6th May 2019
Mortuza Ahmad Faruque

Bangladesh has so far discovered 27 gas fields and Titas Gas Field is the largest among them. Allthegasfields in Bangladesh are locatedinandnearthe frontalfoldedbeltoftheIndo-BurmaRangesintheeasternpart ofBangladesh.Pakistan Shell Oil Company (PSOC) delineated Titas structure after recording single fold seismic lines in 1961. This field is located near Brahmanbaria town, around 100 km northeast of Dhaka. It lies within the northeastern part of the Bengal foredeep in the western zone of the folded flank. The field was discovered by PSOC in 1962. The commercial gas production from this field was commenced in April 1968 and a total of 4.7 Tcf of gas has been produced till March 2019.

 

The Titas Gas Field is owned by the Bangladesh Gas Fields Company Ltd (BGFCL). BGFCL derives its origin from the then Shell Oil Company. With emergence of Bangladesh and promulgation of Petroleum Act 1974, the government of Bangladesh bought five gas fields – Titas, Habiganj, Bakhrabad, Rashidpur and Kailashtila – along with all the shares of Shell Oil Company with effect from August 9, 1975. The company was renamed as Bangladesh Gas Fields Company Ltd on September 12, 1975.

Structure

Titas structureis a subsurface anticlinal structure. The structure is an elongate north-south asymmetrical anticline with a simple four ways dip closuremeasuring about 190 sq km.The eastern flank is relatively steeper in comparison with the western one. The structure lies on the western margin of the Chittagong-Tripura folded belt in the south central part of the Surma Basin.The structure is asymmetric in nature with steeper dip in the eastern flank and gentler dip in the western flank. There has been indication of faulting in the deeper level in the eastern flank, as shown by seismic reflection discontinuities. The structure has no surface geomorphic expression and is covered by Titas-Meghna river floodplain.

 

Stratigraphy

The stratigraphy of the field is related to the stratigraphy of the Surma Basin and is based on the lithological correlation with rocks in the Assam oil fields. The formations that have been reached by wells in the Surma Basin are Dupi-Tila, Tipam, Bokabil and Bhuban. Sediments deposited in the later stages of the Indian Plate collision include the Upper Bhuban and Bokabil units which are overlain by Tipam and Dupi-Tila. This stage is represented by contemporaneous sedimentation with the major phase of continental collision (Late Miocene- Recent), when the main uplift Himalayan and Indo-Burma ranges occurred. Deposition occurred in fluvial-deltaic to estuarine environments during the Miocene-Pliocene, accompanied by extensive channeling and sediment reworking.

 

The reservoir sands in the area are composed of stacked sands which are divided into three groups A, B and C Sands. A  Sandisthemainreservoirholdingalmost85%ofthetotalreservesandgaswatercontactwasobservedinthediscoverywell.This zone consists of sandstone which is lightly grey to white, very fine to fine grain and sub-angular to sub rounded. The siltstone and shales are found to be interbedded with the sandstone. The sandstones are separated by shales and also have shale bedding within them. The major gas sand of A-group is A2 sand.

Reservoir

Like all other reservoirs of the country, Titas reservoirs are mainly sandstones of Miocene age. The gas was trapped in multilayered sandstone reservoirs of Bhuban and Boka Bil formations. According to Shell study, sands were deposited in offshore deltaic environment. The continuous nature of some of the reservoir sands is related to rising sea and the deposition was under shallow marine, delta front environment. The results of isotope analyses of gas indicate that the natural gas is derived from marine source rocks. Titasgas is a sweet gas essentially methane, sulfur free and high calorific value-1032 Btu/cft.

 

Titas Gas Field reservoir sands were named on the basis of the result of the discovery well. Major payzones were named as 'A', 'B' and 'C' sands. Depending on the results of subsequent well sand the gas sands were further sub divided. 'A' Sand is divided into six unit sand named as A1, A2, A2B, A3, A4U and A4L. In the same way 'B' and 'C' sands are divided into B1, B2 B3a and B3 band C1 and C2 sands. Theses sub-division is expanded on the basis of the result of wells drilled later. A part from the main gas sands, in some of the wells localized gas sands were encountered.

 

FieldDevelopment

A total of 27 wells were drilled in this field so far, out of these 9 wells are vertical and the remaining 18 are directional wells. At present daily average gas production is about 505 MMcf  from 26 wells and 425 bbls condensate also produced with gas as by-product. It is processed through glycol dehydration plants & low temperature separation plants and supplied to the high pressure transmission pipelines. Titas well#3 was plugged & abandoned on 25th February 2008 to prevent gas seepage from nearby locality.

 

In 1962, an exploratory well, Titas # 1 was drilled to 3690m by PSOC and drilling was terminated after opening over pressure zone. It was the deepest well in Titas structure.The five zones were tested with indepth of interval 2573-3072m. During drilling in this well, encountered over pressure zone at depth in excess of about 3,600m. Gas production from Titas started in 1968.  PSOC drilled Titas well # 2 in the late 1962. During 1970, they drilled two more wells close to Well # 1. In 1970, well 3 and 4 were added to the producing wells. In 1970, yearly gas production from Titas was 1.0 Bcf which is now about 190Bcf.

BGFCL completed drilling of seven wells (Titas # 5 ~ Titas # 11) from 1981 to 1990.Three more wells were drilled at the end of nineties which were Titas # 12, 13 and14.  Titas #15 and 16 have been drilled during 2005-06.

