Role of the Low Salinity Condensate Water during Steam Injection in Carbonate Reservoirs

Numerous laboratory and pilot tests have verified low salinity waterflooding (LSWF) as a promising enhanced oil recovery (EOR) method in carbonate reservoirs. The multi-ion exchange (MIE) and anhydrite dissolution mechanisms are widely accepted mechanisms for the LSWF. This study investigates the effects of the low salinity condensed water (LSCW) and anhydrite dissolution on oil recovery during steam injection in carbonate reservoirs. The work has been verified using actual laboratory and production data of an existing cyclic steam injection project in carbonate reservoir. Several core samples were extracted from the reservoir under study. The wettability index of two cores was measured. The first core was taken from Well-01 before starting any steam injection in its area. However, the second core was extracted from Well-02, which was drilled in the area affected by steam injection. The analysis of the production data of the 9 oil wells was performed to study the effect of anhydrite percentage on oil recovery. Theanalysis showed that the LSCW could alter wettability in the direction of water wet. The analysis also concluded that although the anhydrite dissolution caused alteration of wettability, the increase of anhydrite percentage could cause a reduction in reservoir quality and oil production.


Introduction
The LSWF is defined as waterflooding technique that decreases the total dissolved salts (TDS) of the injected water and/or modifies the injected water ionic content.The first core flooding experiment in the carbonate reservoir was conducted by Bagci et al. (2001), who documented higher oil recovery up to 18.8% by changing the injected water composition to 2% KCl plus 2% NaCl brine mixture.Furthermore, diluted sea water injection in carbonate plugs showed additional oil recovery up to 11% in several experiments performed by Shariatpanahi et al. (2011), Fathi et al. (2010), and Zekri et al. (2019).
The mechanism of the LSWF in carbonate reservoirs is still debatable.The proposed mechanisms include fine migration, multi-component ion exchange (MIE), calcite dissolution, interfacial tension (IFT) reduction, double-layer expansion, anhydrite dissolution, salting-in, water micro-dispersion, osmosis pressure effect, and surface roughness (Snosy et al. 2021).It should be highlighted that the MIE mechanism is widely accepted as the main controller for the performance of the carbonates' LSWF projects.The carbonate reservoir's MIE mechanism was supposed to be anion exchange and not cation exchange like sandstone reservoirs.The mechanism is attributed to the adsorption of potential determining ion (PDI) SO4 2-, Ca 2+ and/or Mg 2+ onto the rock surface (Snosy et al. 2022a).The MIE evidence of the LSWF in the carbonate reservoirs was proposed by Austad et al. (2005).Then, this mechanism was accepted by Strand et al. (2008a), Ligthelm et al. (2009), Zhang and Austad (2006), Puntervold (2008), and Myint and Firoozabadi (2015).They demonstrated that the incremental oil recovery of sea water injection in carbonates is attributed to the high sulfate concentration.Moreover, Austad et al. (2011) and Pu et al. (2008) proposed that the anhydrite dissolution in the carbonates is a method of generating in-situ SO4 2-which is thought to act as a catalyst in the wettability alteration process.
Furthermore, Al-Saedi et al. (2019) investigated the effect of the condensed steam (low salinity water) during steam injection in sandstone reservoirs.They reported that the wettability was altered towards more water-wet when the steam was turned into low saline (LS) water.In addition, they reported that the LSCW could act as a wettability modifier only without reducing the oil viscosity.
Moreover, there are no available studies for investigating the effect of condensed water during steam injection in carbonate reservoirs.Therefore, this work was performed to study the effect of LSCW and anhydrite dissolution in carbonate reservoirs using laboratory and field data of an existing cyclic steam injection project.

