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Carbonate reservoirs and factors affecting calcium carbonate (limestone) dissolution.

Introduction

Although majority of the oil traps in the North Sea and some other places in the world are found in sand stone reservoirs, carbonate reservoirs also form large oil traps across the globe. In fact about 50% of the world reserves are found in the carbonate reservoirs. They are therefore, of great concern from the global perspectives. While the secondary means of carbonate rocks formation is; by the precipitation of ions (which form the carbonate minerals) due to the super saturation of the sea water with respect to these ions, majority of the carbonate rocks are formed primarily from the broken shells of marine organisms which get compacted and transform in to a carbonate rock over the geological time, these shells could be from either the internal or external skeleton of the marine organisms [1]. Some few carbonate rocks are formed from biochemical processes that are facilitated by the marine organisms, for example some carbonate sedimentary materials are biologically produced when the conditions are favorable, in such process the biotic influence of marine organisms is not effective and the sediment formation is from the reactions between the metabolic byproducts and ions that precipitated in the external environments [2]. The marine organisms commonly associated with the carbonate materials deposition include; bivalves, gastropods, echinoids, crinoids etc [3]. The main depositional environment for most carbonate sediments is either marine or lacustrine environment [1]. The clasts and matrix present in the carbonate sediments are made from various forms of calcium carbonates and are therefore more prone to dissolution than their clastic rocks counterparts. Different marine organisms will precipitate different forms of shelly materials, this will in turn leads to the precipitation of different carbonate minerals, however majority of these carbonate minerals are composed of calcium carbonates, which may exists in different forms [1]. To effectively design a "squeeze" treatment that will be attractive economically and/or otherwise in a carbonate reservoir, a good understanding of the carbonate system based on science is required, this will allow the proper selection of the chemical scale inhibitors to be used, the concentration at which these chemicals should be used, and the required shut in times after the "squeeze" treatment [4]. This paper is therefore devoted to understanding what the carbonate system is, and the processes affecting its chemical reactions. Some detailed chemistry regarding calcium carbonate, its reactions, and factors affecting these reactions have been investigated, finally some common carbonate minerals present in the carbonate systems and their mode of formation have been examined.

Carbonate system chemistry and mineralogy

For processes (such as dissolution, adsorption and/or precipitation) that frequently occur in carbonate rock on contact with the reservoir fluid or injected chemicals such as chemical scale inhibitors to be correctly understood, it is necessary to have a good understanding of some basic chemistry related to the minerals and chemical compounds making up the carbonate rock, this is because, it is primarily these compounds that take parts in these reactions. This section therefore considered some basic chemistry of the most commonly occurring carbonate minerals found in the carbonate reservoir, however, the basic chemical compound present in virtually all the carbonate systems is the calcium carbonate (CaC[O.sub.3]) and it is this compound that accounts for the carbonate system reactivity, therefore a relatively detailed chemistry of this compound have been considered.

Calcium Carbonate (CaC[O.sub.3])

Chemically speaking, carbonate system simply reflects the carbonate species present in an aqueous solution, these species are; carbonate ions, bicarbonate ions and the carbonic acid. These species are either directly or indirectly sourced from the chemical compound called the calcium carbonate (CaC[O.sub.3]) [5]. calcium carbonate represents the main components of the shell materials deposited by the marine organisms, it is a chemical compound commonly found in all the carbonate rocks around the world, for example it exists massively in chalk, limestone and marble deposits, other carbonate systems consisting mainly of calcium carbonate are; Calcite, Aragonite, Vaterite, Chalk, Travertine etc [5]. Chalk is formed from the calcium carbonate skeleton of marine organisms during the cretaceous period about 135 million years ago, limestone is formed from the solution which is supersaturated with calcium carbonate forming ions due to evaporations of the sea water and marble (though metamorphic in nature) is formed due to the deep burial of the limestone within the earth crust which melts and transform in to a marble (a process which is facilitated by heat and pressure). The chemical means of detecting the presence of calcium carbonate in a rock sample is by passing a strong acid in the form of HCl or [H.sub.2]S[O.sub.4] in to the suspected sample, if the sample fizzes and produce C[O.sub.2] and [H.sub.2]O this confirms the presence of CaC[O.sub.3] [5].

