TO LEARN HOW TO CREATE AND MANAGE, AT INTERMEDIATE DEPTH (2000M) IN GRANITE ROCKS, A FRACTURED RESERVOIR WITH PARAMETERS SUCH THAT - IF THE RESERVOIR WERE TO BE EMPLACED AT DEPTHS WHERE THE TEMPERATURE IS 200 CELSIUS DEGREES - IT COULD PROVIDE A COMMERCIALLY VIABLE OUTPOUT. THESE PARAMETERS INCLUDE:
EFFECTIVE SURFACE AREA > 2 MILLION SQ.M;
FLOW RATE > 50 KG/S;
FLOW LOSSES < 10%.
The continuous hot dry rock (HDR) reservoir circulation programme carried out by the Camborne School of Mines in the period 1985 to 1988 at Rosemanowes Quarry in Cornwall is reported. The programme is divided into 3 stages: a gradual stepwise increase in injection flow rate to a level of 35 l/s; the use of a downhole pump to lower the pressure in the main production well to simulate subhydrostatic conditions which are expected in a 6 km deep production well; and a long period of constant injection flow rate (21.5 l/s), during which a flow path characterization experiment was carried out.
The 1987 review of the technology considered the design of a prototype commercial system, and concluded that the current Rosemanowes HDR reservoir was very much smaller than necessary for the prototype. The upper limits of controlled circulation flow rates in the current reservoir have been established, and valuable experience and data have been gained concerning the onset of microseismicity at high injection pressures and flow rates. The tendency for reservoir growth, water losses and microseismicity to increase when production flow rates exceed 0.2 of the production flow rate required of a prototype indicates that the reservoir is far too small. Thermal modelling, microseismic analysis and the preliminary results of the flow path characterization experiment all indicate the presence of a short circuit. Further work is necessary to establish the nature and loation of this short circuit, which may be a combination of a number of preferential flow paths.
The optimum impedence of the Rosemanowes reservoir is higher (0.6 MPa/kg/s) than the target for a prototype, but if the suggestion is correct, that the prototype reservoir should be created by stimulation of a number of segments in parallel, the prototype reservoir should have the low impedence required of it (0.1 MPa/kg/s). The size and number of stimulation operations necessary with be the subject of the conceptual design of the prototype in Phase 3A (1988 to 1990).
THIS IS A CONTINUATION OF WORK ON THE "HOT DRY ROCK" PROBLEM (HDR) AT ROSEMANOWES QUARRY, ON THE CARNMENELLIS GRANITE IN SW ENGLAND. THIS CONTRACT COVERED PHASE IIB OF A PROJECT ESTIMATED TO REQUIRE 10 YEARS FOR COMPLETION. (PHASE IIA - 1980-84 - WAS SUPPORTED UNDER CONTRACT EG-D2-003-UK). THE MAIN TASKS UNDER THE NEW CONTRACT HAVE BEEN:
- THE DRILLING OF A THIRD BOREHOLE TO A DEPTH OF 2800 M, DIRECTIONALLY CONTROLLED TO PASS BELOW THE FIRST TWO WELLS AND TO INTERSECT THE ZONE OF MICROSEISMIC ACTIVITY THOUGHT TO DEFINE THE RESERVOIR CREATED PREVIOUSLY;
- CONNECTION OF THE THIRD BOREHOLE TO THIS RESERVOIR BY HYDRAULIC STIMULATION, USING A VISCOUS FLUID;
- TESTING OF THE PROPERTIES OF THE RESULTING RESERVOIR DURING CONTINUOUS LONG-TERM CIRCULATION AT INCREASING FLOW RATE.
RELAATEDTASKS HAVE INCLUDED A STUDY OF THE FEASIBILITY OF DRILLING AND RESERVOIR CREATION AT DEPTHS APPROACHING 6000 M, AND DEVELOPMENT OF SUITABLE INSTRUMENTATION.