Page 83 - Fundamentals of Gas Shale Reservoirs

OIL AND CONDENSATE WITH SHALE GAS 63
Pyrolysis gas chromatography (Lis et al., 2008) and GC/MS/ nonpolar, C saturated and aromatic hydrocarbons. The polar
20‐
MS (D. Rocher, 2012, QQQ thermal maturity, unpublished materials, resins and asphaltenes, have been largely elimi­
data) have also been used for maturity assessments. nated by thermogenic cracking processes. Of course, the eco­
Advanced well logging techniques include new geochem­ nomics of oil versus gas prices has pushed the development of
ical tools such as extended hydrocarbon analysis, HAWK shale resource systems toward oil in North America.
well‐site pyrolysis, and rock‐oil fingerprinting. The In shale gas systems, GOR increases as a function of
DQ1000™ (Fluid Inclusion Technologies, Broken Arrow, thermal maturity. Oil cracking data from Claypool and
OK, USA) is used to assess product composition, water sat­ Mancini (1989) show a constant gas‐to‐oil ratio (GOR) from
uration, and barriers or seals. It measures petroleum com­ 0.86 to 1.15%R of 1000 scf/stb. However, GOR values do
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pounds up to ten carbon atoms including saturated and vary considerably from 0.60 to 1.20%R with low values
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SM
aromatic hydrocarbons. HAWK (Wildcat Technologies, early in the oil window at about 100 scf/stb and values at
Humble, TX, USA) is an automated pyrolysis instrument 1.20% in the 3500 GOR range depending on kerogen type or
designed for well‐site use to measure oil and kerogen con­ mixing of kerogen types; this assumes no alteration processes
tents and thermal maturity as well as discriminating GOC or other gas input either biogenic or thermogenic.
and NGOC components of organic carbon. Its output There is a small drop in GOR in the early–middle oil
includes mass and volume of TOC and yields in barrels of window. This is the transition phase of gas being derived
oil equivalent. It is the only pyrolysis instrument that has directly from kerogen to that being derived from secondary
multipoint calibration insuring good results over a broad products. Such an effect was noted on pyrolysis of coal with
range of pyrolysis yields and forces recalibration when and without bitumen, where the presence of bitumen resulted
standard values are not obtained. GC‐Tracer™ (Weatherford in a decrease in gas yields (Tiem et al., 2008). Ultimately,
International, Houston, TX, USA) provides continuous sur­ GOR increases exponentially above approximately 1.40%R ;
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face gas analysis during drilling using a membrane extrac­ this is the zone of C alkane cracking, gas cracking, and
20‐
tion to obtain optimum results. PEERI (Covina, CA, USA) demethylation of aromatics. This is obvious from gas
has developed a laser‐based analytical system for carbon chromatographic fingerprints of condensates, where above
isotopic measurement on methane, ethane, and propane at this maturity the ratio of compounds less than 20 carbon
the well site. Geoservices has developed the FLAIR system atoms (C ) accounts for all of the C oil content.
6+
20‐
for quantitative gas concentrations of C –C gases and Condensate production occurs in small amounts with
5
1
methane carbon isotopic measurements in the field (Niemann many of the highly mature shale gas systems. For example,
and Breviere, 2010). the Mitchell Energy T.P. Sims #2 Barnett Shale gas well has a
measured and calculated thermal maturity of about 1.60%R ,
o
but has produced about 7000 barrels of light condensate with
3.14 OIL AND CONDENSATE WITH SHALE GAS a gas‐to‐condensate ratio (GCR) of ca. 250,000:1. Similarly,
some high maturity (2.0%R ) Haynesville Shale wells yield
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The presence of black and even volatile oil tends to diminish small amounts of condensate.
gas production from shales. First, oil is certainly more viscous Prediction of GOR or GCR becomes an important
and has larger molecules some of which are polar meaning consideration when the economics of oil are more favorable
enhanced adsorption in nanodarcy pore throats. Second, if oil than natural gas. A reference table modified from Whitson
remains in the system, the maximum amount of gas has not and Brulé (2000) shows a range of GOR values with various
been generated as the predominant source of gas is not ker­ other chemical properties (Table 3.3). GOR increases expo­
ogen conversion but conversion or cracking of secondary nentially and rapid increase occurs above about 3000 scf/stb.
products, that is, bitumen and oil. Condensates do not signifi­ At very high GOR values, there are still minor amounts of
cantly alter gas production, but such liquids are generally pro­ wet gases and condensable hydrocarbons present. The yield
duced in very low amounts. Condensates are dominantly is the amount of liquids available given the GOR value.

TABLE 3.3 Approximate physicochemical properties of various petroleum products with projected GOR and yields
Product C (%) C + (%) °API GOR (scf/stb) Yield (bbls/mmcf)
1 7
Black oil <60 >35 <39 <1,500
Volatile oil 61–80 15–34 40–49 1,500–3,499 667–285
Gas condensate 80–84 19–10 50–54 3,500–4,999 286–200
Wet gas 85–90 10–0.10 55–66 5,000–99,999 199–10
Dry gas >90 <0.10 >60 >100,000 <10
These properties require calibration to specific source rocks or by organofacies within a given source rock.