Energy & Power

Cover Report

Reserve Growth in Gas Sector
Hoshne A. Banu

At present, energy security is a much- talked about issue in the country as power sector in general and oil & gas sector in particular are in crucial stage. Being the main source of energy, natural gas plays the prime and central role in the country, which can be compared with the sun in the solar system as depicted in Figure-1. Approximately 90% of total electricity produced in the country is generated from gas while the fertilizer sector exclusively depends on gas. Other entities in industrial and commercial sectors directly or indirectly depend on gas. Therefore it is imperative to ensure uninterrupted supply of gas to these sectors in order to make the engine of the country run.

Figure:1- Being the main source of energy, the gas sector plays the central role in the country’s economy.

Unfortunately, the current situation in the country’s gas sector is alarming. It is estimated that the present proved reserves will fail to meet the ever increasing demand of gas after 2011 if no more gas fields are discovered. This might be devastating for the country’s growing economy. Undoubtedly, exploration program should be undertaken without any delay in order to augment the country’s gas reserve. At the same time proper reservoir management program need to be practiced to extract the maximum amount of gas efficiently and economically from the existing reservoirs.

Sources of Future Additional Reserves

At present, the first and foremost task of the sector should be to augment the reserve. Three possible sources may be considered for evaluating the future addition of gas reserves in the country:
a. Additional growth of reserves in existing fields.
b. New fields to be discovered through exploration
c. Unconventional source such as Coal Bed Methane (CBM).

Among the above three sources, reserves growth is the most definite to add a significant volume of gas to proven reserve. Internationally, the issue of reserve growth is being given high importance as worldwide reserve is on depletion.

It might be assumed that more exploration would yield more reserves. But discovery of new fields through exploration is less certain and economically dependent.

With regards to unconventional source, the existence of coal bed methane in the country is not yet clearly understood though Bangladesh is assumed to have a substantial amount of coal deposits. New technology will be required to extract the gas. Countries all over the world are beginning to see the benefits of extracting pure methane from subterranean coal deposit. De-gassing of coal in the vicinity of active mines is also benefiting miners by minimizing the risks associated with CBM explosions in their tunnels.

Opportunities of Reserve Growth in the Existing Fields

Reserves growth in the existing fields can be considered to be a reliable source of additional reserve. In order to obtain the desired growth of the existing fields, the following measures can be taken:

1. Increasing the Recovery Factor (R.F) of the existing reservoirs. It is possible to increase a substantial amount of reserve through the following measures:
a. Using wellhead compressor in the depleting fields.
b. Increasing the length of perforation and using frac-pack technology for thin bedded, tight or highly heterogeneous reservoir.
c. Drilling new wells in the un-drained/ un-extracted area within the existing producing wells.

2. Increasing number of sands in the existing fields:
a. Conducting modern 3D seismic survey in order to identify comparatively smaller sands lying within/around the major sands and also shallow sands which were not noticed before.
b. Using advanced electronic log, such as HMI log, during drilling in order to identify thin bedded sands.
c. Re-examining all the previous logs (mud logs and electronic logs) to identify the minor (viable) sands lying within the main sands.

3. Developing a proper reservoir/well management system in order to ovoid unexpected /premature termination of the existing wells.

Increasing Recovery Factor

In this article, the possibility of growth of reserves in the existing fields by increasing recovery factor will be discussed briefly. Recovery factor, an uncertain parameter (usually a round figure), is used to calculate the amount of gas (recoverable reserve) that would be possible to produce economically from the reservoir. Generally, on discovering a reservoir, the recoverable reserve is calculated from the following formula:

Recoverable Reserve:= volume of the gas bearing sand (AH) X percentage of pore volume (porosity, f) X amount of gas within the pore (Sg)) X Recovery Factor (R.F)

In the above formula, other than R.F, all other components are properties of reservoir which can not be changed, but will be more accurate with time on gathering new data. While R.F varies from 35 % to 85 % depending on the type of drive mechanisms and reservoir characteristics, it can be improved by using modern technology and knowledge.

According to the data from Petrobangla webpage, it is seen that recovery factor estimated for most of the fields is 70%. This figure might be true for a depletion reservoir but it is too high for a water drive reservoir like Habiganj and Titas (drive mechanism of Titas is still controversial)). If the reservoirs are not monitored and maintained properly, the recovery figure may be even less than 70%. In such situation, the present figure of 2P recoverable reserve would be less than expected.

