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Evaluation Of Initial Gas Volume Of Coalbed Methane Using Four Method

Coal bed methane (CBM) is an unconventional energy resources in which the main ingredient is methane gas formed in the process of formation of coal (coalification). Based on research Advance Resource International inc. (ARI), the CBM resource potential is 453 Tcf spread across 11 basins in Indonesia. CBM technology is expected to be one of the alternative energy can play a role in helping to suppress the natural gas needs of oil as one of the energy commodities di Indonesia.This paper discusses about a calculated initial volume of gas in place (Gas in Place) on CBM reservoir “X” field. This initial gas volume calculation first calculating the gas content is becoming an important parameter in calculating the initial volume. This content gas value can be obtained through data analysis proximate such as ash content, moisture content, volatile matter, fixed carbon which such data can be obtained from laboratory results. But there is also the calculation of the gas content which can be obtained by simply using coal density data using several methods of calculation, such as the Mavor method, Bambang, Modified Kim, and Mullen method. In the “X” field there are five wells coalbed methane drilling has been done, namely, SS-1, SS-2, SS 3, SS-4 and SS-5 and there are five layers are seam-1, seam-2, seam-3, seam-4, and seam-5. Based on the calculations were also obtained in the “X” field coal rank, the average is sub-bitminous. Of the five wells will be calculated gas content and the initial volume of gas using a method Mavor, Bambang, Modified Kim, and Mullen.Using four methods then the gas content in the SS well ranged between 200-500 SCF / ton. And assuming that the well drainage area (A) covering an area of 100 acres, the obtained large volumes of coal bed methane wells SS-1 with the four methods ranged from 3-9 BSCF, the SS-2 ranged between 1-3 BSCF, wells SS- 3 are 2-5 BSCF, the SS-4 range between 1-3 BSCF and the SS-5 ranges between 1-3 BSCF.We will be compared the initial gas volume in the Field “X” using the four methods with data from the laboratory result. From that analysis it can be proved that the Bambang method is the most suitable method and represents CBM field in Indonesia. So the initial gas volume in Field “X” is using the result from Bambang method, and the result are 5312.17 MMscf on the SS-1well, 2385.77 MMscf on the SS-2 well, 3330.84 MMscf on the SS-3 well, 1635.44 MMscf on the SS-4 well, and 1608.74 MMscf on the SS-5 well. And the total initial gas volume in the “X” Field is 14273 MMSCF.


I.
BACKGROUND Coal methane gas (CBM) is an unconventional energy resource which the main composition is methane gas formed in the process of coal formation (coalification) Coal Bed Methane (CBM) or methane gas is a methane gas trapped in a coal micropore due to a thermogenic or biogenic process and is associated with coal. Methane is a non-toxic gas that is tasteless, odorless and colorless. Methane is a major element of natural gas that is about 80% to 98% natural gas. At first this methane is often regarded as a danger and high risk in coal mining because it is explosive or burning. Along with the development of science and development of economic aspects, coal methane gas is now widely produced for use in energy and fuel supplies. Based on Advance Resource International research inc. (ARI), CBM resource potential of 453 Tcf spread over 11 basins throughout Indonesia. Its areas are basin of South Sumatera (183 Tcf), Barito (101,6 Tcf), Kutei (89,4 Tcf) and Central Sumatra (52,5 Tcf) for high prospective category. The Tarakan Basin (17.5 Tcf), Berau (8.4 Tcf), Ombilin (0.5 Tcf), Sand / Acid (3.0 Tcf) and Jatibarang (0.8 Tcf) have the modarate prospective category. While the Sulawesi basin (2.0 Tcf) and Bengkulu (3.6 Tcf) are categorized as low prospective. CBM technology is expected to be one of the alternative energy that can play a role in helping natural gas suppress oil needs as one of the energy commodities in Indonesia.

II. BASIC THEORY III. Proximate Analysis
Proximate Coal Analysis is used to determine the characteristics and quality of coal in relation to the use of coal, namely to determine the relative amount of moisture content, volatile matter, ash and contain carbon In coal. This proximate analysis is the most fundamental test in determining the quality of coal.

Coal Rank
Coal rank is the parameter of production success and the amount of reserves that have from a coal bed methane (CBM) field. This is because the coal rank implies potential gas content, permeability, and physical and mechanical properties of coal. The rank of these coal may vary laterally or vertically within a coal seam and may also vary between each seam in a coal group. Generally, to determine the quality of coal, chemical analysis of coal is used, such as proximate analysis. Coal at bituminous rank is the most sought and most preferred coal in coal bed methane (CBM) exploitation because the physical properties and mechanical properties of coal are at optimum levels. Specifically, bituminous to sub-bituminous bituminous coal is best for CBM reservoirs. More gas contained in this rating during the maturation process and gas storage capabilities is also increasing.
Generally, the standard coal-scale parameter parameters that are often found in coal bed methane (CBM) projects can be seen in Table 2.1 (Appendix)

Ash Content
Ash content (ash) is defined as the organic matter left behind after the coal sample is incinerated under standard conditions until a fixed weight is obtained. The ash content is measured by burning in a furnace at a temperature of 815 ° C. The residue formed is ash from coal.

Moisture Content
In coal, moisture content (m) comprises at least one single chemical compound. The form may be in the form of water that can flow rapidly from within a coal sample, an adsorbed compound, or as a chemically bonded compound. Some moisture is a component of mineral substances that are not bound to coal.

