Suitability Analysis of the Potential Gas Fields of Bangladesh for Carbon Dioxide Sequestration and A Simulation Approach for Titas Gas Field

Global warming has become one of the major environmental issues all over the world. The temperature of the world is increasing day by day, and fortunately, everybody wants to mitigate this fearful condition from own perspective. Carbon dioxide sequestration is a worldwide recognized project for making huge amount of carbon inactive, which is the main component of greenhouse gases, by sequestrating it beneath the Earth. The paper aims to dictate the probable success of CO2 sequestration in Bangladesh. As abandoned gas fields are the best options for sequestration, characteristics of the potential gas fields of Bangladesh have been analyzed according to Stefan Bachu's method of classification for CO2 sequestration. Titas gas field shows the best suitability, and accordingly, an Eclipse simulation software basis approach has been taken for this field to determine possible outcomes from probable sequestration project. Simulation result predicts an amount of 2.92 Tcf carbon dioxide gas to be sequestrated after 50 years of simulation. Other criteria such as field pressure, gas saturation are also evaluated.


Introduction
Vast majority of scientists agree that global climate change is occurring and that to prevent it"s most serious effects we must begin immediately to significantly reduce our greenhouse gas (GHG) emissions.One major contributor to climate change is the release of the greenhouse gas carbon dioxide (CO 2 ).As a developing country, carbon dioxide emission rate is explicitly escalating at a very high rate in Bangladesh.The GDP growth rate of Bangladesh increases from 6.5% to 7.1% from the year of 2012 to 2016 (World Bank, 2016) and according to Alam [1], for the growing GDP of Bangladesh, industry and service sectors play a vital role which are also responsible for increasing CO 2 emissions.Also, Bangladesh"s remarkable economic growth is due to high cost of natural resources and health of the environment [2].
Gunter and Rahman [2] have worked together with a vision to provide an alternative projection for Bangladesh's future CO 2 emissions to assess if Bangladesh is likely to achieve 'simple per capita income limit' that would be consistent with the Copenhagen accord by using data for the period of 1972-2005, and their findings are quite important.They have reached to a decision that CO 2 emissions in Bangladesh will be 15 times larger in 2050 than 2005 if there is no improvement in country's energy efficiency.They finally concluded that as poverty reduction is imperative, some increases of CO 2 emissions are unavoidable in Bangladesh and therefore, it is important to minimize this increase as far as possible by providing appropriate technologies [3].
CO 2 emission as a whole and emission rate (metric tons per capita) of Bangladesh from the year of 1994 to 2014 are shown in Fig. 1.Both graphs showing an increase of CO 2 emission.
Bangladesh produces a very small amount of global CO 2 emission.But, the country"s emission scenario has marked a rapid increase of CO 2 emission over a long time.The overall CO 2 emission has been increased to approximately 286% from the year 1994 to 2014 as shown in Fig. 1.Evidently, Bangladesh is trying to implement a lot of initiatives and certain projects to decrease the emission rate and tackling climate changing impacts.Bangladesh has developed climate change strategy and action plan towards mitigation and low carbon development along with other strategic areas [4].Eventually, with feasible economic planning, Bangladesh has a chance to develop a long term plan for CO 2 sequestration in suitable place.Carbon or CO 2 sequestration means putting carbon into long-term storage.It denotes the removal of CO 2 , either