Enhancing Accuracy in Carbonate Characterization and Evaluation of Formation Sands in the Presence of Clay and Feldspars using Multiple Analytical Methods: A Case Study

Knowing the exact mineralogical composition of the formation sand is very important for well treatment designs. Acidizing treatments are designed with various concentrations and mixtures of acids, depending on the composition and concentration of carbonate minerals, such as calcite, dolomite, siderite, and ankerite, in the formation. Accurately identifying carbonate systems in the presence of clay minerals (i.e., muscovite, illite, kaolinite, chlorite, smectite, and mixed layer) and feldspars (i.e., albite and microcline) is always a challenge. To meet this challenge, X-ray diffraction (XRD) analyses based on Rietveld and external standard methods have been widely used to determine the mineralogical compositions of samples. But accuracy of the results depends on various factors, such as the crystalline nature of the sample, the presence of an amorphous inorganic phase, and the presence of organic materials. Sample preparation, sample packing, and even human error in phase identification are also factors responsible for inaccuracy in compositional study. This paper describes an attempt to enhance the accuracy level in analyzing formation samples containing clays, feldspars, carbonates, and quartz with the help of combined analytical methods, such as XRD, thermal gravimetric analysis (TGA), and acid solubility. Some differences were observed during the initial XRD study of the samples and acid solubility data, and the differences were greater with increased concentrations of clay and feldspars in the sample. Hence, for better calculation of carbonate minerals present in the sample, the result from TGA was taken into account. The results derived from TGA were shown to be in line with the solubility data. Final interpretation was drawn on the basis of the combined data obtained from XRD, TGA, and the solubility analysis. For confirmation, the resulting filtrate from the acid solubility test was analyzed using the inductively coupled plasma (ICP) method, and the output data helped in calculating the final concentration of soluble components.