Summary
Accurate calculation of multiphase fluid transfer between the fracture and
matrix in naturally fractured reservoirs is a very crucial issue. In this
paper, we will present the viability of the use of a simple transfer function
to accurately account for fluid exchange resulting from capillary and gravity
forces between fracture and matrix in dual-porosity and dual-permeability
numerical models. With this approach, fracture- and matrix-flow calculations
can be decoupled and solved sequentially, improving the speed and ease of
computation. In fact, the transfer-function equations can be used easily to
calculate the expected oil recovery from a matrix block of any dimension
without the use of a simulator or oil-recovery correlations.
The study was accomplished by conducting a 3-D fine-grid simulation of a
typical matrix block and comparing the results with those obtained through the
use of a single-node simple transfer function for a water-oil system. This
study was similar to a previous study (Alkandari 2002) we had conducted for a
1D gas-oil system.
The transfer functions of this paper are specifically for the sugar-cube
idealization of a matrix block, which can be extended to simulation of a
match-stick idealization in reservoir modeling. The basic data required are:
matrix capillary-pressure curves, densities of the flowing fluids, and matrix
block dimensions.
Introduction
Naturally fractured reservoirs contain a significant amount of the known
petroleum hydrocarbons worldwide and, hence, are an important source of energy
fuels. However, the oil recovery from these reservoirs has been rather low. For
example, the Circle Ridge Field in Wind River Reservation, Wyoming, has been
producing for 50 years, but the oil recovery is less than 15% (Golder
Associates 2004). This low level of oil recovery points to the need for
accurate reservoir characterization, realistic geological modeling, and
accurate flow simulation of naturally fractured reservoirs to determine the
locations of bypassed oil.
Reservoir simulation is the most practical method of studying flow problems
in porous media when dealing with heterogeneity and the simultaneous flow of
different fluids. In modeling fractured systems, a dual-porosity or
dual-permeability concept typically is used to idealize the reservoir on the
global scale. In the dual-porosity concept, fluids transfer between the matrix
and fractures in the grid-cells while flowing through the fracture network to
the wellbore. Furthermore, the bulk of the fluids are stored in the matrix. On
the other hand, in the dual-permeability concept, fluids flow through the
fracture network and between matrix blocks.
In both the dual-porosity and dual-permeability formulations, the fractures
and matrices are linked by transfer functions. The transfer functions account
for fluid exchanges between both media. To understand the details of this fluid
exchange, an elaborate method is used in this study to model flow in a single
matrix block with fractures as boundaries. Our goal is to develop a technique
to produce accurate results for use in large-scale modeling work.
© 2009. Society of Petroleum Engineers
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History
- Original manuscript received:
11 December 2006
- Meeting paper published:
11 March 2007
- Revised manuscript received:
14 November 2008
- Manuscript approved:
27 November 2008
- Published online:
15 April 2009
- Version of record:
15 April 2009
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