Abstract
To study the mechanism of hydraulic fracturing with pulse injection theoretically, in this paper, the transient flow model of fracturing fluid in the pipe string was established, and it was solved by method of characteristics and finite difference method, respectively. Furthermore, the elastodynamic model of reservoir was also established. Based on the finite element method, the dynamic stress distribution in the reservoir was simulated and calculated. In addition, the influence of parameters in the pulse injection scheme on dynamic stress was analyzed. The results indicate that the unsteady injection produces a pulse pressure wave at the wellhead. The pressure wave propagates along the pipe string to the bottom of the well, and its amplitude attenuates due to the resistance loss. When the pressure wave propagates to the bottom of the well, it will be reflected and there is a superposition area of the downward pressure wave and upward reflection wave near the bottom hole. The bottom hole pressure of pulse injection is the sum of stable injection pressure and the above pressure wave. Simultaneously, this fluid pressure with pulse variation will stimulate reservoir to produce dynamic stress in its interior. The pulse adjustment time and adjustment amplitude in the injection scheme have a significant impact on the dynamic stress. The results of this paper are helpful to understand the mechanism of hydraulic fracturing with pulse fluid injection and provide guidance for its parameter design.
Highlights
Hydraulic fracturing has been widely used in the reservoir stimulation, which plays an important role in the exploitation of low permeability reservoirs and the development of geothermal energy resources [1, 2]
The simulation will be carried out according to the operation steps of hydraulic fracturing with pulse fluid injection, and the results of transient fluid flow and reservoir dynamic response will be analyzed
If fluid injection is continued in order to increase the bottom hole pressure, it will break through the safety pressure, which will threaten the service life of the equipment and the safety of personnel. e fluid injection should be stopped immediately
Summary
Hydraulic fracturing has been widely used in the reservoir stimulation, which plays an important role in the exploitation of low permeability reservoirs and the development of geothermal energy resources [1, 2]. During hydraulic fracturing, considering the pressure bearing capacity of surface equipment and the safety of the operator, it is necessary to set safety pressure at wellhead [3, 4]. E safety pressure limits the continuous increase of wellhead pressure. Once the wellhead pressure is close to the safe pressure, the injection should be stopped even if the breakdown pressure of the reservoir is not reached. The fracturing operation is interrupted, stimulation effect is affected, and operation time and cost are increased. To solve the above problems, the pulse injection scheme by alternately stopping and starting the fracturing pumps is proposed in the field operation. Is attempt has achieved considerable results in practical application. There is still a lack of research on its mechanism and theoretical guidance in the application
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