No abstract

Wax precipitation is one of the most challenging flow assurance problems because of its ability to create restrictions to flow inside wellbores, pipelines, and some production facilities. Inaccuracy in predictions of wax appearance temperature (WAT) and amount of precipitated wax makes it necessary to reassess existing thermodynamic models. In addition, most of the current models require accurate description of wax composition from expensive PNA analysis. Here we propose to substitute traditional cubic equations-of-state with Perturbed Chain form of the Statistical Associating Fluid Theory (PC-SAFT). The advantage of PC-SAFT is mainly the accuracy in the estimation of fugacities of heavy components in vapor and liquid mixtures. These fugacities are important because they define the equilibrium between wax, liquid and vapor phases. The novelty of this research was in approach for fluid characterization. Such models as multi-solid or solution-solid were not used. Instead, wax phase was represented as one phase and its amount was taken from inexpensive crosspolarized microscopy data. Therefore, we did not need PNA analysis. To be able to accurately predict with one wax component, reservoir was divided into sectors to determine PC-SAFT parameters for this wax component from calibration of each sector separately. Later, when a new well is drilled, its content of wax can be determined from the cross-polarized experiment and PC-SAFT parameters are same as PC-SAFT parameters of that sector (they were obtained from calibration for that sector before). This data alone is enough to predict amount of precipitated wax at any conditions with high accuracy. First, we validated PC-SAFT with experimental PVT data such as bubble point pressure, gas-to-oil ratio (GOR) and oil formation volume factor (Bo) and compared to the results of Peng Robinson EoS. This PVT data is from one of the fields in the South Caspian Basin. The first validation of wax precipitation itself, however, was performed on experimental data in the literature. Later, the model was calibrated on the oil sample data (composition and wax data from cross-polarized lab experiment) from the field in the South Caspian Basin. Finally, we verified the model with the data from the rest of the wells in this field. The results prove the accuracy of PC-SAFT method and show that costs of PNA analysis can be avoided if cross-polarized microscopy is available.

Previous studies indicated the efficacy of the simplest amino acid, glycine, as an aqueous additive for enhanced water imbibition in carbonate reservoirs. The objective of this research was to investigate the importance of the amino group of glycine in its enhanced water imbibition. To this end, glycine was compared with two carboxylates (acetate and formate) with/without adding hydrogen chloride (HCl) for adjusting the solution pH. Note that the amino group is the only difference between glycine and acetate. Contact-angle experiments on calcite were carried out at 347 K and atmospheric pressure with 68000-ppm reservoir brine (RB), and 4 different concentrations of glycine, acetate, and formate solutions in RB. To test the hypothesis that calcite dissolution is one of the main mechanisms in wettability alteration by glycine, we performed another set of contact angle experiments by adding HCl to brine, acetate, and formate solutions. HCl was added to match the pH of the glycine solution at the same concentration. We also performed imbibition tests with Texas Cream Limestone cores at 347 K with brine, glycine, acetate, and formate solutions (with and without HCl) in RB at 5.0 wt%. Contact-angle results indicated that glycine changed calcite's wettability from oil-wet to water-wet (45°). However, acetate solution was not able to change the wettability to water-wet; and formate moderately decreased the contact angle to 80°. The pH level increased from 6.1 to 7.6 after the contact angle experiment in glycine solution, indicating the consumption of hydrogen ions due to calcite dissolution. The levels of pH in formate and acetate solutions, however, decreased from 8.4 to 7.8. The acidity of glycine above its isoelectric point arises from the deprotonation of the carboxyl group. Imbibition tests with carbonate cores supported the observations from the contact-angle experiments. The oil recovery was 31% for glycine solution, 20% for RB, 21% for formate solution, and 19% for acetate solution. This re-confirmed the effectiveness of glycine as an additive to improve the oil recovery from carbonates. An additional set of imbibition tests revealed that acetate at the pH reduced to the same level as glycine was still not able to recover as much oil as glycine. This showed that glycine recovered oil not only because of the calcite dissolution and the carboxyl group, but also because of the amino group. It is hypothesized that the amino group with its electron donor ability creates a chelation effect that makes glycine entropically more favorable to get attached to the calcite surface than acetate. Another important result is that the formate solution at an adjusted pH resulted in a greater oil recovery than RB or RB at the same pH. This indicates that there is an optimal pH for the carboxyl group to be effective in wettability alteration as also indicated by the pH change during the contact-angle experiment.