Microseismic and cross-well strain are both high-end diagnostic tools that provide insight into hydraulic fracture geometry and stimulation effectiveness. However, both datasets have limitations: for example, microseismic can over or underestimate the true fracture geometry, while cross-well strain is restricted to describing a portion of the fracture geometry it is situated to measure. Furthermore, both datasets can show not only the creation of new hydraulic fractures but also the reactivation of previously created hydraulic fractures. The focus of this study is two projects in which both cross-well strain and microseismic were integrated to characterize the geometry of new hydraulic fractures and understand the interaction with pre-existing fractures during well stimulation.
The first project is a multi-well development in the Meramec formation of the Anadarko Basin. The second project is the Hydraulic Fracturing Test Site 2 (HFTS2) in the Wolfcamp formation of the Delaware Basin. Both projects collected low-frequency DAS using permanent fibers in offset wells and were monitored with borehole microseismic arrays during stimulations. Organizing the data relative to distance from the active stage and time since stage start, i.e., spatiotemporally, was a key step in understanding what the diagnostics measured during stimulation.
Both projects tell a similar story, where wells have extensive interactions with previously created fractures originating from both parent wells and recently completed child wells. This interaction manifests as a quicker arrival and muted strain response in cross-well strain and a more rapid and linear move-out of the triggering front over time in the microseismic. We interpret these signatures to be showing re-dilation of pre-existing fractures. Also visible in the diagnostics are arrivals with slower growth, generating microseismicity with a parabolic move-out of the triggering front over time, and typical strain response with heart-shaped tensional front leading the arrival. This signature is interpreted to be new hydraulic fracture creation and growth. Once this reactivation mechanism is understood for a basin, it can also be noted and described using lower-cost techniques, such as sealed-wellbore pressure monitoring (SWPM).
The main motivation for most microseismic and cross-well strain studies is understanding hydraulic fracture geometry; however, interactions with failed media and analysis of either datatype in isolation can cause misinterpretations far from reality. Understanding the range of possible mechanisms measured by these advanced diagnostics is key not just in accurately characterizing fracture geometry but also in understanding the impact of failed media on hydraulic fracture growth. Once understood, these observations can also be used as a baseline to measure success or failure of mitigation trials.