報告書番号

U90005

タイトル（和文）

高温岩体発電のための岩盤破砕評価技術の開発－肘折地点でのＡＥ観測による人工貯留層評価－

タイトル（英文）

STUDY ON MICROEARTHQUAKES ASSOCIATED WITH HYDRAULIC-FRACTURING EXPERIMENT AT HIJIORI

概要 （図表や脚注は「報告書全文」に掲載しております）

高温岩体発電システムを実用化するためには，水圧破砕で構成された人工貯留層の規模・形状を正確に評価する技術の確立が不可欠である。この方法としては現在のところＡＥ法が最も有効と考えられている・ＡＥ法の有効性を確認するため，ＮＥＤＯが山形県肘折地点で深さ１８００ｍの坑井を用いて行った水圧破砕実験を利用し，地表に８観測点を設置してＡＥ観測を実施し，水圧破砕に伴って発生したＡＥ（３３個）の震源を決定した。その結果ＡＥは注入坑井のやや南側を中心に東西方向に分布し，また，その深さは１５００～１７００ｍ程度である事を確認した。従来ＡＥ法による貯留層の形状評価は深井戸中に設置した３軸ジオフォンを用いて行ってきた。しかし，今回の結果から大量を水を大きな流量で注入すれば，地表ＡＥ観測網でも形状を評価できる見通しを得た。

概要 （英文）

IN OCTOBER 1986,THE HYDRAULIC-FRACTURING EXPERIMENT AT THE HIJIORI HOT DRY ROCK GEOTHERMAL SITE WAS CONDUCTED BY THE NEW ENERGY DEVELOPMENT ORGANIZATION. MICROEARTHQUAKES ASSOCIATED WITHTHE FAILURE TO DEFINE THE NATURE AND EXTENT OF THE ARTIFICIAL HEAT RESERVOIR WERE OBSEWED. EIGHT SURFACE SEISMIC MONITORING STATIONS WERE ARRANGED AROUND THE HYDRAULIC FRACTURE ZONE TO DETERMINE THE MICROEARTHQUAKE HYPOCENTERS PRECISELY. THE MAIN CONCLUSIONS ARE AS FOLLOWS: 1.A TOTAL OF 1080 QUBE M OF WATER WAS INJECTED INTO THE INJECTION WELL. WHEN THE FLOW RATE WAS KEPT AT 2 QUBE M/MIN FOR 2 HOURS AND 4QUBE M/ MIN FOR 2HOURS,NO MICROEARTHQUAKE WAS DETECTED. ON THE OTHER HAND,SEISMIC ACTIVITY OCCURED WHEN THE FLOW RATE WAS INCREASED TO 6 M3/MIN. THIS INDICATES THAT SEISMIC ACTIVITY DEPENDS ON FLOW RATE. 2.FOCAL MECHANISM SOLUTIONS DERIVED FROM THE RADIATION OF THE FIRST P-WAVE SHOWED THAT ALL THE MICROEARTHQUAKES WERE CAUSED BY SHEAR FAILEURE. THERE WAS NO EVIDENCE OFTENSILURE FAILURE. 3.THIRTY-THREE HYPOCENTERS WERE LOCATED USING THE SURFACE NETWORK. THE HYPOCENTER DISTRIBUTION SUGGESTS THAT THE FRACTURE EXTENDED MAINLY UPWARD FROM THE INJECTION POINT IN THE OPEN-HOLE SECTION AND THE DIRECTION WAS NEARLY EAST-WEAT. 4.THIS RESULT DEMONSTRATES THAT A SURFACE-NETWORK MONITORING SYSTEM CAN BE USED TO DEFINE THE GEOMETRY OF HYDROFRACTURES AND CAN PROVIDE USEFUL INFORMATION ON THE MECHANISM OF HYDRAULIC FRACTURING.

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