Horizontal-well developments in unconventional plays have increasingly been featuring multiple downhole gauges to monitor pressure and temperature variations during both the stimulation and the production phase. Pressure variations in the monitor/offset wells during hydraulic stimulation have largely been documented and can range from just a couple psi to over a thousand psi. By modeling the geomechanical impact of a propagating fracture, we were able to demonstrate that some of these observed pressure responses do not represent a hydraulic communication between the fracture and the offset well, but instead a poroelastic response to the mechanical stress induced by the fracturing process.
When a stress load is applied to a fluid-filled porous material, the pressure inside the pores will increase in response to it. In theory, the incremental pore pressure is then progressively dissipated through porous media until equilibrium is achieved. But in a shale formation, dissipation is slow enough that the excess pressure is maintained throughout the stimulation phase (undrained deformation). As a result, the poroelastic responses captured by downhole and surface pressure gauges are directly proportional to the volumetric stress perturbation induced by a propagating hydraulic fracture.
After building a geomechanical model of a propagating tensile fracture in a poro-linear-elastic material, we were able to match the poroelastic response captured in the toe of an offset well during the first stage of the stimulation of a horizontal well, and calculate the height, length, and orientation of the corresponding hydraulic fracture. Following shut-in, the induced fracture proceeds to close thus relaxing the surrounding formation and causing the poroelastic response to fall off. By simulating the coupled leak-off and frac closure process, we were able to calculate the effective permeability of the formation following stimulation.
Poroelastic Response Monitoring is showing tremendous potential in reducing the uncertainties of multi-stage fracture treatments in unconventional plays. Among its many advantages, it is based on simple well-established physical models (linear-poro-elasticity), it is much less sensitive to rock heterogeneities than pressure transient analysis, each stage can be matched separately, and the noise to signal ratio is small. Also unlike microseismic event monitoring, it directly measures the dilation of propagating hydraulic fractures.
