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 Gibbs free energy equation, the effect of pressure can be seen easily, though it may not be too great.