Experimental study to investigate the effects of in situ stress state and rock-fluid interactions on propped fracture conductivity in the Vaca Muerta formation, An

Abstract

Much evidence has been presented in literature proving the importance of proppant testing for hydraulic fracture design. The majority of the research studies published for proppant conductivity present experimental results obtained under a laboratory uniaxial stress state. Under field in situ stress state, however, the fracture is subjected to triaxial stress. Propped fracture conductivity degradation resulting from various damage mechanisms and changes in mechanical properties depends on the stress state applied on the core samples tested and the sample sensitivity to the aqueous fluids. In this research study, the main objective was to measure propped fracture conductivity and changes in ultrasonic wave velocity measurements performed under a triaxial stress state using 2% KCl brine saturated shale core samples from the Vaca Muerta formation in the Neuquén Basin, Argentina. The results of this study could aid in decision making regarding hydraulic fracture treatments and production optimization for operators in the Vaca Muerta shale. Another objective of the study was to quantify the relationship between fracture contact stiffness and shear wave velocity through uniaxial test measurements. This relationship could help estimate the stimulated reservoir volume (SRV) from microseismic data. Fractures in core samples were induced parallel to the natural bedding planes, in the axial direction of core samples to obtain a natural rough fracture surface. Changes in the geomechanical properties due to interaction with KCl brine were measured before conductivity measurements were performed. Samples were saturated in 2% KCl brine for 30 days. Ultrasonic wave velocity measurements were conducted before and after saturation. A uniaxial test was performed utilizing a cubic sample to measure fracture displacement and P and S wave velocity as a function of stress. For conductivity measurements, samples were then placed in a triaxial sample cell where the cell pressure acts as the closure stress on the fracture. The confining stress was increased by stages and flow tests were performed to measure the permeability of the fractured sample at each stage. Acoustic data was also collected as the stress was increased for the calculation of acoustic wave velocities. Finally, after completion of the test, damage mechanisms were studied both through visual inspection and field-emission scanning electron microscopy (FSEM). An insignificant decrease in core sample Young’s moduli was measured with exposure to KCl brine. Fracture permeability was calculated from fracture aperture values obtained from derived relationships between fracture stiffness and shear wave velocity (Vs) obtained from the uniaxial test measurements.. The triaxial test results showed decrease in conductivity with increasing stress in the samples tested. Fracture permeability reduction was sensitive to stress at lower stresses up to 2000 psi while at higher stresses the rate of reduction decreased with increasing stress. Conductivity values at each point of measurement decreased and then stabilized at the second day of exposure to the respective stress condition. Damage mechanisms observed contributing to the conductivity degradation were spalling of the formation into the proppant pack, proppant embedment and fines migration. Compressional wave velocity increased slowly with increasing stress while shear wave velocity was more sensitive to stress increase.