directly from anthropogenic sources, or from the atmosphere, and disposing of it either permanently or for geologically-significant time periods [5].CO 2 can be sequestered in deep underground formations of porous rock at least 2,500 feet below ground surface, but diligent geological site characterization is essential.To be suitable for CCS (Carbon Capture & Storage), porous formations must lie under layers of impermeable rock that will provide a "cap" or seal to prevent upward migration of the CO 2 .Carbon dioxide can be sequestered in geological media by geological (stratigraphic and structural) trapping in depleted oil and gas reservoirs, solubility trapping in reservoir oil and formation water, adsorption trapping in uneconomic coal beds, cavern trapping in salt structures, and by mineral immobilization [6].These media have both the space (porosity) and injectivity (permeability) necessary for CO 2 injection, and, by and large, have the ability to either prevent or delay for geologically significant periods of time the CO 2 return to the atmosphere.Crystalline and metamorphic rocks, such as granite, on continental shields are not suitable for CO 2 storage and sequestration because they lack the porosity and permeability needed for CO injection, and because of their fractured nature.Volcanic areas and orogenic belts (mountains) are also unsuitable mainly because they lack capacity and are unsafe.Carbon dioxide sequestration in gas or oil field is far more attractive than other processes.The possibility of geological sequestration of carbon dioxide in Bangladesh is analyzed in this study.A series of suitability criteria for geological sequestration of CO 2 were previously developed [6], they are: -Basin characteristics, such as tectonism, geology and geothermal and hydrodynamic regimes (these are ""hard"" criteria because they do not change).-Basin resources (hydrocarbons, coal, salt), maturity and infrastructure (these ""semihard"" or ""semi-soft"" criteria because they may change with new discoveries, technological advances and/or economic development).-Societal, such as level of development, economy, political structure and stability, public education and attitude (this is "soft" criteria because they are rapidly change or vary from one region to another).These criteria are developed & applied by Stefan Bachu [5].It can also be applied for small regions like oil & gas reservoirs.Site selection for CO 2 sequestration among four potential gas fields of Bangladesh [7] is analyzed here carefully according to Bachu"s method where 15 important criteria are considered.The criteria are, tectonic setting, size, depth, geology, hydrogeology, geothermal, hydrocarbon potential, maturity, coal & CBM, salts, on/offshore, climate, accessibility, infrastructure, CO 2 sources.S. Bachu [6] assigned weight for all characteristics according to their importance.Thus geology, hydrogeology, geothermal, maturity, on/offshore, climate and CO 2 sources are considered as most important criteria.It can also be easily proved that initial capacity of a gas field, its maturity and nearby CO 2 sources would be the most valuable criteria in case of CO sequestration in a gas field.
The suitability analysis of the gas fields is far more reliable after simulation takes place.The ECLIPSE simulator has been the benchmark for commercial reservoir simulation for more than 25 years, covering the entire spectrum of reservoir models, including black oil, gas field operations, compositional, thermal finite-volume, and streamline simulation.In this research, Schlumberger Eclipse (version 2010.1)-E300portion has been used to carry whole simulation process.The grid portion is prepared by Petrel software.Simulation result analyses are the most important part that should be done carefully to interpret reservoir conditions after simulation.

Materials and Methods
Stefan Bachu"s criteria for suitability analysis of CO 2 sequestration can be divided into two parts, i) variable criteria which are different for the gas fields with respect to their respective existing conditions and ii) constant criteria that are dictated by geological conditions of Bangladesh.

Analyzation of Bachu's variable criteria for potential gas fields of Bangladesh
Criteria under this category are, size, depth, geology, hydrogeology, hydrocarbon potential, maturity, infrastructure and CO 2 sources.i) Size: Titas gas field is one of the major contributors in the supply of gas in Bangladesh.It has a size of 16 km by 8 km.It can be considered as large size reservoir along with Rashidpur gas field, which has an area of 35 km by 7 km.Habiganj gas field has a small area of 11.5 km by 4.5 km whereas Kailashtila gas field has a medium area of 17 km by 5 km [8].The classes are mainly assigned with respect to each other of the gas fields.
ii) Depth, Geological and Hydrogeological condition: Titas gas reservoir includes multiple sandstone layers in the Bhuban and Bokabil formations of Miocene-Pliocene age.The depths of the gas reservoirs [8] range from about 2616 m to 3124 m below the surface.Recently BAPEX (Bangladesh Petroleum Exploration and Production Company Limited) has discovered country"s deepest gas reserve at Titas around 7000 m under the ground.Data analysis suggests that there is no major fault but seismic study indicates relatively small faulting having down throw to the east in the crestal region adjoining the eastern flank [9].The fault affects older section at depth of about 2800 to 3000 m [10].Also, Clayton [11] identified few small faults within depths of around 2650 to 2750 m.Other analysis also suggests the same.Water injection with reservoir depth analysis reveals that aquifer presents in a shallow depth and compaction water flow exists in the reservoir [12].The Habiganj gas field sandstones belonging to the Surma group (Bhuban and Bokabil formations) of Miocene-Pliocene age.There are two gas zones known as upper gas sand (UGS) and lower gas sand (LGS).The upper gas sand lies at a depth of 1320 m where the lower gas sand has a depth of 3000 m.Though no significant fault is yet discovered, but some analysis results indicate that there may be some big features like faults mainly in the upper gas sand of Habiganj gas field [13].As the formation also contains mostly sandstones, it may be considered as "extensively faulted and fractured".Habiganj gas field has already been identified as a strong bottom water drive.Without considering water influx, the conventional material balance (p/z vs. Gp method) for OGIP of upper sand can be determined as 16 Tcf that appears to be overestimation of real volume, and cannot match with the present flow and pressure data.Considering bottom water drive, the Havlena-Odeh method for materials balance analysis has been applied, as the total underground withdrawal is equal to summations of water influx and gas-water expansion.The calculation of water influx by Allard-Chen and Carter-Tracy methods for Havlena-Odeh plot indicate that the average OGIP of that sand is 4.52 Tcf which is acceptable and seems more reliable with respect to the present production data [14].So shallow distance short flow system exists here.The Rashidpur gas field has a depth of 1380-2787 m.The volumes of shale values were low within the hydrocarbon-bearing zones of Rashidpur gas field [15] and also the major fault is of narrower width but there are numerous other small faults running N-S trend on the eastern flank of the structure [16].So the field is classified as "extensively faulted and fractured".It contains significant water bearing zones show in the table 1 for well-4 analysis.In Kailashtila gas field, there are three main gas zone (upper, middle and lower) ranging from 2280 m to 3045 m.It is visible that Kailashtila gas field"s lithology comprises mostly of sandstone with less shale which belongs to the Bhuban & Bokabil formation of Surma group.According to the analysis of the four wells (KTL-2, KTL-3, KTL-4, KTL-5) and considering small amount of shale presence, the gas field may be classified as "moderately faulted and fractured" [17].Fig. 2 shows a great difference between the Analytical method with Aquifer Influx and without Aquifer Influx which indicates an active Aquifer model for the Kailashtila upper gas sand [18].Fig. 2.Analytical Method Comparison with and without Aquifer Model for UGS [18].Fig. 3. Analytical Method Comparison with and without Aquifer Model for MGS [18].Fig. 3 shows difference between the Analytical method with Aquifer Influx and without Aquifer Influx which indicates an active aquifer model for the Kailashtila middle gas sand.Fig. 4 shows difference between the Analytical method with Aquifer Influx and without Aquifer Influx which indicates an active Aquifer model for the Kailashtila lower gas sand.Although significant water flux occurs for upper gas sand, water flux is relatively low for middle and lower gas sand in Kailashtila gas field.So, Kailashtila gas field"s hydrogeology can be classified as "intermediate flow systems".Fig. 4. Analytical method comparison with and without Aquifer model for LGS [18].
iii) Hydrocarbon potential: According to hydrocarbon reserve the gas fields can be classified as giant, large, medium or small size showing in Table 2 [8].
Table 2. Classification of hydrocarbon potential by reserve size.

