gas ratios

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Gas Ratios – Short Overview
A Surface Measurement While Drilling™ Service

2. • Historically gas ratio analyses have provided first look
evaluation of the formation and its fluids. Much of the value
was overlooked as downhole logging tools became prevalent.
• With improved computing power from specialized applications
to integrated A.I. solutions and the need for cost efficient
formation evaluation, gas analysis is proven to be valuable once
more.
• Of the ratios in use across the industry, Diversified routinely
starts its analytic process by calculating:
• Haworth ratios
• Pixler ratios
• Oil Indicator ratios
• Methane % ratios
• All gas ratio calculations have their limitations, but as indicators
of fluid or formation change, they are of great use.
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The Use of Gas Ratios

3. Ch = Hydrocarbon character (Ch) in the original paper.
GWR = Hydrocarbon wetness (Wh) in the original paper.
LHR = Hydrocarbon balance (Bh) in the original paper.
Haworth Gas Ratios
GWR = ((C2+C3+C4+C5) / (C1+C2+C3+C4+C5)) * 100
LHR = (C1 + C2) / (C3 + C4 + C5)
Ch = (C4 + C5) / C3
Still one of the most popular (or well known) methods was designed originally to indicate
the reservoir potential of conventional reservoirs. When used in unconventional
formations the limitations of the method must be appreciated.
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4. Reading the Numbers
LHR Relation GWR Theoretical Hydrocarbon Type
LHR>100 - Dry, light gas. Non-productive like geopressured methane
LHR<100 <0.5 Possible production of light dry gas
GRW<LHR<100 0.5 - 17.5 Productive gas. Better quality as curves come together
LHR>>GWR 0.5 - 17.5 Dry gas. Coal related
LHR>GWR 0.5 - 17.5 Productive wet gas, condensate gas, or light oil with high gravity – high GOR
GWR>LHR 17.5 – 40 Productive oil. Gravity decreases as curves come together
GWR>>LHR 17.5 – 40 Low potential of low gravity oil and low gas saturation
GWR>>>LHR >40 Residual oil. Low gravity, lots of water, unproductive
GWR and LHR should be plotted in the same track on a logarithmic scale.
In a homogenous formation, both curves should be parallel – a good geosteering indicator.
Changes in curve appearance will indicate changes in formation or formation fluid type.
The default set-points shown below will rarely be applicable to every well and need calibration.
Every well is different, every reservoir is different – take care!
The Hydrocarbon Character may be used to further indicate the type of fluid (see flowchart).
This ratio should also be used with care and often does not add to the overall interpretation.
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5. Pixler Gas Ratios
The other well-known gas ratio method is the Pixler method from 1969. Gas values were picked depth by
depth and point by point and ratios (C1/C2, C1/C3, C1/C4, and C1/C5) plotted on a semi-log grid. The grid
was marked to indicate unproductive zones through oil and gas and back to unproductive.
Some rules of thumb include:
• Productive dry gas zones will yield mainly methane.
• Very high ratios may indicate gas in solution in a water zone.
• If C1/C2 indicates oil, but C1/C4 is high – above 45 – the zone may be non-productive.
• If any ratio is lower than the preceding ratio, the zone is probably non-productive.
• If C1/C4 is lower than C1/C3, the zone is probably water wet.
As a quick-look indicator we use:
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C1/C2 Theoretical Fluid Type
< 2 Very low gravity, high density & viscosity, non-productive, residual oil
2 - 4 Low gravity oil – 10-15 API
4 – 8 Medium gravity oil – 15-35 API
8 – 15 High gravity oil – API > 35
10 – 20 Gas condensate
15 - 65 Gas
> 65 Light gas, principally methane, non-productive

6. Oil Indicator & Methane % Ratios
Two other ratios we use are the Oil Indicator (along with the Inverse Oil Indicator) and the simple % Methane ratio.
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OI = (C3 + C4 + C5) / C1 IO = C1 / (C3 + C4 + C5)
OI Hydrocarbon Type IO
0.01 – 0.07 Dry gas. Gas charged water. 100 – 14.3
0.07 – 0.10 Condensate, light oil, high GOR 14.3 - 10
0.10 – 0.40 Oil (undersaturated) 10 – 2.5
0.40 – 1.00 Residual hydrocarbons 2.5 – 1.00
Heavy Oil Medium Oil Light Oil Wet Gas Dry Gas
60 67.5 75 82.5 99.5 100
Note again that all empirical ratios will
give a range of responses that may not
correspond with the actual hydrocarbons
in a well. They do show changes from
lighter to heavier fluids which can suggest
GOR trends, but without local ‘calibration’
of the set-points, nothing is certain.
Methane % Ratios
Oil Indicator Ratio Inverse Oil Indicator Ratio

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Final Words
on Gas Ratios
• Every gas ratio method has default cut-offs which should be used with care.
• Gas ratios are best suited in evaluating contacts, evolutions and trends.
• DWL does not simply state that a formation has gas or oil, but that there are lighter or heavier HC’s that can be seen to change.
• For the gas ratios to have definitive answers in a field, production fluids need to be known. (Gas ratio ‘scales’ need known end-points.)
• The different ratio methods can and will give different results. DWL compares four different ratios and uses crossplots to better evaluate the results.
• Gas ratios work best with at least C1 to C5 being measured and recorded.
• Total gas measurement does not play a part in fluid analysis with gas ratios.
• Good gas extraction at the ditch is a necessity for any gas/fluid evaluation. Insufficient flow, bad ditch design, etc., will render any analysis ‘suspect’.
• Consistency and quality control is everything.