CHEMICAL ENGINEERING TRANSACTIONS VOL. 56, 2017 A publication of The Italian Association of Chemical Engineering Online at www.aidic.it/cet Guest Editors: Jiří Jaromír Klemeš, Peng Yen Liew, Wai Shin Ho, Jeng Shiun Lim Copyright © 2017, AIDIC Servizi S.r.l., ISBN 978-88-95608-47-1; ISSN 2283-9216 Performance Assessment of CO2 Sequestration in a Horizontal Well for Enhanced Coalbed Methane Recovery in Deep Unmineable Coal Seams Syahrir Ridhaa, Edo Pratama*,b, Mohd. Suhaili Ismailb aDepartment of Petroleum Engineering, Faculty of Geosciences and Petroleum Engineering, Universiti Teknologi PETRONAS, 32610 Bandar Seri Iskandar, Perak Darul Ridzuan, Malaysia bDepartment of Geosciences, Faculty of Geosciences and Petroleum Engineering, Universiti Teknologi PETRONAS, 32610 Bandar Seri Iskandar, Perak Darul Ridzuan, Malaysia edo.pratama1@yahoo.com Although the CO2 injection for enhanced coalbed methane (ECBM) recovery is one of the potential coalbed methane production techniques, the effectiveness of the process is greatly dependent on the coal seam and method for CO2 injectivity enhancement which is still becomes one of the technical challenges. This study has therefore aimed to investigate the performance of CO2 sequestration for ECBM recovery through a horizontal well in deep unmineable coal seams. To achieve the objectives, a novel 3D numerical model was developed based on the characteristics of coal seams in Indonesia’s Basins and reservoir simulation study was performed. From the results, the productivity of methane was increased by applying horizontal well instead of vertical well, especially for the coal seams with low permeability. In addition, CO2 sequestration coupled with the use of a horizontal well resulted in the volume of CO2 stored in deep unmineable coal seams increases to three times larger than a vertical well, it depends on the horizontal well length. 1. Introduction Sequestration of carbon dioxide (CO2) in coal seams is benefit to mitigate greenhouse gas emissions and enhanced coalbed methane (ECBM) recovery. For the purpose of CO2 emission reduction, CO2 must be stored in coal permanently, the coal seams used for storing CO2 should be unmineable forever, otherwise, coal mining, combustion, or gasification would release CO2 stored in the coal (Li and Fang, 2014). Thus, unmineable coal seams have the potential to store large volume of CO2 (Corum et al., 2013). DOE’s Midwest Geological Sequestration Consortium (MGSC) defines unmineable coal: all coals at 152-305 m deep, coal seams 0.5-1.1 m thick and so are reasonable sequestration targets (NETL, 2010). At present, CO2 sequestration for the ECBM recovery (CO2-ECBM) has been studied to minimize the CO2 release into the atmosphere, and these projects have been operating all over the world, such as the Fenn-Big Valley project in Canada, with two wells using a “huff and puff” scheme (Gunter et al., 2004), Yubari project in Japan, with a vertical injection well and a producing well (Fujioka et al., 2008). Although the ECBM recovery process is one of the potential coalbed methane (CBM) production enhancement techniques, the method for CO2 injectivity enhancement is still become one of the technical challenge. Horizontal well may be an effective way to increase CO2 injectivity compared with conventional vertical wells (Li and Fang, 2014). However, there is no study performed in assessing the effectiveness of the horizontal well for CO2-ECBM, especially in deep unmineable coal seams. This study has therefore aimed to investigate the performance of CO2 sequestration for ECBM recovery through a horizontal well as the CO2 injectivity enhancing technology in deep unmineable coal seams. In addition, the comparison on production performance of vertical and horizontal wells during ECBM will be examined and analysed by varying the well spacing. Furthermore, an optimization of horizontal well for CO2-ECBM will be carried out by optimizing the well spacing and length of horizontal section. Finally, a sensitivity analysis will be DOI: 10.3303/CET1756099 Please cite this article as: Ridha S., Pratama E., Ismail M.S., 2017, Performance assessment of co2 sequestration in a horizontal well for enhanced coalbed methane recovery in deep unmineable coal seams, Chemical Engineering Transactions, 56, 589-594 DOI:10.3303/CET1756099 589 conducted to evaluate the production performance of CO2-ECBM in a horizontal well based on the different reservoir parameter of the coal seams. 