The effect of pH on the gelation time of xanthan/Cr(III) have been studied. Few previous studies on the effect of this important reservoir parameter have been reported, and the reported results vary substantially. This is probably due to different conditions used in the experiments. Often pH has not been kept constant during the measurement. When pH has been kept constant, buffers, that can affect gelation rate, have been used. In this study pH is kept constant using an autotitrator, and the results shows several interesting features.
Both Cr-nitrate (Cr(NO3)3) and two different forms of Cr-acetate have been used as Cr(III) source. One Cr-acetate system, referred to as Cr(OAc)3, is prepared by mixing acetate with CrCl3 in the ratio 3:1 in brine. The other system is prepared by dissolving basic Cr-acetate (Cr3(OAc)7(OH)2) in brine.
It is shown that the pH affects the gelation time significantly less if pH is kept constant with an autotitrator compared to pH control using buffers or no pH control at all.
All three chromium sources shows different behavior as pH varies, using the autotitrator method. Gelation time changes most rapidly with pH for the Cr(NO3)3 system, and most slowly for the Cr(OAc)3 system. For both these systems the gelation time increases with decreasing pH, and between pH 4 and 6 the Cr(OAc)3 system gels 2–5 times slower than the Cr(NO3)3 system. However for the Cr3(OAc)7(OH)2 system, the gelation time increases with increasing pH when pH is below 5.5, an decreases with increasing pH when pH is above 5.5. Between pH 4 and 6 the Cr3(OAc)7(OH)2 system gels 2–5 times slower than the Cr(OAc)3 system.
The ability to predict gelation time when applying gel treatment is important, especially when treating producers. If the estimated gelation time is shorter than the actual gelation time, the gelant will be back produced. If the estimated gelation time is longer than the actual gelation time, the gelant cannot be placed as deep as predicted, and the well is shut in for a longer period than necessary. The two most important parameters governing the gelation time of Cr(III) based gelants are temperature and pH. For a xanthan/Cr(III) gel it has been shown that gelation time decreases by a factor of approximately 10 when temperature is increased by 20 C. If chromium is protected with malonic acid even stronger temperature dependence is observed. However, the temperature in the reservoir can be determined quite accurately, at least in producing wells, and in the laboratory the temperature can be controlled to avoid large errors in determining gelation time.
With pH it is different. In the reservoir it can be difficult to determine pH since a correct pH value must be measured at reservoir conditions. With a given uncertainty in the reservoir pH it is important to know to which degree gelation time depends on pH. To obtain that, gelation time has to measured at constant pH at different pH values. At the laboratory pH seems difficult to keep constant during gelation. During gelation of HPAM with chromium, pH has been observed to drop by more than one unit. Since for this system the gelation time decreased by a factor of about 10 when pH increased with one unit, buffers were used for pH control. For a xanthan/chromium(III) system, initial rate of crosslink formation has been measured as a function of pH. During these experiments pH was reported to drop during gelation, but no attempt to control pH during gelation was applied. Initial rate of crosslink formation was reported to be inversely proportional to [H+], giving fairly the same effect of pH on gelation time with xanthan as reported above for HPAM. However, for such a comparison it is necessary to assume that the initial crosslink rate is affected by pH the same way as gelation time, which is not always true.