Insights from pressure transients at observation wells near hydraulically fractured wells in a domestic shale play

Abstract

We integrate microseismic data and pressure measurements at far-field observation wells to characterize the relationship between deformation and fluid flow during hydraulic fracturing and production in four horizontal wells in an unconventional shale play. The microseismicity qualitatively delineated, where injection fluid traveled during stimulation. However, there was one clear example of the Kaiser effect, in which a strong pressure signal propagated aseismically over hundreds of feet through fractures that had recently been stimulated around a neighboring well. Analysis suggested that poroelastic pressure changes caused by fracture deformation were minimal because of the high compressibility of the volatile oil formation fluid. Therefore, the pressure changes at the observations wells were likely caused by flow of the injection fluid. Based on this hypothesis, the pressure signals in the observation wells were roughly categorized based on whether the pressure response exceeded the magnitude of the minimum principal stress. The relationship between pressure and the minimum principal stress can be used to identify whether fluid traveled through newly forming hydraulic fractures or through stimulated natural fractures. However, the interpretation was significantly complicated by uncertainty in the magnitude of the stress caused by the prior depletion of the observation wells. In some observation wells, the pressure entered a gradual decline over time, indicating that significant long-term fluid production occurred in the neighborhood of the wellbore. But in the more distant observation wells and in depth intervals more vertically separated from the production wells, long-term depletion was not observed, indicating that significant reservoir depletion did not occur. These pressure gauges were sharply pressurized during stimulation, which implies that injection fluid reached these wells during fracturing, but proppant was not transported to the wells and the unpropped fractures subsequently closed and lost their conductivity.