AN EXPLORATORY REVIEW OF BANGLADESH GAS SECTOR: 
LATEST EVIDENCE AND AREAS OF FURTHER RESEARCH

Title Page   I     II     III     IV     V     VI     VII     VIII     IX     X     XI

IV.   GAS RESERVES ESTIMATES BY DIFFERENT STUDIES (10, 27, 28, 32-34, 35-38)

Reserve estimation is very important for the proper planning of the energy sector of the country. However, it must be recognized that the reserve estimation is a dynamic process and the reserve of a field/country needs to be updated with the exploration, production and development activities. When it comes to reserve, one needs to have clear understanding of some technical terms to avoid confusion. When talking about the reserve, people often get confused by the interchanging use of the terms reserve and the gas initially in place (GIIP). GIIP is the total amount of gas found initially in a reservoir when the reservoir is discovered. However, GIIP needs to be updated with additional information as a result of production and development activities. Reserve is that portion of the GIIP that can be produced from the reservoir under the present technical and economic conditions. Another term that is quite frequently used now a days is reserve growth. Reserve growth is the additional reserve over that of the previous estimate of the discovered fields as a result of additional appraisal cum development activities and/or application of new technology. Finally, resource potential of a country means probability of finding new reserve in addition to that already discovered. Resource potential is a probabilistic estimate, and information like geological data, exploration history of the region, exploration history of similar but mature basins of other region, etc., are used in this kind of study. In this section various studies on reserve and resource potential of the country have been summarized.  

4.1 IKM Study (27)

In 1992, Intercom-Kanata Management Ltd.(IKM), a Canadian petroleum consulting company, conducted a comprehensive geological and reservoir engineering study on eight gas fields of Bangladesh. In this study, IKM conducted geophysical/petrophysical evaluation of the gas fields and updated the subsurface maps of the fields. On reservoir engineering side they conducted well testing and fluid properties evaluation of the gas fields and estimated the gas initially in place (GIIP) and the reserve. The results of IKM study are summarized in Table 4.1. Table 4.1 shows that the GIIP and the recoverable reserve of the eight fields considered are 15.651 TCF and 9.04 TCF, respectively.

TABLE 4.1: SUMMARY OF GAS IN PLACE AND RESERVE ESTIMATION BY IKM (1992)  

                    a. Based on cumulative production up to December 31, 1991.

4.2 Petrobangla Study (38)

Petrobangla engages consultants time to time to perform specific jobs and studies on its behalf for the development of different gas fields. On the exploration and production side, Petrobangla usually engages consultants to conduct and interpret seismic surveys, perform drilling, completion, workover, pressure survey, reserve estimation, etc.

Some of these consulting firms carried out studies to estimate and update the gas in place and reserves on behalf of Petrobangla. Some of the important ones were conducted by Welldrill (UK) Ltd. (32,33), Hydrocarbon Habitat Study (34), IKM Study (27), and BCIF study. Based on the findings of these studies, Reservoir Study Cell of Petrobangla has estimated/updated the gas initially in place (GIIP) and reserve of different gas fields. Summaries of gas initially in place (GIIP)and reserve estimates of different gas fields by Petrobangla are shown in Table 4.2. Table 4.2 shows that the total GIIP and initial recoverable reserve of Bangladesh are 24.745 TCF and 15.51 TCF, respectively. Out of this reserve, 4.07 TCF has been produced already (up to February 2001), and the remaining reserve is 11.42 TCF.

TABLE 4.2: GAS IN PLACE AND RESERVE OF DIFFERENT GAS FIELDS AS DECLARED BY PETROBANGLA  

        Source: Marketing and Production Division, Petrobangla (Revived on 15/02/2001)

4.3 BUET Study (35)

Petroleum and Mineral Resources Engineering Department (PMRE) of Bangladesh University of Engineering and Technology (BUET) conducted a gas in place estimation study recently. In this study gas in place values of all the fields under Petrobangla have been estimated using flowing material balance and volumetric estimation methods. Since sufficient pressure survey data are not available for most of the fields operated by Petrobangla, flowing well material balance method in which flowing well pressure data instead of the static reservoir pressure data have been used in this study. The results have been compared with those of Petrobangla. The comparison shows that for a number of reservoirs estimation of this study is significantly higher than those of Petrobangla study. This happened because in this study some new information revealed by some recently drilled development wells have been used. Data from these recent development wells have showed that some of the reservoirs are much larger than previously thought, for example Titas field. If all the gas fields are systematically developed, it is highly likely that the natural gas reserve of the country would increase from the present value. Summary of BUET Study is provided in Table 4.3. Table 4.3 shows that the GIIP of the material balance and volumetric study are 28.49 TCF and 24.401 TCF, respectively. The difference may be due to under estimation of the reservoir bulk volume or presence of water drive.