CaC[O.sub.3(s)] + 2H[Cl.sub.(aq)] [right arrow] Ca[C.sub.l2(s)] + [H.sub.2][O.sub.(l)] + C[O.sub.2(g)]

Some major primary chemical reactions that calcium carbonate undergoes are as follows;

Reactions with strong acids: (CaC[O.sub.3(s)] + 2H[Cl.sub.(aq)] [right arrow] Ca[Cl.sub.2(s)] + [H.sub.2][O.sub.(l)] + C[O.sub.2(g)])

Decomposition on heating: (CaC[O.sub.3(s)] [right arrow] Ca[O.sub.(s)] + C[O.sub.2(g)]) and

Formation of calcium bicarbonate: (CaC[O.sub.3(s)] + C[O.sub.2(g)] + [H.sub.2][O.sub.(l)] [right arrow] Ca[(HC[O.sub.3]).sub.2(aq)]

Calcium carbonate preparations

Majority of calcium carbonate used industrially is produced from the mined or quarried marble; or can be prepared by calcining the crude calcium carbonate according to the following series of reactions;

CaC[O.sub.3(s)] [right arrow] Ca[O.sub.(s)] + C[O.sub.2(g)]

The quicklime (CaO) formed is then treated with water to form calcium [hydroxide.sup.10];

Ca[O.sub.(s)] + [H.sub.2][O.sub.(l)] [right arrow] Ca[(OH).sub.2(aq)]

On treating the calcium hydroxide with C[O.sub.2] the required calcium carbonate is formed according to the following reaction;

Ca [(OH).sub.2(aq)] + C[O.sub.2] [right arrow] CaC[O.sub.3(s)] + [H.sub.2][O.sub.(l)]

The calcium carbonate formed in this way is commonly called precipitated calcium carbonate (PCC) and has a number of industrial applications which include; its applications in building works, being an important ingredient in cement industries, it is also used in blast furnace for iron ore purifications, and serves as an important ingredient in the paint, ceramics and plastic industries.

Carbonate minerals

In most carbonates rocks, the frequently occurring minerals are calcite (CaC[O.sub.3]), dolomite (CaMg(C[O.sub.3]).sub.2]), and Aragonite (CaC[O.sub.3]), other minerals are; otavite, smithsonite, siderite, magnesite, rhodochrosite, vaterite etc [2]. However, investigation of the phase behavior of the CaC[O.sub.3]-MgC[O.sub.3] series, and its subsequent extrapolation to sedimentary conditions reveals that only calcite, dolomite and magnesite are expected to be stable at normal conditions. The remainder of this section has considered some basic chemistry related to the commonly encountered minerals present in the carbonate formation.

Calcite (CaC[O.sub.3]) Chemistry

This is the most commonly occurring carbonate mineral found in the carbonate formation, its abundance is related to its thermodynamic stability. Greater percentage of this mineral occurs in the limestone, these mineral posses a chemical formula CaC[O.sub.3] and it has a common polymorph called Aragonite (with which it shares the same molecular formula) [2]. Although this mineral may occur in igneous and metamorphic rocks it is predominantly found in carbonate sedimentary rocks [1]. A close analysis of calcite structure reveals that there are the presence of alternating layer of Ca and C[O.sub.3] groups, the Ca atom is coordinated to six oxygen from different C[O.sub.3] groups to form a slightly distorted octahedron, and each oxygen is bonded to one carbon atom and two Ca atoms from adjacent cation layers. The Ca position in calcite can be

substituted by a variety of elements, this leads to the formation of many different calcite isotypes; Ca (calcite), Cd (otavite), Co,Zn (smithsonite), Fe (siderite), Mg (magnesite), Mn(rhodochrosite) etc. Calcites (and most other carbonate minerals) inhibit an unusual chemical behavior called the retrograde solubility; counter to the expected increase in solubility as the temperature increases; calcite tends to be more soluble at lower temperature conditions. For example the [k.sub.sp] (ionic solubility product) for calcite at 0 and 50[degrees]C are [10.sup.-8.02] and [10.sup.-8.63] respectively [6]. The deposition of calcite minerals is influenced by a number of factors such as temperature, Pc[o.sub.2,] and more importantly saturation index of calcite forming ions.

