An Improved Model for Sandstone Acidizing and Study of the Effect of Mineralogy and Temperature on Sandstone Acidizing Treatments and Simulation

Abstract

Sandstone acidizing is a complex operation because the acidizing fluid reacts with a variety of minerals present in the formation that results in a wide range of reaction products. The hydrofluoric acid (HF) reaction rate differs widely from mineral to mineral because of the variation in the reaction rate and the area of contact with the injected fluid. The series of reactions occurring in sandstone makes it all the more difficult to find the exact individual reaction rate constants. An improved model that provides better estimates of the outcome of a sandstone acidizing treatment is developed following a review of previous sandstone acidizing models.

The model follows the lumped mineral methodology and is based mainly on the kinetic approach. The use of accurate reaction-rate laws allows the model to effectively predict the consumption of acidizing fluid during the stimulation treatment. The consideration of a proper equation for the silica gel filming factor accounts for the fact that some clay becomes inaccessible to the acid when silica gel precipitates on their surface. The proposed model is shown here to be valid in extrapolating laboratory coreflood data and predicting the effluent acid concentration at various flow rates.

The damage during sandstone acidizing can be minimized when stimulation treatments are designed according to the percentage of carbonate in the formation, type and amount of clay in the formation and the reservoir bottomhole temperature. Most of the available software for design and evaluation of acidizing treatments do not consider the temperature and mineralogy effects extensively. We studied one such software and developed recommendations to improve the design and evaluation of sandstone acidizing treatments by taking into account the multifaceted effects of temperature and mineralogy in increasingly deep and hot sandstone environments. These recommendations will be of great use in the times to come as most of the wells will have to be drilled at greater depths in search for new reserves.