Poroelastic Pressure Transient Analysis: A New Method for Interpretation of Pressure Communication Between Wells During Hydraulic Fracturing

In conventional reservoirs, pressure communication between wells is ascribed to hydraulic diffusion through the rock matrix. In this work we show that in unconventional (low-permeability) reservoirs, pressure communication due to matrix diffusion is insignificant, and pressure changes observed in an offset monitor well during stimulation of a nearby well are primarily due to poroelastic effects. We quantify the pressure transient response observed through external downhole gauges in monitor wells, when an adjacent well is fractured. Our goal is to model this poroelastic response and obtain important reservoir mechanical and flow properties, as well as hydraulic fracture geometry. A fully-coupled, 3-D, poroelastic, compositional, reservoir-fracturing simulator was used to simulate dynamic fracture propagation from a treatment well and compute the resulting pressure changes at one or more monitor wells. The pressure transient response is shown to depend on the reservoir fluid and formation properties (permeability, Biot's coefficient, stress anisotropy) and reservoir mechanical properties (Young's modulus). The impacts of hydraulic diffusivity versus poroelastic pressure response are compared. Type curves are presented that allow the pressure transient response to be interpreted for any general reservoir and well configuration. These type curves can be used to obtain reservoir mechanical and flow properties and the geometry of the propagating fracture. We show that modeling the fracture as a discrete discontinuity (as opposed to high permeability grid- blocks) is essential to obtain good agreement with field pressure observations. The pressure observed in the monitor well first decreases and then increases over time as the growing fracture interacts poroelastically with the monitor well. It is shown that this pressure transient signature is dominated by poroelastic effects for most unconventional reservoirs. The poroelastic response depends on the reservoir fluid type (gas, oil) and the mechanical properties of the reservoir. To simplify the quantitative interpretation of the pressure transient response we have developed type curves that allow us to determine the rock elastic and flow properties and the evolving geometry of the propagating fracture. If multiple monitor wells are utilized, the relative communication between different vertically separated reservoirs and the effects of the altered stresses in the reservoir induced by prior production / depletion can clearly be observed. We present, for the first time, general type curves for interpreting the pressure transient response of monitoring wells when an adjacent well is being fractured. Our representation of the propagating hydraulic fracture as an explicit discontinuity in a poroelastic medium is crucial to capture the poroelastic response observed. The impacts of reservoir heterogeneity (layering), fracture geometry, reservoir mechanical properties, hydraulic diffusivity and prior depletion on the pressure response are quantified. The interpretation of inter-well pressure interference data using the methods described in this paper presents a powerful new fracture diagnostic method.