Calcite deposition predictions

Generally, the most important factor favoring any mineral deposition is the degree of supersaturation of a solution with respect to that mineral [6] A solution for example, will precipitate calcite when it is supersaturated with respect to calcite, and at the same time the same solution is expected to dissolve calcite when it is undersaturated with respect to calcite 6 it is possible to predict the possible deposition of this mineral and even the amount that can be deposited by calculating the saturation index (SI) of a given solution with respect to calcite mineral, this is done by calculating the relative saturation (RS) using many computer codes, this is then related by the following equation [2];

SI = Log RS

RS is the ratio of calcite activity product (Ka) to calcite solubility product (Ksp), SI value of less than 1 indicates undersaturation, SI of 1 indicates saturation, and SI of more than 1 indicates supersaturation.

Dolomite (CaMg[(C[O.sub.3]).sub.2]) Chemistry

Dolomite is a carbonate rock composed mainly of dolomite mineral; since this rock is porous and can store reasonable amounts of hydrocarbons, it therefore forms a petroleum target. The mineral dolomite has a chemical formula CaMg[(C[O.sub.3]).sub.2] and a molecular weight of 184.4 g/mol, this mineral may have different appearance from white gray to brownish white depending on the depositional environment and the nature of the diagenetic processes that occur during and after its deposition (Figure 1).

[FIGURE 1 OMITTED]

Dolomite may be deposited in many types of environments and can have varying structural, textural and chemical properties [1]. This mineral has a very complicated crystal structure. It is commonly suggested that sea water is supersaturated with respect to dolomite but it does not precipitate, and CaC[O.sub.3] hardly changes to dolomite (via dolomatization process) on the sea floor (due to some unfavorable kinetic factors). At lower temperatures no record of its precise laboratory method of precipitation has being clearly documented. However, some of the following chemical reactions have been suggested [2];

[Ca.sup.2+] + [Mg.sup.2+] + 2([C[O.sub.3].sup.2-]) [right arrow] CaMg[(C[O.sub.3]).sub.2]

Dolomite can be formed by replacing Ca atom in calcite by Mg atom according to;

2CaC[O.sub.3] + [Mg.sup.2+] [right arrow] CaMg[(C[O.sub.3]).sub.2] + [Ca.sub.2+] or

CaC[O.sub.3] + [Mg.sup.2+] + C[O.sub.3.sup.2-] [left and right arrow] CaMg[(C[O.sub.3]).sub.2]

In a solution containing Ca, Mg, and carbonate ions for example will preferentially deposits CaC[O.sub.3] and/or MgC[O.sub.3] rather than CaMg[(C[O.sub.3]).sub.2]. This complication arises because dolomite is a highly ordered mineral; therefore a solution of this type which is supersaturated with respect to dolomite will precipitate a lower ordered mineral such as calcite. However, for dolomite to be formed a temperature of about 150[degrees]C is required, and present investigation suggests that, on the sea bed for example, calcium carbonate mineral will be first precipitated and percolation of sea water which is rich in magnesium ions through this material will lead to the replacements of some calcium ions by the magnesium ions therefore formation of the dolomite. The structure of this mineral is simply viewed by replacing Mg atom in to an alternating layer of Ca atom in calcite structure [7], this replacement can however; introduce a change in the bond strengths and lengths which may cause some displacements of atoms causing dolomite possessing a relatively lower degree of symmetry than calcite. However, a model built by Pokrovsky et al. [8]. Which is based on dolomite surface speciation suggests that there are about 3 (three) distinct species found on the surface of dolomite these are; >MgC[O.sub.3.sup.-], >CaMgC[O.sub.3.sup.+], and MgO[H.sub.2.sup.+]. Pokrovsky et al. [8] further suggests that, for dolomite dissolution to occur there has to be the destruction of the protonated magnesium surface sites (MgO[H.sub.2+]). However, it was suggested, that the concentration of MgO[H.sub.2.sup.+] present on the dolomite surface can be related to the aqueous activity of [Ca.sub.2+] and C[O.sub.3.sup.2-](found on the surface of calcite and other carbonate minerals) if the reactions that occur at the surface of dolomite are to be considered.

>CaMgC[O.sub.3.sup.+][left and right arrow]>MgC[O.sub.3.sup.-] + [Ca.sup.2+]

>MgC[O.sub.3.sup.-] + [H.sub.2]O [left and right arrow] >MgO[H.sub.2.sup.+] + C[O.sub.3.sup2-]

This is perhaps the reason why dolomite posses some physical properties similar to calcite mineral, it however does not undergo some basic chemical reactions under gone by most carbonate minerals (especially) when it is in solid form [5].