TABLE 4.3: ESTIMATED GIIP OF DIFFERENT GAS FIELDS BY BUET STUDY (2001)

                     ^a Proven GIIP only  
                     ^b May be overestimated due to water drive

4.4 Study by Shell (36, 40)

In 1998, Shell Bangladesh Exploration and Development B.V. carried out a study on the gas reserve base in all regions of Bangladesh. In this study five categories of resource base have been considered. These are: Cumulative production, mature reserves (discovered and developed), immature reserves (discovered but not developed), Identified potential, and Unidentified potential.

In the study the undiscovered resource base has been assessed with the concept of 'chance of success'. The chance of success is based on a combination of the historical success rate and geological evaluation of the individual prospect. In this study, each prospect/field has been assigned a probabilistic volume range based on uncertainties in the input parameters. The risked volume is an assessment of the expected success volume which is likely to be obtained from the undiscovered potential of a basin.

This study by Shell estimated the total resource base of the country as 38 TCF. This includes the total reserves of the discovered fields and undiscovered resource potential based on geological evaluation and exploration history of the country. Later in a presentation and in the light of the USGS-Petrobangla study Shell estimated the total resource base between 43 to 64 TCF, in which existing reserve 18.0 TCF, field growth 5-6 TCF, and the undiscovered resource potential 20-40 TCF.  

4.5 UNOCAL Study (37)

Unocal Bangladesh Ltd. conducted a study on Hydrocarbon Resource Base of Bangladesh. This study has estimated the existing recoverable reserve, field growth potential and further resource potential of six blocks considered in this study. In this study existing reserve has been taken as 16.1 TCF including Bibiyana but excepting Moulovibazar.

The second category of their reserve is the field growth which includes additional probable reserve as a result of new technology and enhanced recovery technique applied to the existing fields. The field growth components considered in the study are: 1) 3-D Seismic surveys, 2) Petrophysical thin bed analysis, 3) Compression, and 4) Reservoir management. This study estimated a probable reserve addition of 12.8 TCF of field growth (1.6 TCF from reservoir management, 3.2 TCF from 3-D seismic, 4.8 TCF from thin bed and 3.2 TCF from compression) from existing fields.

The third category that the study considered for the resource base is the potential of new field discovery. The study considered 30 selected prospects from 6 PSC blocks of the country and estimated a mean probability of finding new discoveries to be 13.2 TCF (5.3 TCF for P90 and 22.6 TCF for P10). In the light of the USGS-Petrobangla study Unocal concluded the total hydrocarbon resource base of the country as 61 TCF, in which discovered reserve 16.1 TCF, field growth potential 12.8 TCF and undiscovered resource potential 32.1 TCF.

4.6 USGS- Petrobangla Joint Study on Natural Gas Resources of Bangladesh (28)

A joint team of the United States Geological Survey (USGS) and Petrobangla conducted a study on natural gas resource assessment of Bangladesh. This study, which was funded by the U.S. Agency for International Development (USAID), estimated the natural gas resource potential of the undiscovered gas fields of Bangladesh. The assessment team consisted of six geologists from the USGS's World Energy Resources Assessment Team and seven geologists, geophysicists, geochemists, and a petroleum engineer from Petrobangla. A regional assessment geologist presented the geological and geophysical data needed for the formal assessment. International oil companies, namely, Unocal Corporation, Shell Bangladesh, and Cairn Energy PLC, were invited to present geological background and assessment information and included. The objective of the study was to assess the technically recoverable undiscovered gas resource potential of Bangladesh that might be found in a 30-year period (2000-2030) through a properly conducted exploration program.

