For Original Oil in place (N) calculations, PVT properties (Table 1b.) are required to be interpolated at the reservoir pressures given in Table 1a. Interpolation data is shown below in Table 1d below. The interpolated data and pressure production data indicates that the reservoir is in saturated condition at the initial reservoir pressure. Therefore, for calculation of OOIP, material balance equation for saturated oil reservoir with constant porous volume has been used (Eq. 1a). Calculations are shown in Table 1e.

Eq. 1a.

Table 1d. Interpolated PVT Properties.

Table 1e. Calculation for Original Oil In Place (N).

Plotted Y vs X (mentioned in Table 1e.) to calculate the slope which calculates Original Oil in Place (N).

**Original Oil in Place: 2.44 MMSTB**

**Original Gas in Place: 3.468 BSCF**

Figure 1a.

For the estimation of oil recovery at the abandonment flow rate of 250 STB/day, decline curve model has been used. Initial decline parameter (D) are calculated using production data (Table 1c) and then 1/D is plotted against time to calculate decline rate constant (Di) and b. Later using Eq. 1c, Di and b, simulated production rates are plotted and matched with given production data (Figure 1b.) to tune decline parameters for a good match. With tuned decline parameters, recovery has been calculated using Eq. 1d.

Eq. 1b

for b=0

Eq. 1c

And for recovery

Eq. 1d

Table. 1f: Decline parameters calculation.

Figure 1b: Plot of 1/D (years) vs time

For calculations **qi: 742 STB/Day**

Slope of Figure 1b. Indicates value of b and inverse of the Y-intercept is Decline constant (Di).

**Calculated b: -0.0105**

**Calculated Di: 6.8%**

Value of b is negative which is unusual but possible. We will use **b=0** and continue with exponential decline curve fitting approach using Eq. 1c to tune Di. Simulated data matched with production of data gives **Di=26.0%.**

Using Di=26% and b=0, recovery has also been calculated using Eq 1d.

**Total Recovery @ 250 STB/day: 997.88 MSTB**

Figure 1b. Qoil Vs Time (Production Vs Predicted Data)

For Original Gas in place (G) calculations, PVT properties (Table 2b.) are required to be interpolated at the reservoir pressures given in Table 2a. Interpolation data is shown below in Table 2c below. For calculation of OGIP, material balance equation for gas reservoir with constant porous volume (P/Z method) has been used (Eq. 2a). First calculated P/z for all reservoir pressures and then plotted P/z vs Cumulative Gas production (Gp). X-Intercept of the plot gives the Original Gas In place (G).

Eq. 2a

Using P/Z plot, Original Gas in Place : **94.04 MMMscf.**

Table 2c. Interpolated PVT Properties.

Table 2d. Calculation for Original Gas In Place (G)

Figure 2a. P/Z Vs Gp

**Gas Recovery Estimation using Decline Curve Analysis:**

For the estimation of gas recovery at the abandonment flow rate of 25 MCF/day, decline curve model has been used. For that, Average gas flowrates (Table 2e) has been calculated using Cumulative Production data. **Qi=34.181 **MMscfd has been highlighted as the point from where decline starts. Initial decline parameter (D) are calculated using production data (Table 2e) and then 1/D is plotted against time to calculate decline rate constant (Di) and b. Later using Eq. 1c, Di and b, simulated production rates are plotted and matched with given production data (Figure 2b.) to tune decline parameters for a good match. With tuned decline parameters, recovery has been calculated using Eq. 1d.

Table 2d. Average Gas flowrate Calculation

Table. 2e: Decline parameters calculation.

Figure 2b: Plot of 1/D (years) vs time

Slope of Figure 1b. Indicates value of b and inverse of the Y-intercept is Decline constant (Di).

**Calculated b: -0.0143**

**Calculated Di: 4.5%**

Value of b is negative which is unusual but possible. We will use **b=0** and continue with exponential decline curve fitting approach using Eq. 1c to tune Di. Simulated data matched with production of data gives **Di=15.0%.**

Using Di=15% and b=0, recovery has also been calculated using Eq 1d.