Volatile Matter
Volatile Matter (VM) is the amount of substance lost when the coal sample is heated at a predetermined temperature and time (after being corrected by the moisture content). The temperature is 900oC, with a precise seven-minute warm-up time.

Fixed Carbon
Fixed carbon (FC) represents the amount of carbon contained within Waste material after volatile matter is removed.

Gas Content
One of the determinants of the amount of initial gas content in place in the CBM reservoir is the gas content (gas content). Gas content itself is the amount of gas in the coal matrix. There are several methods for determining the gas content, the difference in yields of each method can lead to differences in the final gas volume in place.

GIP Calculation
Calculation of initial gas volume in place by using gas content calculated by various methods. The use of some gas content calculation methods is intended to compare the initial volume of gas in place with different gas content calculation methods. The calculation of the initial gas volume in place is carried out using the Mavor and Nelson equations as follows:

IV. METODOLOGY
In this study, there are four main steps taken to obtain evaluation and analysis of the final calculation results are: 1. Calculation of ash content, moisture content, volatile matter, and fixed carbon 2. Coal Rank Determination based on calculation data 3. Gas content calculation using 4 methods 4. Calculation of initial gas volume in coal seam using Mavor and Nelson method There are 109 coal core samples conducted by laboratory test, 5 wells and 5 seam which is calculated in this study for evaluation and analysis. In each well the dispersion of seam is uneven, so that in the SS-3 well there is no seam-4, the SS-4 wells are not seam-1, seam-4, seam-5, and SS-5 wells are not seam-1, Seam-4, seam-5.

V. DATA ANALYSIS
In this paper, the authors evaluate CBM reservoir laboratory data in the "X" field to account for the initial volume of gas present in it. Evaluation of initial gas volume is done by analyzing coal core samples contained in the laboratory, then compared with the result of initial gas volume with proximate analysis so that it can get the proximate analysis calculation method which closest to laboratory result. After the appropriate method obtained to calculate the initial volume of the gas, there is no need to do analysis of core samples in laboratories that are time consuming and costly. So to calculate the initial volume of CBM gas using only well logging data that is density log with data of coal density.
In this field, there are 5 layers of Seam-1, Seam-2, Seam-3, Seam-4, and Seam-5. Gas intial in place data obtained from the calculation using 4 methods of Mavor method, Bambang, Modified Kim, and Mullen.

Coal Rank Decision
Proximate analysis is performed to determine the ash content, moisture content, volatile matter and fixed carbon used for calculation of initial volume of gas and coal rank. From the proximate analysis, dry ash, dry volatile, dry fixed carbon, and dry ash free volatile and dry ash free fixed carbon values are found. These results are then classified according to the coal rank tables of the American Society for Testing

Gas Content
To determine the initial volume of gas in a well, must be calculated Gas Content first. In SS-1 wells, gas content was obtained using Mavor et.al method ranging from 491 scf / ton to 522 scf / ton. By using the method Bambang gas content value ranges from 301 scf / ton to 308 scf / ton. Modified Kim ranges from 357 scf / ton to 474 scf / ton. The Mullen method ranges from 381 scf / ton to 449 scf / ton. SS-2 well, obtained the calculation of gas content, using Mavor et.al method ranged from 451 scf / ton to 570 scf / ton. By using Bambang method gas content value ranges from 297 scf / ton to 310 scf / ton. Modified Kim ranges from 341 scf / ton to 416 scf / ton. The Mullen method ranges from 338 scf / ton to 466 scf / ton. The SS-3 well, obtained from gas content calculation, using Mavor et al., Ranged from 413 scf / ton to 545 scf / ton. By using Bambang method, gas content value ranges from 293 scf / ton to 307 scf / ton. Modified Kim ranges from 289 scf / ton to 455 scf / ton. The Mullen method ranges from 296 scf / ton to 440 scf / ton. SS-4 well, obtained from gas content calculation, using Mavor et.al method ranged from 344 scf / ton to 534 scf / ton. By using the method Bambang gas content value ranges from 285 scf / ton to 306 scf / ton. Modified Kim ranges from 324 scf / ton to 448 scf / ton. The Mullen method ranges from 217 scf / ton to 428 scf / ton. SS-5 well, obtained from gas content calculation, using Mavor et.al method ranged from 486 scf / ton to 496 scf / ton. By using Bambang method gas content value ranges from 301 scf / ton to 302 scf / ton. Modified Kim ranges from 382 scf / ton to 409 scf / ton. The Mullen method ranges from 376 scf / ton to 386 scf / ton.

Initial CBM Volume
The initial gas volume at Field "X" is calculated using laboratory results data (Langmuir). As mentioned earlier, the calculation of the initial gas volume in place also uses four methods, namely the Mavor Method, Bambang Method, Kim Modified Method, and also the Mullen Method. These four methods are used as comparators of initial volume of calculated gas from laboratory data (Langmuir). And which will be analyzed which method is the result most closely to the initial volume calculation results with laboratory data. The initial volume of gas in Field X using laboratory data (Langmuir) on well SS-1 was obtained at 5245. MMScf. The ratio of the initial gas volume to each method can be seen in the curve contained in the appendix. After the calculation on the four methods, it can be seen that Bambang's method is the closest to the data results from Laboratorium (Langmuir), this is because the formula in Bambang Method is a modification of the formula that is adjusted with CBM field in Indonesia. So through this calculation can prove that Bambang's method is very suitable and represents and can be applied to any CBM field in Indonesia.