The USGS periodically conducts assessments of hydrocarbon resources of the United States and of the world. As a part of the process, regional geologists present descriptions of the petroleum geology and known hydrocarbon resources of the region to be assessed to the Assessment Team. This information is used to identify and describe Total Petroleum Systems (TPS) within the assessed region. Total Petroleum Systems encompass the natural process that begins with the generation of hydrocarbons from kerogen-rich source rocks, is followed by the migration of hydrocarbons from their source area, and ends with their entrapment within reservoir rocks beneath relatively impervious seals. The area (or country) in which that Total Petroleum System is active is then divided into Assessment Units (AU). Assessment Units are areas of specific geological terrain for hydrocarbon development within a given Total Petroleum System. Available historical oil and gas production data from existing wells and fields and information on discovered prospects and leads are then allocated to each AU. The AU is characterized as to type of hydrocarbon, in this case gas, the minimum field size to be assessed (in this case 42 BCF), the number of discovered fields exceeding the minimum size, and the median size of discovered gas fields. Geologic elements may be risked only when no fields of the minimum field size have been discovered within an AU. In such a hypothetical AU, geologic risk probabilities are determined for adequate petroleum charge, reservoirs, traps, and seals, and the timing of geologic events. These probabilities are multiplied together to determine the geological risk for an accumulation of one deposit equal to or greater than the minimum field size. Accessibility is also risked to account for the possibility of adequate locations to allow for exploration for a field equal to or greater than the minimum field size to be found within a 30-year time frame. Utilizing these data, along with the historical knowledge of world petroleum resources and geology of hydrocarbon accumulations worldwide, the Assessment Team conducts an iterative analysis of each AU until they reach a consensus on the ranges of the numbers and sizes of undiscovered fields (minimum, median, maximum) in that unit. These data are input into a computer simulation program that generates probabilistic forecasts on the undiscovered resources such as natural gas, oil and their co-products contained within the AU.

Previously, the USGS had conducted an assessment of the Ganges-Brahmaputra province, including parts of India and Myanmar and almost all of Bangladesh, as documented in the World Petroleum Assessment 2000 (U.S. Geological Survey World Energy Assessment Team, 2000). The current assessment reflects new information for the time period 1995 to 2000 and utilizes considerable proprietary data.

The study divided the country into six Assessment Units based on their geological attributes. These are: Bang0101 Surma Basin Assessment Unit; Bang0102 Easternmost Extremely Folded Assessment Unit; Bang0103 High Amplitude Faulted Anticlines Assessment Unit; Bang0104 Moderately Folded Anticlines Assessment Unit; Bang0105 Western Slope Assessment Unit; and Bang0106 Western Platform Assessment Unit. The assessment units are shown in Figure 4.1. Table 4.4 shows summary results for the onshore region, offshore region, and the grand total for all of Bangladesh. Table 4.4 shows that Bangladesh has a natural gas resource potential of 8.43 TCF with 95% probability, 65.7 TCF with 5% probability, with a mean potential of 32.12 TCF. The onshore area is more promising and has a potential of finding natural of 5.99 TCF with a 95% probability and of 48.33 TCF with 5% probability with a mean potential of 23.34TCF. The offshore has a potential of 2.44 TCF with a 95% probability and 17.37 TCF with 5% probability with a mean potential of 8.05 TCF.

The resource numbers calculated indicate the range of probable resources that may be discovered if Bangladesh were actively explored during a 30-year time frame. In places, where detailed geologic information is lacking in Bangladesh, the assessment team used geological play types that occur in similar geological provinces elsewhere in the world. In addition to the fairly well understood structural anticlines, which have thus far constituted the main play in Bangladesh, the assessment team recognized the potential offered by stratigraphic traps, plays at depth within the high pressure zone and other possible plays. The study also considered the possible technological advances in the in the fields of exploration and production that may occur within the nest 30 year time frame.

TABLE 4.4: PREDICTION OF THE UNDISCOVERED GAS RESOURCES OF BANGLADESH BY USGS-PETROBANGLA STUDY

                a. From Monte Carlo Simulation
 

4.7    Hydrocarbon Unit and Norwegian Petroleum Directorate Joint Study (39)

A joint team of Hydrocarbon Unit (HCU) of Energy and Mineral Resources Division and Norwegian Petroleum Directorate (NPD) conducted the latest study to estimate the reserve of the discovered fields and undiscovered resource potential of the country. This joint study was conducted by experts of NPD on Norwegian side and those of Petrobangla, BAPEX, BGFCL, SGFL, and GSB on HCU side.

This study re-estimated the reserve of four major gas fields, namely, Titas, Habiganj, Rashidpur and Kailashtilla, by material balance and volumetric methods utilizing latest available data and reviewed the gas reserves of other discovered fields. For estimation of the resource potential, this study divided the country into two petroleum provinces comprising of six petroleum systems. This study identified the prospects, leads, plays, and selected reservoir parameters utilizing latest information and available geological, exploration, reservoir and production data.

This study estimated the proven and probable gas initially in place (GIIP) of the discovered fields as 28.79 TCF and recoverable reserve as 20.44 TCF. Based on recent information, this study showed an increase in GIIP in Titas and Habiganj fields and a decrease in GIIP in Kailashtilla and Rashidpur fields. The study contended that using modern technologies and good reservoir management practices, it is possible to achieve a recovery factor of 70-75% in different gas fields compared to 52-70% used by Petrobangla.

The study estimated the undiscovered resource potential of the country between 18.5 TCF (90% probability) and 63.7 TCF (10%) with a mean of 41.6 TCF.

4.8  Remaining Recoverable Reserves of Gas

According to Petrobangla, the remaining recoverable reserves of 22 gas fields in June 2000 were estimated to be 11.596 TCF. The remaining recoverable reserves of 12 gas fields now producing were 7.325 TCF and these reserves are listed below according to ownership.

Petrobangla:  5.652 TCF (Titas, Bakhrabad, Habigonj, Rashidpur, Kailashtilla, Sylhet,
                                             Beanibazar, Narshingdi, Meghna and Saldanadi)
Shell:             0.777 TCF (Sangu)
UNOCAL:    0.778 TCF (Jalalabad)

The remaining recoverable reserves of 10 non-producing fields are 4.271 TCF and these are listed below according to ownership.

Petrobangla:  1.904 TCF (Chhatak, Kamta, Feni, Shahbazpur, Begumgonj and Fenchugonj)
Shell:             0.566 (Kutubdia and Semutang)
UNOCAL:    2.801 TCF (Bibiyana and Moulavibazar)

Three non-producing fields Chhatak, Kamta and Feni under Petrobangla were taken out of production due to production of excessive water and sand. There is uncertainty with continual production from the Bakhrabad Field due to production of water and sand; and today four out of ten wells are producing gas at a reduced rate. Continued production from this field is uncertain.

Some recent development wells as well as studies show that the remaining recoverable reserves are higher than those officially recognized by Petrobangla. Unfortunately, most of the reserve figures of Petrobangla are based on studies conducted in 1991 or earlier. Petrobangla figures need to be updated based on recent development/appraisal activities and studies.

Table 4.5 summarizes the GIIP, reserve, field growth, and resource potential of various studies conducted so far. It should be observed that scope as well as the results of various studies are quite different. Therefore, it will not be wise to compare the results of one study with those of other without mentioning the limitations. Also, there is a need to evaluate the results of various studies professionally.

TABLE 4.5: SUMMARY OF GIIP, RESERVE AND RESOURCE POTENTIAL OF DIFFERENT STUDIES

                a.  Based on 8 gas fields
                b.  Based on producing gas fields of Petrobangla
                c.  Includes reservoir management, 3D, thin bed and compression
                d.  Based of 30 selected prospects of 6 PSC blocks
                e.  Re-estimated four fields, rest Petrobangla figures
                f.  Includes compression only 

V. USES OF NATURAL GAS

The uses of natural gas in Bangladesh can be broadly divided into the following five categories:

    ·        Power
    ·        Fertilizer ( urea, ammonia and ammonium sulfate)
    ·        Industrial
    ·        Commercial, and
    ·        Domestic.

There are some seasonal users like the brick fields. The consumption pattern during the past decade shows that power sector consumes approximately 45%, fertilizer 35%, and the other sectors (industrial, commercial, domestic and seasonal ) 20% of the gas (19,24).

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