Carbonate Acidizing and Flowback Analysis- A Review and an Evaluation Method

Abstract Acidizing treatments are the primary technique to enhance production of carbonate formations. Analysis of historical matrix treatment flowback, and laboratory assessment provide insights on the reservoir response to the acid recipe, and thus, positively enhance the selection of fluid recipes. The objective of this paper is to share an evaluation method to assess the effectiveness of an HCl-based acid recipe through experimental studies and flowback analysis. Gelled, emulsified, and straight hydrochloric acids (HCl) were used to demonstrate the method. Inductively-Coupled Plasma (ICP) was utilized to characterize the liquid flowback samples. HP/HT filter press test was conducted using oil-based and water-based drilling fluids at 150°F to examine the integrity of filter cake and the removal means. Compatibility between crude oil sample and the acid recipe was conducted under reservoir conditions using HP/HT aging cell. ICP analysis was used to correlate the calcium concentration to the magnitude of the rock dissolved by the selected acid recipe. Filter cake removal using water-based and oil-based drilling fluids showed an effectiveness of 90% and 94.8%, respectively. Clear separation was observed between the oil sample and the acid recipe for the slightly gelled and fresh acid formulas. Calcium and magnesium concentration profile in flowback samples indicated the time when spent acid was recovered. Reduction in calcium concentration suggested calcium-based precipitation occurrence. The pH profile, in combination with density measurements, can assess the optimum shut-in and flowback time for full acid spending and retrieval. Steady pH profile at later times of the flowback revealed the presence of carbonic acid. The iron profile analysis was critical to determine the presence of corrosion products and the origin of iron ions in combination with chromium and manganese ions. Dilution factor of sodium and potassium ion concentration was used to determine water cut in acid flowback samples.