American Oil and Gas Reporter - February 2015 - 78

SpecialReport: Improved Oil Recovery
FIGURE 2
Schematic of Model Showing
How Porosity Systems are Connected

Kerogen

Shale matrix

Inorganic
- Water wet
- No significant
adsorption/desorption
- Low porosity
- Small pore size
- Higher grain density
- Very low permeability

- Organic wet
- Hydrocarbon
adsorption/desorption
- High porosity
- Small pore size
- Lower grain density
- Low permeability

Shale matrix

Mass/Heat
Transfer:

Fracture
- High permeability
(serves as flow path)

a few nanometers up to a micrometer-is thought to play an important role in hydrocarbon transport processes and permeability patterns, given its relatively high porosity.
On one hand, high capillary pressure in oil-wet kerogen related to the small pore throat sizes in shale could cause this media to pull and store hydrocarbons (mainly oil). On the other hand,
a large connected network of highly porous kerogen could reach
a "percolation" threshold and produce a pathway for hydrocarbons (mainly gas) to move within shale. Made mostly of carbonrich material with a high surface ratio, kerogen adsorbs hydrocarbon molecules onto its surface. As reservoir pressure drops,
adsorbed hydrocarbons may desorb and be produced at a later time.
The porosity of the inorganic frame of quartz, clay minerals,
carbonate, pyrite, etc., in the shale matrix appears to be subject
to debate, which is understandable given the considerable variations in the microstructures of various shale plays. This article
assumes that the inorganic matter in which the kerogen grid blocks
are distributed has a slightly lower porosity than the kerogen.
Having a designated inorganic submedium with water-wet characteristics contributes to the ability to better model dynamic waFIGURE 3A
Average Pressure in Shale Matrix and Fracture
40
35

5,000
4,000

25
3,000

20
15

2,000

10
1,000
5
0
1.0E+00

0
1.0E+01

1.0E+02

Inorganic

1.0E+03
1.0E+04
1.0E+05
Dimensionless Time
Kerogen

1.0E+06

Bulk Matrix

78 THE AMERICAN OIL & GAS REPORTER

1.0E+07

Fracture

Pressure (psi)

Pressure (MPa)

ter behavior. Despite kerogen, hydrocarbon adsorption in inorganic matter could be so weak as to be neglected under moisture
conditions. In fact, inorganic matter can store hydrocarbons only
through compressed fluid accumulated in its pores.
Although conventional techniques can be used to model liquid (water or oil) flow in shale, such models tend to inaccurately predict gas flow behavior because of the significance of molecule/wall collisions and the consequent rarefaction effects. The
current model accounts for such deviations from conventional gas
flow behavior by incorporating a second-order slip boundary condition and a correction term expressed as rarefaction coefficient.
This mechanism is considered to occur for the gas phase flow in
both kerogen and inorganic matter, but the coefficients vary, based
on the mean pore sizes.
Inorganic and kerogen grid blocks in the model are surrounded by high-permeability natural or hydraulic fracture grids. For
a natural fracture, the grid network serves as a pathway to connect the shale matrix to the hydraulic fracture system and wellbore. During production, hydrocarbons flow from the matrix into
the natural fracture network, and the pressure within the fracture
network can be depleted immediately to the level of the bottomhole pressure. Numerous kerogen flakes can be crossed by highpermeability fractures and act as hydrocarbon sources for those
pathways. Figure 2 shows a schematic of the mass transfer between different porosity systems.
Shale Matrix Depletion
Figure 3A shows the average pressure in the shale matrix and
fracture using a liquids-rich shale reference case for hydrocarbon production/water imbibition from and into the matrix. Because of the small dimensions of the simulation case, the graph
is plotted using a dimensionless time function to better represent
simulation progress in a more readable time scale.
As shown, pressure in the inorganic matter decreases when
hydrocarbon production starts as oil and gas leave water-wet inorganic matter (with higher gas and organic phase pressures) toward the low-pressure fracture network. At later times, when pressure depletes in the matrix, water phase pressure in the water-wet
inorganic matter falls below the fracture pressure, resulting in
counter-current water imbibition from the fracture into the matrix.
The average pressure in kerogen follows a trend similar to that
of inorganic matter in early simulation time. In the middle stages
of simulation, the pressure decrease in kerogen is slightly steeper than in inorganic matter, indicating a more efficient depletion
process in kerogen as a result of its higher permeability and hydrocarbon movement from kerogen into the fracture.
Oil movement in kerogen follows two flow paths. The first relates to kerogen grid blocks that are directly connected to frac-

American Oil and Gas Reporter - February 2015

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