Geothermal Model of the Katayama Field, Onikobe Area

Abstract

1) Initial Geothermal Model
Geothermal Survey shows that Onikobe Katayama Geothermal Area is situated just above Anticline Axis; NW SE. Besides, during deposition of Omkobe Group, uplift along the Axis continued till deposition of Iwanazawa Formation in Pleistocene.
Electric Resistivity Survey shows that the Geothermal Area is situated above the top of the Horst structure of Electric Basement.
Depth of the Horst top is about 200-250m. Resistivity of the Electrical Basement is above 100fm; Whereas the Top part shows 3-4Ωm.
Fault structure is presumed to be on both sides of the Horst, named F3, F4 fault. Low resistivity Area developed along the Faults and the top of down step faults.
Broadily speaking, F3 and F4 Fault line correspond to Okunoin geothermal alteration zone and Katayama geothermal alteration zone.
Based on these datum, at the first stage of exploration, Initial Geothermal Model was drawn up. It showsthat geothermal fluid rise up through the faults of both sides of the Horst is reserved in the top part of the Horst horizontally. Actually, several exploratory well got steam at 150-300m depth.
With the progress of the exploration, it was recognized that geothermal reservoir did not always existhorizontally, but linearly along a fault.
2) Fracture Type Geothermal Model
In the case of Onikobe Katayama Geothermal Area, low electrical resistivity part corresponds to highlyaltered zone, characterized by clay.
Temperature Gradient is high in this part, but steam production is not got always. If it get steam, theproduction is intermittently.
Production part is deeper than low electric resistivity part. It is situated in the electric basement. It'shigh resistivity is above>100Ωm
Lost circulation points in the drilling are important indicator that show production zone directly.
Figures 11, 12, 13 show lost circulation point and temperature in drilling of the pilot borings and production wells.
Horizontal distribution of production zone is shown along K, Z, Y, R, and another fault. Geothermal fluid rise up from deeper part over 1000m depth, and reserve the tension faults and fractures caused by uplift movement along NW-SE anticline axis, and the faults intersect with them. It is supposed that another end of the faults connects with recharge area.
Geothermal fluid rises through fault system is reserved at 1200m depth (L3 formation; from reflection method), 800m depth (L2 formation), 250-300m depth (L1 formation) on the way.
The past experience shows that the most excellent production zones are close to the intersect points of the fault and L1, L2, L3 formation.
Main production zone of Onikobe Geothermal Power Plant was L1 formation. According to the deeper part, GO-10 (1, 350m depth) and GO-11 (1, 300m depth); which diameters are as same as production well were bored. Both exploratory well were measured high temperature and got steam about 30-40 ton/hr. Actually the temperature of 288°C was measured at 1, 290m depth in GO-11.
But the hot water with steam had very high HC1 acidity, pH 3.3 and 2.6. After 4 months from the first production, pieces of casing material rushed out from GO-11, and the steam product volume decreased to 30% of the former. It was presumed that the bottom part of the borehole collapsed. From these information, it was judged that deep well in Katayama geothermal area did not suit for generation.
Therefore main production zone was decided to L1 formation L2 formation has been used to reinjection zone.
After that, carried various examination, we obtained the conclusion that acid problem can avoid with separated drilling from the Horst.
We drilled several directional wells which length are 1, 150-1, 500m and displacement are 500-700m. It was succeeded to produce steam. The volume of steam from these directional wells amount to 70% of total nowadays. It's pH value is 8.0-8.2.