報告書番号

U98033

タイトル（和文）

高温岩体貯留層評価のためのシミュレーション手法の開発（その３） -ＧＥＯＴＨ３Ｄの改良と雄勝実験場への適用-

タイトル（英文）

Development of Simulation Method for Hot Dry Rock Reservoir Estimation (Part 3) Modification of GEOTH3D and its Application to the Ogachi Test Site

概要 （図表や脚注は「報告書全文」に掲載しております）

当所で開発中の高温岩体貯留層解析コードを雄勝実験場での循環実験に適用していく過程で、実験値との相違や数値解法の収束性などに改良の余地を見出した。これを解決するために、２相流の定式化を単相流へ、また流量境界条件を圧力境界条件へ変更した。これにより、取り扱う行列方程式の収束性が向上し、さらに、雄勝実験場のような２段貯留層の場合においても、境界条件の設定が容易となった。1995年に雄勝で行われた３０日循環試験を対象として解析を行ったところ、生産井流量と注入井流量の時間的な変化は実験とほぼ一致した。さらに、雄勝での熱水回収率の向上方策案を評価したところ、500ｍ四方規模の破砕領域を伴う新生産井により全回収率が、現状の雄勝実験場条件での回収率に比べて約１．７倍に向上すること、およびダウンホールポンプによって生産点圧力を3MPaだけ低減することにより回収率を約２．２倍に向上できることを示した。

概要 （英文）

A simulation code for geothermal reservoir, GEOTH3D, has been developed in the CRIEPI for the purpose of prediction of flow rates and temperature at production well(s). Though the method was effective in simulating hydraulic behavior of primary importance in an HDR reservoir, it was found the code lacks computational robustness, i.e., numerical convergence and applicability to various operational conditions of a real HDR site. In the present study, the authors modified the GEOTH3D code to improve the computational robustness by replacing the two-phase (water-steam) flow formulation with a single-phase (water) formulation and by replacing the formulation of the flow-rate-prescribed boundary condition with that of pressure-prescribed boundary conditions. Employment of single-phase formulation allows us to have simple partial differential equations and to easily solve well-conditioned algebraic equations. The pressure-prescribed boundary condition also turned out to be superior to the flow-rate boundary condition, not only because it is practically impossible to measure flow rate distribution along whole open intervals of injection and production well bores, but also because it is theoretically impossible to know the flow rate distribution along an open interval(s) a priori(before computation) under various operational conditions. Especially the CRIEPI's Ogachi site is the case, because there are two major flow paths from the injection well to the production well, forming double major fracture zones. The modified GEOTH3D code was applied to thirty-day circulation test conducted at the Ogachi site in 1995. The computed injection flow rate and production flow rate were 487 Kg/min. and 174 Kg/min. respectively, while the counterparts of the measurement were 500 Kg/min. and 120 Kg/min. at quasi-steady state. The temporal changes of the computed flow rates were also comparable well to those of the measurement with the similar deviation. The modified GEOTH3D was also used to evaluate two design options for improvement of hot water recovery in the Ogachi site. One option is to drill a new production well and the other is to install a down-hole pump in the existing production well. The results of the former option show that the recovery could be increased by factor of 1.7 if a new production well were drilled with a hypothetical brick-like fracture zone (500m by 500m, and 800m in depth). The results of the latter option show the recovery could be doubled if pressure at the production point were reduced by 3 MPa with a hypothetical down-hole pump.

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