Fracturing Fluid Effects on Young's Modulus and Embedment in the Niobrara Formation

Abstract

Abstract Shale plays and the evolving technologies of horizontal drilling and hydraulic fracturing are driving the petroleum industry in many regions. Due to their low permeability, hydraulic fracturing is necessary for economic production in these shale systems. The success of these reservoirs is dependent on optimized hydraulic fracturing designs, and requires an understanding of the mechanical properties of these reservoirs. One of these properties, Young's modulus, can weaken the formation if it is reduced. This weakening of the formation can, in turn, lead to; increased proppant embedment into the fracture face and a subsequent loss of conductivity. In low permeability shale reservoirs, the conductivity achieved through fracturing is just as critical as in other formations, thus making proppant embedment an issue that needs to be fully understood. This paper focused on how different fracturing fluids impacted the Young's modulus of the Niobrara shale, a major producing formation in the states of Colorado and Wyoming, and how this change affects proppant embedment and conductivity with certain proppants. Nanoindentation technology was used to determine Young's modulus changes in Niobrara core samples after they were saturated for 30 days in certain fracturing fluids, and after they were heated under temperature (180°F) for five days. Treated samples were then subjected to high pressure (3030 psi) with selected proppants, simulating a fracture to test the proppant embedment profiles using scanning acoustic microscope (SAM) and profilometer measurements. The experimental results showed that Young's modulus decreased with fluid exposure regardless of the fracturing fluid type and also increased (rebounded) after a certain saturation time. The magnitude of decrease in Young's modulus values was dependent on fluid type and saturation time. In one extreme case, (when KCl+friction reducer fluid was used as treatment fluid for 30 days) Young's modulus decreased by approximately 80%. The Young's modulus reduction is believed to be mainly caused by a weakening of the calcite minerals. Higher reductions experienced by KCl based fluids could be due to a detrimental chemical reaction between KCl and calcite minerals. Results also showed proppant embedment and crushing are inevitable under the tested circumstances and are dependent on distributed stress on the contact points as well as proppant type and fluid exposure.