A Study on the Effect of Fracture Aperture Variability on Advective Transport in aFractured Shale using Discrete Fracture Network Modeling

Abstract:

Natural gas from unconventional fossil energy sources such as shale and tight gas
formations has a profound impact on US energy independence. The current state of
production of methane and other hydrocarbons from low permeability shale involves
processes such as hydraulic fracturing of rock, multiphase flow, and recovery of the gas
via these fractures. Although hydraulic fracturing has been used for the past couple of
decades, little is known about the underlying mechanisms behind the production curves
that are seen in the field, such as, reasons for 50-60% decline after the first year in typical
production curves.
Numerical experiments on a realistic fractured shale system are presented to
identify the effect of complex flow of gas in fractures and matrix diffusion on the
production curve. For characterizing flow, including the characteristics and geometries
for the fracture networks, we use a methodology that incorporates a recently developed
discrete fracture network meshing approach [1], which is combined with the highly
parallel PFLOTRAN subsurface flow and reactive transport code [2] and a new particle
tracking capability [3]. The results of this reservoir-scale methodology for analyzing the
decline in gas production rates indicate dominant flow in fractures in the initial high
production rate. Increase in matrix diffusivity improves production recovery after the
initial production of gas from fractures. Moreover, it is observed that increasing aperture
variability within a single fracture has little effect on the production compared to
variations of the mean fracture aperture from fracture to fracture in a fracture network.

[1] Hyman, J.D., Gable C.W., Painter S.L., and Makedonska N., Conforming
Delaunay Triangulation of Stochastically Generated Three Dimensional Discrete
Fracture Networks: a Feature Rejection Algorithm f or Meshing Strategy, SIAM J.
Sci. Comput, 2014 (in press).

[2] Lichtner, P.C., Hammond G.E., Lu C., Karra S., Bisht G., Mills R.T., and Kumar
J., PFLOTRAN User’s Manual: A Massively Parallel Reactive Flow and
Transport Model for describing Surface and Subsurface Processes, 2014.

[3] Makedonska N., Painter S.L., Karra S., and Gable C.W., Numerical
Experiments on Advective Transport in Large Three-Dimensional DFNs,
Abstract H53A-1398 ,2013, AGU, San-Francisco, CA, 9-13 Dec.