With the recent development of a comprehensive theory for the complete chemistry and kinetics of sandstone acidizing, it was appropriate to develop useful radial models for exploring and simplifying the application of the new technology. These radial models were designed to accommodate both the positive and negative aspects of the recent discoveries. These discoveries included ion-exchange transformation of brines, decomposition of clays in hydrochloric acid, precipitation of fluosilicates, removal of carbonate to prevent the precipitation of complex aluminum fluorides, silica-gel filming, colloidal silica-gel precipitation, precipitation of the various complex aluminum fluorides, mixing between the various stages of the treatment, damage removal, permeability modeling, and skin evolution. Calculations were initially performed with a comprehensive workstation-based computer model so that the magnitude of the various effects could be measured. These calculations showed that many negative effects could be minimized or eliminated by special design considerations. A PC-based model was then written that could help design engineers avoid serious problems when acidizing sandstone formations and provide them with visual guidance for some of the other issues that were relevant. The "avoiding problems" approach for the PC-based radial model provided a powerful basis for introducing advanced designs for sandstone acidizing that have dramatically improved the success rates of field treatments. In addition to choosing the correct HF fluid to avoid secondary precipitations, considerations for choice of preflushes to condition the formation proved essential. Proper choice of preflush is based on acid stabilities of the clays, carbonate removal, and the avoidance of deep clay swelling. Appropriate acid-preflush volumes help ensure carbonate removal for at least 24 in. and provide sufficient fluid spacing between formation brine and the spent HF fluids at the end of the treatment.
The global success rates for HF acidizing have historically ranged from 20 to 80%, depending on the criteria used by individual operators.1 The values that operators report are influenced by candidate selection procedures, the type of HF acid used, previous successes in a region. and the oil company's definition of "success." In some regions, success rates are quite high, perhaps greater than 80%, while in other regions it is "known" that HF acid fails every time. Recent studies into the chemistry and kinetics of HF acidizing have made it clear that chemistry plays a key role in the success of sandstone acidizing treatments. Improved success rates for sandstone acidizing, therefore, require a full knowledge of the relevant reactants and their chemistry. The reactants include not only the HF acid pumped, but also the downhole reactants or formation minerals. The chemistry of this process is very complex, but it can readily explain many of the historical failures of HF acidizing.
Interestingly, the development of a laboratory experiment that can study HF-acidizing chemistry also provided opportunities for discovering the impacts of other interaction processes between injected fluids and a formation. These processes include (1) ion-exchange transformation of brines in the near-wellbore area, (2) HCl decomposition of clays forming plugging colloidal silica gel, (3) precipitation of complex aluminum fluorides on carbonates, and (4) precipitation of complex aluminum fluorides by mixing with formation water. While many of these processes were known, their magnitude and consequences had not yet been observed and measured by a laboratory experimental model. Once these processes were understood, the knowledge was applied in a way that could improve the success rates of sandstone acidizing treatments.
Because the downhole chemistry was inherently complex, a workstation-based radial computer model was written that would allow all the various aspects of the fluid/formation interactions to occur and be studied. P. 327^