Introduction

Water flooding is one of the choice methods used to enhance and increase oil production from the oil/gas reservoirs to support reservoir pressure (Jreou 2012), and/or to push oil towards the producer well. However, a new method for waterflood injection is being established. Injection water with significantly lower salinity or presence of ions than sea water or surface water has been shown to have beneficial effects on oil recovery (Morrow and Buckley 2011). The problem associated with dissolved undesirable minerals and gas in the water injection that affected this method (Erany 2016).

The mineralogy problem is usually caused by chemical reactions of the water injection with the sensitive clay, which swells or becomes dislodged and blocks the reservoir pores. Water quality can be affected by several types of contaminants, including suspended solids, scale, bacteria, corrosion products, and marine organisms. Chemical and biological analyses can provide a useful indicator of future incompatibility, corrosion, and bacterial problems (Mitchell and Bowyer 1982).

Precipitation of mineral scales causes many problems in oil and gas production operations, such as formation damage, production losses, increased workload for producers and injectors, poor injection water quality, and equipment failure due to under-deposit corrosion. Geochemical reactions between fluids and carbonate rocks can cause significant scaling in and around injection and production wells, leading to high operating costs. The most common mineral scales are sulfate- and carbonate-based minerals. However, scale problems are not limited to these minerals, and there have been recent reports of unusual scale types, such as zinc and lead sulfides (Collins and Jordan 2003).

The aims of this project were to simulate the compatibility between Mishrif formation water with three various kinds of surface waters: Sea water (SW), Euphrates River (ERW), and the Main Outfall Drain water (MODW), and to predict the amount of possible scaling results from the mixing of different rations by using a hydrogeochemical PHEEQC Vr 3, 2015 computer software program to calculate mineral saturation indices. This calculation is based upon the Pitzer theory of electrolytes, which is generally believed to be the best approach to calculate the effect of high temperature, pressure, and total dissolved solids (TDS) in the composition of activity coefficients using Pitzer equations (Pitzer 1991).

Although the South Oil Company in Basrah province may become accustomed to the use of Qarmat Ali water for injections, it is also hoped that the company can use the Main Outfall Drain water. Sea water is commonly used as a source of injecting water, especially in offshore field areas, as in the Basrah oil fields. However, injecting natural sea water is recognized to have no significant effects on the oil field recovery. The high content of hardness ions, such as SO42− and CO32−, are believed to induce more scale depositions when in contact with formation water. Costly treatments and environmental issues are a constraint when applying conventional water injections.

Starting in 1975, the Mishrif reservoir in the West Qurna Oilfield started to produce without any energy supply, and most producers ceased to produce because of low reservoir pressure. Therefore, a water injection pilot was conducted starting in 2010, and the expanded water injection began in 2013. After 2 years of water injections, several producers are back to production, and the production rate of several wells is increasing significantly. However, a number of problems have arisen, the most significant of which being scaling issues. Geochemical reactions between fluids and carbonate rocks can cause significant scaling in and around injection and production wells, leading to high operating costs (Atiaa and Handhel 2009).

In order to manage this problem, this study highlights the hydrogeochemistry of the oilfield water and the expected damage as a result of sea water, Euphrates River water, and Main Outfall Drain water as injections to the Mishrif reservoir by using a hydrogeochemical PHREEQC Vr 3, 2015 computer software program to calculate mineral saturation indices. This calculation is based upon the Pitzer theory of electrolytes, which is generally believed to be the best approach to calculate the effect of high temperature, pressure, and TDS on the composition of activity coefficients using Pitzer equations (Pitzer 1991).

Mechanisms of scaling and types

The main sources of oil field scales result from the mixing of incompatible waters, as precipitation minerals could be found as an outcome of this process (Merdhah and Yassin 2008). A typical example of incompatible waters is sea water with a high concentration of SO42− and low concentrations of Ca2+, Ba2+/Sr2+, and formation waters with very low concentrations of SO42−, but high concentrations of Ca2+, Ba2+ and Sr2+. The mixing of these waters, therefore, causes precipitation of CaSO4, BaSO4, and/or SrSO4 (Paulo et al. 2001).

