New perspectives on ionic control of produced water and low sulphate seawater: Wettability insights from MFT

Introduction Over the last decades, enhanced oil recovery (EOR) has been revealing itself as a critical research topic in industry and academia. Various EOR methods, from chemical to thermal, have been implemented, however the oil price downturn and its slow and fragile recovery have induced the necessity of new and cost-efficient methods that will enable the oil recovery optimization from existing fields, therefore reducing the reservoir residual oil saturation. Reservoir waterflooding is frequently applied at a secondary recovery stage for pressure maintenance, after primary recovery to overcome reservoir subsidence problems. However, this process is normally inefficient leaving 45% of the original oil in place unrecovered (Thomas, 2008). Ionically modified waterflooding have been investigated as an alternative to the conventional waterflooding aiming to achieve higher sweeping efficiencies. This method is rooted in the understanding of the rock wettability and its possible alteration by the means of chemically modified water injection; meaning the control of ionic composition of injected water to achieve optimum wetting conditions at a reservoir scale. Wettability can be defined as a measure of the affinity of one of the fluids to wet the interstitial surfaces of the porous medium in the presence of the other fluid (Donaldson and Alam, 2008). Accordingly, a higher water-wetness of the reservoir translates itself into less crude oil wetting the rock surfaces, therefore, ready to be displaced to the production wells. Modified flotation technique (MFT) is a newly developed, reliable and fast method to quantitively estimate the wettability of the rock. MFT is capable of evaluating the controling parameters affecting the rock preferential wettability for each crude oil-brine (s.l.)-rock (CBR) system. Additionally, Flotation Wettability Index (FWI) defined by Sohal (2016), which works identically to the industry well-known Amott-Harvey Wettability Index, can be used to determine the optimum wetting conditions for the system under investigation. The FWI value varies between -1 and +1 that represents completely oil-wet to completely water-wet conditions, respectively; zero indicates the mixed-wet or neutral-wet conditions (Sohal, 2016). Following the increasing environmental consiousness and responsibility, the possible application of produced water and low sulphate seawater, combined with synthetic brines (s.l.) in ionically controlled waterflooding was investigated. The success of this study can add value to the produced water and open new perspectives regarding its utilization, possibly avoiding its discharge, minimizing the chemicals consumption and therefore reducing the environmental footprint associated with hydrocarbons production. Materials and Methods Carbonate reservoirs constitute most of the currently known worldwide reservoirs rocks. In the current study, two chalk rock samples were investigated. Dan chalk (DnK) was provided by dankalk K/S and an Unknown chalk (UnK) has an undisclosed location. All samples in this study are known to be outcrop rock samples. A Rigaku supermini 200 X-ray fluorescence (XRF) analyser was used to obtain the chalk chemical compositions (Table 1). The brines (s.l.) considered for the analysis are either synthetic or from a North Sea field. EFW and seawater (SW) are synthetic, whereas produced water (PW) and low sulphate seawater have its origin in a North Sea field. Similarly, the crude oil sample has its origin in the same field and is characterized by a density (at 25ºC) of 0.834 g/cm3 and acid/base number of 0.2 mgKOH/g. Laboratory experiments were carried out at a constant temperature of 115ºC following the MFT procedure illustrated in Figure 1. Detailed experimental procedure can be found in Godinho (2017). All samples were firstly aged with synthetic Ekofisk Formation Water (EFW), followed by crude oil aging and later by advanced brine aging, also called smart brine or smart water. This advanced brine corresponds to the designed brine, in which the ionic composition has been selected to allow maximum water-wetness of the rock sample. Component Dan Chalk Unknown Chalk CaO [mass %] 95.300 92.200 MgO [mass %] - 0.851 Al2O3 [mass %] 0.439 0.515 SiO2 [mass %] 3.300 5.190 P2O5 [mass %] 0.346 0.317 K2O [mass %] 0.127 0.115 SrO [mass %] 0.432 0.362 Fe2O3 [mass %] - 0.323 SO3 [mass %] 0.059 0.064 Cl [mass %] 0.013 0.015 Total [mass %] 100.016 99.952 The MFT is able to provide results within one week. It is relevant to mention that the method is sensitive, requiring an experienced technician to obtain consistent and accurate results. Figure 1 Modified Flotation Technique (MFT) schematic and process based on Sohal (2016). The MFT diagram depicts all the stages encompassed by the method. Blue drop shape represents the addition of synthetic Ekofisk Formation water and the brown drop shape represents the addition of crude oil. Adapted from Godinho (2017). Chalk Wettability and Chemical Composition Dependency As previously mentioned, the wettability quantification was performed for two chalk samples. From Table 1, it can be seen that the unknown chalk is characterized by a higher dolomitic (MgO) fraction, meaning