The calculation of site-level basic storage capacity (Level B) takes the CO2 geological storage engineering site as the evaluation unit. It usually requires numerical simulation of the conditions of CO2 injection into the reservoir to obtain an accurate storage life of the engineering site. . Rich and accurate data are required in the calculation process. The basic flow of basic storage calculation is shown in Figure 2-2.

Figure 2-2 Basic flow chart for basic storage capacity evaluation

(1) Geological modeling of storage site

1. Stratum characteristics description

< p>Use the obtained site geological background data and drilling data to conduct a detailed description of the stratigraphic lithology, occurrence changes, geological structure and distribution characteristics of the calculation unit.

2. Generalization of reservoirs and caprocks

Evaluation units are evaluated based on factors such as site geological structure, stratigraphic lithology, reservoir and caprock combinations and spatial distribution, hydrogeological conditions, resource status, etc. The reservoirs and caprocks within the reservoir are refined and generalized, and a high-precision three-dimensional geological model is established.

3. Reservoir parameter generalization

Reservoir parameter generalization requires complete parameters and accurate description of the reservoir and caprock. Carry out detailed parameter partitioning for reservoir porosity, permeability, reservoir type, water saturation in the reservoir, water chemistry type, etc., and overlay each parameter partition to perform calculations for each parameter partition.

(2) Calculation method

Compared with Level C control potential evaluation, the calculation scope of site-level basic storage capacity (Level B) is limited to the site level, requiring complete and precise parameters. , the calculation method is accurate. When calculating, the theoretical storage capacity of the site must first be calculated, and then the effective storage capacity, which is the B-level basic storage capacity, must be calculated based on static storage capacity.

1. Deep saline aquifer

First, calculate the storage capacity of the three parts: structural stratigraphic storage, bound gas storage and dissolved storage, and then calculate the site-level basic storage capacity (level B). Storage capacity.

(1) Structural stratigraphic storage mechanism

The storage of CO2 in deep saline aquifer structural stratigraphic traps is similar to the storage of CO2 in depleted oil and gas reservoirs. The difference is that the traps are filled with of water rather than hydrocarbons. The calculation formula is shown in formula (2-37).

Overview of carbon dioxide geological storage technology methods

In the formula: is the theoretical storage capacity of CO2 in structural stratigraphic traps in deep saline aquifers, 106t; Φ is a certain amount of rock in deep saline aquifers. The average porosity of a point, %; Swirr is the residual water saturation of a certain point of deep saline aquifer rocks, %; is the density of CO2 under formation conditions, kg/m3; x, y, z are spatial variables, m. It is a function of formation pressure and temperature. During the CO2 injection process, the formation pressure changes between the original formation pressure p1 and the caprock threshold pressure pmnx. Therefore,

(2) Bound gas mechanism

When calculating the B-level basic storage capacity, the porosity Φ and relative permeability (related to saturation) can be obtained through core analysis tests. Because the CO2 saturation and the rock volume ΔVtrap that was originally saturated with CO2 and then immersed in water can be determined through numerical simulation methods (Kumar et al., 2005; Juanes, 2006). When the accuracy of the obtained parameters meets the requirements, if the numerical simulation method is used to determine △Vt rap and then the calculated results can be used as the effective storage volume of the bound gas mechanism. The calculation formula is shown in formula (2-15).

(3) Dissolution mechanism

The CO2 content when the original formation water is saturated depends on factors such as the pressure, temperature and salinity of the aquifer. If the pressure, temperature and salinity distribution data of the aquifer are available at the site, it can be calculated using Equation (2-38).

Overview of carbon dioxide geological storage technology methods

In the formula: is the theoretical storage capacity of C-level control potential for dissolution and storage of CO2 in deep saline aquifers, 106t; Φ is the deep saline aquifers. The porosity of the rock, %; ρs is the density of the formation water when it is saturated with CO2, kg/m3; ρi is the initial density of the formation water, kg/m3 is the mass fraction of CO2 in the formation water when the formation water is saturated with CO2, %; is the mass fraction of original CO2 in formation water, %; x, y, z are spatial variables, m.

