Low Salinity Waterflooding for a Carbonate Reservoir Experimental Evaluation and Numerical Interpretation

Abstract

Several laboratory tests have already demonstrated the potential of lowering/manipulating the injected brine salinity and composition to improve oil recovery in carbonate reservoirs. However, laboratory SCAL tests are still required to screen low salinity waterflood (LSF) for a particular field to (i) ensure that there is LSF response in the studied rock/oil/brine system, (ii) find the optimal brine salinity, (iii) extract relative permeability curves to be used in the reservoir simulation model and quantify the benefit of LSF and (iv) examine the compatibility of injected brine with formation brine and rock to de-risk any potential formation damage or scaling. This paper presents an extensive LSF SCAL study for one of the carbonate reservoirs and the numerical interpretation of the tests. The experiments were performed at reservoir conditions using representative reservoir core plugs, crude oil and synthetic brines. The rock was characterized using different measurements and techniques such as porosity, permeability, semi-quantitative X-ray diffraction (XRD), scanning electron microscopy (SEM), and mercury intrusion capillary pressure (MICP). The characterization work showed that the plugs can be classified into two groups (uni-modal and bi-modal) based on porosity/permeability correlation and pore throat size distribution. The SCAL experiments were divided in two categories. Firstly, spontaneous imbibition and qualitative unsteady-state (USS) experiments were performed to demonstrate the effect of low salinity brines. In addition, these experiments helped to screen different brines (seawater and different dilutions of seawater) in order to choose the optimal brine composition that showed the most promising effect. Secondly, quantitative unsteady-state (USS) experiments were conducted and modeled using numerical simulation to extract relative permeability curves for high salinity and low salinity brines by history-matching production and pressure data. Moreover, the pressure drop was monitored during all tests to evaluate any risk of formation damage. The main conclusions of the study: 1- The spontaneous imbibition and qualitative USS experiments showed extra oil production due to wettability alteration when switching from formation brine to seawater or diluted seawater subsequently, 2- Oil recovery by LSF can be maximized by injection of brine at a certain salinity threshold, at which lowering the brines salinity further may not lead to additional recovery improvement, 3- The LSF effect and optimal brine salinity varied in different layers of the reservoir, 4- The quantitative USS showed that LSF can improve the oil recovery factor by up to 7% at core scale compared to formation brine injection. This paper proves the potential of LSF to improve oil recovery in carbonate rock. However, the results demonstrate that the effect of LSF may vary in different layers within the same carbonate reservoir, which indicates that LSF effect is very dependent on the rock properties/mineralogy.

Ion Composition Effect on Spontaneous Imbibition in Limestone Cores

“…With the SO 4 2− ions adsorbing first, considering the rock surface is positively charged due to excess Ca 2+ ions, the EDL also expands as the electrostatic repulsion between the interfaces increases. 5,7,24 As was also discussed by Yousef et al, 17 Nasralla et al, 39 and Guo et al, 24 reducing Na + and Cl − ions content in the diffuse layer of the thin water film makes it easy for divalent ions in SSW to access the solid surface where polar components in crude oil are strongly adsorbed. This makes it possible for Ca 2+ and Mg 2+ ions to form complexes with the adsorbed carboxylic groups according to the mechanism suggested by Zhang et al, 11 a process that makes the rock−fluid interface more water wet.…”

Adsorption- and Diffusion-Controlled Wettability Change in Modified Salinity Water Flooding

“…This understanding needs to be translated to computationally feasible mathematical models to be utilized in the simulation and optimization of MSW flooding. Many experimental studies are performed to identify the underlying mechanisms responsible for the improvement of oil recovery by MSW flooding (e.g., refs − ). Atomic force microscopy, − contact angle, − spontaneous imbibition, − and core flooding − measurements have been employed to investigate the impact of MSW flooding in carbonate from the molecular to core scale.…”

The Impact of Spatially Correlated Heterogeneity and Adsorption on Modified Salinity Water in Carbonates

“…Modified salinity water (MSW) flooding is a developing enhanced oil recovery (EOR) technique that aims at decreasing the residual oil saturation and speeding up the rate of oil production. − MSW is fabricated through modification of ionic composition or/and salinity reduction of injecting brine. − Extensive studies have been conducted on the framework of MSW flooding in sandstone and carbonate rocks to explain the principal mechanisms for the enhancement of oil recovery. ,− The forced core flooding test is one of the most commonly used techniques that is served as the primary evidence for the efficiency of MSW on the improvement of oil recovery at the core scale. MSW core flooding tests are usually studied in two different ways, namely, secondary and tertiary modes.…”