**Total Recovery @ 25 Mscf/day: 101.346 Bscf***

**Recovery after decline started @ 34.181 MMscfd @804 days: 83.119 Bscf***

*****Calculations are shown in Spreadsheet (Case-2b).

Figure 1b. Qg (History and Simulated) Vs Time

For verification of the conceptual model that all six wells are draining from same reservoir, we have used concept of flowing material balance (Pfw/z). Initially, the PVT properties are interpolated for given reservoir pressures in Table 3a. Interpolation results are shown Table 3c. Later the Pfw/z has been calculated and plotted against the Gp. It is observed that all wells expect Z-206 is draining from same reservoir. But **Well X-206** depletion trend deviated from others showing that either a sealing fault or boundary is separating the reservoir (Figure 3c.) So the conceptual model that all wells are draining from same reservoir doesn’t hold true with the production data.

Table 3c. Interpolated PVT Properties.

Figure 3a. Pfw/z Vs Gp

With concept of flowing material balance, slope is calculated from the Pfw/z vs Gp Plot (Figure 3b.)

**Slope: -13.696**

Now according to flowing material balance, the slope of Pfw/z vs Gp is the same as the slope of P/z (Using shutin reservoir pressure) Vs Gp i.e. .

So now using the **slope= -13.696** and taking Initial Reservoir Pressure (**Pi=3993 psi**) calculate Pi/zi = **4338.22 psi**.

Now with the concept of P/z with the equation below:

Where:

Slope:

Y-intercept:

Plot another line with the and slope (Figure. 3c)

The X-intercept of this line shows **Original Gas in place (G) for Anaconda Reservoir Compartment containing (X-201 to X-205)= 316.75 Bscf**

Similar approach is used and for the reservoir compartment with **X-206** has **OGIP: 81.61 Bscf**

Figure 3b. Pfw/z Vs Gp

Figure 3c. Pfw/z and P/z Vs Gp

Figure 4a: Plot of Qoil (MSTB/day) vs time

The possible redevelopment process may include infill drilling (possibly 8 wells indicated by arrows in Figure 4a), which shows increase in production, but after 8th well, the production is in the do nothing case. Possible with the infill drilling, water flooding would also be the source of pressure maintenance.

**Water Flooding**

During water flooding process, water is being injected into the reservoir for pressure maintenance and to increase displacement efficiency. The injected water not only push water towards the well but also replace the pore volume of the oil which results in pressure maintenance and higher recoveries.

**Gas Flooding**

To increase recovery in oil reservoirs, pressure maintenance is also done by injection of gas. Gas not only maintenance the reservoir pressure by replaceing the pore volume but also lightens the oil and helps its displacement much better and effective way.

**Infill Drilling**

Infill drilling improves the oil recovery by sweeping the undrained oil from the reservoir. With the decrease in the well spacing and increase in well density, maximum volume of oil is being produced resulting in higher recovery factors. This practice not only accelerates the recovery but proves to be very effective field development strategy in heterogeneous reservoirs.

**Chemical Injection**

Chemical EOR methods injects chemicals with water slug which altering the fluid to fluid and fluid to rock interaction and increases the effectiveness of the water sweeping resulting in increased recovery. This process includes lowering of IFT between water and oil or an increment of viscosity of injected water to alter mobility properties and conformance control. Chemical injects also results in alteration of wettability of rock in oil wet reservoirs.

**Incremental reserves using Decline Curve Analysis:**

For the estimation of oil recovery at the abandonment flow rate of 1000 STB/day, decline curve model has been used. **Qi=323.761 **MSTB has been selected as the point from where decline starts. Initial decline parameter (D) are calculated using production data and then 1/D is plotted against time to calculate decline rate constant Di and b (Calculations are available in Case-4 b).

Later using Eq. 1b, Di and b, simulated production rates are plotted and matched with given production data (Figure 4a.) to tune decline parameters for a good match. With tuned decline parameters, recovery has been calculated using Eq. 1d.

Figure 4b: Plot of 1/D (years) vs time

Slope of Figure 4a. Indicates value of b and inverse of the Y-intercept is Decline constant (Di).

**Calculated b: 0.0206**

**Calculated Di: 24.5%**

We will use **b=0.0206** and continue with hyperbolic decline curve fitting approach using Eq. 1c to tune Di. Simulated data matched with production of data gives **Di=33.0%.**

Using Di=33.0% and b=0.0206, recovery has also been calculated using Eq. 1d.

**Incremental Recovery @ 1000 STB/day: 9696 Mscf***

Figure 4c: Plot of 1/D (years) vs time

- Larry W.Lake, (2003).
*A Generalized Approach to Primary Hydrocarbon Recovery,*1^{st}Amsterdam Netherland: Elsevier Ltd, p. 380-381, 453-456, 552-563.

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