Material Balance and Reserve Estimation - Reservoir Engineering - Petroleum Engineering Previous Year Questions

1

2016 · Petroleum Engineering · Reservoir Engineering · Material Balance and Reserve Estimation

Petroleum Engineering (PE) 2016

The production of a gas well was found to decline exponentially. The observed production rate on 1^st January, 2014 was 0.6×10¹⁰ SCF/month and on 1^st January, 2015, it was 0.4×10¹⁰ SCF/month. The economic production limit for the well is estimated to be 0.002×10¹⁰ SCF/month.
The remaining reserves for the well as on 1^st January, 2015 were _____ × 10¹⁰ SCF.

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2

2016 · Petroleum Engineering · Reservoir Engineering · Material Balance and Reserve Estimation

Petroleum Engineering (PE) 2016

A 30 ft thick gas reservoir has an area of 3,000 acres (1 acre = 43,560 ft²). The porosity of the reservoir is 15% and the connate water saturation is 20%. Initial reservoir pressure and temperature are 2,600 psig and 150°F (= 610°R), respectively. The compressibility factor (Z) at initial conditions is 0.82. The gas in the reservoir can be produced till it attains the final pressure of 1,000 psig (Z = 0.88) under isothermal conditions.
The gas recovery factor is _____ %.

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3

2017 · Petroleum Engineering · Reservoir Engineering · Material Balance and Reserve Estimation

Petroleum Engineering (PE) 2017

An oil reservoir of \(1000\ \text{m}^2\) area and thickness of 10 m has a porosity of 30%. The connate water saturation is 20%. Initial formation volume factor \(B_{oi} = 1.2\ \frac{\text{reservoir}\ \text{m}^3}{\text{stock}\ \text{tank}\ \text{m}^3}\). Assuming average oil flow rate of \(2\ \text{m}^3/\text{day}\) (at surface condition), the life of reservoir is _____ days.

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4

2018 · Petroleum Engineering · Reservoir Engineering · Material Balance and Reserve Estimation

Petroleum Engineering (PE) 2018

Plot of ratio of pressure to gas compressibility factor (P/Z) vs. cumulative gas production (G_p) for a gas reservoir (represented by solid curve in the figure) was shown to a reservoir engineering student.
The student made the following statements:
(I) A water aquifer is attached to this gas reservoir.
(II) P/Z vs. G_p curve must always be a straight line for water encroachment in a gas reservoir.
(III) The ultimate gas recovery is diminished due to water encroachment.
Which of the above statements are TRUE?

A

Only I and II

B

Only II and III

C

Only I and III

D

I, II, and III

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5

2018 · Petroleum Engineering · Reservoir Engineering · Material Balance and Reserve Estimation

Petroleum Engineering (PE) 2018

Exponential decline curve is to be used to estimate the oil reserves of a well. The current oil production rate is 1000 m³ per day and yearly decline rate is 6% per year. If the minimum oil flow rate economically sustainable for the well is 1 m³ per day, the reserves (economically producible) associated with the well are __________× 10⁶ m³. (rounded-off to two decimal places. Use 1 year = 365 days)

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6

2018 · Petroleum Engineering · Reservoir Engineering · Material Balance and Reserve Estimation

Petroleum Engineering (PE) 2018

An oil well (producing under expansion drive only) in a reservoir is subjected to two pressure build-up tests. The average formation thickness of the reservoir is 13 ft, the total compressibility is \( 1 \times 10^{-5} \) \( \text{psia}^{-1} \), and porosity is 0.2. The average formation volume factor of oil is 1.3 reservoir-bbl/STB. Average reservoir pressure during the first test and the second test was found to be 3500 psia and 3200 psia, respectively.
If the oil produced between the two pressure build-up tests in 180 days is 250 STB/day, the area of the reservoir is __________ acres. (rounded-off to two decimal places)
(Use: 1 acre = 43560 \( \text{ft}^2 \), 1 bbl = 5.615 \( \text{ft}^3 \))