(4) The total theoretical storage capacity of CO2 in the deep saline aquifer

The calculation formula for the total theoretical storage capacity of CO2 in the deep saline aquifer is shown in Equation (2-39) .

Overview of carbon dioxide geological storage technology methods

In the formula: is the theoretical storage capacity of CO2 in deep saline aquifers, 106t; is the structural stratigraphic trap of CO2 in deep saline aquifers is the storage capacity of CO2 bound gas in deep saline aquifers, 106t; is the storage capacity of CO2 dissolved and stored in deep saline aquifers, 106t.

(5) Effective storage capacity

Based on the actual conditions of the site, consider that during the storage process of CO2 in deep saline water layers, it is affected by the heterogeneity of the reservoir, the buoyancy of CO2, and the influence of CO2 Affected by factors such as efficiency and the spatial dispersion and dissolution of CO2 throughout the deep salt water layer, the calculation formula for the effective storage capacity of the deep salt water layer is shown in Equation (2-40).

Overview of carbon dioxide geological storage technology methods

In the formula: is the effective storage capacity of CO2 in deep saline aquifers, 106t; is the theoretical storage capacity of CO2 in deep saline aquifers , 106t; Ce is the effective storage coefficient, with a dimension of 1. This value should be determined according to the specific actual situation. At the same time, it can be determined through numerical simulation or actual engineering experience.

2. Oil fields

When calculating the basic storage capacity at the site level, the production status of different oil fields should be treated separately, and the calculation method cannot be completely based on the storage capacity calculation method of depleted oil reservoirs. Oilfields suitable for CO2 flooding should consider the CO2 geological reserves if this technology is used.

(1) Calculation of theoretical storage capacity of depleted oil reservoirs

This method is used to calculate CO2 geological reserves of depleted oil reservoirs on site. The basic assumption is that CO2 is injected into the depleted oil reservoir until the reservoir pressure returns to the original reservoir pressure, that is, the space vacated by oil and gas production is used for CO2 geological storage. The calculation formulas can be found in Equation (2-2), Equation (2-3) and Equation (2-18).

(2) Calculation of theoretical storage capacity of CO2 injection enhanced oil recovery (CO2-EOR) reservoir

If the oil reservoir on the site has not reached depletion state, it is still being exploited , and if CO2 flooding technology is used, the theoretical storage capacity can be calculated using the storage capacity calculation method of CO2 enhanced oil recovery (CO2-EOR) reservoirs.

1) Without considering injection water, produced water and dissolution mechanism issues, and using CO2 to improve oil recovery, the theoretical storage capacity of CO2 in the reservoir can be calculated by equation (2-41) and equation ( 2-42) Two formula calculations.

①Before the CO2 breakthrough:

Overview of carbon dioxide geological storage technology methods

②After the CO2 breakthrough:

Carbon dioxide geological storage technology Overview of the method

2) When injection water and produced water are considered, but the dissolution mechanism is not considered, two formulas, Equation (2-43) and Equation (2-44), are used for calculation.

①Before the CO2 breakthrough:

Overview of carbon dioxide geological storage technology methods

②After the CO2 breakthrough:

Carbon dioxide geological storage technology Method Overview

3) When considering injection water, produced water and dissolution mechanism issues, use the two formulas Equation (2-45) and Equation (2-46) to calculate.

①Before the CO2 breakthrough:

Overview of carbon dioxide geological storage technology methods

②After the CO2 breakthrough:

Carbon dioxide geological storage technology Overview of the method

In the formula: is the theoretical storage capacity of CO2 in the reservoir, 105t; is the density of CO2 under reservoir conditions, kg/m 3; N is the storage capacity of crude oil, 109m3; B0 is crude oil Volume coefficient, m3/m3; ERBT is the recovery rate of crude oil before CO2 breakthrough, %; ERHCPV is the recovery rate of crude oil when CO2 is injected into a certain hydrocarbon pore volume (HCPV), %; Viw is the recovery rate of injected water reservoir Water volume, 109m3; Vpw is the water volume produced from the oil reservoir, 109m3; Cws is the solubility coefficient of CO2 in water, with a dimension of 1; Cas is the solubility coefficient of CO2 in crude oil, with a dimension of 1.

