23
The engineering challenges that had to be overcome to produce hydrocarbons
from shale were formidable. Shale is a dual-porosity system, with most of the pore
volume located within the matrix, and less than 1% in natural fractures (Soeder
1988). Thus, the matrix pores provide storage for hydrocarbons, while the fracture
porosity provides flowpaths. It is difficult for oil or gas to move out of the matrix
pores – some of these are so small that the motion is by molecular diffusion rather
than flow, and hydrocarbon migration from some of the smaller pores may even take
place molecule by molecule. However, to produce economical amounts of O&G
from shale, hydrocarbons trapped in the tiny matrix pores must be recovered.
Overpressured gas in the natural fracture system typically provides high levels of
initial production that drops off quickly as the fractures deplete. The long-term production of shale wells requires hydrocarbons to move from the matrix and into the
production well. The goal of the reservoir stimulation process is to make it easier for
hydrocarbons to flow out of the matrix and into permeable pathways like fractures
that are connected to the wellbore.
A number of researchers have explored the pore structures of shale, and the processes of liquid and gas movement through these rocks (i.e. Josh et al. 2012). Shale
pores are generally classified as follows: (1) interparticle porosity between grains,
crystals or clay flakes, (2) intraparticle porosity within pyrite framboids, clay aggregates, dissolution pores on the rims of crystals, and moldic pores within fossils,
pellets, or crystals, (3) porosity within kerogen or other organic matter, and (4)
microfracture porosity (Loucks et al. 2012). Many of these pores are less than a
micrometer to only a few nanometers in size (Rodriguez et al. 2014).
Darcy’s Law allows for a limited number of adjustments to be made on the variables to increase q, the discharge rate of fluids at very low permeability (k) values.
Higher q values can be obtained by increasing the cross-sectional surface area (A),
reducing the flowpath length (L), decreasing the viscosity (μ) of the fluid, and boosting the differential pressure (ΔP). Although the viscosity of oil in a reservoir can be
altered a number of different ways, changing the viscosity of natural gas contained
within a rock pore system is not practical. The engineers could only work with A, L,
and ΔP in their attempts to develop shale gas resources.
This is where fracking became important. Hydraulic fracturing was used to create closely-spaced, high-permeability flowpaths into the rock. These reduced the
distance or flowpath length (L) that the hydrocarbons had to follow to exit the
matrix, which according to Darcy’s Law increases q. The flat hydraulic fracture
faces penetrating the rock also expanded the surface area (A) of the matrix in contact
with high permeability flowpaths, again increasing q. Finally, lower pressures in the
fracture system connected to the production well raised the differential pressure
(ΔP) between the fracture and the matrix, also increasing q.
Thus, all of these factors together enabled the hydrocarbons to flow more easily
from the shale matrix, and when a sufficient volume of rock had been treated, economical quantities of oil and gas could be recovered from a well. This had been
known in theory for quite some time, but achieving it in practice turned out to be
immensely challenging (Soeder 2017).
2.2 Why Frack?
The engineering challenges that had to be overcome to produce hydrocarbons
from shale were formidable. Shale is a dual-porosity system, with most of the pore
volume located within the matrix, and less than 1% in natural fractures (Soeder
1988). Thus, the matrix pores provide storage for hydrocarbons, while the fracture
porosity provides flowpaths. It is difficult for oil or gas to move out of the matrix
pores – some of these are so small that the motion is by molecular diffusion rather
than flow, and hydrocarbon migration from some of the smaller pores may even take
place molecule by molecule. However, to produce economical amounts of O&G
from shale, hydrocarbons trapped in the tiny matrix pores must be recovered.
Overpressured gas in the natural fracture system typically provides high levels of
initial production that drops off quickly as the fractures deplete. The long-term production of shale wells requires hydrocarbons to move from the matrix and into the
production well. The goal of the reservoir stimulation process is to make it easier for
hydrocarbons to flow out of the matrix and into permeable pathways like fractures
that are connected to the wellbore.
A number of researchers have explored the pore structures of shale, and the processes of liquid and gas movement through these rocks (i.e. Josh et al. 2012). Shale
pores are generally classified as follows: (1) interparticle porosity between grains,
crystals or clay flakes, (2) intraparticle porosity within pyrite framboids, clay aggregates, dissolution pores on the rims of crystals, and moldic pores within fossils,
pellets, or crystals, (3) porosity within kerogen or other organic matter, and (4)
microfracture porosity (Loucks et al. 2012). Many of these pores are less than a
micrometer to only a few nanometers in size (Rodriguez et al. 2014).
Darcy’s Law allows for a limited number of adjustments to be made on the variables to increase q, the discharge rate of fluids at very low permeability (k) values.
Higher q values can be obtained by increasing the cross-sectional surface area (A),
reducing the flowpath length (L), decreasing the viscosity (μ) of the fluid, and boosting the differential pressure (ΔP). Although the viscosity of oil in a reservoir can be
altered a number of different ways, changing the viscosity of natural gas contained
within a rock pore system is not practical. The engineers could only work with A, L,
and ΔP in their attempts to develop shale gas resources.
This is where fracking became important. Hydraulic fracturing was used to create closely-spaced, high-permeability flowpaths into the rock. These reduced the
distance or flowpath length (L) that the hydrocarbons had to follow to exit the
matrix, which according to Darcy’s Law increases q. The flat hydraulic fracture
faces penetrating the rock also expanded the surface area (A) of the matrix in contact
with high permeability flowpaths, again increasing q. Finally, lower pressures in the
fracture system connected to the production well raised the differential pressure
(ΔP) between the fracture and the matrix, also increasing q.
Thus, all of these factors together enabled the hydrocarbons to flow more easily
from the shale matrix, and when a sufficient volume of rock had been treated, economical quantities of oil and gas could be recovered from a well. This had been
known in theory for quite some time, but achieving it in practice turned out to be
immensely challenging (Soeder 2017).
2.2 Why Frack?