Abstract

The authors have noted a common problem in deep well acidizing. The problem is that, as the wells become deeper, the quantity of unspent hydrochloric acid increases. This fact in some cases renders the well stimulation useless. In this paper we look at the pressure and temperature effects on the quantity of unspent acid in low permeability - high pressure wells. In the past, only the temperature effect has been critically studied and the effect of pressure has been neglected. By adding the pressure terms of the ideal gas law to the pressure terms of the ideal gas law to the Gibbs free energy equation, the effect of pressure can be seen easily, though it may not be pressure can be seen easily, though it may not be too great.

Introduction

In the past the dissociation in carbonate reactions has been assumed to be independent of pressure. On the basis of this dissociation, pressure. On the basis of this dissociation, only the liquid phase was taken into account in calculating the equilibrium constant. Of course, this logic had its source in the empirical and experimental results. For example, CO2 was considered to have been dissolved in the solution at pressures greater than 600 PSI, forming an aqueous product. However, the PSI, forming an aqueous product. However, the problem becomes apparent if not all the CO2 problem becomes apparent if not all the CO2 goes into the solution and/or a great deal of CO2 already exists in the carbonate rock. For example, only 1.15 moles of CO2 can be dissolved in 1000 grams of water at 2205 PSI and 212F, but if the reaction of 20% of HCl and carbonate at 90% conversion produces 3.089g-moles of CO2 at the same pressure and temperature condition, what becomes of 1.93g-moles of CO2 that will exist in the system? So much for the problem related to pressure effect. As for as the problem of temperature pressure effect. As for as the problem of temperature effect on the quantity of the unspent acid is concerned, there seems to be no disagreement between our analysis and those offered by the previous researchers in this field. previous researchers in this field. With the above in mind, it is the objective of this paper to shed light on a different approach taken paper to shed light on a different approach taken in analyzing the effect of pressure and temperature on the quantity of unspent acid in carbonate acidizing.

THEORY

To develop the theory in an orderly manner, it is first assumed that the CO2 is in the gas phase in the hydrochloric acid-carbonate reaction. The chemical equation for this reaction is:

CaCo3(s) + 2HCl (aq)->CaCl2 (aq) + H2O (aq) + CO2 (g)..(1)

In our analysis the thermodynamic properties are extremely important because each phase, that is, aqueous (aq), gas (g) or solid (s) phases, obviously will have its own set of values which are used in the calculations.

Secondly the thermodynamic consideration lead us to a calculation of Gibbs free energy under certain temperature and pressure. The Gibbs free energy equation is:

..........................(2)

The derivation of equation (2) is based on the following:

..................(3)

which defines the equilibrium constant on the basis of change in temperature (T1 and T2)

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