Heat Transfer: 2018 #69

Problem 69: A 5.8-mile flowline moves 10 MMscf/D of natural gas of SG 0.7. Indirect fired heaters along the flowline to prevent hydrates. Using flowline temperature & pressure sensors and charts, the NG enthalpy should drop from 234 to 200 Btu/lbm between matching flowline heaters. The flowline heat transfer coefficient is 1.2 Btu/hr-ft^2-deg F. The true rating of the line heaters (MBtu/hr) is closest to: (A) 747; (B) 757; (C) 767; (D) 777.

This can be solved using conversions found on 8 FAC 9 & 9 PVT 1. A similar, more difficult problem can be found in Guide to Professional Registration for Petroleum Engineers, 1991.

This problem is merely a heat-transfer calculation, and so many don't find this problem type applicable to the modern petroleum PE exam. I generally agree. However, I include it because a) it's fair game, and b) it's good practice for heaters & enthalpy, which can find their way into facility-type problems. In the solution, you need to calculate q in lbm/hr to use with the given enthapy:

1. m=0.7(28.97 lbm/mol)(10MM scf/D)/(379.49 mol/scf * 24 hr/D) = 22,267 lbm/hr.
2. q = m(h2-h1) = 22,267 lbm/hr (234-200 Btu/lbm) = 757,031 Btu/hr (B).

It's all dimensional analysis; fast work if you know the needed constants on 8 FAC9 & 9 PVT 1.

NG Processing: 2018 #30

Problem 30. The statement most FALSE about natural gas treating and processing is:

(A) In long distance transmission of sales gas by pipeline, pressure is usually less than 1,000 psig.
(B) A potential cause for treated natural gas “going sour” is too low of an inlet gas rate.
(C) All raw natural gas is fully saturated with water vapor when produced from an...
(D) There are four glycols that are used in removing water vapor from natural gas or in...

This question can be answered quickly knowing the basics of natural gas processing. Or you can quickly glean the answer from HS III, pages 186, 192, and 198. Overall, a pretty easy question, but it can take time if you don't have much experience, or lack the right resources.

Watch the wording like a hawk. It's always best to find a direct quote from an SPE source for these types of problems if the wording seems vague.

Choke: 2018 #1

Problem 1. NG, SG 0.70 through ½" choke at 500 psig, 140 dF. 2,900 MCF/day is closest to: (A) 20% high (B) 10% high (C) Accurate (D) 10% low.

This solution is shown in the Guidebook 8 FAC 1-2. At said conditions, z = ~0.95 (find using 9 PVT 2 with preferred z chart; the Guidebook walks one through the psuedocritcal calculations).

Once you have z, flow computes to 2,900 MCF/day or (C). Note the Guidebook comment how estimating z = 1 gets you close, but this problem demands an exact answer, so you can't risk estimating.

Natural Gas: 2005 #61 (similar)

The Guidebook doesn't include complicated graphs; it uses tables for speed. There are several graphs that were too complex to convert into table format that you should bring to the exam.

One (naturally) is a z-chart. I use the one in TS8 (or TS1). Another is the equilibrium water vapor content for natural gas (I use Fig 5.7 in HS3-199). You should own and bring (at a minimum) TS1 and HS3, so just use those charts. Practice using them until you are fast.

A typical natural gas problem? Water vapor (lbs) to be removed from a saturated gas (MMscf).

Just go to the chart at any temperature and pressure to get saturated lbs water/ MM gas (at say 100 deg F & 1,000 psig you get about 60 lbm water). If given the minimum water allowed for sales (say 10 lbs/MM) you must thus remove 50 lbm for each 1 MM sold (for my example). Simple.

Always glance over your answer choices before finding "exact" chart numbers. Work fast and estimate. Remember, it's all about speed. This is why tables are much faster; just scan down the table and make estimates between numbers.

Wet Gas: 2005 #48 (similar)

Equations for wet gas are found on 13 RES 9.

Q1) Given API or SG of oil, what is the molecular weight?

A1) Given API, MW = 5954/(API - 8.8).

Q2) Given the GOR and SG of gas and oil, along with the Mol wgt (say 50,000 scf/STB, 0.7, 0.83, and 197), what is the SG of the fluid mix back in the reservoir?

Q3) What is the "Gaseous Equivalent" of a stock-tank liquid?
A3) 133,300(SG_oil / MW_oil)

The numbers for Q2 above are easy to play with. If your GOR was 15,000, SG 0.67 & 0.76, and a standard mol wgt, you calculate a SG of the reservoir wet gas in the mid 0.8's. This sort of problem is easy if you check your units. And don't panic when seeing terms like condensate, wet gas, etc.