Fracture extension created by fluid injection in a hot dry rock reservoir at the depth of 4 km and the temperature of 300 °C is studied using a finite-element heat and mass transfer code. In this paper, the attention is focused on the relative extension of two competing fractures with different closure characteristics.
On etudie prolongement d'une fissure produit dans un reservoir du roche chaud et sec per injection liquide à un fond de 4km et une temperature de 300 °C avec une code de transfer d'element fini de chaleur et de masse. On met au point la prolongement relative de deux fissures concourant avec divers caractères de clôture.
Spaltverlangerung ersghafft mit flussigen Einspritzen in ein heisees, trockenes gesteinreservoir bei 4km tiefe und 300 °C ist untersucht mit einem Warme- und massen transport koden von begrenzten elementen. Hier ist die emphase auf die relative verlangerung von zwai konkurrierten bruchen mit verschiedenen verschlusseigenshaften.
At Los Alamos National Laboratory, more than ten hydraulic fracturing experiments have been conducted to stimulate a hot dry rock reservoir. Two hydraulic fracturing attempts, Exp.2059 (Kelkar 1985) and exp.2062(Robinson 1985), successfully established a large fracture system connecting an Injection well, EE-3A, and a production well, EE-2. The initial closed loop test was carried out from May through June,1986 to get information on volume, impedance and temperature of the reservoir. The fracture systems created by the two experiments show the characteristics that are different from each other. For example, higher pumping pressure was required to extend fractures in Exp. 2062 than in Exp. 2059. In order to understand the combined behavior of two fracture, the computer simulation techniques were used. The parameters for the simulation are chosen to fit, the experimental data. Two models are introduced to obtain a good match between the simulation and the experiment. The extension of fractures is discussed. The fracture radius is predicted when water is pumped into the Combined two-layer fracture Systems.
Experiment 2059 was conducted in May. 1985. A packer was run to 3.519m and the bottom hole was sanded back to a depth of 3.722m. In Exp.2062, which was performed in July, 1985, a packer was set at 3,653m and the total depth was 3.837m. The test intervals are illustrated in Fig.1. The pressure histories and flow rate in these tests are shown in Fig.2(a) and (b) Total injected water volume was about 1.600m3 in Exp.2059 and about 5.722m3 in Exp.2062. From these figures the relation of flow rate to fracture extension pressure is obtained and the results are plotted in Fig.3. The fracture extension pressure in Exp.2059 was lower than that in Exp.2062 at the same flow rate. Temperature surveys were run in the open hole section of EE-3A. Fig.4 shows post-experiment logs of these experiments. Major depressions exist at 3.570m and 3.660m and small anomalies appear at 3.700m in Exp.2059. Fractures at 3.660a and 3.690m took the bulk of fluid during EXP.2062 and in addition a new fracture was stimulated at 3.750m. The fracture system created by each experiment consists of several fractures. But in the simulation model, each fracture system is represented by one major fracture, because it is difficult to characterize each fracture independently for modeling.
A model of the fracture system showing the two layer fractures is given in Fig.5. A packer is placed at 3.500m. The upper fracture represents those fractures stimulated in Exp. 2059, while the lower fracture represents those fractures stimulated in Exp. 2062. A finite element grid in a radial geometry is shown In Fig. 6. It is very important to characterize these two fracture systems for the computer simulation. In this paper, the permeability was chosen as the main parameter to characterize the fracture system. In the first model the permeability is assumed to be constant and independent of the local pressure (linear permeability model). In the other model, the aperture is assumed to be related to the local pressure and the permeability is expressed as a function of the aperture (Kelkar 1986) (non-linear permeability model). The Finite Element Heat and Mass Transfer Code (FEHM) (Zyvoloski 1983) is used for the simulation. In this computer code, heat and mass transfer is allowed among the fracture, the wellbore and the strata. All boundaries are fixed as "no flow". The parameters used in this simulation are listed in Table 1. Flow rate is kept at 0.8m3/min for the simulation.
