The Application of P/Z Methods in Evaluation of Initial Gas in Place of X Reservoir

Reservoir X is located in West Tanjung Jabung Regency, Jambi, Indonesia, with initial pressure is 2,028 psia, and a temperature is 226°F. The reservoir has been produced from February 2018 to the present (February 2021), with cumulative gas production of 5.17 Bscf. Based on the Plan Of Development (POD) study in 2016, the initial gas in place was determined volumetrically to be 49,92 Bscf. To compare the initial gas in place, it is deemed necessary to recalculate the initial gas in place by utilizing another method. The calculation of Initial Gas In Place (IGIP) is conducted by using the P/Z method. In this method, the Initial Gas in Place is obtained 49.01 Bscf. Based on analyzing the phase diagrams and the prevailing reservoir conditions, reservoir X indicated a dry gas reservoir. Such reservoir drive mechanism is a depletion drive, determined from the P/Z vs Gp plot, generating a straight line. Based on the assumed abandonment pressure value of 300 psia, the estimated ultimate recovery value is 41.93 Bscf, with the recovery factor value of 87.65% and the remaining reserves of 36.76 Bscf.


INTRODUCTION
In developing a natural gas field, several important factors must be determined accurately, one of which is determining the Initial Gas in Place (IGIP). The determination of IGIP provides an important role in basic decisions for the development and operation of a reservoir.
Based on the Plan Of Development (POD) study in 2016, the initial gas in reservoir X was determined volumetrically to be 49,92 Bscf. To obtain a comparison, the initial gas in place was recalculated by utilizing another method.
Reservoir X contains 5 production wells (SA-3 SUT-1, SAD-2, SAD-3, and SAD-5). Reservoir X has been producing from February 2018 to the present, with cumulative gas production of 5.17 Bscf (February 2021). The data obtained from the 5 above wells (Production well data, Pressure well data, PVT data) is used to determine Initial Gas In place. IGIP calculation is determined by various methods. However, in this research, the P/Z vs Gp material balance method was used. In addition to calculating IGIP, the recovery factor, estimated ultimate recovery, and recovery factor are also determined.

METHODOLOGY
In the early stages of the study, data was collected, including well pressure data, production data, and lab PVT data. Further, an analysis is conducted on the type of reservoir. Afterward, to obtain the reservoir pressure data monthly, pressure interpolation was carried out from the existing wells. The z factor for each pressure was then determined. By Gopal method Afterward, the drive mechanism and initial gas in place were determined with the results of the Pz vs Gp plot. Further, the value of the recovery factor estimated ultimate recovery, and remaining reserves are determined.

RESULTS AND DISCUSSION
Reservoir X is a gas reservoir situated in West Tanjung Jabung Regency, Jambi, Indonesia. Reservoir X has an initial pressure of 2028 psia and a temperature of 226°F. To determine the initial gas in place, the type of reservoir needs to be determined first.Determination of Reservoir Type Based on the PVP data, the reservoir type is determined by analysis using PVTP software to obtain the phase diagram as depicted in Fig. 1. Figure 1 Reservoir Types against Reservoir X.

Condition
The above Figure 1 indicates a reduction in pressure and temperature from the initial condition to the separator conditions, the path of depletion is completely outside the phase envelope. Its means that the fluid does not undergo phase changes, and therefore no liquids are formed either in the reservoir or production facilities. Based on the analysis, reservoir X was classified in the dry gas type. Thus, in determining the initial gas in place in reservoir x the p/z method could be used. Therefore, data such as pressure, production data are required to apply this method.

Pressure on Reservoir X
The pressure in Reservoir X is only available from the four wells at a certain time (table 1); thus, it is necessary to do a regression from the existing pressure data to draw a trendline. From the table, a graph of reservoir pressure against time is made and extrapolated to obtain the unknown monthly pressure from February 2018 to February 2021 (Fig 2).

Determination of Z Factor
Determination of the Z factor is conducted by using the Gopal method. The Gopal method is used because it is easier to determine z at more data various reservoir pressures using the equation of Gopal rather than graphically (Standing and Katz).
The following is an example of determining the Z factor using the Gopal method:  The results of the Z Factor calculation based on the Gopal method can be seen in Table 2 in Appendix.

Determination of Drive Mechanism
The drive mechanism is determined by using the material balance method P/Z Vs Gp by plotting the value of Gp on the x-axis and P/Z on the y-axis. (Table 2 at Appendix) is depicted in Fig.3 Based on the P/Z Vs Gp graph analysis (Fig  3), which shows the plot of the P/Z vs Gp is a straight line, indicating that reservoir X is the depletion drive.

Calculation of Initial Gas In Place
The initial gas in Place of reservoir X is calculated using the P/Z Vs Gp material balance method. Figure 3 indicates the line equation to find the initial gas In Place value, whose equation is y = -43.268x+2120.9 and R2 = 0.9982. The initial gas in place is obtained based on the equation of the line obtained when P/Z=0. Thus, y = -43.268x+2120.9 where the y-axis is P/Z and the x-axis is Gp, in which: y = -43.268x+2120.9 43.268 = 2120.9/ G G = 2120.9/ 43.268 G = 49.01 Bscf The gas initial in place is 49.01 Bscf.

Determination of Estimated Ultimate Recovery
Estimated Ultimate Recovery gas depends on abandonment pressure. Abandonment pressure conforms to company policies. and in the study conducted by the author, it was assumed that the abandonment pressure is 300 psia.
Based on assumed abandonment pressure, the calculations are carried out to obtain the value of Za. Gopal

Determination of Recovery Factor
The recovery Factor of Reservoir X is determined by the EUR value divided by Initial Gas in Place. Thus, the value of the recovery factor is obtained by: