A Field Study on Simulation of CO 2 injection-ECBM Production and Prediction of CO 2 Storage Capacity in Un-mineable Coal Seam

Studies have shown that, coal seam, an un-conventional reservoir, is considered as a promising option for CO2 sequestration because of its large CO2 sequestration capacity, long time CO2 trapping and extra enhanced coal-bed methane (ECBM) production benefits. However, unlike conventional reservoirs, gas flow in the coal seam can cause the cleat permeability and porosity change during the injection/production process. In other words, for example, once gas is injected and adsorbed on coal matrix, the matrix will swell, and the cleat permeability and porosity will decrease correspondently. Because of its special features and the nature of gas retention in CBM reservoirs, simulating the production and injection will have more complexity compared to conventional resources. In this paper, a real project is studied and is used to simulate the primary and secondary coal bed methane production and CO2 injection. The key geological and reservoir parameters that are germane to driving ECBM and sequestration processes, including cleat permeability, cleat porosity, CH4 adsorption time, CO2 adsorption time, CH4 Langmuir Isotherm, CO2 Langmuir Isotherm, Palmer and Mansoori parameters, have been analyzed and varied within a reasonable range consistent with typical coal seam performance through sensitivity analysis. The final simulation results of the CBM/ECBM production and CO2 injection show a perfect match with


Introduction
Fossil fuels are currently playing a significant role in the whole world's energy supply.
However, its damage to the environment, especially the CO2 emission resulting in the green house effect, has gotten more and more attention.At present, more technologies have been studied on CO2 sequestration to decrease the CO2 release into the atmosphere.Un-mineable coal seams (seams too deep or too thin to be mined economically) are one of the main types of geological formations targeted for the sequestration of captured CO2 and enhanced recovery of coal-bed methane.
Similar to conventional naturally fractured reservoirs, coal is characterized as a dual-porosity system consisting of matrix and cleat, in which majority of the gas is stored within the coal matrix by a process of adsorption and a small amount of free gas exists in the cleats or fractures [1] .When CO2 is injected into the coal seam, because of its higher affinity to the coal matrix than methane, it will be held by coal and displace the methane to boost extra natural gas production (Fig. 1).It is estimated by laboratory measurements that, this process, known as CO2-enhanced coal bed methane, can store twice as much CO2 as the methane desorbed or even more [2] .

Fig.1 CO2 injection-ECBM production Phenomenon
The entire gas flow mechanism can be summarized in three steps: 1) once free gas or water existing in cleats is produced from coal seams, the pressure will drop in fracture and micro-pores, then the adsorbed gas will be released from the matrix 2) due to the gas molecular concentration difference, gas will diffuse from matrix to cleats 3) Finally, all the gas will flow from the cleats to the wellbore by Darcy's flow (Fig. 2).
On the other side, if CO 2 is injected into a coal seam, it will store within the seam because of its higher affinity, the methane adsorbed on the matrix will be displaced, which results in enhanced methane production.Nowadays, the numerical reservoir simulator becomes the most popular tool to predict coal seam performance, which provides a good understanding of how gas flows from the reservoir to the wellbore [3] .
In this case, QUICK CBM SETUP in CMG is used for modeling.The theories behind this process are included into the simulation model to mimic the real condition inside the coal seam: As shown in Fig. 3, Langmuir Volume (V L ) is the maximum amount of gas that can be adsorbed on a piece of coal at infinite pressure.Langmuir Pressure (P L ) is the pressure at which the Langmuir volume can be adsorbed.V(P) is the amount of gas at different pressure, also known as gas content (scf/ton).Whenever the Langmuir volume and Langmuir pressure is known, the adsorbed gas amount can be calculated at any pressure.Fig. 3. Langmuir Isotherm Function

2) Diffusion
Diffusion is the fact that particles move/spread from high concentration to low concentration region.Diffusion of gas out of the coal matrix can be expressed by a simple diffusion equation.The diffusion process in coal seams can be described by either diffusion coefficient or coal desorption time input in the simulator [3] .D: Diffusion coefficient ̅ : Average gas concentration in the matrix τ: desorption time 3) Coal shrinkage and swelling One of the unique characteristics of coal seam is the phenomenon of pressure dependent permeability.As the production from the reservoir take places, two distinct phenomena occur.First, the reservoir pressure declines, causes the pressure in the fractures to decline as well, which leads to an increase in the effective stress within the cleats causing the cleats to be more compactable, so the cleat permeability will decrease.At the same time, the gas that has been desorbed is coming out of the matrix, which causes the matrix to shrink and the cleats to open-up; thereby the cleat permeability will be increased.As a function of the pressure drop, compressibility dominates in early time and shrinkage dominates in the late time [4] .Palmer and Mansoori model [5] is used to simulate the permeability change process during production/injection in this model.These wells have been producing coalbed methane since 2004.The center wells (MH18 and MH20) have been converted to CO2 injection wells since September 2009 [5] .20,000 short tons are required to be injected through well MH18 and MH20 in two years (Fig. 4).