2. Methodology A numerical modelling simulation was used to model the coalbed methane reservoir using Generalized Equation of State Model-Computer Modeling Group (GEM-CMG) compositional simulator. Modelling developed by combining all of supporting data in terms of geology and reservoir, then the next step is to conduct the initialization to validate the reservoir model. Having obtained the valid model, the CO2 storage capacity is estimated and a horizontal well is then designed and modelled to produce coalbed methane with the primary recovery method. Afterwards, the CO2 injector wells (vertical and horizontal wells) are designed and modelled to inject CO2 for the ECBM recovery. Subsequently, the comparison of primary CBM production and ECBM methods is analysed by performing production forecasting for 30 y. A sensitivity study is then conducted in order to examine and analyse the performance of CO2-ECBM through vertical and horizontal injector wells by varying the well spacing. This is followed by an optimization of horizontal well for CO2-ECBM by varying CO2 injection scenario which are well-spacing between well producer and well injector and length of horizontal section of the injector well. The CBM production performance resulted from several scenarios will be compared and analysed for looking the optimum production and the optimum CO2 injection model is then determined. Having determined the CO2 injection model, a parametric study on the numerical model is conducted to assess the production performance based on a wide range of reservoir parameter of the coal seams. To understand the methodology, Figure 1 shows the study workflow. Figure 1: The study workflow 3. Model Development A 24 x 23 x 6 (3312 grid) model which covers 670 acres of unmineable coal seams lying ± 1,050 m below the ground surface with total thickness of 6 m was considered for the model development. The model parameters used in this study based on the coal seams characteristic in South Sumatera Basin (Steven and Hadiyanto, 2004), Barito Basin (Sapiie et al., 2014) and Kutai Basin (Apriyani et al., 2014), Indonesia. Storage and compositional properties (Sosrowidjojo, 2013) and gas composition (Mazumder et al., 2010) from CBM wells in South Sumatera Basin were also considered during model construction. Having constructed a novel 3D numerical model, the model was then validated by initializing the results of GIP with volumetric method and initial reservoir pressure (Pi) from model with actual pressure data. The GIP resulted from model is about 240.76 MMm3 while GIP from volumetric computation is estimated about 235.38 MMm3, thus, the differences of about 2.35 %. Furthermore, initial reservoir pressure at reference depth of 1,051 m resulted from model is about 10,700 KPa, it has differences of about 1.90 % from actual pressure data (10,500 KPa at 1,051 m). The difference of IGIP between volumetric and simulator model as well as Pi between actual pressure data and simulator model 590 below 5 % are considered good match and acceptable in the reservoir engineering practice. Therefore, the developed CBM reservoir model is valid and it is then used to estimate the CO2 storage capacity. According to the results of the model, the total CO2 sequestration capacity is estimated of about 222.86 MMm3. 4. Coalbed Methane (CBM) Recovery Process 4.1 Comparison of Primary CBM and Enhanced CBM Recovery Primary methane (CH4) production capacity from the unmineable coal seams was examined using a horizontal well with length of horizontal section of 560 m during 30 y of simulation. The CH4 production performance from primary production was then analysed and compared to the CO2-ECBM technique. For CO2-ECBM purposes, vertical and horizontal injector wells were modelled with the well-spacing between CBM producer and CO2 injector of about 140 m. The horizontal well has length of horizontal section of 280 m. The CO2-ECBM technique was examined by injecting CO2 into the coal seams at maximum of 15,000 kPa injection pressure and injection rate of 10,000 m3/d. According to the production simulation results from 2016 until 2046 (Figure 