Due to the increases in completion costs demand for production improvements, fracturing through double casing in upper reservoirs for mature wells and refracturing early stimulated wells to change the completion design, has become more and more popular. One of the most common technologies used to re-stimulate previously fracked wells, is to run a second, smaller casing or tubular inside of the existing and already perforated pipes of the completed well. The new inner and old outer casing are isolated from each other by a cement layer, which prevents any hydraulic communication between the pre-existing and new perforations, as well as between adjacent new perforations. For these smaller inner casing diameters, specially tailored and designed re-fracturing perforation systems are deployed, which can shoot casing entrance holes of very similar size through both casings, nearly independent of the phasing and still capable of creating tunnels reaching beyond the cement layer into the natural rock formation. Although discussing on the API RP-19B section VII test format has recently been initiated and many companies have started to test multiple casing scenarios and charge performance, not much is known about the complex flow through two radially aligned holes in dual casings. In the paper we will look in detail at the parameters which influence the flow, especially the Coefficient of Discharge of such a dual casing setup. We will evaluate how much the near wellbore pressure drop is affected by the hole's sizes in the first and second casing, respectively the difference between them and investigate how the cement layer is influenced by turbulences, which might build up in the annulus. The results will enhance the design and provide a better understanding of fracturing or refracturing through double casings for hydraulic fracturing specialists and both operation and services companies.

Hydraulic fracturing or fracking is a well stimulation technique for extracting hydrocarbons from naturally low (extra low) permeable oil and gas reservoirs. In this process water, proppant and chemicals are injected through the wellbore and from the perforation hole into the reservoir. The main goal of this treatment is to create artificial fluid path conduits in the formation and finally increase the permeability (and productivity) of the reservoirs. One of the important factors which affects the near wellbore fluid pressure drop is the coefficient of discharge (Cd) which is a characteristic of the perforated hole in the wellbore tubular. The coefficient of discharge is defined as the ratio of the measured mass flow to the theoretical mass flow. The Cd depends on many factors and may change with time due to erosion caused by the injected sand, which was pumped into the formation. In this research, we investigated some of the factors that can affect the coefficient of discharge like the erosion of the perforated hole and the backpressure given by the fracture. For this purpose, we have developed a new high-pressure high-flow rate setup for examining the effect of the following parameters which can alter the coefficient of discharge. More specifically we have investigated the effect of perforation hole size, perforation hole geometry and perforation shape on the Cd value at ambient conditions and with backpressure, before and after sand erosion. To do so, in a first step we have used machined holes with a clearly defined geometry and then compared the results with real perforated holes which were generated using various shaped charge designs. The coefficient of discharge was measured using water or gelled water with different pressure differentials and back-pressures. In our study, we have injected sand slurry for 30 minutes with a constant concentration. The flow rate and pressure drop were also recorded simultaneously during the injection of the sand. Our results show how the erosion directly affects the Cd value and the subsequent pressure drop near the perforated hole. A clear increase of the Cd magnitude becomes visible only due to a change of the inlet geometry without changing the diameter. Also, the backpressure, which represents real fracking conditions, leads to a significant increase compared to the measurements at ambient outlet pressures. The measured values before and after the erosion for real perforation holes differ from simple drilled holes. From the recorded results, it also seems that certain perforation shapes or geometries are more effected by erosion than others.

Numerous papers have dealt with the description and measurements of the erosion of perforation holes during a hydraulic fracturing treatment in single casing completions, but not much is known about the erosion of perforations in dual casing setups. This study addresses this topic and compares it to the erosion rate of single casing scenarios and how this is influenced by the backpressure, which is created by the fracture closure pressure. The API 19B norm provides a guideline on how to test perforators under the most realistic downhole conditions. All casings used in our experiments were perforated in such a Section IV test set-up and subsequently installed in a specially designed high pressure flow apparatus. The casing holes were carefully measured, their hydraulic resistance was determined by a flow test and successively eroded by a slurry using high pressure pumping equipment. After each test, the holes were again geometrically measured, and their flow resistance was tested. In addition, the sand grain sizes were analyzed before and after the tests. Our tests revealed a significant difference in the erosion characteristic of dual casing compared to single casing setups. Especially the diameter of the hole in the inner casing is critical for the progress of the erosion and the final hole diameters. Equal holes on both casings provide a better control of the treating pressures, especially after the first minutes of the treatment. The back pressure, which is created by the fluid in the fractures, influences mainly the flow rate through the perforation. For identical flow rates, the pressure differential becomes less with back pressure, however the erosion rate as a function of the cumulative energy pumped through the perforation, remains similar. Finally, the application and design of a bigger test cell was evaluated and will be discussed as well. Although many perforating companies have started testing the charge performance for multiple casing completions, not much is known about the flow and erosion of two radially aligned holes in dual cemented casings during the fracture treatment and the influence of the back pressure created by the reservoir. The results will enhance the completion design and provide a better understanding of fracturing or refracturing through double-casings for hydraulic fracturing specialists and both operation and services companies.

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.

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