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7

2020 · Petroleum Engineering · Reservoir Engineering · Material Balance and Reserve Estimation

Petroleum Engineering (PE) 2020

A gas reservoir without aquifer is at 300 bar (absolute) and 90°C. The GIIP (gas initial in place) is 10⁷ m³ (at surface conditions). Neglect formation and water compressibility.
Given:
Surface pressure = 1 bar (absolute)
Surface temperature = 25°C
Gas compressibility factor, Z (at surface condition) = 1
Z (at 300 bar (absolute) and 90°C) = 0.88
Z (at 100 bar (absolute) and 90°C) = 0.83
If the reservoir pressure reduces to 100 bar (absolute) under isothermal conditions, the total volume of gas (at surface conditions) produced from the reservoir (rounded off to two decimal places) is ______ ×10⁶ m³.

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8

2021 · Petroleum Engineering · Reservoir Engineering · Material Balance and Reserve Estimation

Petroleum Engineering (PE) 2021

A cylindrical crude oil reservoir with a radius of 3000 ft is under water influx from a cylindrical aquifer with an estimated radius of 9000 ft. The reservoir has the following properties.
Aquifer thickness, $h = 40$ ft
Porosity, $\phi = 15\%$
Formation compressibility, $C_f = 4.5 \times 10^{-6}$ psi⁻¹
Water compressibility, $C_w = 4.0 \times 10^{-6}$ psi⁻¹
Assuming a pot reservoir model with fractional encroachment angle as unity, the water influx into the reservoir for a pressure drop of 700 psi is __________ MMbbl (million barrels) (round off to two decimal places).
($\pi = 3.14$, 1 bbl = 5.615 ft³)

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9

2021 · Petroleum Engineering · Reservoir Engineering · Material Balance and Reserve Estimation

Petroleum Engineering (PE) 2021

The fluid flow through an under-saturated oil reservoir is driven by solution gas drive mechanism. The reservoir parameters are as given below.
Compressibility of water, \(c_w = 1 \times 10^{-6}\) psi⁻¹
Compressibility of formation, \(c_f = 1 \times 10^{-5}\) psi⁻¹
Connate water saturation, \(S_{wc} = 0.2\)
Initial reservoir pressure, \(p_i = 4000\) psi
Reservoir pressure at bubble-point, \(p_b = 3000\) psi
Oil formation volume factor, \(B_{oi} = 1.24\) rb/STB
Formation volume factor at bubble point pressure, \(B_{ob} = 1.26\) rb/STB
The percentage of oil recovered as a fraction of the Original Oil in Place (OOIP) is __________% (round off to one decimal place).

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10

2021 · Petroleum Engineering · Reservoir Engineering · Material Balance and Reserve Estimation

Petroleum Engineering (PE) 2021

A depleted and shut in oil reservoir originally contained \( 25 \times 10^6 \) STB of oil with a formation volume factor of 1.35 res bbl/STB and a connate water saturation of 0.25. Cumulative oil production to date has been \( 2.5 \times 10^6 \) STB of oil. The oil formation volume factor is now 1.25 res bbl/STB. Assuming no water influx, the gas saturation in the reservoir is __________ % (round off to one decimal place).

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11

2022 · Petroleum Engineering · Reservoir Engineering · Material Balance and Reserve Estimation

Petroleum Engineering (PE) 2022

A reservoir is producing oil at 7000 stb/day with a producing gas to oil ratio (GOR) of 2000 scf/stb. At a certain point of time, the reservoir pressure is monitored and decided to be maintained at a constant pressure of 2500 psi using water injection. The PVT properties estimated at 2500 psi are:

• Bubble point pressure = 3000 psi
• Oil formation volume factor = 1.2 rb/stb
• Water formation volume factor = 1.0 rb/stb
• Gas formation volume factor = 0.0012 rb/scf
• Solution GOR = 300 scf/stb

The initial water injection rate (stb/day) required to maintain oil production at 7000 stb/day is ________ (rounded off to the nearest integer).