2), total cumulative CH4 production with primary CBM production is about 129.04 MMm3 with recovery factor of 53.6 %. While simulation results of CO2-ECBM, the model forecast showed that total cumulative CH4 production with the vertical well injector of 149.85 MMm3 with recovery factor of 62.23 % and the horizontal well injector resulted in 202.96 MMm3 of cumulative CH4 production with recovery factor of 84.30 %. The simulation results for each CBM recovery method is summarized in Table 1. From the results, application of CO2 sequestration in a vertical well for ECBM can obtain additional recovery factor of about 8.63 % while through a horizontal well can achieve additional recovery factor of about 30.7 % with total incremental reserves compared to primary production of about 73.92 MMm3. In addition, by applying a horizontal well for CO2-ECBM can result in the total CO2 stored in the coal seams increases to three times larger than a vertical well. It further will be examined and analyzed by varying the well spacing and horizontal well length in the next chapter. Figure 2: The comparison of production performance of Primary CBM and CO2-ECBM Recovery Table 1: Summary of the simulation results for each CBM Recovery Method Production Method Volume of CO2 Stored (MMm3) Peak Methane Production Rate (Mm3/d) 30 Years Cumulative CH4 Production (MMm3) 30 Years Recovery Factor (%) Primary No injection 43.13 129.04 53.60 CO2-ECBM (Vertical Well) 30.81 43.02 149.85 62.23 CO2-ECBM (Horizontal Well) 93.11 43.90 202.96 84.30 591 4.2 Vertical and Horizontal Wells Performance in CO2-ECBM Recovery Process A sensitivity study was performed in order to examine and analyse the performance of CO2-ECBM through vertical and horizontal wells by varying the well spacing between CBM producer and CO2 injector wells (70 m, 140 m, 210 m, 350 m, and 490 m). The simulation results of production performance and volume of CO2 stored were observed and analysed in order to compare both of vertical and horizontal wells performance for different well spacings. Table 2 summarizes the simulation results for vertical well performance. From the results, it was observed that the volume of CO2 stored in unmineable coal seams for a period of 30 y was increased from 23 MMm3 to 55.20 MMm3 when the well spacing was increased from 70 m to 490 m. The maximum cumulative CH4 production was achieved for the well spacing of 350 m with recovery factor of about 63.83 %. For the horizontal well (length of horizontal section of 280 m), it is observed that decreasing well spacing from 490 m to 70 m increases the cumulative CH4 production for 30 y from 184.73 MMm3 to 205.98 MMm3 (Table 3). The well spacing of 70 m resulted in the largest cumulative CH4 production with recovery factor of 85.56 % while storing the smallest volume of CO2 in unmineable coal seams which is 88.68 MMm3. Table 2: Summary of the simulation results for each well spacing of the vertical well Well Spacing (m) Volume of CO2 Stored (MMm3) Peak Methane Production Rate (Mm3/d) 30 Years Cumulative CH4 Production (MMm3) 30 Years Recovery Factor (%) 70 m 23.00 43.33 147.48 61.26 140 m 30.81 43.02 149.85 62.24 210 m 39.30 43.26 152.75 63.44 350 m 50.12 44.70 153.67 63.83 490 m 55.20 43.65 150.63 62.56 Table 3: Summary of the simulation results for each well spacing of the horizontal well Well Spacing (m) Volume of CO2 Stored (MMm3) Peak Methane Production Rate (Mm3/d) 30 Years Cumulative CH4 Production (MMm3) 30 Years Recovery Factor (%) 70 m 88.68 44.25 205.98 85.56 140 m 93.11 43.90 202.96 84.30 210 m 99.25 44.48 201.61 83.74 350 m 105.21 46.73 194.23 80.67 490 m 107.14 45.17 184.73 76.73 4.3 Optimization of Horizontal Well for CO2-ECBM Recovery An optimization of CO2 sequestration in a horizontal well for ECBM was carried out by varying length of horizontal section. In the sensitivity study of horizontal well length, the selected 70 m well spacing was investigated by varying lateral length from 210 m to 560 m. Volume of CO2 stored and production performance of 30 y for each case are summarized in Table 4. The results of methane production rate and cumulative production obtained from the simulation for each length are plotted in Figure 3. It is also note that the vertical length (surface to the starting point of horizontal) of each horizontal well is constant. It is observed that increasing length of horizontal well from 210 m to 560 m increases the cumulative CH4 production for 30 y from 195.16 MMm3 to 219.23 MMm3. In addition, volume of CO2 stored in the coal seams also increases from 76.83 MMm3 to 104.23 MMm3. Table 4: Summary of the simulation results for each horizontal well length Horizontal Well Length (m) Volume of CO2 Stored (MMm3) Peak Methane Production Rate (Mm3/d) 30 Years Cumulative CH4 Production (MMm3) 30 Years Recovery Factor (%) 210 m 76.83 43.94 195.16 81.06 280 m 88.68 44.22 205.98 85.56 350 m 95.77 44.52 212.52 88.27 420 m 100.27 44.80 216.55 89.94 490 m 102.80 45.19 218.57 90.78 560 m 104.23 45.24 219.23 91.06 592 Figure 3: Production performance of various horizontal well lengths 4.4 Sensitivity Analysis A sensitivity analysis was carried out to examine the influences of different reservoir parameter on the numerical model in order to assess the performance of CO2-ECBM. The ‘High’, ‘Low’ and ‘Base’ cases were designed for the value of each uncertain parameter, which were quantified through the sensitivity analysis. In this analysis, the infuences of the reservoir condition on cumulative methane production for the CO2 sequestration in a horizontal well (well spacing 70 m and length of horizontal section 560 m) was investigated using the values of each parameter assisgned from ‘High’, ‘Low’ and ‘Base’ cases. The values assisgned in each case are summarized in Table 5. Table 5: Parameter used in sensitivity analysis Reservoir Parameter Low Case Base Case High Case Fracture permeability (mD) 2 4 6 Matrix permeability (mD) 0.1 1 4 Reservoir temperature (oC) 35 40 45 Skin Factor +2 0 -2 Figure 4: Tornado plot indicating the influences of reservoir parameters on cumulative CH4 production 219.23 221.14 216.01 186.21 219.23 217.39 223.54 235.08 170 180 190 200 210 220 230 240 250 Matrix permeability Reservoir temperature Skin Factor Fracture permeability Cumulative Methane Production (MM m3) High Case Low Case 593 The results of sensitivity analysis is presented in the tornado plot in order to show the comparison of the sensitivities of each parameter. Figure 4 shows the results obtained from this sensitivity analysis which orderly indicates the impact of each parameter on cumulative methane production. The axis in the middle of this graph represents the base case selected for the study which has a cumulative CH4 production of 219.23 MMm3. As shown in the tornado chart, fracture permeability is plotted on the top since it has the most significant effect on methane production. It is followed by skin factor and reservoir temperature of the coal seams which are also influential parameters on methane recovery. However, there is no effect of matrix permeability on methane production. 5. Conclusions Through the numerical simulation study which assesses the performance of CO2 sequestration for ECBM in deep unmineable coal seams, it has proven that the productivity of methane was increased by applying horizontal well instead of vertical well, especially for the coal seams with low permeability. In addition, CO2 sequestration coupled with the use of a horizontal well could result in the volume of CO2 stored in deep unmineable coal seams increases to three times larger than a vertical well, it depends on the horizontal well length. From the results of sensitivity analysis, fracture permeability and skin factor have a significant impact on methane production and it therefore these parameters should be considered on application of horizontal well for CO2-ECBM recovery process in unmineable coal seams. Acknowledgments The authors express their gratitude to Universiti Teknologi PETRONAS through Yayasan Universiti Teknologi PETRONAS (YUTP) financial assistance on this project No. 0153AA-E27. References Apriyani N., Suharmono, Momen M., Djaelani S., Sodli A., Satria A., Murtani A.S., 2014, Integrated Cleat Analysis and Coal Coality on CBM Exploration at Sangatta II PSC, Kutai Basin, Indonesia, AAPG International Conference & Exhibition, 14-17 September 2014, Istanbul, Turkey. Corum M.D., Jones K.B., Warwick P.D., 2013, CO2 Sequestration Potential of Unmineable Coal – State of Knowledge, Energy Procedia 37, 5134 – 5140. 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