A Description of Chemical Precipitation Mechanisms and Their Role in Formation Damage During Stimulation by Hydrofluoric Acid
Authors: Walsh Mark P. , Lake Larry W. , Schechter Robert S. 
Published: December 2007

Summary
A geochemical simulator that allows for mineral dissolution, precipitation, ion-pair complexation and flow with dispersion in a linear porous medium has been developed. The calculations are based on the thermodynamic equilibrium between aqueous species of the fluid phase and mineral assemblages of the solid phase. The simulator has been used to study sandstone acidizing by use of hydrofluoric/hydrochloric (HF/HCl) acid mixtures and specifically to examine the permeability loss commonly encountered when formations are first contacted with these mixtures. Included were 30 different aqueous species and 14 possible mineral species. it was found that when acids are introduced into sandstone media containing aluminosilicate minerals, the formation of chemical precipitates takes place if a maximum allowable HF level for a given HCl level is exceeded. Further, the permeability losses attending the precipitation were shown to correlate with experimental precipitation were shown to correlate with experimental observations. Model calculations confirmed the precipitation of colloidal silica, as proposed by others, precipitation of colloidal silica, as proposed by others, and identified other, further complicating precipitates. Various formation mineralogies were examined to identify the critical and sensitive parameters that promote reaction-induced formation damage. The clay and promote reaction-induced formation damage. The clay and carbonate mineral contents were found to be the most critical variables. Only small amounts of acid-consuming carbonate minerals were required to alter significantly the design of an acid treatment. The clay mineral content was shown to determine greatly the extent of damage when the maximum HF level is exceeded. The design and effectiveness of the acidization preflush was shown to be crucial to the success of an acid preflush was shown to be crucial to the success of an acid treatment. The simulations have provided definite guidelines for the selection of acid formulations that minimize precipitation for argillaceous sandstones. These are precipitation for argillaceous sandstones. These are discussed and recommendations are made.

Introduction
Matrix acidizing is the process by which acids and acid mixtures are used to dissolve minerals near the wellbore to increase the permeability in that region. Where the dissolution of clay or quartz is desired, mixtures of HF and HCl acids are used. The flowing acid reacts with some or all of the minerals present in the formation, and, as the reaction proceeds, its composition changes as a function both of time and of position in the formation. Despite this complexity, considerable progress has been made in modeling the acidizing process. Model development has led to design procedures that determine the appropriate acid injection rate and the total volume of acid to be used. These models, while very sophisticated in their approach, all fail to account for the precipitous decline in permeability usually observed when sandstone cores are permeability usually observed when sandstone cores are first contacted with acid. This important issue is ignored largely because the origins of the observed permeability reduction are uncertain. The most common permeability reduction are uncertain. The most common explanation concerns the release, migration, and subsequent clogging of pores by fines (clays, colloidal silica) that are released when the acid contacts the core. A second mechanism that could cause a permeability loss is the generation of a CO2-rich gaseous phase that then would reduce the relative permeability to acid. The CO2 is produced by the reactions of acid with carbonates. Labrid has proposed that precipitation of colloidal silica from spent acid solutions may be responsible for the permeability reduction. He performed batch experiments in which kaolinite and silica were dissolved in HCl/HF acid mixtures and showed that, if Si(OH)4 had a limited concentration, thermodynamic predictions correlated with the observed solution compositions.
JPT
P. 2097