The chemical reactions between mud acid (HF/HCl mixtures) and sandstone minerals are numerous and complex. To obtain tractable models of mud acid treatments it is common practice to approximate the chemistry as the dissolution of two "lumped" minerals. Examination of coreflood data shows that this approximation does not consistently account for HF consumption. In particular, this approach cannot account correctly for the effect of flow velocity. Consequently this model cannot reliably extrapolate laboratory data to different operating conditions. Neither can it be used with confidence in the simulation of well treatments, since local flow velocity decreases with distance from a well.
This paper presents an improved chemistry model that includes a secondary dissolution/precipitation reaction recently established in the literature. The improved model provides a consistent prediction of acid consumption. Thus this model provides a more reliable basis for extrapolating laboratory data and for simulating matrix acid treatments. The improved model also accounts for porosity changes more reliably, thereby providing a stronger basis for calculating productivity changes using permeability/porosity correlations.