Pressure transient analyses using discrete fracture network models can provide insights to fracture length, stimulated reservoir volume, and fracture-matrix interaction. These questions are critical to understanding the production behaviors of unconventional reservoirs. This paper presents the results of Discrete Fracture Network (DFN) simulations using
finite-conductivity fractures with constant spacing and size and
more realistic fracture networks based on stimulated natural fracture networks.
These are compared with production data from Eagle Ford wells that are deconvoluted to produce equivalent constant-rate pressure derivatives.
Simulations of finite-conductivity hydraulic fractures with constant length and spacing produce distinct flow regimes. When the matrix has low permeability, the pressure derivative has half-slope, linear-flow periods corresponding to flow from the fractures and then the flow from the matrix blocks to the fractures. The transition between fracture and matrix flow has a steeper, near unit slope, which represents the depletion of the fracture before significant matrix flow occurs. When the matrix is relatively permeable, the effect of the matrix appears before the end of the fracture-only flow and the result is quarter-slope bilinear flow. Intermediate matrix permeabilities produce transitional behaviors between trilinear and bilinear flow. The transitions between fracture and matrix flow provide a basis for assessing fracture length, provided the matrix permeability is known; however, the transitions occur very early in production, within a day or much less time.
A DFN model provides a more realistic simulation of production from the Eagle Ford shale in southwest Texas. The model uses fracture orientations and intensities from seismic and outcrop data. The fractures are assumed to be non-conductive prior to stimulation. The stimulated network model is calibrated to microseismic data. Unlike the simple simulations with constant matrix block sizes, realistic networks have a distribution of block sizes that produce derivative slopes between on half and one.. Production data from Eagle Ford wells were analyzed using pressure deconvolution methods to obtain equivalent constant-rate pressure derivatives. Late time derivatives show similar slopes to the complex DFN fracture simulations.
The quality of the early-time deconvolution transformations of normal production data is not adequate for assessing fracture length due to primarily to low early-time data density. Understanding fracture geometries and behaviors may require several days of build-up data from wells that are shut in after production has started.