An Improved Model of Mud Acid/Sandstone Chemistry

Abstract

Abstract 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 providing a stronger basis for calculating productivity changes using permeability/porosity correlations. Introduction Mud acid, a mixture of hydrofluoric (HF) and hydrochloric (HCl) acids, has long been used in efforts to restore the productivity of sandstone formations. The rationale for productivity of sandstone formations. The rationale for using mud acid is that it will dissolve most minerals found in sandstone, as well as many of the solids that can be introduced into the near-wellbore formation by drilling, completion or workover operations. Dissolution of solids re-opens or enlarges microscopic flow paths and thus improves near-wellbore permeability. A mathematical model of mud acid treatments is useful in the design and optimisation of such treatments. Although the parameter of real interest is the formation permeability, the foundation of any treatment model permeability, the foundation of any treatment model must be the chemistry of the mud acid/sandstone system. If these chemical reactions are not accounted for correctly, then the model is unlikely to provide reliable predictions of the permeability changes induced by the predictions of the permeability changes induced by the reactions. This paper describes an improved model of mud acid/sandstone chemistry. The development of an improved model is motivated by the discovery that the "standard" chemistry model cannot account for acid consumption consistently. Evidence for this inconsistency will be presented along with a discussion of its implications. It will be seen that the improved model addresses the shortcomings of the standard model. Though the improved model offers a stronger basis for calculating permeability changes, an explicit discussion of permeability changes lies outside the scope of this paper. paper. THE "STANDARD" CHEMISTRY MODEL The improved model of mud acid/sandstone chemistry is an extension of existing lumped-mineral dissolution models. The use of two lumped minerals has gained fairly wide acceptance in the industry, and for convenience it will be referred to as the "standard" model. A brief review of the standard model follows. P. 385