Estimation of fracture flow parameters through numerical analysis of hydromechanical pressure pulses

Abstract

[1] The flow parameters of a natural fracture were estimated by modeling in situ pressure pulses. The pulses were generated in two horizontal boreholes spaced 1 m apart vertically and intersecting a near-vertical highly permeable fracture located within a shallow-fractured carbonate reservoir. Fracture hydromechanical response was monitored using specialized fiber-optic borehole equipment that could simultaneously measure fluid pressure and fracture displacements. Measurements indicated a significant time lag between the pressure peak at the injection point and the one at the second measuring point, located 1 m away. The pressure pulse dilated and contracted the fracture. Field data were analyzed through hydraulic and coupled hydromechanical simulations using different governing flow laws. In matching the time lag between the pressure peaks at the two measuring points, our hydraulic models indicated that (1) flow was channeled in the fracture, (2) the hydraulic conductivity tensor was highly anisotropic, and (3) the radius of pulse influence was asymmetric in that the pulse traveled faster vertically than horizontally. Moreover, our parametric study demonstrated that the fluid pressure diffusion through the fracture was quite sensitive to the spacing and orientation of channels, hydraulic aperture, storativity, and hydraulic conductivity. Comparison between hydraulic and hydromechanical models showed that the deformation significantly affected fracture permeability and storativity and, consequently, the fluid pressure propagation, suggesting that the simultaneous measurements of pressure and mechanical displacement signals could substantially improve the interpretation of pulse tests during reservoir characterization.

Fracture hydromechanical response measured by fiber optic distributed acoustic sensing at milliHertz frequencies

“…Hydraulic response lags strain response because the mechanical strain in the fracture propagates in advance of diffusion of fluid pressure. A similar phase difference between pressure and displacement was noted in cross‐hole pulse/strain measurements measured in a fractured carbonate rock [ Cappa et al ., ]. A fracture normal compliance can be estimated from the ratio of effective stress, as measured by the fluid pressure, and strain, as measured by DAS.…”

“…The measurement of displacement was repeatable and quantitative, i.e., well above noise, for hydraulic oscillation frequencies ranging from 0.94 mHz (18 min period) to 7.8 mHz (2 min period). Although these displacements are about 1000 times smaller than those measured in previous cross‐hole experiments [ Burbey et al ., ; Cappa et al ., , ], they are as yet uncalibrated to mechanical measurements by strain meters and should be considered preliminary. The uncalibrated DAS strain measurements on the order of a nanometer are comparable to measurements made with borehole extensometers used for hydromechanical observations [ Hisz et al ., ].…”

Self‐healing of cement fractures under dynamic flow of CO2‐rich brine

“…This leads to the formation of tortuous reacted channels, as observed from the postexperiment images (Figure 3) discussed above. The impact of fracture flow channeling on the effective permeability of fracture networks has been well recognized [Cappa et al, 2008;Long and Witherspoon, 1985;Paluszny and Matthai, 2010;Tsang and Neretnieks, 1998;Zimmerman et al, 1992]. The 2-D model in this study assumes homogeneous flow through one conductive path.…”

Modeling spatial fracture intensity as a control on flow in fractured rock

“…Fluid flow in fractured rock is often complex because of heterogeneities, both at the scale of a single fracture and the entire fracture network (Cappa et al 2008).…”