SPE International Conference on CO2 Capture, Storage, and Utilization,
10-12 November 2010,
New Orleans, Louisiana, USA
The process of modeling ultra low permeability and desorption-controlled shale
gas reservoirs has always been challenging. Desorption is an important issue in
recovery of many shale plays. Although a large amount of gas in place comes
from adsorbed gas, ultra tight matrix and high bottom hole pressure may not
allow this gas be produced.
In this paper, an integrated workflow is described, which demonstrates a
quantitative platform for shale gas production optimization through capturing
the essential characteristics of shale gas reservoirs. Because organic matter
has a greater sorption affinity for CO2 than methane, a comprehensive
feasibility study has been performed to evaluate the applicability and
significance of CO2 injection (with simultaneous production of methane) on
expedition of desorption process and also CO2 sequestration in shale.
Modeling of complex fracture networks is a very important step in simulation of
shale reservoirs. Discrete fracture network (DFN) models can be used to
generate and validate multiple realizations for a quantitative measure of
uncertainty. A 3-D discrete fracture network using typical shale properties was
generated stochastically, based on different realizations (homogenous and
heterogeneous fracture properties).The complex DFNs were upscaled in order to
simulate fluid flow through the system. In addition, hydraulic fractures were
introduced to the model. Sensitivity analysis has been performed on key matrix
and fracture properties for both natural and hydraulic fractures.
Numerical simulation was performed using appropriate gridding (logarithmic
local grid refinement around the hydraulic fractures) and, in order to capture
long transient gas flow from matrix to fracture, multi porosity model with
matrix sub grids has been employed. This step was followed by a history
matching process and the fracture and reservoir properties were varied within a
range that appears consistent with actual typical shale gas well performance.
One model, which gave a better approximation of the actual production profile,
was selected for CO2 injection study.
In order to understand the economic effect of matrix swelling on permeability
and injectivity in the presence of CO2, different injection strategies
(patterns, injection time interval and rate) were defined in order to recommend
the best scenario for maximum and economic recovery.