Aragonite (CaC[O.sub.3]) Chemistry

This is one of the two most commonly occurring calcium carbonate polymorph, it is more thermodynamically stable than the vaterite but less stable than the calcite mineral, it is unstable at standard temperature and pressure and tends to convert to calcite mineral easily. The deposition of aragonite is believed to be influenced by high pressure conditions and some other chemical factors [2]. The aragonite mineral can be white or colorless with usually passive shades of yellow or orange, it has a transparent to translucent crystal. Aragonite is a stable phase only below 75[degrees]K at atmospheric pressure, it is known to be very difficult to synthesize since it easily converts to a more stable calcite.

Structurally, aragonite is orthorhombic and therefore different from rhombohedral calcite and dolomite; however the C[O.sub.3] group in aragonite is identical to that in the rhombohedral carbonates [2]. Aragonite has a triangular carbonate ions groups (C[O.sub.3.sup.2-]) with a carbon at the centre of the triangle and three oxygen atoms at each corner (Figure 2). Unlike in calcite the carbonate ions do not lie in a single plane pointing in the same direction, instead they lie in the two plane that point in opposite direction which destroy the trigonal symmetry of the calcite structure.

[FIGURE 2 OMITTED]

This mineral is commonly stable at very high pressures, and it is formed under certain conditions where the formation of calcite is not favorable [2].

Methodology

Solubility of CaC[O.sub.3] as a function of some physical parameters; concentration and partial pressure of C[O.sub.2] (pC[O.sub.2]), and pH of solution was investigated using a Microsoft spreadsheet model. The model works based on the available chemical and mathematical equations (detailed under results and discussion) for carbonate dissolution. All calculations were carried out at 25[degrees]C, taking the standard Ksp value of CaC[O.sub.3] as 4.47 x [10.sup.-9] [mol.sup.2][L.sup.-2.]

Analysis of the effect of C[O.sub.2] concentration was carried out by varying the concentration from 0 to 0.6 ppm at constant initial pH of 3.0. The model output in this case are final pH values, [[Ca.sup.2+]],[HC [O.sub.3.sup.-]] and [CaC[O.sub.3]](i.e. dissolved or deposited). On the other hand, we have varied the pC[O.sub.2] from 1.0 x [10.sup.-12] to 10.0 atm for the partial pressure studies, and the model outputs are resulting pH and [[Ca.sup.2+]]. pH studies were carried out by considering both de-ionised and ionized water as media. The pH values were generally varied from 1.0 to 8.0 at constant temperature. All outcomes of the model were treated according to standard statistical methods.

Results of the Experiment

The results of the various investigations are presented in Tables 1 to 5 and Figures 3 and 5.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

Discussion of the Results

Calcium carbonate solubility and the model equations

Some common chemical processes occurring in carbonate systems are the dissolution and precipitation of the calcium carbonate, dissolution of this compound specifically occurs during 'squeeze' treatments [9]. This is because the inhibitor slugs commonly used are applied at very low pH values. Some factors affecting these dissolution and precipitation processes have been investigated; this include pH, partial pressure of carbon dioxide, temperature etc. calcium carbonate has poor solubility in pure water or alkaline solution; the solubility equation is;

CaC[O.sub.3](s) [left right arrow] [Ca.sup.2+.sub.(aq)] + C[O.sub.3(aq).sup.2-]

The solubility product ([K.sub.sp]) of the two ions ([Ca.sup.2+] and C[O.sub.3.sup.2-]) varies from 3.7x[10.sup.-9] to 8.7x[10.sup.-9] and this suggests that the product of the molar concentration of these ions in any solution should not exceed the [K.sub.sp] value [5], however when the molar concentration product of dissolved [Ca.sup.+2] and C[O.sub.3] exceeds [K.sub.sp] then it implies that the solution is supersaturated with these ions and will therefore precipitates CaC[O.sub.3], reverse will be the case for a molar concentration product of less than [K.sub.sp], where dissolution of calcium carbonate is the likely process to occur [10], an example of this phenomenon is in sea water where the [Ca.sup.+2] and C[O.sub.3.sup.-2] ionic concentration product at 25[degrees]C is about 3 and 5 times higher than the [K.sub.sp] (for aragonite and calcite respectively) and calcium carbonate precipitation is therefore expected in the form of these minerals at standard conditions [10]. Some of the C[O.sub.3.sup.2-] ions present in the solution will combine with the [H.sup.+] ions to form bicarbonate which is a very soluble entity;