Reserve size
Field category >3 Tcf giant >1 Tcf -3 Tcf large >300 Bcf -1 Tcf medium <300 Bcf small Titas gas field has been an example in which significant reserve growth has been demonstrated.Recently 1 Tcf of natural gas has been found in the gas field.Now overall Gas Initially In Place (GIIP) of the field is 8.05 Tcf gas.It is one of the giant gas reserves for Bangladesh.Petrobangla published a revised estimate of Habiganj gas field in which shows estimated GIIP of 3.68 Tcf and an initial reserve of 2.63 Tcf [8].Based on the reserve size, it can be considered as "large" hydrocarbon potential gas field.Also, Kailashtila is a large hydrocarbon potential reservoir with 2.76 Tcf initial reserve of gas and 109 million barrels of oil [17].The gas field is in production since 1983 and almost Tcf reserve remaining according to SGFL (Sylhet Gas Fields Limited).Rashidpur gas field is also regarded as large hydrocarbon potential reservoir as it has a huge hydrocarbon reserve of 1.481 Tcf.iv) Maturity: Although Titas gas field started production in 1968, the field is however yet to enter into a mature state of development and most of the 16 production/appraisal wells drilled are concentrated at the crestal part of structure [8].Production data analysis from Table 3 also suggests that it is still producing at a significant rate and it is the second largest supplier of gas field (after Bibiyana) to the national grid.Approximately 4383 billion cubic feet gas or 57.8 % of reserves has been recovered.So the gas field can classify as "developing" in the criteria of "maturity".Table 3. Daily production of Titas gas field (1st April, 2009) [19] Daily production rate analysis of Habiganj gas field for the year of 2007 (Table 4) and 2017 (Table 5) shows a significant decrease in producing gas.Production of water also increases at a high rate in this time period.Also near 85 % of the reserves of this field has been recovered already.So the field is in its way to mature stage.For Rashidpur gas field, an analysis shows that if wells produce at a constant flow rate, then Rashidpur-1, 3, 4, 7 wells will run for next 58.90,16.90, 32.72 and 32.67 year respectively [22].Thus it may consider as developing reservoir.
v) Infrastructure and CO 2 sources: The overall infrastructure is in moderate condition for Titas gas field.Significant number of wells are producing in this giant gas field.The gas field also surrounded by major carbon dioxide sources as it is near Dhaka.Big Ashuganj power plant is situated at a near distance approximately 22 km and other small sources also available.Moderate infrastructure with good well condition is assumed for Hobiganj gas field.It places at a near distance of Titas gas field and surrounds with major CO 2 sources like Shahjibazar power plant & rental, Habiganj power plant and other small sources.However, Kailashtila gas field has only one big (fenchuganj) power plant around and also has little build-in infrastructure.For Rashidpur gas field, minor infrastructure presents due to only 8 wells for this large gas field and also as a result of developing reservoir, all facilities will not be yet included.Moderate classification for CO 2 sources (i.e.Shahjibazar power & rental, Habiganj power plant, Ashuganj power plant) as they situated at a distance not too close.

Analyzation of Bachu's constant criteria for potential gas fields of Bangladesh
Geological condition of Bangladesh dictates constant criteria; tectonic setting, geothermal condition, coal & CBM, salts, on/off shore, climate, accessibility.
The tectonic framework of Bangladesh may be broadly divided into two main units: 1) stable platform in the northwest and 2) deep (geosynclinal) basin to the east and southeast.A narrow northeast-southeast trending "Hinge zone" separate the above two units diagonally.The geosynclinal basin is subdivided into two parts: 1) fold belt in the east, and 2) foredeep in the west (Fig. 5).The Bengal Basin is located at a junction point of the three lithospheric plates viz. the Indian Plate, the Eurasian Plate and the Burma Plate posing high seismic susceptibility in the region according to West Bengal Disaster Management Department.The earthquakes in the Tripura fold belt are much more frequent than in the Eocene Hinge zone because of the plate boundary activity.In addition, adjacent Indo-Burma subduction zone helps identifying the fold belt region as "Convergent Oceanic".The fold structures favoring gas accumulation occur in the eastern fold belt part and thus most of the gas fields are located in the eastern part of the country which are mainly onshore gas reservoirs.No salt beds or dome is present in the gas reservoirs of Bangladesh.Also, a huge amount of CO 2 sources present in this region.Total CO 2 emissions from large point sources in Bangladesh in the IEAGHG R&D program database amount to some 17 Mt CO 2 .The coal fields occupy in the northwest stable platform.Average temperature of this region is said to be 25 C.The geothermal gradient [5] of the fold belt region of Bangladesh ranges from 20 C /km to 30 C /km.This results to an overall P-T diagram like Fig. 6.It relates to warm basin behavior according to phase behavior of CO 2 [6].

