Page 145 - Fundamentals of Gas Shale Reservoirs

WELL LOG ANALYSIS OF GAS SHALE RESERVOIRS 125
limited data points at low maturity conditions, and more data The effect of CEC on (the) shale conductivity depends on the
are required to find a general relationship for different types salinity of the formation water. If the formation water salinity
of kerogen. is greater than sea water salinity, the effect of excess conduc­
tivity due to clay minerals is small (Passey et al., 2010).
Anisotropy of the gas shale is an effective parameter in
6.4 WELL LOG ANALYSIS OF GAS SHALE the interpretation of resistivity log and water saturation
RESERVOIRS estimation in shale layers. Chemali et al. (1987) reported a
disparity between laterolog and induction resistivity
Well log data are valuable sources of information for reservoir measurements in shales. Induction devices are sensitive
characterization. Like the other parts of gas shale evaluation; only to the horizontal resistivity (R ) of the formation, while
h
well log analysis of these reservoirs is complex, and it needs laterolog measures a combination of both horizontal and
unconventional as well as the routine conventional well logs. In vertical resistivity (R ). Due to the vertical transverse
v
this section, there is a brief explanation of the well log signatures isotropy (VTI), R is expected to be higher than R . Now the
h
v
and well log interpretation in the gas shale reservoirs. challenge is to find whether the true resistivity of gas shale is
closer to R or R (Miller, 2010).
h
v
Taking these limitations into account, using resistivity log
6.4.1 Well Log Signatures of Gas Shale Formations in gas shale layers needs closer attention, and it is not
6.4.1.1 Resistivity Log The measurement of formation possible to predict a universal response for the resistivity log
resistivity is of primary importance in well logging since it is in the gas shale compared to conventional reservoirs.
a definitive method for identifying hydrocarbons and quanti­ 6.4.1.2 Gamma Ray Log The gamma ray (GR) log is a
fying the water saturation. The resistivity of the rocks type of tool that is used to measure the formation of natural
depends on the following:
GR radioactivity. There are basically two main types of GR
tools:
• Fluid resistivity in the pore spaces • Natural GR tool (NGT): It measures the general GR
• Fluid saturation emissions of all the radioactive elements (potassium,
• Rock lithology and the percentage of conductive min­ uranium, and thorium) together.
erals and the rock anisotropy • Spectral GR tool (SGR): It differentiates GR emissions
• Overburden pressure and pore pressure from (the) three main individual radioactive elements.
• Temperature
Amongst the sediments, shales have by far the strongest GR
Usually in the gas shale reservoirs, it is expected that the radiation. Due to this fact, the GR log is principally used to
resistivity of the rock will increase due to the presence of derive shale volume quantitatively.
hydrocarbon and organic matter. This assumption is correct The potassium content of the clay mineral species varies
only if the thermal maturation of the formation is high considerably. Illite contains the greatest amount of potassium,
enough to result in hydrocarbon generation. Conversely, while kaolinite has very little or none (Dresser Atlas, 1983).
Anderson et al. (2008) showed that some gas producing The consequence of this is that clay mixtures with a high kao­
shale layers can have high electric permittivities or lower linite or high smectite content will have lower potassium
resistivity. The cause of the high permittivity has been attrib­ radioactivity than clays made up essentially of illite. However,
uted to the presence of conductive minerals, such as pyrite or since most clays are mixtures of several clay minerals, the
graphite, that build up as a result of kerogen transformation differences discussed earlier are muted. The average shale
and exposure to elevated temperature and pressure. has a potassium content of about 2–3.5% (Rider, 1991).
Interestingly, so far this observation has yielded mixed Uranium forms unstable soluble salts that are present in
results: some gas shale samples have not shown such high sea water and rivers. Uranium content has a positive rela­
permittivities while others have. This might be due to the tionship with the TOC deposited under marine conditions
different depositional environment of the gas shale layers or (Fertl and Reike, 1980). In lacustrine settings, due to the
different thermal history of the formation. paucity of uranium there is not any relationship between
Cation‐exchange capacity (CEC) of the clay minerals is uranium and TOC (Bohacs and Miskell‐Gerhardt, 1998);
another property that has an effect on the resistivity of the therefore, in these cases, GR could be used as a clay volume
shale layers. CEC value varies with the surface area of the indicator but not TOC content. It should be noted that the use
clays. This means that the difference between the conductivity of uranium is suitable for gas shale reservoirs that do not
of clay species should be related to surface area (Rider, 1991). have uranium‐enriched minerals like apatite (Kochenov and
Smectite has a far greater specific surface area than the other Baturin, 2002). In these reservoirs, elevated uranium could
clays and is therefore more conductive (Passey et al., 2010). not be used to predict TOC.