Reservoir Description
The reservoir under study is made up of dolomite, anhydrite nodules, shell fragments, and argillaceous materials.The dolomite is tannish gray, dark gray, light brown, brown, tannish brown, medium-hard to very hard, fine -cryptocrystalline, anhidrotic in parts, sandy in parts, glauconitic at rare parts, and vuggy in parts.While, the anhydrite is off white, milky white, white, light brown, colorless, medium-hard to hard, finecryptocrystalline translucent -opaque, with sucrose texture in parts.
The reservoir contains heavy oil accumulations and could not produce cold production.It has been producing under cyclic steam stimulation.Most of the trials performed to produce this zone as cold production were failed until the first cyclic steam trail was made.The formation contains extra heavy oil with 12-15 o API and viscosity of 4000 cP at standard conditions, 10% H2S, 10% CO2, and 15% asphaltene contents.The geochemical analysis and studies indicated that the biodegradation is due to bacterial action.Table 1 summarizes the reservoir's average properties.

Results and Discussion
Effect of LSCW on the Wettability.Two cores from two wells were used to study the effect of LSCW.The two cores have the same rock compositions: dolomite, anhydrite nodules, shell fragments, and argillaceous materials.The first core was taken from Well-01 before starting any steam injection in its area.However, the second core was taken from Well-02, which was drilled in the area affected by steam injection.Amott wettability index for several core samples from the two wells (Well-01 and Well-02) was measured as shown in Table 2.The core samples of Well-01 reported Amott wettability index ranges from -0.10 to -0.51 with an average value of -0.29.These values reveal that the core samples have oil to neutral wettability index.While the core samples of Well-02 reported Amott wettability index ranges from -0.11 to -0.26 with an average value of -0.15.These values reveal that the core samples have a neutral wettability index.These results reveal that the LSCW alters the rock wettability from oil-neutral wet to neutral wet.The average Amott index changed from an average of -0.28 in Well-01, which had no effect of steam, to -0.15 average in Well-02, which has an effect of steam injection.It should be highlighted that these results are consistent with those obtained by Hjelmeland and Larrondo (1986), Wang and Gupta (1995), Punase et al. (2014), andRao (1999).They reported that carbonate became water-wet with temperature increase.Furthermore, the results of this study agree with those of Blevins et al. (1984), who suggested that the rock wettability of the Qarn Alam field in Oman became more water-wet with an increase in temperature.
It should be highlighted that the alteration of wettability was attributed to the presence of LSCW in carbonate reservoirs.This can be clarified as a result of two mechanisms which were proposed to study the effects of sulfate anion in carbonates: anhydrite dissolution and MIE mechanisms.
For the anhydrite dissolution mechanism, Worden and Smalley (1996) documented that anhydrite and hydrocarbons were reacted together, in carbonate reservoirs of hotter than 140°C, to produce calcite and H2S.It should be stated that the sharp increase of H2S in produced oil in the current study after steam injection in all wells up to 10% was a strong evidence for anhydrite dissolution.Austad et al. (2011) stated that the anhydrite (CaSO4) dissolution generates in-situ SO4 2-ions which act as a catalyst agent in the wettability alteration process.
The MIE mechanism was attributed to the adsorption of potential determining ion (PDI) SO4 2-, Ca 2+ and/or Mg 2+ onto the rock surface.Adsorption of SO4 2-decreases the positive charge density on the rock surface.It minimizes electrostatic repulsive force and causes co-adsorption of Ca 2+ and Mg 2+ on the rock surface.Ca 2+ reaction with carboxylic acid groups breaks the attractive interactions between the oil and rock interface, which changes rock surface into more water wet (Snosy et al. 2022b).
Furthermore, Fathi et al. (2010) proposed that the high temperature (above 90 o C) could cause substitution of Ca 2+ ions on the carbonate surface by Mg 2+ ions from the injected water.Therefore, the displaced Ca 2+ ions bond to carboxylic acid molecules and cause absorption of them in the form of calcium-carboxylate complexes.Thereby further improving oil recovery is achieved.Moreover, Strand et al. (2008b) and Hognesen et al. (2005) concluded that sulfates could act as a catalyst at high temperature and below a certain concentration (1.0 g/L and 2.31 g/L, respectively).That will cause a decrease in IFT and alteration of wettability to more water wet.
Effect of Anhydrite on the Oil Recovery.The oil production data of 9 wells were collected.The 9 wells have almost the same reservoir characteristics and same completion strategy.The wells were cased hole wells, drilled in the same period, and showed encouraging production behavior.These 9 wells are selected because they have less heterogeneity and minor natural fracture.It is believed that the oil production from the wells has occurred through matrix dominant with minor natural fracture.The anhydrite percentage varies significantly in the selected wells as it ranges from 2% up to 10%.Table 3 shows the rock properties, anhydrite percentage, fracture density, and well production data in the selected wells.
Figure 1 presents the relationship between the anhydrite percentage and the oil production rate before and after acidizing and steam injection in the selected wells.The acid stimulation was performed to remove skin damage after drilling and open channels for steam injection.However, Figure 2 represents the relationship between the anhydrite percentage and (1) the 5 years cumulative oil production, and (2) 5 years cumulative oil production per net pay thickness.Figures 1 and 2 document that increasing of the anhydrite content in the reservoir causes reduction of the oil recovery as a result of reservoir quality decrease.It is obviously clear that the anhydrite amount in the reservoir has a negative impact on the oil production rate.The oil wells which are located in reservoir area of high anhydrite percentage show less oil production rate and less cumulative oil production.
It is concluded that the anhydrite dissolution causes alteration of wettability.However, the increase of the anhydrite percentage could cause a reduction in the reservoir quality and oil production.

