Fluid and proppant are injected into a shale reservoir during a hydraulic stimulation, causing changes in rock properties. Over time fluid and pressure bleed off into the reservoir causing further changes. We measured these changes as well as the height of the hydraulic fracture at 1.5-hour intervals using single source point seismic recordings.
A distributed acoustic sensor (DAS) and pressure gauges were installed in a vertical well to monitor the hydraulic stimulation of several horizontal wells. In the vertical well we conducted microseismic recordings using geophones, tiltmeter measurements, strain measurements from DAS, distributed temperature sensor (DTS) readings, and several monitor walk-away time-lapse VSPs (vertical seismic profiles) along with repeated single offset source VSPs. The single source VSP was acquired every 1.5 hours over three days and was oriented so that the direct arrival passed through a single stage in one of the horizontal wells. We estimated the height of the p-wave velocity change due to the hydraulic fracture by measuring travel time changes in the direct arrival. The changes in height and velocity due to the deflation of the pressure over time was also measured. The fracture height was comparable with estimates from microseismic, DAS, and tiltmeters.
In this paper we describe a method to better highlight the geometry of altered rock from a hydraulic stimulation within the Spraberry Formation of the Midland Basin in West Texas. Pioneer Natural Resources is currently developing significant unconventional resources within the basin and methods like those noted here enable an understanding of fracture geometry and well interaction during hydraulic stimulation that are important in developing unconventional resources. By acquiring several different types of data, a more accurate picture of the fracturing process can be observed and field development and geomechanical models can be adjusted accordingly. The use of DAS/DTS fiber allows for a very cost-effective and rapid acquisition of vertical seismic profiles. Pioneer has used time-lapse, fiber-based VSPs in the past with good results (Meek, 2017). Meadows (1994) observed changes in travel time during a hydraulic fracture using geophones. Recently, Byerley et al (2018) described a time-lapse experiment to monitor a hydraulic fracture during each stage into a horizontal fiber. They observed that the time delay diminished over a few days. It is thought that this time-delay was caused by fractures opening during the completion and decreasing the velocity around the well bore. Fluid and pressure leaking off over time then results in an increase in velocity of the altered rock. Understanding this pressure build up and later diffusion is important to understanding the interaction of offset well fracture stages which may influence well spacing decisions. It is also useful in determining how long adjacent wells that were shut in during completion can be placed back on production. Beyond the use of microseismic, imaging the hydraulic completion from surface geophysical techniques has been challenging. As a result we have begun to utilize subsurface imaging techniques like VSPs to gain further insight into the dynamics of the stimulation. Here we demonstrate the usefulness of the VSP by recording data into the vertical fiber only. Unfortunately, with a horizontal fiber it is difficult to obtain the height and width of the fracture using reflection energy. Experiments are currently being conducted using downward continuation of reflection energy from horizontal fibers to image around the well bore (Fuller, 2019).