Enhancing Accuracy in Carbonate Characterization and Evaluation of Formation Sands in the Presence of Clay and Feldspars using Multiple Analytical Methods: A Case Study
Authors: Nanda Jajati Keshari 
Published: May 2016

Abstract
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.

Introduction
Previous studies (Nanda et al. 2011) reveal good comparative results between TGA, XRD, and acid solubility data in cases of carbonate stones (calcite and dolomite). When this method was applied to formation samples containing carbonates in the presence of clay and feldspars, some deviation was observed between the XRD results and the acid solubility data.
Some formation (shale) sand samples were received from a field location for characterization and evaluation of mineral composition, and the samples showed deviations in the results for acid solubility and XRD study, indicating soluble phases. According to composition studies from XRD, the samples were mixtures of chlorite, calcite, dolomite, feldspars, clay, and quartz. Among these, chlorite, calcite, and dolomite should have been soluble in dilute hydrochloric (HCl) acid. Weight loss as a result of the acid soluble contents should have been in line with chlorite and carbonates (calcite and dolomite). But in this case, deviations were observed, which could have been a result of errors in calculating the soluble phases by XRD in the presence of feldspars and clays, as there were too many peaks to consider. So, for confirmation, an XRD study was also conducted on the samples after an acid solubility test, which showed wiped off calcite, dolomite, and chlorite peaks.
To obtain an actual quantification of carbonates in the presence of a soluble chlorite phase, a TGA was performed on the samples in the temperature range of 400 to 950°C (Gunasekaran and Anbalagan 2007). The TGA revealed the weight loss was caused by carbon dioxide formation of calcium/magnesium oxide from the calcite and dolomite phases. These data were back calculated to obtain the calcite and dolomite content in the samples. The calcite and dolomite content obtained from TGA data was added to the chlorite content for the acid soluble phases.