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

This bicarbonate ion can continuously react with the hydrogen ions present in the solution according to; HC [O.sub.3(aq).sup.-] + [H.sup.+.sub.(aq)] [left and right arrow] [H.sub.2]C[O.sub.3(aq)][ K.sub.al] = 2.5x [10.sup.-4] at 25[degrees]C

The carbonic acid ([H.sub.2]C[O.sub.3]) formed may split to form water and dissolved carbon dioxide;

[H.sub.2]C[O.sub.3(aq)] [left and right arrow] [H.sub.2][O.sub.(l)] + C[O.sub.2(aq)][k.sub.h] = 1.7 x 10 - 3 at 25[degrees]C

For any aqueous solution, dissociation of water molecule occurs according to;

[H.sub.2][O.sub.(l)] [left and right arrow] [H.sup.+.sub.(aq)] + O[H.sup.-.sub.(aq)]K = [10.sup.-14] at 25[degrees]C

And for a solution to be in equilibrium, such a solution has to be electrically neutral, that is;

2[Ca.sup.2+.sub(aq)] + [H.sup.+.sub(aq)] [left and right arrow] HC[O.sub.3(aq).sup.-] + 2C[O.sub.3(aq).sup.2-]+O [H.sup.-.sub.(aq)]

The above series of equations will allow the determination of the unknown ionic concentrations of calcium, carbonate and bicarbonate ions present in the solution; In summary the three principal reactions governing the dissolution of calcium carbonate are;

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

In any carbonate sediment containing pore water and/or sea water, the kinetics of calcium carbonate dissolution is given by the equation;

[R.sub.d] = [K.sub.d] [(1-[OMEGA]).sup.n], where [OMEGA] = [[Ca.sup.2+]] [C[O.sub.3.sup2-]]/K

Where [R.sub.d] & [K.sub.d] are the calcite dissolution rate & calcite dissolution constant respectively, and [OMEGA] describe the degree of saturation, and n is constant. Therefore, in any solution that contains carbonates the following equilbria exists;

[[H.sup.+]] [[??]C[O.sub.3.sup.-]] [[H.sub.2]C[O.sub.3]] = [K.sub.1]

[[H.sup.+]] [[??][O.sub.3.sup.2-]] [HC[O.sub.3.sup.-]] = [K.sub.2]

[[H.sup.+]] [O[H.sup.-]] = [K.sub.w]

[C.sub.t] = [[H.sub.2]C[O.sub.3]] +[HC[O.sub.3.sup.-]] +[C[O.sub.3.sup.2-]] (Mass balance)

[[H.sub.+]] - [HC[O.sub.3.sup.-]] - 2[C[O.sub.3.sup.2-]] - [O[H.sup.-]] = O (Charge balance)

These mathematical expressions formed the basis of how the model works.

Effects of pC[O.sub.2] on CaC[O.sub.3] solubility

An important factor affecting the dissolution of calcium carbonate in any aqueous solution is the rate of conversion of C[O.sub.2] in to [H.sup.+] and HC[O.sub.3.sup.-] ions; this process increases the acidity of the solution and therefore increasing the solubility of the carbonate itself [10]. Carbon dioxide can be generated either from oxidation of the organic matter or from gaseous exchange with the atmosphere. When a solution is in contact with the atmosphere, the partial pressure of C[O.sub.2] determines the concentration of carbonic acid that will be present in the solution therefore dictating the solubility of that system. In general for a geological carbonate formation, it is assumed that the formation brine is in equilibrium with the partial pressure of carbon dioxide in the atmosphere, and in such an open system the following chemical reactions occur; Formation of carbonic acid upon dissolution of gaseous carbon dioxide;

C[O.sub.2(g)] + [H.sub.2][0.sub.(l)] [left and right arrow] [H.sub.2]C[O.sub.3(aq)]

Formation of bicarbonate ion upon donation of proton by the carbonic acid;

[H.sub.2][CO.sub.3](aq) [left and right arrow] H[.sup.+](aq) + HC[O.sub.3(aq).sup.-]

Formation of carbonate ion upon donation of the last proton by the bicarbonate ion;