Simulation process
A simulation model is one which shows the main features of a real system, or resembles it in its behavior, but is simple enough to make calculations on [23].To establish a tangential real system, huge amount of data is required to provide in data file.Our data file is generated by using RUNSPEC, GRID, PROPS, SOLUTION, SUMMARY and SCHEDULE section in GASWAT (i.e., modeling gas phase/aqueous phase) option within "Eclipse 300" portion [24].
The simulation is done for Titas gas field.Analysis of Haq [25] indicates a complete ending of Titas gas field"s life approximately in the year of 2060.The planning for carbon sequestration in this study is as such planned to start from 2060.Six injection wells (well no. 2, 5, 6, 7, 11 & 13) are used in this model to inject carbon dioxide gas in "A" sand for 50 years.Injection rates of the wells are determined according to their respective production rates.Injection locations are distributed into five layers (A1, A2, A3, A4U, A4L) of "A" sand such that no well will face shut-in situation in future.Table 6 contains information about injections wells.Table 6.Injection well conditions.
Fracture pressure is determined to set injection well shut-in pressure value.According to Haq [25], initial pressure at an average depth of 8800 ft of Titas gas field is 4005.9psia.This value is added by minimum matrix stress to find overall fracture pressure of "A" sand.Matrix stress ratio (0.775) and vertical overburden stress (0.96 for 100 µs/ft at 8900 ft [26]) are calculated from Pennebaker correlation [27].Then the value of minimum matrix stress will be 3442 psia and fracture pressure will be 7448 psia.For safety, injection well shut-in pressure is set to the limit of 6000 psia.
The entire cartesian grid of "A" sand is prepared using Petrel software.The simulation model is a non-orthogonal corner-point model."A" sand contains 50 grid blocks in X direction with average dimension 252 m and 100 grid blocks in Y direction with average dimension 264 m in each of the five layers.Average thickness of layers A1, A2, A3, A4U and A4L are 24, 178, 112, 79, and 50 ft, respectively.The permeability distribution ranges from 12 to 30 mD in section A that is determined by a hydraulic flow element study [28].
Four components, C1(CH 4 ), C2(C 2 H 6 ), CO 2 , H 2 O have to be considered with their critical properties, acentric factor, Molecular Weight (MW) for PROPS section.All the properties are recorded in field units with essential command within simulator.The ZMFVD function of PROPS section is assembled with approximate values assumed with respect to abandonment condition of the gas field.Abandoned reservoir temperature is considered similar to present condition (188℉).GSF (gas saturation factor) and WSF (water saturation factor) values are prepared by SCAL.
Initial equilibration conditions, which are the characteristics of abandonment condition in this case, are specified in solution section.Summary file of the simulation process is generated in summary section.

Suitability result
As stated in the introduction chapter: geology, hydrogeology, geothermal, maturity, on/off shore, climate, CO 2 sources are the most important criteria in selecting suitable location for carbon dioxide sequestration.Only hydrogeological condition of Kailashtila gas field and maturity of Habiganj gas field are better than Titas gas field.For all other criteria, Titas gas field provides the best condition.Also from scoring and ranking of Hoque et al. [30], Titas gas field has been proved to be geologically better gas field for carbon sequestration among the gas fields of Bangladesh.

Eclipse simulation result
Gas saturation of the field (SGAS), field gas injection total (FGIT), field gas in place (FGIP) and field pressure( FPR) have been analyzed after 50 years of carbon dioxide simulation process.Carbon dioxide accumulates in the lower part of the reservoir as expected.It sweeps existing hydrocarbon gases upwards.As a result, saturation of gas in the bottom portion of the reservoir is relatively small than upper part of the reservoir (Fig. 7).
A total of 2.92 Tcf carbon dioxide gas has been sequestrated after 50 years of time period (Fig. 8).This huge amount of gas has been injected without any break of continuation.2.58 Tcf of existing (abandon) hydrocarbon gas and 2.92 Tcf of carbon dioxide gas sums up a total of 5.5 Tcf gas in the reservoir (Fig. 9).It is lower than the capacity of Titas gas field.FGIT vs time, FGIP vs time graphs are showed here which are prepared by using office option within eclipse.
Field pressure of the gas field has been escalated to an amount of 2410 psia after the ending of carbon dioxide injection (Fig. 10).The value is much lower than the fracture pressure (7530 psia) and as such, reservoir shouldn"t face any fracture problem within this time.Fig. 7. Gas saturation 3D model of Titas gas field after carbon dioxide simulation (using FloViz option-front view).

Discussion
Titas gas field is one of the main gas fields of Bangladesh which serves to the energy sector of this country at large in present time.The geological criteria for carbon dioxide sequestration are fit to this field more perfectly than other gas fields.Huge storage capacity of this gas field should be used for environmental purpose.The field will be abandoned approximately around the year of 2060 and immediately after that period, carbon dioxide sequestration project can be undertaken using the existing wells.

Fig. 6 .
Fig. 6.Overall P-T diagram for fold belt region of Bangladesh.