Petro physical parameter studies for characterization of gas reservoir of Narsingdi gas field, Bangladesh

The study focused on quantitative analysis of petrophysical parameters to characterize the reservoir of Narsingdi Gas Field, Bangladesh using well log data. 12 permeable zones were identified, out of which 5 zones were gas-bearing. Shale volume of those zones were averaged at ranged from 14% to 25% indicate reservoir consists of sand dominated lithology and less influence the water saturation for the study well. Porosity of these zones were averaged as 27 % which was within the ranged for very good gas accumulation. Water saturation values were less than 35 % which calculated by using three equations. Gas saturation values were more than 65 % revealed that high gas accumulation in these gas reservoirs. Permeability was averaged at ranged from 32 mD to 55 mD which was within limit for good gas accumulation. Bulk volume of water was averaged at ranged from 0.030 to 0.038 indicate fine grained sand occupied by gas reservoirs were more or less at irreducible water condition. Gas was moveable as indicated by gas moveability index < 0.7. The study suggests that gas reservoir is potential for gas production and accumulation. The study results will contribute in future gas field development programme.


Introduction
Petrophysical parameter studies are very important for the development and production of the well and estimation of the hydrocarbon reserves in any gas field. The geological model of gas reservoir is based on the estimates of reservoir properties such as lithology, porosity, permeability and fluid type [1]. Some studies were carried out regarding petrophysical analyses of shaly-sand reservoir of Bengal Basin, Bangladesh [2][3][4][5]. Rahman et al. [6] revealed to evaluate the reservoir sand of Titas-15 well, Bangladesh. Recent study was conducted by Islam et al. [7][8] to assess the gas reservoir of Rashidpur and Meghna Gas Field, Bengal basin using well log interpretation. These earlier studies were motivated by the fact that gas-bearing reservoir of Bangladesh was taken particular attention due to its highly economic interest. In this context, the study was emphasized on the quantitative analysis of petrophysical parameters of gas reservoir in the Miocene sequence encountered in the Narsingdi Gas Field, Bangladesh using well log data. The aim of the study was to characterize the reservoir quality of Narsingdi Gas Field. It is located in the Narsingdi district adjacent to the Dhaka-Sylhet highway about 45 km away of northern most from capital city of Bangladesh, Dhaka (Fig. 1).

Geological framework
Bengal Basin of Bangladesh is a young prolific depositional basin in the world [8]. This basin meets the entire geological requirement for accumulation of natural gas in the subsurface [6]. Greater Bakhrabad Structure of Bengal Basin is an elongated close anticline and is about 67 km long and 6 km wide [10]. Belabo Structure of Narsingdi Gas Field is a one of the part of greater Bakhrabad structure which randomly egg like shaped anticline whose run slightly longer N-S axis [11]. It is located in the crestal region of Bakhrabad anticline. Belabo structure is the smaller part of other Bakhrabad and Morichakandi structures forming the greater Bakhrabad anticlinal complex. The greater Bakhrabad Structure lies on the southern fringes of the Sylhet Trough of Bengal Basin.
Well log data was collected from the data Centre, Petro Bangla. So, hand copy of well logs including gamma ray (GR), selfpotential (SP), resistivity logs (MSFL and ILD), density log (RHOB) and neutron log (NPHI) etc. were digitized manually keeping restraint to regenerate for the study well Bakhrabad-10. Primary study is related to data quality, depth correction, sand and shale zone identification were carried out on paper based logs. The study gas field was covered with the depth ranges of 867-3440 m. Permeable zones were identified with the help of GR, SP, resistivity, neutron and density composite log responses. Well log data were analyzed using MS-Excel software. Detailed petro physical parameters were estimated by the following equation for the study.

Shale volume
Before the calculating the shale volume, the gamma ray index was the first step to estimate with the help of gamma ray log (GR) using Schlumberger [14] formula. (1) Where, = Gamma ray index, =Gamma ray reading of formation, =Minimum gamma ray (for clean sand), = Maximum gamma ray (for shale). Shale volume ( was calculated from the gamma ray index ( for tertiary unconsolidated rocks Dresser Atlas [13]. So, the study reservoirs are made of unconsolidated sandstone rock. The following equation was used to calculate shale volume and written as: (2)

Porosity
After calculation of the shale volume ( is to correct the shale effect for calculation of porosity. The following equation proposed by Dresser Atlas [13] was used to estimate porosity using available neutron and density logs: = -[( ] (5) Both neutron-density logs were also used to calculated porosity ( [14]. Where, = density derived porosity, = neutron derived porosity, =matrix density (sandstone), =bulk density (log reading), =fluid density (for water=1.0g/m and for gas=0.7g/m), =shale density.

Gas saturation
Gas saturation is depend on water saturation and is the part of pore volume in a formation occupied by gas. It was calculated by the following equation.

Permeability
Permeability has been estimated from the following equation by Coates and Dumanoir [19]. K= (12) Where, W= (13) c=23+46 (14) Permeability was calculated by using the Wyllie and Rose [20] equation which is given: = saturation at irreducible water saturation, = deep resistivity at irreducible water resistivity, SSP= Static Self Potential, w and c=constant on hydrocarbon density, = gas density (0.7 for gas)

Gas moveability index
Gas moveability index is expressed of the following equation by Archie [21]: Where, =flushed zone water saturation, =mud filtrate resistivity, = formation shallow resistivity.

