| Abstract |
The Saint-Petersburg geothermal project is developing for heat-supply system of the big greenhouse Complex A/S ìLetoî near of international airport ìPulkovoî. Geothermal gradient 0.03 ìC/m was confmed by exploration well, drilled trough 300-m sedimentary layers in granite-gneiss basement. The first stage of the project creation includes five hydrofracturing zones (equiv. radius of 675 m between inclined (45î) intervals of injection and production wells on depth 2.2 km with rock temperature 75 ìC, as upper reservoir of the cascade Geothermal Circulation System -GCS).. Total heat- exchange surface of 14 km2 and water flow 0.2 m3/sec guarantee 37.3 MW thermal power of hot-water supply system during 20 Years. Investments by our economic- mathematical model, with risk-factor 1.3, will be 5.6 M US$. In two Years of circulation with yearly production 1.072 M GJ the net profit is well to create, without bank credits, the second stage of cascade deeper reservoir - 3 zones of hydrofractures (R2= 790 m) on depth 3 km with temperature 95?C (2.5 M US$). Special wells construction allows to pass some part of flow rate with 70?C (after upper fractured zone) for overheating up to 90?C in deeper reservoir. Seasonal changing the overheating part of flow rate allows to rise the maximum thermal power of Cascade GCS from 37.3 up to 53.7 MW. Yearly production will be increased to 1.41 M GJ. After 3 Years (pay back time) clear cost of geothermal energy will be less then 1 US $ / GJ and the net production value- NPV, discounted for 20 Year, estimated about 28 M US$. High level of economic efficiency connected with unusual high level of flow rate and thermal production. It became possible, thanking our new methods of fractures treatment (inside pressure 70 up to 360 MPa) for theirs aperture expansion to 3 - 5 mm. Pressure drops in fractures are reduced to 0.026 MPa and hydraulic impedance - to 0.26 MPa.s/l. Same flow rate and pressure drops possible to reach in case of natural reservoir with seam thickness 1.5 km and permeability not less, than 2 Darcy (unique case !). The Saint Petersburg Project shows, that improving the hydrofracture treatment methods before circulation can guarantee effective heat mining from GCS with power of 50 - 60 MW per two wells even for case of moderate geothermal gradients. |