Fracturing fluid effects on Young's modulus and embedment in the Niobrara Formation

Abstract

New discoveries of shale plays and their abundant resources have directed the petroleum industry towards their development. Production from shale reservoirs has grown significantly in the past few decades spurred by successful development of plays such as the Barnett and Bakken Shales. Due to their low permeability, hydraulic fracturing is a necessity for economic production in these reservoirs, and the success of these reservoirs is dependent on optimizing hydraulic fracturing designs which are in turn dictated by an understanding of the mechanical properties of these reservoirs. The geometric growth and conductivity of fractures in these reservoirs are influenced by many factors. Young's modulus reduction, which essentially means weakening of the formation, is one of these factors. This reduction damages the reservoir and makes embedment of proppants into the fracture face a detrimental effect on fracturing success. The goal of this research project is to focus on how different fracturing fluids change the Young's modulus of the Niobrara shale and how this change affect the proppant embedment and conductivity when commonly used proppants are subjected to pressure tests in the effected Niobrara shale samples. A series of tests and associated methodology were developed and applied to investigate how the Niobrara shale's Young's modulus is affected by various fracturing fluids and to quantify associated proppant embedment. In order to achieve this, core samples at various depths from the same well were selected from the cores. The selection was made based on mineralogy data. Samples containing the highest, lowest, and median calcite percentage and porosity were selected to be imaged with QEMSCAN in order to understand the mineralogy of the samples being used. Eight selected samples were then saturated with four fracturing fluids (freshwater, KCl, KCl+friction reducer, and freshwater+KCl substitute). Four of these samples were the "expansion" samples to see if the results were more dependent on the fluid selections. Two additional samples were heated with freshwater and KCl at 180°F for five days to investigate the heat effect on Young's modulus and embedment in comparison to room temperature experiments. In order to compare in-situ saturated Young's modulus values to dry Young's modulus values of cores obtained by nanoindenter, the Gassmann fluid substitution equation was used. This method showed significant Young's modulus difference between in-situ saturated and dry core values. The saturated/room temperature and saturated/heated cores were then subjected to 3030 psi for one hour while selected proppants were placed in between them. Four different proppant types (16/30 Brady, 20/40 Ottawa, 20/40 Ceramic, and 20/40 RCS) were used in this research. Various combinations of cores affected by fluids and proppants were made in order to simulate actual treatment designs. The cores were also scanned by scanning acoustic microscope (SAM) before and after they were subjected to pressure with proppants, so that a comparison could be made and embedment profiles could be observed. The very last method was to quantify the proppant embedment caused by proppants for cores that were treated by fracturing fluids. A profilometer was used in order to achieve this. In addition to developing the study methodology, experiments showed that Young's modulus decreased with fluid exposure regardless of the fracturing fluid type and also increased after some time of saturation. The magnitude of decrease in Young's modulus values was dependent on fluid type and saturation time and very significant reaching up to approximately 80%. The Young's modulus reduction is believed to be happening due to calcite minerals dissolving in the fluids. Higher reductions experienced by KCl based fluids are believed to be due to KCl causing a detrimental chemical reaction with the calcite minerals in the samples, or possibly due to the nanoindenter measuring salt precipitates and friction reducer residue on samples. Results also showed proppant embedment and crushing are inevitable under the tested circumstances and are related to stress contact effect as well as proppant type and fluid exposure.