HC[O.sub.3(aq).sup.-] [left and right arrow] H[.sup.+.sub.(aq)] + C[O.sub.3(aq).sup.-]

This is in contrary to what is observed in a closed system where no carbon dioxide gain or loss is considered. Table 1 presents the result of calcium ions dissolved as a function of partial pressure of C[O.sub.2] (where [K.sub.sp] = 4.47X[10.sup.-9] is used) from this table it can be deduce that;

Decrease in the partial pressure of C[O.sub.2] may cause an increase in the alkalinity of the solution, therefore increasing the precipitation tendency of calcium carbonate. This is because at this condition dissolved carbon dioxide has to escape to the atmosphere leaving a highly concentrated solution of carbonate ions, therefore precipitating calcium carbonate.

Conversely, increasing the ambient pC[O.sub.2] will leads to the decrease in pH making the solution more acidic, this results in the conversion of carbonate in to bicarbonate ions which is capable of producing more soluble salt of calcium carbonate [5].

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

Effects of C[O.sub.2] Concentration on CaC[O.sub.3] Solubility

Table 2 above presents clearly the effect of aqueous carbon dioxide concentration on calcium carbonate dissolution, from this table it is evident that increasing the C[O.sub.2(aq)] concentration in the solution leads to a higher calcium carbonate solubility; this is because the more the dissolved aqueous C[O.sub.2] in the solution the higher would be the acidity of that solution and this will lead to the conversion of carbonate in to the bicarbonate ions leading to higher solubility of calcium carbonate (Figure 3) [9, 10]. The overall effects of C[O.sub.2(aq)] concentration on carbonate dissolution is;

CaC[O.sub.3(s)] + C [O.sub.2(aq)] + [H.sub.2][O.sub.(l)] [right arrow] Ca[(HC[O.sub.3]).sub.2(aq)]

The product formed is a very high soluble entity which is more soluble than the CaC[O.sub.3].

Another interesting fact is illustrated in Figure 4 above. As the concentration of C[O.sub.2] in the solution increases, the amounts of bicarbonate ions produced increase (i.e. the reaction shifted to the right). The consequence is the availability of more [H.sup.+] necessary to keep the bicarbonate ions in solution. Chemically, for a normal system, the concentration of these species must be equal. This implies that, at high C[O.sub.2] concentrations more protons are produced, leading to increased calcium carbonate solubility.

Effects of pH on CaC[O.sub.3] solubility

pH of the solution is an important factor affecting the solubility of calcium carbonate in aqueous medium [9]. Adjusting the pH alone without necessarily altering the partial pressure of C[O.sub.2] may lead to the change in the solubility of calcium carbonate. Example of this situation is in a swimming pool where the pH of the water is adjusted by the addition of NaHS[O.sub.4] or NaHC[O.sub.3] to maintain a neutral pH, in this case the ambient pC[O.sub.2] and atmospheric pC[O.sub.2] are in equilibrium [4,5]. Table 3 indicates a situation where pure water is used as solvent; it is evident from this table that the dissolution of calcium carbonate occurs at values typically from 2.1mg to 50,305mg at the corresponding pH values of 7.0 and 1.0 respectively (Figure 5). pH affects the carbonate dissolution since at lower pH there is usually more [H.sup.+] ions (from the acid) present in the solution that allows the conversion of carbonate in to bicarbonate ions which increases the solubility of this compound [10]. However, presence of ions in the solution (such as [Ca.sup.2+,] HC[O.sub.3.sup.-] and dissolved C[O.sub.2(aq)]) may alter the degree of calcium carbonate dissolution, this is indicated in Table 4,where water is used with some ionic concentrations present ([Ca.sub.2+]= 0.01M, HC[O.sub.3.sup.-]= 0.01M and C[O.sub.2(aq)] = 0.001M) and Table 5 ([Ca.sup.2+]= 0.1M, HC[O.sub.3.sup.-] = 0.1M and C[O.sub.2(aq)] = 0.01M). From these tables, it is observed that; contrary to what is observed in table 3, although similar pH values were used, some degree of calcium carbonate precipitations occurred; this is because the solution is supersaturated with respect to CaC[O.sub.3] and at higher pH the calcium carbonate had to be deposited. The simple explanation as to how pH affects the dissolution/precipitation of calcium carbonate is that low pH condition favors the conversion of carbonate in to bicarbonate ions according to;

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

This reaction considerably lowers the concentration of C[O.sub.3.sup.2-] ions present in the solution leading to the increased solubility of calcium carbonate; reverse of this process is expected at high pH values.