(3) Calculation of reservoir effective storage capacity based on material balance method

The calculation formula is similar to the C-level control potential evaluation, see formula (2-32).

(4) Calculation of oil field effective storage capacity based on analogy method

The calculation method of actual data obtained in the project of using CO2 to improve oil recovery is to calculate the effective storage capacity by introducing the CO2 utilization coefficient Storage capacity. The calculation formula is shown in formula (2-47).

Overview of carbon dioxide geological storage technology methods

In the formula: is the effective storage capacity of CO2 in the oil reservoir, 106t;N. is the increased crude oil volume due to CO2 injection, 109m3; N is the storage volume of crude oil, 109m3; RCO2 is the CO2 utilization coefficient, the ratio of net CO2 injection volume to crude oil production volume, t/bb1

1bb1=159L. .

The CO2 utilization coefficients are different in different regions (Ecofys, 2004). As shown in Table 2-2, the variation range is relatively large, ranging from 0.1 to 0.8t/bbl. Ecofys (2004) proposed using the highest, middle and lowest levels to represent the CO2 utilization coefficient, with corresponding values ??of 0.8, 0.45 and 0.15t/bbl respectively. The calculation of Np value is shown in formula (2-48).

Overview of carbon dioxide geological storage technology methods

In the formula: EEXTRA is the additional recovery rate obtained due to CO2 injection, %; Nc is the original crude oil geological reserves in contact with CC2, 109m3.

Table 2-2 Statistical table of CO2 utilization coefficients of various oil field projects

Stevens et al. (1999) determined the relationship between crude oil weight and CO2 utilization coefficient due to CO2 injection based on the data of 7 oil displacement projects. Empirical relationship between enhanced oil recovery (Figure 2-3).

Ecofys (2004) divided the additional recovery value due to CO2 injection into three levels according to Figure 2-3, namely the highest, middle and lowest levels, with values ??of 5, 12 and 20 respectively.

Figure 2-3 Relationship between crude oil gravity and CO2 enhanced oil recovery

N. It means that it is impossible for all CO2 to come into contact with crude oil during the injection of CO2, so the contact coefficient is introduced, then N. It can be expressed as formula (2-49).

Overview of carbon dioxide geological storage technology methods

In the formula: N is the original crude oil geological reserve, 109m3; C is the contact coefficient, the dimension is 1.

It is generally believed that during the CO2 flooding process, the contact coefficient between CO2 and crude oil is 0.75. Foreign countries usually only know the recoverable storage volume, but not the geological reserves. The original crude oil geological reserves can be calculated using Equation (2-50).

Overview of carbon dioxide geological storage technology methods

In the formula: NR is the final recoverable storage capacity, 109m3; C is the contact coefficient, the dimension is 1; API is the crude oil gravity, API= 141.5/γ. -131.5/γ. is the relative density of crude oil.

Scope of application: Mainly used to calculate the basic storage capacity of CO2 in reservoirs where CO2 is injected to improve oil recovery. Correlation coefficients are obtained from extensive practice in actual oil fields.

3. Gas field

The calculation formula is similar to the C-level control potential, see formula (2-4). The calculation accuracy should be significantly improved.

4. Coal seam

The calculation formula is similar to the C-level control potential, see formula (2-34). The calculation accuracy should be significantly improved.

(3) Application of numerical simulation methods

In the calculation of site-level basic storage capacity, it is necessary to numerically simulate the CO2 migration conditions injected into the reservoir. Numerical simulation can predict the distribution of CO2 in the reservoir in a certain period of time in the future, and then verify the calculation results of CO2 geological reserves. The calculation methods mentioned above are all based on the total amount of CO2 that is finally soluble in the reservoir. However, the storage process of CO2 in the reservoir has a certain time scale, and the amount that can be injected in different periods is different, so numerical simulation is required. Technology to solve this problem, see Chapter 9 Numerical Simulation Technology Methods for details.