Fig.4 Schematic graphs for project description
A dual-porosity model is generated when all necessary basic information such as structure map, thickness map, well trajectory, initial reservoir condition are ready (Fig. 5).Some coal properties, such as initial coal permeability, porosity, Langmuir isotherm equation, and Palmer & Mansoori equation, are difficult to obtain and will be assumed in a reasonable range.Langmuir strain and Langmuir volume.For instance, if CH4 desorption time increases, it will make desorption process slower, then matrix shrinkage process becomes slower, thereby the cleat permeability increase rate will be slower, and finally gas will be produced slower in limited time.There is no doubt that gas production of slower producing rate will be less than that of faster producing rate in the same range of time.As shown in Fig. 7, the rose red dots and the red line represents the actual daily gas rate and cumulative gas production, respectively; the blue line and the green line illustrates the simulated gas rate and cumulative gas production, respectively.Similarly, the red dots and red line in Fig. 8 shows the actual CO2 injection rate and CO2 cumulative injection amount, respectively; while the blue dots and blue dashed line presents the simulated results of CO2 injection rate and CO2 cumulative injection amount, respectively.
In the history matched model, two different regions for MH18_inj and MH20_inj have been set.Most parameters, such as desorption time, gas content, Langmuir volume, Langmuir pressure, Langmuir strain, fracture spacing, Young's modulus &Poisson ratio, matrix porosity, matrix permeability, have been set in the reasonable range consistent with typical CBM reservoir performance according to literature review In order to simplify the calculation, two assumptions have been made: 1) the coal reservoir is depleted without any other gas or water, so it can be full of injected CO2; 2) since most of the gas is adsorbed on the coal matrix and the free gas only accounts for small portion, the free gas is ignored in this case.So the adsorption of CO2 capacity can be calculated as:

CO2 Injection Prediction
Based on the calculation, it can be seen that the CO2 sequestration capacity is more than the required 20,000 tons.Prediction on CO2 injection has been performed thereafter to figure out when the 20,000 tons of CO2 injection will be fulfilled.
Currently, the West Virginia Environmental Protection Agency has recommended a maximum injection pressure of 933 psi and it should not be exceeded during injection.
This value was obtained through a mechanical integrity test of the formation based on the fracture pressure.In other words, it provides a safety factor.Therefore, different prediction scenarios have been done using maximum injection pressure of 933psi and different maximum injection gas rate as constraints.The prediction time is from 2011-1-10 to 2030-1-10.All injection scenarios are shown below in Table 3:

Fig. 2
Fig.2 Gas flow mechanism in coal seam 1) Langmuir Isotherm the stress-dependent permeability term; ( )   ): the matrix shrinkage term Фi: Initial fracture porosity Cf: pore volume compressibility P: initial pressure M: axial modulus K: bulk modulus Є: Langmuir strain P L : Langmuir pressure Project Description From 2009, the CO2 sequestration with ECBM production project began in Marshall County, West Virginia.The objective of this DOE sponsored project was to help mitigate climate change by providing an effective and economic way to permanently store CO2 in un-minable coal seams.In advance of CO2 injection, four horizontal coalbed methane wells (MH5, MH11, MH18 and MH20) were drilled into the un-minable Upper Freeport coal seam, which are 1,200 to 1,800 feet below ground.

Fig. 5 3
Fig.5 3-D view of the coal seam model (CO2) 470ft3/ton Palmer&Mansoori exponent 3 Initial gas content(CH4) 180ft3/ton Sensitivity Analysis Sensitivity analysis is the study of how the variation (uncertainty) in the output of a mathematical model can be apportioned, qualitatively or quantitatively affected by the change of different variations in the input of the model.In this case, several parameters including cleat permeability, CH 4 desorption time, CO 2 desorption time, CH 4 Langmuir volume, CO 2 Langmuir volume and Palmer & Mansoori parameters varied in a reasonable range have been tested as shown in Fig.6.

Fig. 6
Fig. 6 Sensitivity analysis on different reservoir parameters

Fig. 7
Fig. 7 History matching for CBM/ECBM production Injection prediction scenariosAs shown in Fig.9and Fig.10, MH18_inj is the main contributor on CO2 injection with the maximum injection gas rate of 100,000scfd.Its cumulative CO2 injection amount reaches 11,102tons by 2030, which is far more than 700tons in well20_inj.Until 2030, the total maximum CO2 injection amount is 11,802 tons.In other words, it is impossible to inject 20,000 tons of CO2 into the coal seam during two years, even in twenty years.In fact, our model shows that the cumulative CO2 injection is about 2,570tons in two years.Referring to the actual injection data, the total injection amount is only 2,506 tons in two wells from September 2009 to October 2011, which verifies our simulation results again.

Table 1
summarizes all the initial input values.All the initial parameter values are constant within a reasonable range of typical coal seam performance, because it can become much simpler for us to perform each simulation run in order to find out how different parameters affect the final interested outcome, which is, in this case, CO2 injection and CBM/ECBM production.Of course, the final history matched model won't be homogeneous in this real case.

Table 1
Initial reservoir parameter setting