Other important parameters that affect solubility of calcium carbonate are the temperature and degree of oxidation of organic matter. Although the present model could not account for these factors, some useful details were supplied based on available literature. Counter to the normal situations where increase in temperature increases the solubility of a solute substance present in the solution, calcium carbonate tends to be less soluble at high temperature conditions. For example on heating hard water, a deposit of calcium carbonate in the form of lime scale is often observed, this is because rise in temperature favors the conversion of bicarbonate in to the carbonate ions according to [10];

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

Presence of carbonate ions in the solution increases the tendencies of the carbonate deposition due to its high insolubility, and as the temperature rises more and more carbonate ions are formed leading to the precipitation of calcium carbonate in the form of lime scale. On the other hand, the reduction of sulfate to sulfite ions during the decay of organic matter plays an important role in monitoring carbonate solubility. It is scientifically proved that; both dissolution and precipitation of calcium carbonate could occur concurrently during these reactions. Biological degradation of organic matter generally leads to the evolution of carbon dioxide [3]. This in turn lowers the pH of the solution due to the generation of bicarbonate ions, and therefore may cause the dissolution of already precipitated calcium carbonate. However at the earlier stage of this reaction, the reduction of sulfate to sulfite ions could leads to the rise in pH which may reverse the dissolution back to the precipitation of calcium carbonate.

1/53[(C [Hsub.2]O).sub.106] [(N[H.sub.3]).sub.16] [H.sub.3]P[O.sub.4] + S[O.sub.4.sup.2-][left and right arrow] C[O.sub.2] + HC[O.sub.3.sup.-] + H[S.sup.-] + 16/53N[H.sub.3] + 1/53[H.sub.3]P[O.sub.4] + [H.sub.2]O

The above reaction indicates the biological oxidation of marine plankton (organic matter) via sulfate reduction. Some other less significant biological processes that could lead to the carbonate dissolution are; oxidation of methane, nitration reduction and fermentation process.

Conclusion

Since the first chemical process observed on injection of chemical scale inhibitor in to the carbonate formation is the dissolution of the carbonate material due to the high reactivity of calcium carbonate (CaC[O.sub.3]); this work has investigated the factors that govern this process, detailed analysis of calcium carbonate dissolution/precipitation suggests that parameters such as pH, partial pressure of carbon dioxide, and temperature are the major factors. Generally, situation that lowers the pH of the solution is necessary to ensure desired inhibition of CaC[O.sub.3] precipitation during inhibition treatment.

To this end, it is important to appreciate the fact that, optimum oil production is only achievable from the carbonate reservoirs 'if and only if the mineral scale problems are managed effectively, and the only way of managing this, is by correctly designing an effective "squeeze" treatment, and for an effective "squeeze" treatment to be designed, a good understanding of the carbonate system chemistry is required, which had been tailored in this research.

Acknowledgement

Full Postgraduate scholarship funding to Garba Mohammed and Ahmad Galadima by the Petroleum Technology Development Fund (PTDF), Nigeria, under the Overseas Scholarship Scheme (OSS) is highly appreciated. PhD Scholarship to Leke Luter by ETF, Nigeria is also acknowledged.

References

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[7] Tennant, C.B., and Berger, R.W.; X-ray determination of dolomite -calcite ratio of a carbonate rock. 17th May, 1956.

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[9] Kan A.T., FU, G., Thomson (2005) Prediction of scale inhibitor squeeze and return in calcite bearing formation. SPE 93265, SPE international symposium on oil field chemistry, Houston, Texas. 2-4 February 2005.

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(1,2) Mohammed G., (3) Galadima A. * and (3) Leke L.

(1) Insititute of Petroleum Engineering, Herriot Watt University, Edinburgh, United Kingdom

(2) Cement Company of Northern Nigeria Plc. P.M.B 2166, Kalambaina Road, Sokoto, Sokoto State, Nigeria

(3) Surface Chemistry and Catalysis Research Group, University of Aberdeen, AB24 3UE, Aberdeen, United Kingdom.

