Study of Ionically Modified Water Performance in the Carbonate Reservoir System by Multivariate Data Analysis

Abstract

Adjustment of the ionic composition and strength of injected or imbibed brine for enhanced oil recovery (EOR) in carbonate reservoirs has been an area of active research for the past two decades. Despite many successful laboratory and field applications, the method has been reported ineffective in other cases. Most of the published results attributed EOR to improved water wetness in initially oil-wet carbonates. Nevertheless, in a few studies, EOR was observed without apparent wettability alteration. We undertake the analysis of a large set of published recovery experiments to try to identify the critical mechanisms at the pore scale. Better pore scale physicochemical understanding will guide the formulation of accurate reservoir-scale models. This paper presents a comprehensive meta-analysis of the proposed mechanisms using multivariate data analysis. Detailed review of the subject, including mechanisms with supporting and contradictory evidence, has been presented by Sohal et al. In this study, the significance of each contributing factor to EOR was quantified and subjected to rigorous multivariate statistical analysis. The analysis was limited because there is no uniform methodology or sampling protocol. We confined our analyses to experiments with sufficient data so as many potential contributing factors as possible could be evaluated. We were able to identify the most important factors that control increased recovery in chalk and limestone experiments. We also found the most critical measurements to provide the best basis for interpretation of the experiments that guide future recovery experiments and provide better results.

Low Salinity Waterflooding in Carbonate Reservoirs: Review of Interfacial Mechanisms

“…For limestone core plugs to restore the reservoir wettability, Yi and Sarma suggested that, according to properties of the rock core and fluids, a minimum aging time of three weeks is required [80]: they suggested that a deficiency in aging time may result in improved oil recovery in the secondary oil recovery mode, but limited wettability modification in the tertiary mode. In contrast, it was suggested that aging time and temperature have a negligible impact on oil recovery from chalks, as the oil-rock-brine system reaches equilibrium quickly [111,112]. Pressure may also affect wettability of carbonate rock reservoirs, due to changes in solubility of compounds in the crude oil.…”

Probing the Effects of Ca2+, Mg2+, and SO42– on Calcite–Oil Interactions by “Soft Tip” Atomic Force Microscopy (AFM)

“…12,13 However, these mechanisms are still under debate, especially Karimi et al 13 observed no evidence of SO 4 2− ions adsorbing on the calcite surface using Fourier transform infrared spectroscopy and thermogravimetric analysis, making the surface charge change mechanism questionable. There are also examples showing opposite results against the existing mechanisms; for example, the study of Sohal et al 21 showed that the enhanced oil recovery by smart water flooding is not correlated to the effects of SO 4 2− ions, or the smart waters rich in Ca 2+ ions cannot enhance the water-wettability of carbonate rocks. 22,23 To date, the effects of Ca 2+ , Mg 2+ , and SO 4 2− on carbonate wettability are still not well understood.…”

“…In recent years, numerous studies have shown that the oil recovery from carbonate reservoirs can be significantly enhanced by “smart water” flooding (e.g., 5–25% incremental recoveries). − The reason for the enhanced oil recovery can be attributed to an alteration of rock wettability (i.e., from oil-wet to water-wet) under the influences of Ca 2+ , Mg 2+ , and SO 4 2– ions in smart waters. , It was reported that the smart waters containing both SO 4 2– and Ca 2+ (and/or Mg 2+ ) ions are very effective in altering the wettability of calcite surface. − Some studies even reported that SO 4 2– or Mg 2+ ion alone can change the wettability of calcite surface, − especially at certain pH values, Mg 2+ ion becomes more effective than SO 4 2– in enhancing the water-wetness of calcite surface. − The effects of Ca 2+ , Mg 2+ , and SO 4 2– on the wettability alteration of carbonate rocks are complex at molecular perspective, which have been attributed to calcite dissolution, surface charge change, , the combination effect of calcite dissolution and surface charge change mechanisms, or the bridging effect of Mg 2+ and SO 4 2– with the surface complexations of calcium and carboxylic group. , However, these mechanisms are still under debate, especially Karimi et al observed no evidence of SO 4 2– ions adsorbing on the calcite surface using Fourier transform infrared spectroscopy and thermogravimetric analysis, making the surface charge change mechanism questionable. There are also examples showing opposite results against the existing mechanisms; for example, the study of Sohal et al showed that the enhanced oil recovery by smart water flooding is not correlated to the effects of SO 4 2– ions, or the smart waters rich in Ca 2+ ions cannot enhance the water-wettability of carbonate rocks. , To date, the effects of Ca 2+ , Mg 2+ , and SO 4 2– on carbonate wettability are still not well understood.…”

Impacts of Smart Waters on Calcite–Crude Oil Interactions Quantified by “Soft Tip” Atomic Force Microscopy (AFM) and Surface Complexation Modeling (SCM)

“…The performance of smart water flooding is closely related to the ionic chemistry of smart waters. In recent years, many studies showed that the oil recovery of carbonate rocks had been improved by sulfate-rich seawaters when the sulfate concentration (denoted [SO 4 2– ]) in seawater was increased to 2–4 times (up to 0.072 M), whereas the oil recovery started to decrease beyond the limit of 4 times (or 4 × [SO 4 2– ]). , Interestingly, the statistical analysis by Sohal et al suggested that this improvement in oil recovery was not correlated to [SO 4 2– ] but to magnesium concentration (denoted [Mg 2+ ]) in smart waters, although the exact roles of Mg 2+ ions in carbonate recovery are still under debate. The studies by Austad’s group suggested that Mg 2+ ions shall be used with SO 4 2– ions and especially at high temperatures (>90 °C) to enhance the oil recovery of carbonate rocks.…”

“…2− ]). 16,17 Interestingly, the statistical analysis by Sohal et al suggested that this improvement in oil recovery was not correlated to [SO 4 2− ] but to magnesium concentration (denoted [Mg 2+ ]) in smart waters, 18 although the exact roles of Mg 2+ ions in carbonate recovery are still under debate. The studies by Austad's group suggested that Mg 2+ ions shall be used with SO 4 2− ions and especially at high temperatures (>90 °C) to enhance the oil recovery of carbonate rocks.…”