Gas seepage is being observed near southeastern part of the location 3 of Titas gas field since mid November 2006. The seepages are spreading out over an area of about 3 to 4 square kilometers from numerous cracks, hundreds of holes in the paddy fields, Loisha beel, Shyampur village and also in Titas river under Mozlishpur union of Brahmanbaria district. Water is flowing out of five hand tube-wells at the village including one in the beel area round the clock. Villagers said gas leakages started to occur a few years back on small scale. 

Some gas samples have been collected and tested in the laboratories of Bapex and Buet. The presence of methane ranges from 95.1% to 95.5%, ethane 1.4% to 1.9%, propane 0.44% to 0.59%, iso-butane 0.11% to 0.16%, normal butane 0.05% to 0.08%, iso-pentane 0.02% to 0.06%, and normal pentane 0.16% to 0.55%. The laboratory analyses confirmed that the gas is mature formation gas (Thermogenic). The collected water samples were also analyzed in the labs of Bapex and BGFCL. The chloride content ranges between 500ppm and 1000ppm.

Observation

Titas Gas Field is the major and most promising onshore discovery in Bangladesh. Bangladesh Gas Fields Company Ltd. (BGFCL) operates this field. It is the largest gas production company in the country. It originated as a Shell subsidiary in early 1960s. After the independence of Bangladesh, it became a national company. Today, BGFCL also operates Habiganj, Bakhrabad, Narsingdi and Meghna gas fields. BGFCL processes its own gas before supplying it to transmission companies. The Pakistan Shell Oil Company discovered Titas Gas Field in 1962. Sixteen wells have been drilled so far in Titas structure. Of them, eleven wells are directional. The angle of deviation varies from 9 to 50 degrees. The Titas sands are classified into two categories --major and minor sands. Major sands include A2, A3, A4, B3 and C3 while minor sands include A1, B0, B1, B2, C1, C2 etc. Thirteen wells are encountered in the A group of sands and three wells in the B and C group. At present about 435 mmcfd gas is being produced but optimum production rate was 475 mmcfd. The estimated recoverable reserve is 5.1 Tcf of which 2.6 Tcf already been produced. 

The gas seepage and water streaming out area is near the location 3 of Titas gas field and the surface location of Titas well 6, 8, 9 & 10 is in the same location. It was appearing from different logs of well 10 that presence of cement between casing and formation is very poor, especially at the interval between 3005m to 3162m in gas sands. Titas well 10 was drilled by BGFCL under the supervision of Oil and Mining Services Ltd (OMS) during 1988. The well was drilled as a directional hole to a sub-surface location to the north and completed in the B and C gas sands. The well was kicked off at a depth of 853 feet and the hole angle was built to a maximum of 50 degrees. A consulting company Oil and Mining Services (OMS) of London was engaged for drilling supervision by BGFCL. Just after drilling, OMS suggested to BGFCL that production should be started after the completion of cement squeeze due to bad cement in well 10. Against OMS recommendation, cement squeeze was not performed and the gas is being produced from this well since September 1990.

In well 10, the cement bond against 7-inch casing was found moderately good in B & C Gas Sands. Reservoir pressure of A Gas Sands decreases about 1000 to 1200psi due to long-run gas production. There is a chance of cross flow between the water-saturated lower part of A Gas Sands formation and upper part of water saturated formation in the cement free section of 7-inch casing annulus. This gas might come to the surface through different channels (minor fault/fracture). 

The surface location of Titas well 9 is in location 3 of Titas field. The well was drilled in 1988 and directional hole to a sub-surface location to the southeast of location 3. All seepages and continuous water flow from the tube-wells are lying along the sub-surface drilling path of Titas well 9. The well was completed in B & C Gas Sands and producing from these sands. The cement bond against 7-inch casing and formation was found good. Reservoir pressure of B & C Gas Sands decreases similar to A Gas Sands of well 10. Besides these, reservoir pressure of water-saturated top sands remains unchanged. Under these circumstances, it is very difficult to make comments that cement bond might break due to water pressure without conducting new logs. 

Water Production Problem of Titas well 12, 13 & 14

During 1999-200 Titas well 12, 13 and 14 were drilled and completed in A group of sands. All these wells are deviated wells. Well 12 was completed as gas producer in August 1999. In March 2000 the well was opened for running PLT log and during this time excessive water production was recorded. The well remained shut down for some time. In April 2002, production started and remained restricted to flow at the rate of 22-23 mmcfd and water production rate was about 30 bbbl/mmcf. CBL-VDL log of Titas well 12 shows that the entire interval covering the wet sand overlying the gas sand is poorly cemented. Only interval 2765-2778m shows some fair cement bond. For the rest of the section both casing and formation arrival is quite clear. The interval representing poor cement bond is most likely to work as a conduit for the water from the overlying water saturated sandstone to flow into the underlying partially depleted gas saturated section. Negative pressure difference between the pressure of overlying wet sand and underlying gas sand could accelerate the process. There will always be a possibility of enhancement of this conduct due to water flow during the period started from the day when cementation was completed. Water dropping down from the overlying wet sand is entering the well bore through the perforations and flowing along with the produced gas. Certain portion of the water might invade the gas sands at the lower level of A Gas Sands. Initial formation pressure at the top of A group of sands was 4452psi at 3073m. Reservoir pressure of A Gas Sands gradually reduced due to continuous gas production.

