MBE: 2018 #51

Problem 51: A 150-degF gas reservoir, originally 4,000 psia...known water drive...10 Mbbl of water and 2 MMMCF of gas has been produced...the percentage recovered to date is closest to: 

This problem is wordy (I've cut most for the blog post) but is just plug -and-chug. Using 13 RES 2:

Bi = 5.04(150+460)0.9/(4000) = 0.692
Bf = 5.04(150+460)0.85/(2500) = 1.045
Delta B = 0.354 bbl/MCF
G = [GpBf – We + BwWp]/(delta B) = ((2000000*1.0453)-1000000+(10000*1))*(1/0.354)
G = [(2 MMMCF(1.0453 bbl/MCF) – 1 MMbbl + (10 Mbbl*1 bbl/STB)]*(1/0.354 bbl/MCF = 3.1 MMMCF
2/3.1 = 64%

MBE Drive Indices: 2018 #10

Problem 10. Material balance equations are used to derive indices to compare the relative strengths of drive mechanisms. The statement regarding these indices most FALSE is: 

(A) Drive indices definitions are subjective and arbitrary. 
(B) Water drive indices must use...
(C) The Sills drive indices attempted to define the source...
(D) When comparing Pirson and Sills drive indices...

The  solution to this problem (or any indices problem you will likely see) are found on 13 RES 5 of the Guidebook, where I source both Slider and Towler. The key point? If  (A) is true, this makes (B) obviously false; how can one thing be "subjective and arbitrary" yet require something else?

Towler explains this well in SPE TS8 P48. Having read quite a few reservoir texts over the years, be warned that while SPE TS8 is not popular it is the "official" SPE reservoir reference, so it's good to know for these kinds of tricky word problems. However, now extra resources are not allowed it's important to learn the material rather than just locate it for future use.
 

MBE: 2005 #51 (similar)

An example of a saturated, below BP (gas cap) MBE problem typically requires Rp and m. Also usual is to be given a reservoir oil and gas volume to calculate m:

Rp = Gp/Np = 468 MMCF / 260 M bbl = 1.8
m = Vgas/Voil = 3,000 ac-ft / 10,000 ac-ft = 0.3

Next assume the following reservoir properties.
Bgi & Bg = 1.2 & 1.4 bbl/STB
Boi & Bo = 1.45 & 1.4 bbl/STB
Rsi & Rs = 900 & 800 scf/STB

The single page required in the Guidebook is 13 RES 3.
It's critical to list out your known values WITH PROPER UNITS.
On the exam, look at the variable list in the sidebar and convert to those units.
---------------------------------------------------

----------------------------------------------------- 
Bt = 1.4 bbl/STB + (900-800 scf/STB)0.0014 bbl/scf = 1.4 + 0.14 = 1.54 bbl/STB
Bti = 1.45 bbl/STB (note Bti = Boi)

N = 260,000 STB [1.54 bbl/STB+((1.8-0.9 MCF/STB)1.4 bbl/MCF)] = 730M bbl
              ...divided by...
1.54 bbl/STB - 1.45 bbl/STB + 0.3(1.45 bbl/STB)[(1.4 - 1.2)/1.2)] = 0.16 bbl/STB
N =  4.5 MMSTB

Consider skipping detailed problems like this. Dozens of ways to go wrong.
The difficulty? To figure this out in 30 seconds before investing too much time!

MBE: 2005 #51 (similar)

An example of a saturated, below BP (gas cap) MBE problem typically requires Rp and m.

Also usual is to be given a reservoir oil and gas volume to calculate m:

Rp = Gp/Np = 468 MMCF / 260 M bbl = 1.8
m = Vgas/Voil = 3,000 ac-ft / 10,000 ac-ft = 0.3

Next assume the following reservoir properties.
Bgi & Bg = 1.2 & 1.4 bbl/STB
Boi & Bo = 1.45 & 1.4 bbl/STB
Rsi & Rs = 900 & 800 scf/STB

The single page required in the Guidebook is 13 RES 3.
It's critical to list out your known values WITH PROPER UNITS.
On the exam, look at the variable list in the sidebar and convert to those units.
---------------------------------------------------

----------------------------------------------------- 
Bt = 1.4 bbl/STB + (900-800 scf/STB)0.0014 bbl/scf = 1.4 + 0.14 = 1.54 bbl/STB
Bti = 1.45 bbl/STB (note Bti = Boi)

N = 260,000 STB [1.54 bbl/STB+((1.8-0.9 MCF/STB)1.4 bbl/MCF)] = 730M bbl
              ...divided by...
1.54 bbl/STB - 1.45 bbl/STB + 0.3(1.45 bbl/STB)[(1.4 - 1.2)/1.2)] = 0.16 bbl/STB
N =  4.5 MMSTB

Consider skipping detailed problems like this. Dozens of ways to go wrong.
The difficulty? To figure this out in 30 seconds before investing too much time!

MBE: 2005 #45 (similar)

The Guidebook uses Slider's MBE format (13 RES 3) for speed and simplicity.

The SPE Textbook Series #8 reservoir book (Towler) uses different MBE terminology. No matter what you use personally, be familiar with Expansion (Et) and Withdrawal (Fo) terms (included in the GB on 13 RES 3 below):


These terms are often used for MBE line plots. There are lots of possible plot types.
For example, F/Et vs We/Et creates a straight line. Note Et is in both denominators; therefore Et measurement error would still produce a line.

Not so regarding We (influx) or F (withdrawal). When these are in error, the line would curve. If We is low, the line turns up; if We is high, it turns down (follow the math). It's opposite for withdrawal (low, the line turns down, high, it turns up).

Most questions, however, revolve around the reservoir (e.g. influx) since we can't measure it directly.

MBE: 2005 #44 (similar)

Know your material balance equation. In the Guidebook, it's 13 RES 3.



At pressures above the bubble point, pore volume drive is a dominate denominator term. Compressibility is important at pressures > BP, for both liquid and formation.

Water drive, however, is unaffected by the BP, and Rp (produced GOR cumulative) changes very slowly over the life of the field.

MBE: 2005 #43 (similar)

Know your material balance equation. In the Guidebook, this is 13 RES 3.

Note We is negative, so with influx N calculates to be less.
Forget to include influx? Calculated MBE N will be higher than it actually is in real life.