An Experimental Study on Interactions between Imbibed Fracturing Fluid and Organic-Rich Tight Carbonate Source Rocks

Abstract

Abstract Carbonate reservoirs dominate 70% of oil and 90% of gas reserves in Middle East region, and imbibition is the main mechanism for fracturing fluid up-take during hydraulic fracturing stimulation process. Due to highly heterogeneous nature of tight carbonate source rocks, it is crucial to understand effects of the imbibed fluid on the mechanical, morphological and flow properties of the carbonate rocks. While the influence of imbibed fluids on the wettability of carbonate reservoir has been studied intensively, the research on effects of imbibed fluids on the texture and mineralogy of the carbonate rocks is very limited. This paper aims to provide a conceptual approach and workflow to characterize and quantify microstructure and mineralogy changes resulting from the imbibed fluids. A thin-section of low permeability organic-rich carbonate rock sample with a dimension of 7mm × 7mm × 0.3mm (length × width × thickness) was used in the study. The sample was submerged into 2% KCl (pH = 7.1) fluid from one end to simulate the spontaneous imbibition process. Scanning Electron Microscope (SEM) was used to capture the sample’s morphological change before and after spontaneous imbibition. Energy Dispersive Spectroscopy (EDS) mapping was used to study mineralogy changes (dissolution and precipitation) before and after fluid treatment. Inductively coupled plasma (ICP) equipped with optical emission spectrometer (OES) detector has been used to quantify dissolved ion concentrations in the treatment fluid. Permeability and porosity were measured using core plugs (1" in diameter × 1.5" in length) before and after imbibition process with half-length of the sample submerged into the treatment fluid. The SEM images for the thin-section sample show three zones with distinct fluid up-take characters. In Zone I, which was submerged into the testing fluid, considerable mineral dissolution has been observed. In Zone III, which was above the testing fluid level, considerable mineral precipitation was detected. While in the transition zone (Zone II, which was between the above two zones around the water-air level), minor amount of mineral dissolution was observed. The mineralogy changes resulting from the dissolution and precipitation have been identified by EDS analysis in all three zones. Gypsum and calcite were found to be dissolved in the imbibed fluids, while gypsum was found to be deposited on the rock surface in zones above fluid level. The observed gypsum deposition might result from the dissolution of the gypsum and calcite and re-precipitaion later from the imbibition experiment due to water evaporation and/or from sample drying process. Absolute permeability and porosity measurements for core plug samples show that both increased after the imbibition process.