In dolomite rocks, the pH of the solution is determined by the chemical composition of the aqueous solution. Aqueous solutions that are not highly buffered by dissolved salts have values of pH on the order of 9 to 10 when displaced through the dolomite porous matrix.
Chromium may also be retained by ion exchange or adsorption. Since the loss of chromium in previous studies in porous rocks exceeded the estimated ion exchange capacity of the rocks, the primary mechanism for loss of chromium is believed to be precipitation.
Further reaction of the chromium acetate-polymer complex leads to the formation of the gel structure. The importance of precipitation in this process depends upon the relative rate of chromium uptake by the polymer to the rate that chromium precipitates from solution in the porous matrix when exposed to the pH environment within the pore space. The kinetics of the chromium acetate-polymer reaction is not known. Consequently, it is not possible to determine at this time which reaction controls the transport of chromium acetate in a porous matrix during the gelation process.
A number of research groups have investigated the transport of chromium(III) and chromium(III)-crosslinked gel systems in porous cores containing carbonate minerals and in carbonate rocks. Garver et al.1 measured the retention in Berea sandstone cores of chromium(III) from chromium chloride solutions, with and without xanthan polymer, during displacement experiments conducted with continuous flow and with shut-in periods. Significant retention was observed in all cases. Effluent chromium concentration and pH profiles were simulated with the UTChem simulator using a cation-exchange mechanism for chromium retention. The results indicated that cation exchange was not sufficient to describe the entire loss of chromium during its transport through the cores. The authors concluded that the "loss" of chromium to clays is detrimental to the in-depth placement of gelants that use chromium chloride for the crosslinker.
McCool et al.2 studied the interaction between dolomite rocks and a xanthan-chromium(III) gel system. Transport of both chromium( III) chloride and chromium(III) acetate solutions was studied at 35°C. At the injected chromium(III) concentration of 50 ppm (by weight), only 50% was recovered in the effluent after 4 pore volume (PV) of chromium acetate injection, while essentially no chromium was recovered when the same amount of chromium chloride was injected. Chromium was retained faster when chloride was the counter-ion. The authors suggested that chromium was removed from solution by precipitation.
Bryant et al.3 studied chromium(III) retention in Berea sandstone cores. The cores were shut in for 24 hours after injecting 4 PV of chromium solution. Chromium retention was a function of the injected pH, temperature, core lithology, and the chromium ligand. Bryant et al. pointed out that chromium retention was mainly caused by a precipitation process, but reversible adsorption and ion exchange processes might be involved as well.
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