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12

2025 · Petroleum Engineering · Reservoir Engineering · Material Balance and Reserve Estimation

Petroleum Engineering (PE) 2025

For a hydrocarbon reservoir, the following parameters are used in the general material balance equation (MBE).
\(N\) = Initial (original) oil in place, stb
\(G\) = Initial volume of gas cap, scf
\(m\) = Ratio of initial volume of gas cap to volume of oil initial in place, rb/rb
\(S_{wi}\) = Initial water saturation
\(S_{oi}\) = Initial oil saturation
\(B_{oi}\) = Initial oil formation volume factor, rb/stb
\(B_{gi}\) = Initial gas formation volume factor, rb/scf
The total pore volume (in rb) of the reservoir is

A

\(\frac{G B_{gi} (1 + m)}{1 - S_{oi}}\)

B

\(\frac{N B_{oi} (1 - m)}{1 - S_{oi}}\)

C

\(\frac{N B_{oi} (1 + m)}{1 - S_{wi}}\)

D

\(\frac{G B_{gi} (1 - m)}{1 - S_{wi}}\)

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13

2023 · Petroleum Engineering · Reservoir Engineering · Material Balance and Reserve Estimation

Petroleum Engineering (PE) 2023

A highly permeable reservoir with initial reservoir pressure of 3000 psi is under active water drive from a surrounding large aquifer. The final stabilized reservoir pressure is 2500 psi. Following data associated with the reservoir at 2500 psi are given.
Oil production rate = 30,000 STB/day
Water production rate = 0 STB/day
Oil formation volume factor, B_o = 1.5 bbl/STB
Gas formation volume factor, B_g = 0.00070 bbl/scf
Water formation volume factor, B_w = 1 bbl/STB
Producing Gas to Oil Ratio, GOR = 850 scf/STB
Gas solubility, R_s = 700 scf/STB
(bbl: reservoir barrel, STB: Stock tank barrel, scf: standard cubic feet)
If the reservoir pressure and the reservoir production rates remain constant, the water influx rate is________bbl/day (rounded to nearest integer).

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14

2023 · Petroleum Engineering · Reservoir Engineering · Material Balance and Reserve Estimation

Petroleum Engineering (PE) 2023

A volumetric undersaturated solution gas drive reservoir (without gas cap, no water influx, and with no initial gas saturation) has an initial water saturation of 15% which remains unchanged during production. After the production of 10% of the initial oil (measured at surface conditions), the oil formation volume factor (B_o) is reduced from its initial value of 1.4 bbl/STB to 1.2 bbl/STB.
(bbl: reservoir barrel, STB: Stock tank barrel)
The final gas saturation in percentage is ________ (rounded to one decimal place).

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15

2026 · Petroleum Engineering · Reservoir Engineering · Material Balance and Reserve Estimation

Petroleum Engineering (PE) 2026

Oil is being produced from an undersaturated oil reservoir (with no initial gas cap). The reservoir rock compressibility and water saturation are negligible. There is no water influx, and there is no water production from the well. The initial oil formation volume factor was 1.24 RB/STB. After production of oil for a short period of time, the reservoir pressure is still above the bubble-point pressure, and the oil formation volume factor has changed to 1.25 RB/STB.
The calculated oil recovery factor (in fraction) is ________. (Rounded off to three decimal places)
[RB: Reservoir Barrel; STB: Stock Tank Barrel]

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16

2026 · Petroleum Engineering · Reservoir Engineering · Material Balance and Reserve Estimation

Petroleum Engineering (PE) 2026

A gas reservoir has bulk volume of 10000 ft³, connate water saturation of 0.2 and porosity of 0.2. There is no oil present in the reservoir. The gas formation volume factor, B_g is 0.005 RCF/SCF. The volume of gas in-place for this reservoir (in SCF) is ______×10⁴. (Answer in integer) [RCF: Reservoir Cubic Feet; SCF: Standard Cubic Feet]

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