 

Petrobangla took initiative to drill 11 new wells at Titas Gas Field to enhance its gas production. Out of these, Russian oil and gas company Gazprom has drilled four development wells  during 2013-14.  BAPEX drilled three wells and four wells were drilled by Sinopec, China.

 

In last few years, both the reservoir pressure and wellhead pressure at Titas Field is declining. Under the circumstances, installation of gas compressors is required with the existing processing facilities within short period to continue gas production by raising pressure. Initiative has been taken for installation of Gas Compressor at Titas. BGFCL has taken a program to drill four new wells at the eastern side of the river Titas to increase gas production. A bridge over the river Titas including necessary infrastructures to be required for implementation of this program.

 

3D Seismic Survey

During 2011 to 2012, 3D seismic survey of 335 sq km at Titas Gas Field has been carried out by BAPEX. The world famous geophysical company CGG Veritas of France has been engaged by BGFCL and CGG given all sort of technical support to BAPEX during data acquisition, processing and interpretation. Gas reserve and resources of Titas have been increased substantially after conducting 3D in Titas. BAPEX has identified new gas resources in different layers at the depth below 4000m and within 7500m which is the country's deepest gas resource potentials.

Gas Reserve and Resource

Various methods were used to estimate the gas reserves of the Titas reservoirs depending on the availability of data and developments tage of this field. The methods were used predominantly volumetric and material balance but decline curve and reservoir simulation also used.The Gas Initially in Place (GIIP) of theTitas Gas Field has been estimated mostly using by volumetric and material balance method.

 

A good number of studies on Titas were undertaken by Petrobangla to update the reserve and resource base. Most of the reports were prepared by third party consultants and different agencies.

 

Reserve Estimation by BAPEX

According to reserve estimation by BAPEX in 2012, GIIP (P1+P2) stands at 8.05 Tcf and 494 Bcf (P3 category) and 1.6 Tcf resources. Earlier, Titas Gas Field had 10 gas layers but after 3D survey, another eight layers were identified – of which, two layers are in the low pressure zone while six potential prospects have been identified in the deeper depth. 

 

Considering reserve and resources, a total of 11 wells have been proposed for drilling of future exploration, appraisal and development wells. Among them, one is very deeper prospect, which is not recoverable at this stage because of prospective gas zones that are mostly present below the high pressure zone. The drilling of well by crossing high pressure zone is difficult and costly. The drilling across and below the high pressure zones is definitely very difficult by BAPEX and it wouldn`t be wise to engage them for this venture. It could be done by engaging reputed international oil and gas companies having experience of deep drilling.

 

Deep Exploration Wells in Bangladesh (4000m – 5000m)

A total of 23 deep exploration wells were drilled so far in Bangladesh considering the depth below 4000m and up to 5000m. Out of which, 14 wells are in the onshore and 9 wells in the offshore. There were discovery of four gas fields in the onshore which are Bibiyana, Fenchuganj, Kailashtila and Semutang, The deepest well was Fenchuganj well no-2 where drilled up to 4977m by Petrobangla. During drilling experienced high pressure zone at the depth from 3150m and continued up to  the end of drilling. The target drilling depth was 5500m but couldn`t reach due to some drilling hazards. It was the only well in Bangladesh where drilled 1827m across the high pressure zone but couldn’t cross the zone.

 

Challenges of Deep Drilling

The drilling in the high pressure zone is very much critical and risky due to its complexity and higher drilling cost as compared to drill above high pressure zone. Besides, it needs more drilling time, availability of higher capacity drilling rig and equipment such as draw works, wellhead equipment, mud pump etc. Overcoming a high pressure zone is a great challenge for any drilling company, as it requires additional skill, appropriate technology and equipment support. A drilling rig with a capacity of 3000 hp is required to drill a well up to the depth of 7500m at Titas Gas Field. Wellhead equipment with a capacity of 10,000 to15,000 psi will be necessary to control well pressure. Higher capacity of mud pump needed for circulating system and to manage high specific gravity of drilling fluid during drilling across the high pressure zone.

Way Forward

Titas Gas Field is likely to have huge potential to expand its reserve provided deeper sand zones are explored and drilled. At least two deep exploration wells needed to be drilled at Titas Field for a discovery and to be followed by a number of development wells if commercially viable gas or oil is found. The government/ Petrobangla should take proper initiative for drilling the deeper prospects at Titas Gas Field in search of oil and gas. The formulation of some special incentives will be needed to attract IOCs for carrying out deep exploration drilling at Titas Gas Field across and below the high pressure zone. Petrobangla should encourage any potential oil and gas company having experience in deep drilling for exploring gas in the deeper horizons of Titas Gas Field as well as other prospective structures such asFenchuganj, Patharia, Sitakund, Kasalong etc. may be considered for oil and gas prospect. 

 

The drilling below the high pressure zones is definitely very difficult for the BAPEX and it is not suggested to engage them drillingacross the high pressure zone or for drilling in the deep. The cost of drilling in the deeper prospects will be quite high though it would be much lower than the cost of offshore drilling. By engaging reputed international oil and gas company, drilling may be carried out under PSC/ G to G contract /JVA.

 

Mortuza Ahmad Faruque;

Energy Specialist and Former Managing Director, BAPEX

 

  


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