1992 Volume 14 Issue 3 Pages 273-287
Field tests for the development of Hot Dry Rock have been conducted at Hijiori in Yamagata prefecture. In these injection tests, the flow rates were usually kept constant and the associated wellhead pressures were measured. In this paper, the relations between wellhead pressures and flow rates are summarized for the experiments from 1985 through 1990. Then, the bottomhole pressures were estimated by numerical simulation using the flow rates and wellhead pressures for each experiment. From the relation between bottomhole pressures and flow rates, following results were obtained.(1) Even after the hydraulic fracturing test conducted in 1986, the bottomhole presusre required to inject water at a given flow rate was decreased by repeating injection. (2) An assumption was made that there existed a fractured zone with a thickness of l4m around the bottomhole of the injection well. The permeability of the fractured zone was calculated from the relations between bottomhole pressure and flow rates. In each experiment, permeability was not constant but increasing with increasing bottomhole pressure. The permeability at a given flow rate increased very much after the experiment Exp. 8604, Exp. 8802 and Exp. 8901, in which a large amount of water compared to the former experiments was injected under high pressure. (3) A fracture with constant thickness around the wellbore was then assumed and this thickness was estimated from the relation between bottomhole pressures and flow rates. All the experiments were divided into four terms, i.e. term 1(Exp. 8501-Exp. 8603), term 2 (Exp. 8604-Exp. 8801), term 3(Exp. 8802-Exp. 8901) and term 4 (Exp. 8902 to Exp. 9004). At the beginning of each term, the experiments with large amount of water under high injection pressure were conducted. There existed linear tendencies between bottomhole differential pressure and fracture width for each term. The least square method was applied to obtain the initial fracture width. As a result, the initial fracture width was found to be increasing up to 0.15mm with time.