There are many types of scale-like sulfates, such as calcium sulfate (anhydrites, gypsum), barium sulfate (barite), strontium sulfate (celestite) and calcium carbonate (calcite, dolomite). Iron oxides, iron sulfides, and iron carbonates are the less common scales Table 1 (Moghadasi et al. 2003). Calcium carbonate is the predominant scale deposition from natural waters, especially in cooling and heating systems (El-Said et al. 2009). Carbonate scales can impair production by blocking tubing and flow lines, fouling equipment, and concealing corrosion (Zhang et al. 2001). The variety of ions that exist in the formation and injection waters, such as Mg2+, Ba2+, Sr2+, Mn2+ and SO42−, can affect CaCO3 growth rate (Akin and Lagerwerff 1965).

Table 1 Most common oilfield scales (Moghadasi et al. 2003)
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Factors affected the scaling precipitation

The total dissolved solids and pH affect the permeability of the main reservoir. Clay minerals (like kaolinite and Montmorellonite) are considered as the main reasons behind the damage of oil formations. They can affect negatively formation characteristics through their dispersion and swelling due to the changes of chemical characteristics of the contact water (Scheuerman and Bergersen 1989). Clay mineral types will vary in their influence on the permeability of the formation depending on the basic silica structure, which varies from mineral to another (Al-Mallah 2014).

Increasing the temperature may increase or decrease the solubility of minerals, depending on the nature of the system. For example, if the dissolution process is exothermic, then the solubility decreases with increased temperature; if endothermic, then the solubility increases with temperature. The factors that affect scale precipitation, deposition, and crystal growth are super saturation, temperature, pressure, ionic strength, evaporation, contact time, and pH (Burchette 1993).

Geological and stratigraphic setting

The Mishrif formation is considered as one of the main productive reservoirs in southern Iraq that comprises an important place into the stratigraphic column of the middle Cretaceous period. It reflects the continuous deposition on a shallow carbonate platform developed during the Upper Cenomanian to Early Turonian period (Al-Mohammad 2012). Petrographic study and microfacies analyses revealed five main sub-environments: shallow restricted, shallow open marine, shoal, Rudist biostrom, and deep marine environments. Following a geologic log interpretation and microfacies analysis, the Mishrif formation was subdivided into five reservoir units (mA, mB1-1, mB1-2, mB2-1, and mB2-2). These reservoir units are characterized by high total porosity [18]. Figure 1 shows the stratigraphic column of the South Iraq, Basrah region, with key source rocks, seals, and structural events (Al-Khafaji et al. 2017).

Fig. 1
figure 1

Stratigraphic column of South Iraq, Basrah region showing key source rocks, seals, and structural events (Al-Ameri et al. 2011)

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Methodology

The chemical composition of Mishrif formation water were taken from an internal report of the South Oil company (South Oil Company (SOC) 2012), aside from the surface water (sea water and Euphrates River water), and average physicochemical composition of five Main Drain Outfall stations (Table 2; Fig. 2) (Al-Mallah 2014). This was executed in order to evaluate the simulation compatibility between above surface waters with Mishrif formation water, and to predict the scaling precipitation, in addition to evaluating the mineral saturation indices, resulting from the mixing of different ratios of injection waters and formation water under reservoir conditions.

Table 2 Basic characteristics of Mishrif formation water and present surface waters
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Fig. 2
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Study area location map

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A hydrogeochemical PHEEQC Vr 3, 2015 computer software program (South Oil Company (SOC) 2012), which was written in the C programming language, was used to calculate the mineral saturation indices using Pitzer equations for the calculation of the activity coefficients in highly saline waters (Pitzer 1991). Mathematically, it can be calculated through the following equation:

$${\text{Saturation indices }}\left( {{\text{SI}}} \right){\text{ }}=\frac{{IAP}}{{Ksp}}$$
(1)

where product, Ksp is the equilibrium constant called solubility product, and IAP is the ion activity product. For example, in the case of gypsum dissolution, it would be

$${\text{IAP }}={\text{ }}\left[ {{\text{C}}{{\text{a}}^{{\text{2}}+}}} \right]\left[ {{\text{S}}{{\text{O}}_{\text{4}}}^{{{\text{2}} - }}} \right]$$
(2)

where brackets denote activities.

Water is considered as an equilibrium with respect to a given mineral; when IAP  =  Ksp, the saturation index value is 0. Water is supersaturated and the mineral should precipitate when the saturation index is greater than 0. If the saturation index value less than 0, the water is undersaturated and the mineral should dissolve (of course, if the mineral is present in a solid phase in contact with the water) (South Oil Company (SOC) 2012).