Bulk water volume
The bulk water volume (BVW) was calculated using Morris and Biggs [22]

Results
Petrophysical parameters such as lithology, shale volume, porosity, saturation, bulk water volume and permeability were used to evaluate the gas-bearing reservoir characteristics. In the study gas field, 12 permeable zones were identified from the analyses of composite log response (Table 1). Among these permeable zones, five zones were identified as gas-bearing while remaining is water-bearing.

Shale volume
Shale volume ( is used to evaluate the shale distribution of a reservoir. The result of shale volumes was shown graphically in the fig. 6, 7, 8, & 9.

Porosity and permeability
The porosity calculation is a very important step for reservoir characterization in any gas field. Porosity and permeability distribution were shown graphically in the fig. 6, 7, 8 and 9.

Water and gas saturation
Water saturation of gas-bearing zones has not been calculated using Archie [21] equation which is valid for clean sandstone. All of the calculated saturation value is affected by shale and porosity distribution. Three saturation equations were proposed by Simondoux [15], Fertl [16] and Schlumberger [17] to use in this present study (Fig. 6,7, 8 and 9).Water saturation values were converted to gas saturation values which was determined by subtracting water saturation from the total saturation (100%). The permeable zone with more than 65 % gas saturation values were commonly treated as gas-bearing reservoir [23].

Gas moveability index and bulk volume of water
If gas moveability index is equal to or > 1.0, then hydrocarbon would not move and the ratio is < 0.7 (for sandstone) indicate gas moveable in the well bore [24]. From the calculated result, it was interpreted that gas-bearing zones are moveable because all the movability values are less than 0.7 in the study well (Table 2). BVW indicate the formation is at irreducible water state or not. If the BVW was at ranged from 0.035 to 0.07, the grain size of the sand is fine to very fine-grained [25].

Discussions
The study gas field potentiality was evaluated by petrophysical analysis using log data of well Bakhrabad-10.  (Table 1). Low gamma ray (GR) responses and high SP responses indicate gas bearing reservoir zones. Higher true resistivity responses of these zones with respect to lower water resistivity responses and vice versa indicate that hydrocarbon type fluid is gas bearing. Low density responses and very low neutron responses in the gas bearing zones show that gas bearing reservoir (Fig. 2, 3, 4 and 5). Shale volume values were averaged at ranged from 14 % to 25 % that revised to sand volume of these gas zones were ranged from 75 % to 84 % revealed that reservoir is mainly sand predominant lithology in the well Bakhrabad-10 ( Fig. 6, 7, 8 and 9). Similar observation of the average shale volume was made by Islam et al. [8] in the Well Bakhrabad-09. It indicates that the reservoir is evidence of sand development [26]. Porosity of those zones was averaged and their values ranged from 23 % to 31 % which indicate very good gas accumulation. According to porosity calculation, zone-4 is the most prospective gas-bearing zone with respect to zone1, 2, 3, and 5 in the study well. Some recent studies were found also similar average porosity ranged in the Laja Oil Field, Rickie Field, KN Field and Y Field, Niger delta [27][28][29][30]. For assessing Narsingdi gas productivity, the calculated average permeability values of these zones were ranged from 28 mD to 90 mD by Coates and Domanior [19] equation while average permeability values ranged From 28 mD to 40 mD was followed by Wyllie and Rose [20] equation. According to Coates permeability equation was found better result of reservoir characterization except zone-3 and zone-5 than Wyllie equation. So, the ratio of average porosity and permeability values were 27 %/ 46 mD (58.69 %) indicate good enough to permit free fluid flow of these gas bearing zones. There were positive correlation between porosity and permeability in the study well Bakhrabad-10 ( Figure 6, 7, 8 and 9). The average gas saturation values of these zones were estimated by three equations at 68 % by Simondoux [15] followed by 88 % Fertl, [16] and 90 % using by Schlumberger [17] equation. All the calculated more than 65 % values of the gas saturation suggested that reservoir was assembled very high gas accumulation. These results are very close to previous finding elsewhere gas field in the Bengal Basin [3][4][5].
Gas moveability index values were averaged at range from 0.13 to 0.16 which was less than 0.7 in the study well Bakhrabad-10 that suggest gas in the well will move freely. BVW values were averaged at range from 0.030 to 0.038 indicates sand in reservoir is fine grained ( Table 2). Calculated values of BVW are constant or very close to constant, it indicate that the zones is homogenous at irreducible water condition [31]. Both gas moveability index and BVW were found acceptable for gas production. Titas Gas Field and Shahbazpur Gas Gield were found similar finding by Islam et al [2][3].

Conclusion
The Quantitative analysis of petrophysical parameters were used to characterize the gas reservoir quality of the Narsingdi Gas Field by using well logs such as GR logs, SP logs, resistivity logs, neutron logs and density logs. 5 gas-bearing zones were identified in the study well Bakhrabad-10 on the basis of low GR, high SP, high resistivity, low neutron and low density log responses. The gas reservoirs were shown an average shale volume of about 19.5 % and average porosity of 27. All the calculated values of gas saturation were more than 65% treated as gas-bearing zones. The average permeability of these gas zones were estimated at 32 mD to 55 mD by using two permeability equations. It revealed that there were positive correlations between porosity and permeability of gas-bearing zones in the study well. The composite log response and all the calculated petrophysical parameters of gas-bearing zones suggest gas reservoir is potential for commercial gas production and accumulation. The study recommended that 3-D seismic study integrated with well log study will depict more detailed picture of the gas reservoir in the study gas field.