Файл: Магистерская диссертация тема работы Потенциал закачки со 2 в истощенные месторождения васюганской свиты Томской.pdf

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∆???????? = �????????
????????
− ????????
????????????????
2
� ∙ ???????? ∙ ℎ,
(1) where ∆???????? is the pressure drop at the seal boundary, Pа;
????????
????????
, ????????
CO2
is the density of water and carbon dioxide respectively, kg/m
3
;
g is the gravitational acceleration, m/s
2
;
h is the height of CO
2 plume, m.
All rocks (maybe except for salts only) are somehow permeable, but their pore throats can be so narrow that they create an effective barrier to CO
2
migration. The essential concept here is capillary pressure. The value of capillary pressure depends on the radius of the pore throats and the value of the interfacial tension [37].
Clays and evaporites are characterized by small pore throats radii, due to which they have a high inlet capillary pressure, which prevents further CO
2
migration. For clays, porosity varies from 1 to 12%, the average radius of the pore throat varies from
5 to 100 nm, and the permeability is 10
-6
– 10
-4
mD. Typically, the capillary capacity of seal rocks is determined in the laboratory through experiments using mercury as a non-wetting phase and through microscopic analysis of pore throats.
To determine the quality of a seal rock, we should introduce these two parameters: the sealing number and the stability number.
The sealing number refers to the ratio of capillary breakthrough pressure to the pressure drop across the seal due to buoyancy forces
????????
1
=
????????
????????
∗
∆????????
=
????????∙????????
????????
∙????????????????????????????????∙????????
????????
∙????????
????????
2∙????????∙�????????
????????
−????????
????????????????2
�∙????????∙ℎ
(2)
The second parameter of a seal rock quality is its mechanical stability:
????????
2
=
????????`
????????????????
∆????????
=
(????????
????????
∙????????∙????????
????????
− ????????∙????????∙????????
????????????????????????????????
) − ????????
????????
�????????
????????
− ????????
????????????????2
�∙????????∙ℎ
(3)
where ????????`
????????????????
is initial vertical effective stress at the reservoir depth;
????????
????????
is the height of the water column above the seafloor, m;
????????
????????????????????????????????
is the bulk mass density of the sediment, kg/m
3
; z is the reservoir depth, m;

127
????????
????????
is the initial fluid pressure at the reservoir-seal interface, Pа.
Reservoir pressure changes affect the distribution of stresses, which can cause natural rock fractures to open. Mechanical stability characterizes the ability of the rock matrix to resist changes that occur due to additional pressure drop during injection.
A high-quality seal is characterized by π
1
>>1 value and a high π
2
value. The distribution of these parameters for some of the CCUS projects is presented below
(Figure 8). As it can be seen, the seal of Sleipner CCUS project is characterized by relatively low-quality sealing parameters, since CO
2
injection was carried out into a shallow reservoir with high reservoir properties and large reservoir thickness, due to which a significant pressure drop has developed at the seal-reservoir interface.
However, the reliability of CO
2
storage of Sleipner project is provided by a large seal thickness (50-150 meters) [37].
Figure 8. The distribution of seal quality parameters for different CCUS projects
Despite aforementioned sealing parameters it is necessary to select potential injection targets with the most powerful seals – the generally accepted value is 20 meters. This is due to the fact that even with a high-quality seal diffusion of CO
2
occurs through it at a rate of about 10 meters per 1000 years. In this case, the leakage of carbon dioxide is approximately 3 kg/m
2
year. Carbon dioxide passing through the seal

128 reduces the pH of the environment, which leads to additional degradation of the seal rock. Fluid seal thickness of 20 m provides a secure CO
2
storage for a sufficient time during which a significant part of carbon dioxide chemically traps due to interaction with water and rock (Figure 9).
Injected CO
2
can be physically trapped in a structural or stratigraphic traps or as residual gas because of the relative permeability hysteresis. Geochemically, CO
2
can be trapped by adsorption onto organic material or through dissolution into the formation brine (solubility trapping), where it can interact with the rock matrix and eventually precipitate into stable carbonate minerals (mineral trapping). Hydrodynamic trapping of CO
2
is a process that is affected by a complex combination of the physical and geochemical trapping mechanisms. Each of the trapping mechanisms and processes takes place on a different timescale and therefore has a different degree of importance at different scales. During the injection period and immediately thereafter, the primary trapping mechanism is physical trapping either in stratigraphic or structural traps. In the absence of a significant trap, hydrodynamic trapping will be the primary trapping process [28].
Figure 9. The contribution of CO
2
trapping mechanisms vs time
All of these trapping mechanisms and the complex interactions they have with each other over the lifespan of a CCUS project, must be carefully taken into

129 consideration. When it comes to determining CO
2
storage capacity, the processes that take place on the short-term to midterm timeframe are of primary importance. These processes vary depending on the target, but in most cases, the primary short-term trapping mechanisms are physical and hydrodynamic. Residual gas trapping and mineral trapping do not significantly add to the overall storage capacity of a target formation but rather increase the security of the trapping [24].
The geological object selected for injection must be of sufficient volume to store the approved share of CO
2
emissions during the entire life of the project.
The classification of CO
2
storage resources is made analogous to petroleum industry, since there is a lot in common. The resources classification simplifies work in the following areas:
1. Providing specific criteria for making financial decisions;
2. Accounting for CO
2
storage assets on the state balance;
3. Increasing the efficiency of project management.
Today, the most generally recognized and widespread classification of hydrocarbon reserves is the Petroleum Resources Management System (PRMS) developed by the Society of Petroleum Engineers (SPE). This classification (Figure
10) provides a more accurate reflection of total reserves, since it is more related to the geological characteristics of the reservoir rather than to the predicted production rates.
The PRMS technique takes into account not only the probability of finding hydrocarbons, but also the economic viability of its production [24].
Despite the fact that the process of searching for CO
2 storage formations is in many ways similar to the discovery of oil and gas traps, there are some differences. For example, if a hydrocarbon reservoir has been discovered, then this automatically confirms that this structure can retain fluid for a long time. In the case of CO
2
storage, a similar conclusion cannot be made, since the quality of the seal rock must be additionally confirmed. Another difference is that during oil production, fluid is recovered from the pore space, whereas during CO
2
injection pressure increases and reservoir fluid is displaced into the unexplored part of the reservoir, which increases uncertainty.

130
Figure 10. Petroleum Resources Management System [24]
In order to standardize CO
2
storage potential, Society of Petroleum Engineers has adapted a version of the PRMS classification for CO
2
injection (Figure 11). As illustrated in Figure 11, development projects (and their associated storable quantities) may be subclassified according to project maturity levels and the associated actions required to move a project toward commercial injection.
Figure 11. Subclasses based on project maturity [36]

131
A standardized classification of CO
2
storage resources makes possible the comparison between different projects and gives an understanding of the storage site at each stage. Currently the classification of CO
2
storage resources is based on the classification of oil reserves, but it is expected that it will be improved and eventually become independent.

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