On the other hand, if some advanced techniques are adopted, it may be possible to achieve 90% or even more recovery. Based on data from Petrobangla webpage, a preliminary analysis for 10 fields is made and shown in Table-1. It is seen that approximately 3.68 Tcf additional amount of gas can be produced from the fields if 90% of GIIP (Gas Initially In Place) is recovered, while 1.7 Tcf can be extracted if 80% of GIIP is recovered.

The following three measures can be adopted at different gas fields in order to extract maximum gas economically from the reservoirs.

Using wellhead compressor:

It is worldwide recognized that the use of wellhead compressors allows optimum recovery of gas from a reservoir. Differential pressure, the pressure difference between two points, is the force, which drives the gas to flow from the wells to pipe line. No gas will flow from the wells when the wellhead pressure declines to a level of sales line pressure (abandonment pressure), forcing the well to be abandoned. For Bangladesh the abandonment pressure is normally assumed to be about1000 psia.

In such situation, compressor can be used to boost the wellhead pressure in order to keep the well running until its pressure falls down to100-300 psia, depending on the type of compressor to be used. Approximately 20% additional gas can be recovered from the reservoir using this method as described in chart-1 below. For a certain reservoir with 50 Bcf of GIIP and 1000 psia of abandonment pressure condition, only 35 Bcf of gas can be recovered primarily from the reservoir. However, 45 Bcf of gas can be recovered from the reservoir if a wellhead compressor is used to assist the well to flow until the abandonment pressure is 350 psi.

Chart 1: Material Balance Analysis showing that it is possible to extract 90% of the GIIP if wellhead compression is used while only 70% of it can be recovered if no measure is taken.

Use of compressor not only increases the recovery of gas from the reservoir, it also increases the production rate and maintains plateau production for a long time. It also protects the wells from liquid loading problem, when the wells faces water accumulation in the wellbore due to low gas flow rate.

Table-2 shows a preliminary economic analysis of installing 15 wellhead compressors (positive displacement reciprocating) in various gas fields. This number of compressors can be used in 10 or more fields as it can be shifted from one field to another as per requirement.

According to the table, considering a gas price of USD 1.5 per Mscf, the net profit after installing 15 compressors would be 5,501.0 million USD if recovery is 90% while that would be 2,555.0 million USD if the recovery is 80%.

Increasing the length of perforation and using frac-pack technology:

Generally for a homogeneous reservoir like shown in Figure 3.a, gas flows towards the perforations from all directions. On the other hand, for thin bedded reservoirs, all the layers will not contribute in flowing gas as the flow might be hindered by the thin shale layers as shown in Figure-3.b. The portion of the reservoir, which was not perforated, will remain un-extracted. Therefore, in that situation it is essential to perforate entire depth of the reservoir and create artificial fracture through which the gas can flow from all directions. In this way, maximum amount of gas can be extracted from the thin bedded or tight reservoirs.

Figure- 3.a Figure-3.b

Figure-3.a shows gas flows towards the perforation from all directions in a homogeneous reservoir, while gas can not flow freely from all directions in a thin bedded reservoir as shown in figure 3.b.

Drilling new wells in the un-drained/ un-extracted area:

As the reservoirs are usually not homogeneous in nature, different wells behave differently even when they are in the same sand layer. Drainage boundary, size and shape of a space that can be effectively drained by one well, will be different for different wells as shown in Figure-4. The size and shape of the drainage boundary can be estimated by using well test data.

Figure-4: Different wells have different drainage area.

It is possible to estimate the amount of gas remaining un-extracted in the sand by calculating drainage boundary of each well. Therefore, the un-extracted volume of gas which could not be extracted through the existing wells can be recovered by drilling a new well or re-drilling (side tracking) the existing wells.

Conclusion

Exploration program using advanced technology should be started without delay in order to find out new gas reserves, at the same time, high importance must be given to the issue of reservoir exploitation/ management system. It is possible to add approximately 3.6 Tcf of additional volume of gas to the present reserve if some measures as discussed above are taken. These measures are less expensive and more certain for obtaining additional reserves. Moreover, no new pipeline network or gas dehydrating facilities will be required for marketing this additional gas. It will be very detrimental to the country if the importance of reservoir management issues is not recognized and appropriate measures are not taken at this crucial stage.

Hoshne A. Banu: Deputy Manager-Petroleum, Niko Resources (Bangladesh) Ltd.