This section is concerned with fluid flow in the bulk of the RS Gas Reservoir. The integration of geology and well test analyses will be very helpful [1] . An understanding of the mechanisms controlling fluid displacement will help in maximizing the technically recoverable reserves through

1) Planning reservoir development strategies.

2) Optimizing off take rates at field and reservoir layer level.

3) Marking initial well locations.

4) Designing initial well completions and identifying subsequent interventions.

Knowledge of reservoir drive mechanism is necessary in order to prepare dynamic model to calculate initial gas in-place [2] . Normally, gas reservoirs are produced by expansion of the gas contained in the reservoir. The high compressibility of the gas relative to the water in the reservoir (either connate water or underlying aquifer) makes the gas expansion the dominant drive mechanism. But this depletion drive mechanism changes to water drive mechanism by different indices in different reservoirs depending upon the properties and geometries of the reservoir and aquifer.

To investigate the drive mechanism of RS Gas Reservoir the P/Z Vs G_p Plot (Figure 3) was generated by using the production and pressure history as given in Table 1.

For a volumetric reservoir, the relationship between (P/Z) and G_p is essentially linear because of volumetric depletion of gas reservoir and by extrapolation of the straight line to abscissa, i.e., at P/Z = 0, gives the value of the gas initially in place as G = G_p [3] . But the graphical representation of pressure and production history of RS Reservoir shows the presence of water influx, as shown Figure 1 where the plot of (P/Z) versus G_p deviates from the linear relationship, it indicates the presence of water encroachment [4] [5] .

After confirming by P/Z Vs G_p plot that there is water encroachment in the reservoir [6] [7] , the next step is to generate the Cole plot which is more sensitive reservoir drive mechanism diagnostic plot [8] .

Cole plot of RS Gas Reservoir (Figure 4) shows that reservoir has water drive mechanism means it is has some degree of pressure maintenance due to water encroachment into the reservoir from the aquifer. After validation of reservoir drive mechanism now the question arises about the strength of aquifer support. Accurate aquifer modeling will be done in next step by verifying the history matching and production and pressure simulation. If the comparison of P/Z Vs G_p plot (Figure 3) and Cole plot (Figure 4) is made with their benchmark plots (Figure 5) it reveals that RS Gas Reservoir has moderate water dive support or in other words it has partial water drive mechanism.

Material Balance Method is adopted to estimate the initial gas in place of Jin Gas Reservoir by using the pressure values obtained by well test interpretation using the commercial software. The material balance is based on the principle of the conservation of mass:

Mass of fluids originally in place = Fluids produced + Remaining fluids in place.

The material balance program uses a conceptual model of the reservoir to predict the reservoir behavior based on the effects of reservoir fluids production [9] . The material balance equation is zero-dimensional, meaning that it is based on a tank model and does not take into account the geometry of the reservoir, the drainage areas, the position and orientation of the wells, etc [10] . However, the material balance approach proved to be a very useful tool in this study in performing many tasks i.e. in quantifying different parameters of a reservoir such as hydrocarbon in place, in determining the presence, the type and size of an aquifer, encroachment angle, etc., in predicting the reservoir performance and manifold back pressures for a given production schedule and also in predicting the reservoir performance and well production for a given manifold pressure schedule [11] [12] .

Havlena-Odeh graphical method was adopted to perform material balance calculations which matched with Hurst-Everdingen Dake Radial Aquifer model [13] [14] [15] . An iterative non linear regression was used to automatically find the best mathematical fit for a given model showed original gas in place of 104 BSCF (Figure 6).

Reservoir Driver Mechanism indices are also calculated and plotted (Figure 7) which translates current average water saturation as 56.6%, which was 48.7% at initial condition. Initial water drive index was 25% but now it is 50%, which means gas expansion within the reservoir has reduced to 50%.

Material balance estimates are also validated by history matching of production and pressures which showed good match (Figure 8). A simulation of production is run to check the validity of the results. Gas and water relative per- meabilities are estimated from historical WGR.

The accurate and perfect history matching is achieved through simulation which indicates the authenticity and validity of results of different parameters of reservoir as well as aquifer [16] . The models which are used for reservoir and aquifer are correct and the set of pressure used in this modeling also has been proved accurate. This set of pressure has been obtained through the well testing interpretation. As per the matched material balance dynamic model, the yearly water encroachment into the reservoir is given in Table 2. The cumulative water production into the reservoir is estimated 23 MMRB. The corrected P/Z Vs G_p plot has been generated manually too by using Microsoft Excel as shown in Figure 9. In this corrected P/Z Vs G_p plot volume of encroached water is incorporated and subtracted.

The Initial Gas In-Place of RS Gas Field calculated by Material Balance is also verified by Volumetric Reserve Estimation Method. Volumetric Method uses

static properties of the reservoir and Material Balance Method is the dynamic model of the reservoir [17] . The following two approaches are used to calculate the IGIP of RS Gas Field.

Firstly the reservoir is considered as rectangular in shape (Figure 10) as classical Reservoir Engineering and petro-physical model employ in the following renowned equation.

where G means gas in place (SCF), A is reservoir area (acres), h shows reservoir thickness (feet), θ indicates porosity (fractions), S_wi is water saturation (fractions) and B_gi is gas formation volume factor (ft³/SCF). NTG in the Figure 10 stands for Net to Gross ratio of thickness

By using Equation (1)