Conclusions
1.The wettability index measurements for several core samples indicate that the rock wettability may be alerted after steam injection to the direction of water wet.2. The alteration of wettability is attributed to the presence of low salinity condensed water after.3. The multi-component ion exchange mechanism may be the primary mechanism of wettability alteration due to LSCW. 4. The anhydrite dissolution in the reservoir under study may help in wettability alteration. 5.Although the anhydrite dissolution causes alteration of wettability, increasing anhydrite percentage reduces the reservoir quality and oil production.

Figure 1 -
Figure 1-Relationship Between the Anhydrite Content and the Oil Production Rate Before and After Acid and Steam Injection for the Selected Oil Wells.

Figure 2 -
Figure 2-Relationship Between the Anhydrite Content and the Oil Production Data of the Selected Wells.

Table 3 -Average Properties of the Selected Wells
..Well-10 are drilled at the same time and completed with the same strategy.However, Well-01 was drilled earlier and completed with different technique (Open hole section) so it is not included in this analysis. * Abu El Ela is an SPE member, and currently he is the Faculty Advisor for the SPE Student Chapter at Cairo University.Banbi is a professor of petroleum engineering and chair of the Petroleum Engineering Department at the American University in Cairo.He previously worked for Cairo University.Prior to that, El-Banbi worked for Schlumberger, where he held a variety of technical and managerial positions in five countries.He has considerable experience in managing multidisciplinary teams and performing integrated reservoir studies.El-Banbi authored or coauthored one book, two book chapters, and more than 90 journal and conference papers, and holds one US patent.He holds BS and MS degrees from Cairo University, and MS and PhD degrees from Texas A&M University; all in petroleum engineering.El-Banbi has been a member of numerous SPE committees, was the program chair for the 2015 SPE North Africa Technical Conference and Exhibition and a reviewer for many journals.HelmySayyouh obtained his B.Sc. and MS.Degrees in petroleum engineering from Cairo University in 1970 and 1974 respectively and Ph.D. from Penn State University, USA in 1979 and became an assistant professor of reservoir engineering.He rose to the rank of associate professor in 1984 and full professor in 1989.He was the Chairman of the Department of Mining, Petroleum and Metallurgical Engineering, Faculty of Engineering at Cairo University from 2005 to 2008.Presently, he is the professor of Petroleum ReservoirEngineering. Since 1986, he had been a Consultant Engineer in the areas of petroleum reservoir engineering, enhanced oil recovery, reservoir simulation and natural gas engineering.He was an active member in the Egyptian High Production Committee (EGPC) from 1995 to 2001 with the objective of proposing and evaluating means of maximizing recovery and optimizing production from Egyptian fields, and identifying and solving common problems faced by oil companies.Dr. Sayyouh is a member of SPE of AIME, Egyptian Society of Engineering Professions, and New York Academy of Science.Dr. Sayyouh supervised more than 60 M.Sc.and Ph.D. thesis and published over 150 technical papers in specialized international journals and conference proceedings all over the world.