* Corresponding Author E-mail: ahmadgldm@yahoo.com
Table 1: [Ca.sup.2+] ion solubility as a function
of pC[O.sub.2] at 25[degrees]c ([K.sub.sp]
= 4.47X[10.sup.-9]).

pC[O.sub.2], (atm) pH [Ca.sup.2+] (mol/L)

1E-12 12 5.19 x [10.sup.-3]
1E-10 11.3 1.12 x [10.sup.-3]
0.00000001 10.7 2.55 x [10.sup.-4]
0.000001 9.83 1.20 x [10.sup.-4]
0.0001 8.62 3.16 x [10.sup.-4]
0.00035 8.27 4.70 x [10.sup.-4]
0.001 7.96 6.62 x [10.sup.-4]
0.01 7.3 1.42 x [10.sup.-3]
0.1 6.63 3.05 x [10.sup.-3]
1 5.96 6.58 x [10.sup.-2]
10 5.3 1.42 x [10.sup.-3]

Table 2: Effects of C[O.sub.2] concentration on
CaC[O.sub.3.sup.-] dissolution.

 [Ca.sup.2+] = 0, HC[O.sub.3.sup.-]
 = 0,HC[O.sub.3.sup.-] = 0

initial C[O.sub.2] pH, [Ca.sup.2+]
pH, (aq) (ppm)

3 0 7.86 39.03
3 0.000002 7.84 39.1
3 0.0002 7.66 45.62
3 0.002 6.97 86.45
3 0.02 6.14 206.17
3 0.2 5.43 453.84
3 0.4 5.22 571.84
3 0.6 5.11 654.3

initial HC[O.sub.3.sup.-] CaCO3 (Mg)
pH, (ppm) Dissolve+/
 Deposited-

3 57.31 97.47
3 57.52 97.64
3 77.45 113.92
3 201.99 215.88
3 566.68 514.86
3 1321 1133.35
3 1680.42 1428.01
3 1931.61 1633.93

Table 3: CaC[O.sup.3] dissolution as a function of pH (de-ionised
water as media).

[Ca.sup.2+] = 0,
HC[O.sub.3.sup.-] = 0,
C[O.sub.2] = 0

Initial Final [Ca.sup.2+] HC[O.sub.3.sup.-] CaC[O.sub.3](mg)
pH, pH, (ppm) (ppm) Dissolved(+),
 /Deposited(-)

1 4.4 20144.6 321 50305
2 5.4 2104.7 306 5255.9
3 6.43 272.9 220 681.6
4 7.86 39.03 57 97.4
5 9.51 6.24 6.5 15.59
6 9.79 4.55 4.2 3.2
7 9.82 4.43 4.1 2.1
8 9.84 4.2 4 2

Table 4: CaC[O.sub.3] dissolution as a function of pH (Water
containing ions as media).

[Ca.sup.2+] = 0.01,
HC[O.sub.3.sup.-] = 0.01,
C[O.sub.2] = 0.001

Initial Final [Ca.sup.2+] HC[O.sub.3.sup.-] CaC[O.sub.3] (mg)
pH, pH, (ppm) (ppm) Dissolved(+),
 Deposited(-)

1 4.3 20345 321.4 49805
2 5.35 2306.4 311.2 4758.9
3 6.08 490.5 273.1 223.9
4 6.33 302 248.4 -246.7
5 6.37 282.7 244.5 -294.9
6 6.37 280.7 244.05 -299.7
7 6.37 280.5 244.01 -300.23
8 6.37 280.55 244 -300.28

Table 5: CaC[O.sub.3] dissolution as a function of pH
(Water containing ions as media).

[Ca.sup.2+] = 0.01,
HC[O.sub.3.sup.-] = 0.01,
C[O.sub.2] = 0.001

Initial Final [Ca.sup.2+] HC[O.sub.3.sup.-] CaC[O.sub.3](mg)
pH, pH, (ppm) (ppm) Dissolved(+),
 Deposited(-)

1 3.94 402227 455.1 367.66
2 4.13 22219.5 538.9 -44601
3 4.15 20420.9 553.8 -49092.8
4 4.16 20241.1 555.5 -49541.9
5 4.16 20221 555.69 -49541.3
6 4.16 20221.1 555.69 -49591.77
7 4.16 20221.11 555.69 -49591.8
8 4.16 20221 555.69 -49591.87
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Author:Mohammed, G.; Galadima, A.; Leke, L.
Publication:International Journal of Petroleum Science and Technology
Date:Dec 1, 2011
Words:6252
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