Discussion of early isotopic analyses conducted on the fluids from the HGP-A well

Kroopnick, Peter M.

Thomas, Donald M.

eVols at University of Hawaii at Manoa

Isotopes and gases in a Hawaiian geothermal system : HGP-A

ScholarSpace at University of Hawai'i at Manoa

Geothermal, ResoUY'aes Counail" TRANSACTIONS, Vol,. 2 Jul,y 1978 ISOTOPES AND GASES IN A HAWAIIAN GEOTHERMAL SYSTEM: HGP-A Donald Thomas ------Hawaii Institute of Geophysics, Univ. of Hawaii Gases from the Hawaii Geothermal Project well-A (HGP-A) were sampled over the course of twelve months to determine their isotopic and chemical composition. Samples of steam and gas were col-lected and condensed at the well head through a twin cyclone steam/water separator during steam discharge of the well. Gas separations were carried out on standard gas chromatographic columns with thermistor and mass spectrometric detection of the chromatogram peaks. Isotopic analyses were carried out on several individual components of the gas phase: C02 (013C), N2, He, H20 (0 180). Results. The results of the chemical analyses made for the geothermal gases sampled are summa-rized in Table l~ The most important aspect of the data presented here is that there appears to be a significant increase in the concentration of geothermal gases in the well output during each successive discharge of the well. Hydrogen and hydrogen sulfide appear to be increasing in concen-tration relative to the nitrogen and carbon dioxide fraction of the well gases. It is felt that these changes reflect the progressive maturation of the well toward a steady state composition of the geo-thermal fluids. Helium analyses were also made on the well gases. It was found that the helium:nitrogen ratio in the well was at least six times the atmospheric concen-tration although analytical difficulties prevented an exact determination of the helium concentra-tions. A summary of the isotopic analyses made on,HGP-A gases is presented in Table 2. The carbon, nitro-gen, and helium analyses were made on gases ob-tained during steam discharge while several oxygen analyses were made on downhole water samples. The interpretation of the carbon isotope data is considerably complicated by the presence of 14C at a concentration of 20% of modern carbon in the well gases, thus indicating that no less than 20% of the carbon dioxide in HGP-A is from atmospheric or biogenic sources. The origin, true age, and iso-tope composition of the biogenic fraction of the carbon in the well is presently unknown. Further testing of the well gases is presently planned in order to determine the relative contributions of the several possible sources of carbon in HGP-A. H. I. G. Contributor No. 886 Peter M. Kroopnick Hawaii Institute of Geophysics, Univ. of Hawaii The oxygen isotope data presented in Table 2 indi-cate only slight variations with depth in the well. This can be interpreted in one of two ways: either the water in the thermal reservoir (and the well bore) is very well mixed with depth, or one of the several possible fluid sources for HGP-A is provi-ding nearly all the water in the well. It is pre-sently felt that the latter possibility is the more likely. The olsN values observed for HGP-A are richer in lSN than any nitrogen ratios reported in the liter-ature to date. Whether these values reflect the true isotopic composition of HGP-A nitrogen or whe-ther they are an artifact of contamination is un-certain at present. Analyses by gas chromatogra-phy-mass spectrometry are presently underway to resolve this question. The helium isotope ratios presented in Table 2 are seen to be quite high and are similar to values observed for helium in fumarole gases taken from the Kilauea summit. This is a very strong indica-tion that there is a large component of magmatic gas in th~ geothermal well fluids. One can con-clude from this that the heat source from which HGP-A taps its fluids is in close communication with the Kilauea volcanic system. It is believed that the inter-connection is laterally through the east rift zone rather than directly to the deep magma reservoir beneath Kilauea (R. Koyanagi, per-sonal communication). The conclusions which can be drawn from the above reported results are as follows: (1) There are at least 3, and possibly more, sources of geothermal fluids in HGP-A. The relative contributions of the fluids has been tentatively set at: ocean water, 10%; meteor'ic water, 65%; hydrothermal fluids, 25%. (2) The rift zone in which HGP-A is situated is in close contact with the Kilauea volcanic system. Thus it is possible that the exploitable lifetime of this heat source will be much longer than would be expected for an isolated geothermal system. 653 Thomas and Kroopnick TABLE 1 SUMMARY OF GAS ANALYSES FOR HGP-A Total Gas Content Composition Sampling Temp. (mg/kg of d1s- (wt %) Mole Ratio Oa te/Tne of Sam21e Pressure (2s1) (·e) charge) .'eQ2 !l.2~ ~2 !l.2 .'eQ21.!!2~ ~21.!!2 29 Jan. 1977; Total 50 30 991* 72 10 18 0.1 5.5 19.5 Condensate from Cooled Cyclone Separator April 1977; Steam 165 185 1604 59 20 21 0.5 2.3 3.0 from Cyclone Separator May 1977; Steam 167 188 1912 57 21 21 2.1 2.4 from Cyclone Separator 40 188 2697 65 24 10 .4 2.1 1.8 19 July 1977; 25 180 1337 71 18 11 .5 3.0 1.6 Steam from Cyclone Separator Best estimate for 180 2100 60.3 21.7 17.3 .6 2.1 2.1 total gas content of HGP-A *The 29 Jan. 1977 values most closely reflect the composition of the HGP-A stack emissions. Da te Sampled 7-22-76 11-8-76 11-16-76 1-30-77 2 - 77 Date Sampled 8-19-76 8-19-76 8-19-76 8-19-76 10-30-76 10-30-76 2-9-77 2-9-77 Date Sampled 5-8-77 5-8-77 7-19-77 7-19-77 Date Sampled 1-25-77 1-25-77 2-9-77 TABLE 2 Isotope Analysis Results 513C %. vs POB (C02) - 3.62 - 2.35 - 3.28 - 1.97 - 2.29 Comments Carbon dioxide gas samples taken during steam discharge. Mean - 2.7 standard deviation 0.87 Depth Sampled 1000 ft 4300 ft 5800 ft 6300 ft 2270 ft 5800 ft top I steam top / steam 015N (N2) + 55.7 + 56.7 + 55.9 + 57.6 Dep th Sampled 3500 ft 5800 ft top / steam Kilauea fumarole gas Atmospheric Air 654 5180 (H20 ) - 3.79 - 4.5 - 4.2 - 4.56 - 4.14 - 3.83 - 5.58 - 3.60 Nitrogen gas sampled during steam discharge--all samples treated with CuD to remove CO and -CH20. 3He/4He » 8.25 x 10-6 8.25 x 10-6 1. 7 x 10-5 2.0 x 10-~ 1.4 x 10-

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Isotopes and gases in a Hawaiian geothermal system : HGP-A

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