Titas well 13 and 14 started production in April 2000. In May 2005, production from Titas well 13 and 14 were increased to 35-36 mmcfd to meet up the country’s gas demand. On November 6, 2006, the well 14 was shut down due to excessive water production rate observed more than 100 bbl/mmcf of gas production.

In Titas well 13, gas reservoirs (A Sands) extend from 3062m to 3135m. There is reservoir pressure drop due to gas production in well 13 and the difference between initial reservoir pressure and current reservoir pressure is almost similar to that of well 12. Below the base of the overlying wet sand a band of fair to poor cement bond was observed within interval 3048m to 3060m. Negative differential pressure between the overlying wet sand and gas sand can make communication between overlying wet sand and overlying gas sand. In this well, CBL-VDL log shows overall poor cement bond from 3062m to 3135m. Top of gas sand was identified at 3058m. 

In Titas well 14, gas reservoirs (A Sands) extend from 2596 to 3130m. Reservoir pressure of A group of sands also gradually reduced in this well due to gas production. CBL-VDL log shows that entire reservoir section including overlying and under lying water bearing sands are poorly cemented. This situation could result in water dumping from overlying wet sand into underlying gas sand. During flow a part of this water is flowing along with the gas. 

It appears that in all the three wells water is flowing from water saturated sandstone horizon overlying the gas reservoirs. The cased hole logs including CBL-VDL and cement squeeze operation is essential to be carried out to shut off water flow from overlying sands. 

The gas seepages are coming from the producing formation of Titas Field and possibly from A group of sands. It is leading to be a disaster like fire hazard, land slides, water pollution in the river and also in the paddy lands. Petrobangla/BGFCL should take immediate necessary measures to overcome and minimize the situation. Of course, we never experienced this kind of situation in any other gas fields in Bangladesh. It is absolutely different from Magurchhara or Tengratila incidents. 

An upward path is necessary for approaching the formation gas of sub-surface to the surface. Usually this kind of upward path may create channels due to fault or fracture. The Titas Gas Field is located in the west of eastern belt. The tectonic activity in this area is presently active. It is also appearing the presence of minor faults in the shallower depth from the seismic sections. The compaction of sands up to the depth of 800m to 1000m is unconsolidated and absence of continuous clay or shale layers in this depth. As a result, formation gas from A Gas Sands or from B & C Gas Sands might come upward movement up to certain distance through the annulus.

A six-member committee was formed by Petrobangla on January 9 to identify the reasons of gas seepages and to suggest remedial steps for minimizing the seepage. Three former officials of Petrobangla M.A. Matin (Geologist), Mir Moinul Huq (Geologist) and Hasnat Ali Chowdhury (Drilling Engineer) assist the committee.

Possible Reason of Gas Seepage

In well 10, there was no cement bond from upper part of `A’ sand against 7-inch casing. This resulted in a passage through the annulus for the formation gas to flow upward until finding fault/fracture by which gas come to the surface. The surface seepages are located above the well trajectory (sub-surface well path) of well 9. The gas and water are spreading out in the large area due to unconsolidated sand and presence of minor faults/fractures at the shallower depth in the central part of Titas structure. 

The major recommendations by the committee are as follows

1) To carry out immediate remedial actions, Petrobangla/BGFCL should engage a Foreign Drilling Engineer/Consultants having sound knowledge and experiences of such remedial work. It is suggested to take assistance from the PSC operators presently working in Bangladesh. 

2) Petrobangla/BGFCL should engage a logging company through restricted tendering procedures for conducting different logs viz. Cement Bond Log, Variable Density Log, Temperature Log etc. in the well 8, 9 and 10. The company should be international reputed such as Schlumberger/Halliburton Energy Services/Baker Atlas etc. 

3) Remedial works should be carried out on the basis of findings from logs, which will be conducted in the well 8, 9 and 10.

4) A reputed international company should be engaged immediately for carrying out cement squeeze and other relevant work in the different wells. This job can be carried out by BJ Services which will save a lot of time because the company presently working under BGFCL. BJ Services of USA is one of the reputed international cementing companies in the world. 

5) Petrobangla/BGFCL to instruct Bapex to mobilize a suitable rig for carrying out the work under the supervision of suggested Consultant. 

6) A social survey on public movable property should be carried out immediately covering effected area. 

Remarks

BGFCL/Petrobangla should think about the temporary shifting of few houses and its habitants from Shyampur village into the safe area to avoid any incident or hazard, until completion of remedial work and overcome the present alarming situation. 

We should keep in our mind that Magurchhara or Tengratila disaster is completely different case. In Titas field, presently fifteen producing gas wells are releasing pressure all the time from every wells which might save us from any serious disaster like Magurchhara. But we need to keep close eye on the situation.