Objectives and Scope: The geometry and extent of the hydraulic-fracture network produced during well stimulation are generally not well known. Indirect methods such as microseismic monitoring may provide information about the network, but they are not definitive. The Hydraulic Fracture Test Site (HFTS) project was designed to recover core from a stimulated volume, thus providing direct information about hydraulic fractures. Here we present methods and some results from a fracture description of the HFTS slant well cores.
Methods: The 4-inch-diameter cores were contained within an aluminum tube and were examined prior to slabbing. We developed criteria for distinguishing between hydraulic, natural, drilling-induced, and core-handling fractures by examining the orientation and surface features of all fractures, making use of a CT scan of the core.
Results: More than 700 fractures total were found in 600 ft of core, including hydraulic fractures, two sets of calcite-sealed natural fractures, and drilling-induced and core-handling fractures. Hydraulic fractures were found to be most abundant in the section of core closest to the stimulated wells.
Significance: Benefits from an experiment of this type are:
Hydraulic fractures are observed directly, allowing fracture density and spatial distribution to be quantified.
Twist hackles on hydraulic-fracture surfaces may provide information about the direction of fracture propagation.
The degree to which natural, sealed fractures have been reactivated may be assessed.
Proppant distribution in the fractures can be examined.
Details of the observed hydraulic-fracture network can be used to verify, or provide input data for, models of hydraulic-fracture growth and proppant distribution.
Findings can provide ground truth for indirect diagnostic techniques such as microseismic monitoring.
Fractures in core and image log in the same well can be compared, and findings used to help calibrate other horizontal well image logs.
Knowledge of the extent and geometry of hydraulic-fracture (HF) networks in unconventional reservoirs is generally limited. Operators instead rely on indirect observation of HF growth through tracking of microseismic activity (e.g., Fisher and Warpinski, 2012), and replication of HF growth and interaction with weak planes in geomechanical experiments (e.g., Lee et al., 2015). Whereas these techniques provide many useful insights, verification of results by direct observation of hydraulic fractures is needed, and the best method of achieving this verification is to take core through intervals that have been hydraulically fractured. Previous studies adopting this method have examined HFs in tight-gas sandstones, including Warpinski et al. (1993) and Branagan et al. (1996) at the DOE MultiWell site in Colorado and Fast et al. (1994) in Lost Hills field, California. Raterman et al. (2017) reported a direct-sampling experiment in the Eagle Ford Formation, but in that case natural fractures were rare. The Hydraulic Fracture Test Site (HFTS) project described here acquired six cores comprising four continuous cores in the Upper Wolfcamp and two continuous cores in the Middle Wolfcamp from the Midland Basin, West Texas. In this case, calcite-filled natural fractures are abundant. Courtier et al. (2018) provide background on the wider project, whereas this paper presents a qualitative description of fractures in the six cores from the slant well. We characterize natural, drilling-induced, and hydraulic fractures in the slant core, and comment on reactivation of sealed natural fractures and bedding planes. Material including proppant in open fractures was collected and analyzed at the same time as the fracture description, and it is presented in a separate paper (Elliott et al., 2018).