it has a relatively more impure character if compared to the Dan Chalk. The Figure 2 presents the gathered results in terms of FWI achieved for the performed experiments using different brines (s.l.) containing 20% formation water (EFW) and 80% advanced brine. The advanced brine is represented by various combination ratios of produced water (PW), low sulphate seawater (LSSW) and synthetic seawater (SW). The analysis of the results demonstrates that unknown chalk is characterized by a higher water-wetness (FWI ~0.25 to 0.45) compared to Dan Chalk (FWI~0.1 to 0.3). This tendency brings to light the importance of performing wettability studies for each field case, in order to get accurate estimations, once the average water-wet percentage is influenced by intrinsic rock heterogeneities and can reach values up to 15% in difference. When the attention for EOR applies for a given field it has usually already been exposed to waterflooding and has become a depleted zone. The original formation water has been displaced by any external fluids that have been introduced into the reservoir attempting to increase the reservoir pressure and maintain flow assurance. Though, it is of significance to know the current state on a given production well and the nearby injectors to understand the current state schematic of all parameters influencing the recovery of the oil. Zhang et al. (2006) state that there is no known dominator for the wettability effect, which implies the importance of defining primary parameters such as ionic composition and presence of organic compounds. The MFT can be used as a screening tool for a range of variations in order to determine the current state of the wettability and the influences of currently present ions. Figure 2 Flotation Wettability Index (FWI) or Wetting Index presented by Dan Chalk (DnK) and Unknown Chalk (UnK) aged with Ekofisk Formation Water and Various Smart brines at a constant temperature of 115ºC. Adapted from Godinho, 2017. Advanced waterflooding: Produced Water, low sulphate seawater and synthetic seawater Current work has been suggesting that injection of produced water, optimally conjugated with different brines (s.l.) is able to improve waterflooding sweeping efficiency, turning the reservoir more water-wet, therefore decreasing residual oil saturation. Such evidence provides additional value to the produced water. From Figure 3, one can verify that Dan chalk maximum water-wet percentage (~60%) is achieved for a combination of EFW + (75%PW+25%LSSW), although a similar value is obtained if FW +PW is used. However, if the unknown chalk is considered the maximum wettability (approximately 67%) is acquired from the interaction of the rock with EFW + (50%PW+50%SW) (Godinho, 2017). These wetting trends are thought to be related to potential determining ions (PDIs), especially Ca2+ and Mg2+. Additionally, Sohal 2016, 2017 and Steffensen, 2018 have shown the importance of considering the potential scaling forming ions (PSFIs) in wettability studies. These are largely present in formation brines and might cause reduction in the achieved water-wetness. Its exclusion from laboratory experiments can lead to unrealistic and misleading results. Figure 3 Average water-wet fraction presented by Dan Chalk (DnK) aged with Ekofisk Formation Water and various Smart brines at a constant temperature of 115ºC. The error bars represent the standard deviation of the experimental data in percent. Adapted from Godinho, 2017. Conclusions The selection of the optimum ionic composition for the injection fluid shall be investigated at a laboratory scale, using a reservoir rock and crude oil sample from the field under investigation. In this way, one can provide a field-optimized design fluid, which will minimize the uncertainty associated to the wettability estimations. From the present investigation it was possible to understand that microvariations and rock heterogeneities can lead up to 15% difference in wettability. Also, to understand the wettability alteration mechanisms attention should be paid to the concentration of PDIs and PSFIs. Exclusion of PSFIs from laboratory experiments will lead to inaccurate and unrealistic wettability estimations. References Donaldson, E.C. and Alam, W. [2008] Wettability. Gulf Publishing Company, 336pp, ISBN:9781933762296. Godinho, E.S. [2017] Effectiveness of Produced Water in Advanced Waterflooding. Unpublished Semester Report in Enhanced Oil Recovery. Aalborg University. Sohal, M.A.N., Godinho, E., Thyne, G., Steffensen, K. and Søgaard, E. G. [2017] Effect of temperature and potential ions on North Sea chalk wettability. DHRTC conference 2017, Kolding, Denmark. Sohal, M. A. N. [2016] Wettability Modification in Chalk: Systematic Evaluation of Salinity, Brine Composition and Temperature Effects. PhD Thesis. Aalborg University. DOI: 10.5278/vbn.phd.engsci.00177. Steffensen, K. B. [2018] Optimization of Injection Fluids with Scope of Reservoir Wettability. Unpublished Master Thesis. Aalborg University. Thomas, S. [2008] Enhanced Oil Recovery – An Overview. Oil & Gas Science and Technology – Rev. IFP 2008, 63 (1), 9-19. Zhang, P., Tweheyo, M.T., Austad, T. [2006] Wettability Alteration and Improved Oil Recovery in Chalk: The Effect of Calcium in the Presence of Sulfate. Energy & Fuels, 20, 2056-2062