بررسي رفتار منابع سنگ داغ خشك (HDR) در حين تحريك هيدروليكي با استفاده از روش‌هاي عددي

Application of Numerical Modeling in Simulation of HDR Reservoir Behavior during Hydraulic Stimulation

استخراج انرژي در سنگ‌هاي داغ و خشك HDR (Hot Dry Rock)، كه اغلب در همه جا در اعماق زمين يافت ميشود، از روش‌هاي نوين استخراج انرژي زمينگرمايي است. اين منابع به صورت طبيعي با رخدادهاي لرزه‌يي يا به صورت مصنوعي با تحريك هيدروليكي ميتوانند به يك ناحيه‌ي نفوذپذير تبديل شوند. آگاهي از شكل، اندازه و نفوذپذيري ناحيه‌يي كه توسط تحريك هيدروليكي ايجاد مي‌شود، براي ارزيابي عملي‌بودن پروژه و تعيين بهترين محل چاه‌هاي تزريق و توليد اهميت زيادي دارد. در اين پژوهش سعي شده است تا با استفاده از روش عددي المان مجزا، كه در برگيرنده‌ي فرآيندهاي توامان هيدرومكانيك نيز است، پاسخ مكانيكي سنگ به تحريك هيدروليكي مورد بررسي قرار گيرد. همچنين پارامترهاي موثر براي ارزيابي عملكرد هيدروليكي منبع همچون مقاومت جريان، اتلاف سيال و تزريقپذيري منبع استخراج و تحليل مي‌شود. نتايج حاصل از تحليل‌ها مويد آن است كه با افزايش فشار، مقاومت جريان كاهش، ولي اتلاف سيال افزايش يافته است. همچنين با افزايش دبي تزريق، مقاومت جريان كاهش مييابد. اين نتايج با رفتار مشاهده‌شده در آزمايش‌هاي واقعي تحريك هيدروليكي در منابع زمين‌گرمايي تطابق خوبي دارد.

Hot Dry Rock geothermal reservoirs differ considerably from conventional geothermal reservoirs which exist only in the geologically privileged regions. In fact, HDR reservoirs are artificially created reservoirs which can be considered at any accessible depth of the Earth’s crust with a suitable volume of hot dry rock. One of the main characteristics of HDR reservoirs is enough permeable paths to allow water flow in the region between injection and production boreholes. Hydraulic stimulation is a technique employed to create these permeable paths in such regions. A pair of wells is drilled into the rock, terminating several hundred meters apart. Water is circulated down the injection well and through the HDR reservoir, which acts as a heat exchanger. The fluid then returns to the surface through the production well and, thus, transfers the heat to the surface as steam or hot water. Knowledge of the shape, size and permeability of the fractured zone created by stimulation is important for assessment of the feasibility of the project and determination of the optimum relative locations of the injection and production borehole. Improving permeability allows the system to reduce its hydraulic resistance. This operational hydraulic parameter, which is called impedance, measures the pressure amount required to force water at unit flow rate through the reservoirs. Another important parameter called the water loss rate is the difference between the rates of water injected and produced. Since stimulation experiments conducted at great depths are very expensive, numerical modeling can play a significant role. In this research, the hydraulic and mechanical response of jointed rock mass in a HDR reservoir is treated under high pressure hydraulic injections using a distinct element code. The code is capable of considering Hydro-Mechanical coupling. A parametric study on different joint specifications is carried out to illustrate how the impedance and water loss rate are varied.