5. Reservoir and depletion strategy

5.1. Reservoir Characteristics

The reservoir is at depth -2600 m to -3300 m below seabed. Its length is 12 km and its width 2.5 km and it consists of carbonate rock of the Miocene period. The main characteristic of the reservoir is its high pressure and high temperature. The hydrocarbons in place are estimated using the gross rock volume and other relevant data from the exploration activities (Table 3).

 Low MediumHigh
GRV Oil  6.95×108 m³ 7.72×108  m³ 8.49×108  m³
GRV Gas1.45×1010  m³1.60×1010  m³ 1.77×1010  m³
Porosity (Ø) 0.1 0.225 0.25
Sw0.20 0.25 0.35
RF Oil 0.250.350.45
RF Gas 0.750.775 0.80
Pressure≈ 6000 psia  
Temperature ≈ 150 °C  
FVF Oil (Bo)≈ 1.6  
FVF Gas (Bg) ≈ 0.00385  

Table 3: Resevoir data

The shape of the reservoir rock is shown below.


5.2. Hydrocarbons in place

The table shows the low, medium and high Estimated Ultimate Recovery (EUR) for the fields in consideration. 

 Low MediumHigh
SfR_gas (TCF) 5.4 Tcf
(150 billion m³) 
 6.5 Tcf
(185 billion m³)
7.8 Tcf
(220 billion m³)
SfR_oil (mmbbl)40 mmbbl 58 mmbbl85 mmbbl

Table4: Estimated Ultimate Recovery

To estimate low/medium/high value for the ultimate recovery we use the following formulas:

STOIIP = GRV x N/G x Φ x So / Bo
GIIP = GRV x N/G x Φ x Sg / Bg
SfR_oil = GIIP x RF
SfR_gas = GIIP x RF

A method to estimate the low/medium/high value of the product of two parameters is to multiply the low, medium and high values in a matrix and take the mean value for each set of three products as show in figure 3 (Frank Jahn M. C., 1998). In the same way the product of four parameters is estimated by combining the low, medium and high value of the product of two pairs. By applying the previous method, we derive the low/medium/high EUR (Estimated Ultimate Recovery) for the reservoir of Block 21.


5.3. Depletion Strategy

Before finalizing the development plan, the JV should negotiate with the main gas consumers in Vietnam and make gas purchase agreements. The field development plan will then adapt, so that the agreed quantities can be delivered to the costumers. For the purpose of this study, the delivery quantities of a potential gas purchase agreement are assumed to be:

Daily contract quantity (DCQ):  800 mmscf
Swing factor:    1.3
Contact duration:    13 years

For the mid case scenario of 6.5 Tcf, the DCQ is satisfied if a plateau production of 800 mmscf/d is maintained for 13 years. Around 65 % of the reserves will be produced on the plateau production.

Daily plateau  production = (0.65•6.5) Tcf /(13•365) days ≈ 800 mmscf/d (≈ 23 million m³/d)

Gas field depletion strategy

The field includes a marginal amount of oil. The main part of the oil will be produced in the first five years using two horizontal (combination) wells. Then, those wells will be converted in order or produce gas from the upper part of the reservoir.  The plateau oil production is 20’000 bopd.


At the beginning of the production the wells have high production capacity. As more area is drained, the well pressure drops and the production capacity reduces. To keep the production capacity at a level higher than the required for fulfilling the daily contract quantity, a second drilling round will be contacted at a later stage (after approx. 10 year). If the high estimate for the ultimate recovery is realized, then the plateau production will be extended to 25 years using compression to enhance recovery.


Leave a Comment

Scroll to Top