Flow Modeling of Alkaline Dissolution By a Thermodynamic or By a Kinetic Approach
- Jean Labrid (Inst. Francais du Petrole) | Brigitte Bazin (Inst. Francais du Petrole)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Engineering
- Publication Date
- May 1993
- Document Type
- Journal Paper
- 151 - 159
- 1993. Society of Petroleum Engineers
- 5.3.2 Multiphase Flow, 5.4.7 Chemical Flooding Methods (e.g., Polymer, Solvent, Nitrogen, Immiscible CO2, Surfactant, Vapex), 5.2 Reservoir Fluid Dynamics, 4.3.4 Scale, 5.1.1 Exploration, Development, Structural Geology, 2.4.3 Sand/Solids Control, 2.5.2 Fracturing Materials (Fluids, Proppant)
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This paper presents a calculation of the propagation of basic pH in areservoir rock based on either a kinetically controlled reaction or athermodynamic equilibrium assumption. Results demonstrate that the kineticapproach is the only way to analyze the interactions of alkaline chemicals withclayey sandstones properly.
Prediction of alkali consumption is of great importance to ensure Predictionof alkali consumption is of great importance to ensure the success of an EORprocess that uses alkaline additives. Two approaches were followed. In thefirst, reaction times at field scale were supposed to be long enough to reachthermodynamic equilibrium at each point of space and time. This thermodynamicapproach, including the formation of a new mineral (the sodium zeolite,analcime), was carried out by use of solubility data. In the second approach,it was considered that dissolution is controlled by kinetic limitations. Thekey effects of the adsorption of aluminum and silicon species on solidsurfaces, which reduces the reactivity of the minerals, and the precipitationof analcime formed during the attack were incorporated into the rate equationsused to describe the dissolution.
A model was developed to calculate the propagation of pH on the basis ofeither a kinetically controlled reaction or a thermodynamic equilibriumassumption. Hydroxide consumption by ion exchange was also considered. A directcomparison of the two approaches was made as a function of such differentparameters as composition of the alkaline solution, concentration, and slugsize.
The data clearly demonstrated that, when considering local thermodynamicequilibria, an alkaline solution cannot propagate be- cause of the completetransformation of clays into analcime under alkaline conditions. The kineticapproach, which gives results in close agreement with published experimentaldata, is the only way to analyze the interactions of alkaline chemicals withclayey sand- stones.
Use of alkaline chemicals for EOR has been documented widely. Recently, theuse of these chemicals, along with surfactants or polymers, has been suggestedto improve the oil recovery performance polymers, has been suggested to improvethe oil recovery performance of these processes. Because of the varioustechnical problems connected with the injection of alkali into a reservoir,solid/liquid interactions are considered the most critical factor because theyare the main cause of alkali consumption.
Rock/alkali reactions involve two different mechanisms: ion exchange androck dissolution. Several authors have tried to quantify these mechanisms andhave proposed flow models that allow better estimates of long-term consumptioneffects. As far as ion exchange is concerned, the theory of chromatography iswell-adapted to calculate the transport of alkaline buffers in reservoir rock.On the other hand, there is controversy about the way to approach the problemof rock dissolution.
Some workers assume that, because of the slow propagation rate of the fluidsinto the reservoir, dissolution can be considered to be at equilibrium. In thisrespect, mathematical theories, based on the equilibrium postulate, weredeveloped to describe reactive flow involving precipitation/dissolutionprocesses. These theories, including ion exchange, have been processes. Thesetheories, including ion exchange, have been used to model the propagation ofalkaline additives. However, the specific mineral transformations that occur inbasic pH with the resulting hydroxide consumption were not introducedclearly.
Other authors deduced from laboratory experiments that dissolution iskinetically limited. These investigators describe the dissolution rate withempirical relations involving irreversible first-order or pseudo-first-orderreactions for silica precipitation/dissolution. Our previous work onrock/alkali precipitation/dissolution. Our previous work on rock/alkalireactions demonstrated the key role of clays in quartz dissolution and thatadsorption processes drastically reduce the proper reactivities of eachmineral.
In this paper, we present a flow model that describes the transport ofalkali through a porous medium to evaluate the consumption of the alkalineagent in conditions in which precipitation/dissolution of minerals andhydrogen/sodium ion exchange can occur. The main objectives of the model are totake into account the dissolution of quartz and clays under alkaline conditionsto produce analcime and to use kinetic equations based on extensiveexperimental work on the dissolution mechanisms of sandstones by alkalis. Themain advantage of the model is its capacity to compare the behavior of thealkaline agent either when dissolution of minerals is subjected to kineticlimitations or when solid/liquid local thermodynamic equilibria are assumed toexist. The transport equations and the equilibrium relations in solution arewritten for both assumptions; i.e., thermodynamic equilibrium and kineticrepresentation. Results of simulations are compared and discussed byconsidering continuous injection of alkali and a slug injection. The effect ofthe kinetic factors on the alkaline slug propagation is emphasized in cases ofboth high- and low-pH chemicals. Conclusions are given on the conditions of pHpropagation at field scale.
Modeling of Alkaline Dissolution
It is assumed that the porous medium is made of a quartzitic sandstoneassociated with kaolinite. Previous results have demonstrated that, within alarge range of experimental conditions covering the field of practicalapplications, the dissolution of kaolinite and quartz by alkalis leads toanalcime precipitation. As a matter of fact, in strongly alkaline conditionsand at high Na content, the sodium zeolite, analcime, is stable with respect toother clay minerals. The transformation of kaolinite into analcime occurs witha consumption of alkali according to
This is why we considered quartz, kaolinite, and analcime.
Two hypotheses can be made to describe the alkaline dissolu-tion/precipitation process: the process either is kinetically limited or isgoverned by thermodynamics. Other general assumptions include the following:(1) the medium is ID, homogeneous, and of constant porosity; (2) precipitatescannot migrate by entrainment in the flowing phase; (3) species in solution areat chemical equilibrium; and (4) physical dispersion is neglected. All theconcentrations of the chemical species, in solution and on the solid surfacesor precipitated, refer to the aqueous phase.
Subject to these assumptions, the transport of species through the porousmedium is described with a set of partial-differential equations expressingmass conservation. The system is described with five mass equations relative toaluminum, silicon, sodium, chloride, and carbon (when the alkali isNa2CO3).
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