Chemical composition of formation water, and present surface waters

The chemical composition of the Mishrif Formation water and all the present studied waters are shown in Table 2. All the present studied water samples are characterized by weak alkaline types. According to (Parkhurst and Appelo 2013) Table 3, pH is considered as an important parameter to control the solubility of many salts in a system (Komatina 2004). Therefore, the results may have negative effects on the reservoir due to the deposition of carbonate components, which reduces the permeability.

Table 3 Classification of water quality depending on the value of the pH (Komatina 2004)
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The total dissolved solids of Mishrif formation water is classified as Brine water (high saline water) due to the high value of Ca2+ and Na2+ concentration, which increases the salinity. Sea water, Euphrates River water, and Main Drain Outfall water are shown to be slightly Brakish waters according to (Hem 2005), whereas it classified as Brakish water according to (Todd 2009), (Table 4).

Table 4 Classification of groundwater according to the TDS (mg/L)
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Cation concentrations results Table 2, show that Sodium Na+ is predominant in Mishrif formation water because of the dissolution of halite and evaporate rocks under the reservoir conditions. In addition, all the oil fields contain more sodium than any other cations due to their marine origin (Collins 1975). Sodium concentration also is the main cation in all the present water samples, which reflects different anthropogenic sources, including industrial, domestic, and animal wastes (Freeze and Cherry 1982).

The anion concentration results reflect that Cl− is the most common anion in Mishrif formation water, which reflects the presence of gypsum-bearing sulfate sedimentary rocks (Janardhana Raju et al. 2011). Main Drain Outfall water shows the same results as the Mishrif formation water, due to the percolation from the sewage and the results of irrigation activities (Bhatia 2003). Sulfate SO42− was the main anion in sea water and Euphrates River water because of the high salinity and leaching of industrial and domestic sewage, which led to an increase in its concentration (Ramkumar et al. 2010).

Stiff pattern diagram is one of the hydrogeochemical graphical analysis methods used to represent chemical analyses by four parallel axes; concentrations of cations are plotted to the left of the vertical zero axis, and anions are plotted to the right (Todd 2009). The studied water samples are illustrated in Fig. 3, in which all the patterns show that all studied water samples under NaCl emphasized increasing the evaporation processes and sewage of agricultural and domestic activities that resulted towards the water systems, in addition to increasing the weathering of magnesium and sodium sulfate minerals (Esmaeili and Moore 2012). Dissolution of evaporates and carbonate rocks under the reservoir condition will increase ion solubility in water formation, in addition to the presence of gypsum-bearing sulphate sedimentary rocks (Janardhana Raju et al. 2011).

Fig. 3
figure 3

Stiff diagram of present waters

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Hydrochemical facies

Piper diagrams are a geochemical graphic analysis method used in surface and groundwater studies. They can assist in evaluating the mechanisms of flow and transport in addition to ascertaining paleoenvironmental information (Ghalib 2017). A piper diagram of the studied water samples is shown in Fig. 4, in which all present water samples fell under Sodium chloride type (NaCl), except in the Mishrif formation water, which was located between NaCl and MixCaMgCl type, which represents Na+-K+-Cl−-SO42−. From the cationic and anionic triangular field of the piper diagram, Mishrif is observed to be located in the zone of sodium type with no dominate anion type.

Fig. 4
figure 4

Piper diagram of water samples of the present study

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The hydrogeochemical facies that could be recognized from Fig. 7 show that Mishrif formation water falls under the mixed CaMgCl type, which suggests the dissolution of carbonate and evaporate minerals. In general, alkalis (Na + K) significantly dominate more than the alkali earth elements (Ca2+ and Mg2+), and strong acids (Cl− and SO42−) dominate more than weak acids (CO3−and HCO3−).

Results and discussion

The calculations of mineral saturation indices and the establishment of equilibrium conditions of selected minerals resulting from mixing sea water in Mishrif formation water with different mixing ratios under reservoir conditions are shown in Table 5 (Fig. 5). In general, the results show a supersaturation of Geothite, Dolomite, Calcite, Barite, Strontianite, and Celestite, whereas the predicted results show an undersaturation with Gypsum and Sedirite in any mixing ratios between sea water and Mishrif formation water. Anhydrite saturation indices show a supersaturation of 10 and 30%, 90 and 70% of sea water and Mishrif Formation water, respectively. These changes to undersaturation when mixing 50, 70, 90% of Gulf Sea water, and 50, 30, 10% of Mishrif Formation water, respectively.

