Hydraulic fracturing is the most popular well stimulation technique for extracting hydrocarbons from unconventional oil and natural gas reservoirs. During this process the stimulation fluid is injected into the reservoir from the wellbore with a pressure higher than the breakdown pressure of the reservoir in order to create fractures in the formation.
The pressures needed for hydraulic fracturing depend on many factors such as injection pressure and flow-rate, fluid density, fluid viscosity and the perforation hole. One of the important factors affecting the perforation pressure loss is the Coefficient of Discharge (Cd). This work looks deeper into the factors, which determine the magnitude of this value. Especially for a perforation hole, many of these factors are still not fully understood today and need further research.
As part of this study a new high pressure, high flow test vessel was built, which is compatible with our API19B Section IV test setup, in order to investigate some of the factors that could affect the Cd and subsequently the perforation pressure loss in the fracturing treatment. CFD simulations have been carried out to compare our experimental results with numerical models. In addition, we investigate the effect of the perforation hole size (area) by using different charges, the length of the fluid flow path, the hole geometry (shape), the effect of injecting high viscous fluid and finally the effect of Burr and Cement on the magnitude of the Cd magnitude for the perforated holes.
We developed a simple setup to deduce Cd values from perforations which were created in API19B Section II or Section IV test vessel. The values were measured for different pressure differentials, back-pressures and flow rates. The results show that the above-mentioned parameters directly affect the Cd value and subsequently the near wellbore pressure loss near the perforated hole. The values measured for real perforation holes differ significantly from simple drilled bores. Burrs on the inside and outside of the casing effect the magnitude as well as the length of the flow path.
Our new data sheds new light on the benefit of accurate measurements of Cd values for every shaped charge which helps to efficiently design the hydraulic fracturing stimulation treatment for oil and gas well.