Origins of Gas Shows

Origins of Gas Shows
 

A gas show can be defined as a significant occurrence of hydrocarbons gases
detected from the mud stream and identifiable as being the result of the drilling of
specific interval of formation. This definition is apparently very simple and
readily understood. It is in fact ambiguous and if taken literally, can lead to
confusion.
The magnitude of a gas reading seen at the surface is not a true mark of its
significance. Nor is the fact that a gas reading can be identified as coming from a
volume of formation quantitative evidence of the volume of gas in place in that
formation or even liberated from it while drilling.
The object of good mudlogging is to plot those gas readings produced by gases
liberated from drilled formation in conjunction with the data relevant to their
interpretation. The object of mud log analysis is to reconstruct from these the
composition and mobility of reservoir hydrocarbons. In performing these tasks it
is necessary to appreciate the physicochemical processes active in the formation,
the mud circulation system, the gas sample extraction and analysis equipment.
The amount of gas, i.e., the number of “gas units” or “percentage of gas in air,”
detected by the gas detection equipment from a given formation is the result of a
complex of interacting variables. These factors begin to act before the formation
has been drilled and continue their effect until the gas sample enters the gas
detectors.
In order to reconstruct a picture of the fluids in place in a formation and the type
of fluid the formation may produce, it is necessary to study gas magnitude and
composition in the mud stream and cuttings, oil and water themselves, and
changes in the drilling process and circulation system which may affect or be 

affected by formation fluid behavior

Definitions
 

Prior to considering these complex factors, it is necessary to define the usage of
the term “gas show” as it is to be used in this manual. To do this it will be
necessary to define also the terms “background gas” and “circulation gas.”
Although apparently self-explanatory, these terms are commonly given multiple
uses in the industry.
 

Circulation Gas
 

This is the value of gas seen by a gas detector when circulating under normal
conditions, meaning a clean, balanced borehole with drill pipe in the hole and
rotating but with the bit off bottom and with no vertical movement. Under such
conditions some gas will be present in the sample drawn from the mud stream, but
it will represent only contamination or recycled hydrocarbons in the mud
 

Background Gas
 

When drilling through a consistent lithology, it is common for a consistent gas
value to be recorded. Certain lithologies (for example, overpressured shales) may
show considerable variation in the background gas. Background gas may be
observed to vary with drilling, mud or surface conditions without any change in
lithology or formation hydrocarbons. This should always be given consideration
in both formation and hydrocarbon evaluation
 

Gas Show
 

This any deviation in gas, amount or composition, from the established
background. This may or may not accompany a change in lithology, may or may
not indicate a significant or economic hydrocarbon accumulation. In other words,
“Gas Show” is a term describing an observed response on the gas detector, having
no causal or interpretive significance.
It is common for the surface data logger to be asked, “What is a good gas show?”
The answer to this is complex and relates to many factors beyond the simple
number of gas units seen. To decide whether a gas show is “good” or “poor”,
i.e., whether or not a significant hydrocarbon accumulation is indicated, requires a
total evaluation of all mudlogging parameters plus consideration of the many
variable system conditions.
The manner and extent to which shows will manifest themselves varies so greatly
among the many different regions that it is impossible to set even a general set of
requirements which must be met to qualify a show for additional evaluation or to

determine to what that evaluation should be. To a large extent, evaluation of
interesting zones becomes a matter of comparison of likes and differences.
Parameters of the log through the sections in question are compared with the same
parameters in adjacent known barren zones for perception of the degree to which
they changed or did not change. Generally speaking, the key to interpretation lies
not in the magnitude of the reading reached, but in the extent to which it did
change

  

Sources of Gas in Mud
 

Gas detected in the mud stream may originate from the formation via a number of
mechanisms. It is necessary for the surface data logger to isolate and attribute
these causes in order to draw the appropriate conclusions. Gas originating from
other sources or only indirectly from the formation will also be seen in the mud

 stream. This must, if possible, be recognized and removed from consideration

Gas from Drilling
 

This is often referred to as “liberated” gas since it is liberated from the crushed
formation produced by the drilling process. This should not be taken to imply that
the total volume of gas in place in the formation is liberated to the mud stream and
detected at the flowline. As discussed below, not all of the gas seen at the surface
will be present in the formation as free gas. Conversely, not all of the free gas in
place in the formation will be liberated at the surface. Some free gas will remain
trapped in the drill cuttings due to lack of permeability. Some gas, especially the
lighter hydrocarbons, will remain dissolved in the drilling fluid and not be
released at the surface. Thus, though it can be said that the gas liberated by drilling
reflects the composition and saturation of gas in place, no direct relationship can
be drawn
 

Free Gas and Liquefied Gas
 

Methane and ethane, the two lightest paraffins, have critical temperatures of
-82.5oC and 32.3oC, respectively. Since most petroleum reservoirs will have
temperatures in excess of this, the two will always exist in the formation only as
gases. All other hydrocarbons and water may exist in both liquid and gaseous
forms in equilibrium, dependent upon the temperature and pressure of the
formation.
Since the vapor pressures and critical temperatures of all hydrocarbons are
different, the composition of the free gas in the reservoir will differ from that of
the oil with which it is associated. Similarly, the composition and volume of the
gas detected at surface temperature and pressure will be different to that in place.

  

Dissolved Gas
 

Much of the gas detected at the surface is present in place in solution in oil and
water. This solubility depends upon the temperature and pressure of the reservoir.
Only when the oil and water are fully saturated with dissolved gas will a free gas
phase be present in the reservoir. Some gas will be liberated and detected from the
solution when carried to the surface. Other gases will remain in solution in oil and
water and be retained in the mud system.
The presence of gas as a liquefied, dissolved or free gas phase in the reservoir has
major influence on the relative permeability of the reservoir and its material
balance, i.e., the type of production and productivity. Similarly, due to the
differing solubilities of various hydrocarbons, the solution of gas in recovered oil
and water and mud filtrate will have an effect in reducing the amount of the gas
detected at the surface and in changing its composition