Table 5 Predicted saturation index for all possible scales resulting from mixing between Mishrif formation water and Gulf sea water under reservoir conditions (software output)
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Fig. 5
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Saturation indices of the mixture of Mishrif Formation water and Sea water at the reservoir conditions

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Table 6 shows scale probability depending on saturation ratio (South Oil Company (SOC) 2012). The scale-predicted model for mixing sea water and Mishrif Formation water reveals that only Geothite has a saturation ratio between 4.0 and 10.0, suggesting that scales will possibly precipitate. Dolomite, Barite, Calcite, and Aragonite were predicted to be supersaturated where these waters mixed, and these scales are unlikely or minor to precipitate due to the lower expected saturation ratios, which are between 1.0 and 4.0. Therefore, a periodic inhibition treatment should be carried out through mixing.

Table 6 The scale probability according to scale type and ratio (South Oil Company (SOC) 2012)
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The mixing predicted model of the Euphrates River water injection in Mishrif Formation water tends to precipitate Geothite, Dolomite, Calcite, Aragonite, Barite, and Strontianite, with a decrease in their saturation ratio and an increase of the Euphrates River water ratio Table 7 (Fig. 6), in which all these scales are supersaturated under reservoir conditions. Celestite tends to precipitate through mixing 50% Euphrates River water with 50% Mishrif Formation water, then tends dissolve in 70 and 90% of Euphrates River water with 30 and 10% of Mishrif Formation water. Anhydrite scales results show an undersaturation when the two waters mix, in addition to Gypsum, Halite, and Siderite scales.

Table 7 Predicted scale tendency for all possible scales resulting from mixing between Mishrif formation water and Euphrates River water under reservoir conditions (software output)
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Fig. 6
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Saturation indices of the mixture of Mishrif formation water and Euphrates River water at the reservoir conditions

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According to Table 6, the scale predicted model for mixing Euphrates River water and Mishrif Formation water shows the same results as when injecting sea water, as only Geothite has the possibility to precipitate. Dolomite, Calcite, Aragonite, and Barite were predicted to be supersaturated when these waters mixed, and these scales are unlikely or minor to precipitate. However, there was no problem with Strontionite, Celestite, and Anhydrite, as no inhibition treatment would be needed because no precipitation will be found.

Table 8 shows the results of the simulation mixing ratio results of injecting the Main Drain Outfall water in the Mishrif Formation. The predicted saturation index results from the mixing model show that Geothite, Dolomite, Calcite, Aragonite, Barite, Strontianite, and Celestite scales were supersaturated under all mixing ratios between Main Drain Outfall and Mishrif Formation waters, while a deposition of Gypsum and Siderite scales that could be predicted as an undersaturated condition of these scales was found in various mixing ratios under the reservoir condition (Fig. 7). Therefore, a high saturation ratio of Geothite-predicted scales will be possible, and a permanent inhibition treatment should be implemented. However, there is no need for Strontionite and Celestite scale inhibition treatment, due to their low saturation ratios, whereas Dolomite, Calcite, Aragonite, and Barite should undergo periodic inhibition treatments because their scales have an unlikely or minor occurrence to precipitate.

Table 8 Predicted scale tendency for all possible scales resulting from mixing between Mishrif formation water and Main Drain water under reservoir conditions (software output)
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Fig. 7
figure 7

Saturation indices of the mixture of Mishrif formation water and Main Drain water the reservoir conditions

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Conclusion

  • For the Mishrif formation using sea water for injection, the results of the scale prediction model show that the most common oilfield scales represented by carbonate and sulfate minerals had a possibility to precipitate. Anhydrite and gypsum is unlikely or a minor occurrence.

  • Using Euphrates water in injections were found to generate the same results as the sea water prediction model. The carbonate and sulfate minerals were the main precipitated scales, whereas other predicted scales were not expected to cause major problems as they dissolved under reservoir conditions.

  • Scaling simulation results of mixing Main Drain Outfall water with different ratios showed supersaturation and a possibility of precipitating calcite and barite minerals. However, the other predicted scales were not expected to cause major problems under reservoir conditions.

  • Anhydrite, Gypsum, and Sederite scales in any mixing ratios showed no need for any inhibition treatments because of a low saturation index while tending to be undersaturated.

  • All the scale-predicting models of the surface water injection results revealed compatibility with formation water, with the consideration of implementing a chemical inhibition treatment through mixing to avoid or reduce Calcite and Barite scale precipitation.