Keywords: Coanda effect, perforations, wellbore, formation, etc.

Transcription

1 Study and Application of Transient Characteristic of Fluid Phenomena for Downhole (Oil and Gas) Pulsating Devices Using FLUENT ANSYS Bharat Pawar, Koustubh Kumbhar, Tim Hunter, and Harit Naik, Halliburton Abbreviations: pulsating devices (PD), barrel of fluid per minute (bbl/min), near wellbore (NWB), computational fluid dynamics (CFD), outer diameter (OD), two-dimensional (2-D), three-dimensional (3- D), fast fourier transformation (FFT) Keywords: Coanda effect, perforations, wellbore, formation, etc. Abstract A fluctuating fluid flow in the wellbore can provide intangible benefits in well stimulation applications. A pulsating device (PD) is a device with no moving parts that generates an oscillating jet of fluid at predesigned frequencies. The conceptual basis of a PD is the Coanda effect 1, which is the tendency of a fluid jet to be attracted to a nearby surface. Fig. 1 illustrates the PD producing emissions of alternating bursts of fluid that create pulsating pressure waves within the wellbore and perforations (if present). As each burst is ejected, it forms a compressive wave within the wellbore fluid. As the wave passes through the formation and reflects back, it incurs a tensile loading on the skin damage. Continued cycling of these pressure waves causes the skin damage to reach its fatigue failure point. Scale is fatigued to failure then removed as a result of the fluidic oscillation, helping restore and enhance the permeability of the perforations in the near wellbore (NWB) area. Operation of this device is based on the Coanda effect. The Coanda or wall attachment effect was discovered by Romanian aerodynamicist Henri-Marie Coanda in He observed that a stream of fluid emerging from a nozzle tends to follow a nearby curved or angled surface if the curvature of the surface or angle is not too sharp. A narrow inlet throat is used to create a jet, which exits into the interior of the device. The formed jet adheres to one of the angled interior surfaces by virtue of the Coanda effect described. After the jet attaches to a surface, the fluid that forms the jet travels along the surface until it hits the entrance to the feedback loop. A larger portion of the fluid enters the feedback loop and exits the outlet port attached to the feedback loop. Some of the fluid travels past the outlet port and continues through the feedback loop, where it disrupts the attached jet and deflects it toward the other angled surface. The jet then attaches to the opposite surface, and the process continuously repeats, creating a steady oscillation of flow between the outlet ports. Introduction This paper reviews efforts to simulate the behavior of fluid using a transient numerical computational fluid dynamics (CFD) simulation, FLUENT, and examines the study and application of transient characteristics of fluid phenomena for downhole PDs. Design parameters were selected and fined-tuned based on simulation results to achieve the desired frequency, mass flow rate (bbl/min), pressure drop, and outlet vector plot, etc. Virtual CFD simulation provided the discussed knowledge in advance. The fine-tuned design was prototyped and lab-tested for a wide range of inlet data. The testing results show consistent correlation with transient CFD simulation outcome. The newly developed device has improved performance in all respects. The methodology adopted is in close agreement with CFD numerical tool and was immensely beneficial to the development of this technology.

2 Fig. 1: Pulsating Pressure Waves (PD 2 ) Problem Definition During the life cycle of oil or gas wells, over a period of time, a loss of production or injectivity can occur caused by NWB conductivity problems. Typical examples of those types of formation damages are Process Methodology This paper addresses the study of fluid flow characteristics and optimizing the design of a device for a desired oscillation frequency by selecting design parameters and including conceptual shapes. CFD analysis software FLUENT-ANSYS is extensively used to study fluid behavior with respect to time. Devices of several different sizes and configurations were simulated over a wide range of flow rates. Fig. 3 shows the overall process methodology adopted in this work 3. Scales Paraffin Migrating fines Asphaltene deposits Lost or dehydrated drilling mud Fig. 2 illustrates an example of perforation blockage. These above mentioned examples can result in either a well not producing at maximum potential or ineffective stimulation treatment (e.g., acidizing, fracturing, etc.) caused by NWB blockage or restrictions. Therefore, it is important to develop a geometry that will provide appropriate frequency of oscillations. Fig. 2: Removal of Migrating Fines from Perforations Fig. 3: Process Methodology This analysis allowed for suitable changes to the geometry of the device, which improved performance and helped ensure reliable operation over the range of flow rates. FLUENT ANSYS CFD Setup Fig. 4 illustrates a typical computational domain considered for a replica model of a PD. Transient solution is preferred to capture the inherent transient fluid flow oscillation phenomenon described. The boundary conditions for the model consist of constant pressure applied at the inlet to the PD and zero pressure applied to the outlets. The properties of the working fluid used in the modeling were those of water. Pressures ranging from 500 to 2,000 psi were used, resulting in flow rates

3 between a few gallons per minute to some bbl/min, depending on the outer diameter (OD) size of device. Fig. 4: Computational Domain Fig. 5 shows the CFD software specific settings that were used. For quick results, a twodimensional (2-D) profile of a fluid flow path was simulated, and, later, the detailed analysis was carried out with the help of a three-dimensional (3-D) simulation in FLUENT. Time discretization errors were avoided by reducing the time step size. The time step size used for this simulation was 1e-5 sec. The lesser the time step size, the more the solution time; therefore, appropriate selection of time step size is important for transient CFD simulations. Mostly, time step size is calculated with the help of the empirical relation shown (i.e., Courant number = (velocity * time step size) / mesh size). Keeping a courant number (e.g., 0.5) and a given inlet velocity, time step size for each mesh size can be calculated. Fig. 6 demonstrates a plot of mass flow through both outlet ports of the device for definite pressure differential. It is known and was observed in this study that, as pressure differential is increased, the mass flow rate and oscillations frequency also increases. Fig. 6: Mass Flow at Outlet Ports Fast fourier transformation (FFT) of the discussed time domain data provides the oscillation frequency imparted by the device. Fig. 7 shows an FFT plot. Fig. 5: FLUENT Input Parameters Fig. 7: Oscillation Frequency

4 To visualize the oscillatory flow through the device, incremental snap shots of relative fluid velocity were saved during the transient response simulation. Fig. 8 is an example of these snap shots sequences. Fig. 8: Transient Modeling of Pulsating Device Prototype and Testing Two-stage prototyping was used, as mentioned, in the process methodology. At Stage I, rapid prototyping of the pulsating device along with the insert and housing was created and tested for city water pressure. This method of rapid prototyping is very quick and involves less cost. The steel prototype was the next stage of prototyping. Manufacturability, cost, and product lifecycle were all factors considered during the design changes of the PD. Along with the goal of definite oscillation frequency, geometry was developed that allowed for a replaceable insert, metal-to-metal seals, and adaptability for use with other in-house device systems. During test setup, a specific fluid pump rate was established and held constant while the device was tested. A high-speed camera was used to capture the details of the fluid spray behavior. The video was captured at a rate of a few thousand frames per second, which was later used to calculate oscillation frequency. Conclusions The technique of using wellbore cleanout and NWB stimulation in conjunction with a PD has been known to the industry for many years. The effectiveness of the device was further enhanced by means of a CFD study of the conventional devices, as shown in Fig. 10. Fig. 10: Experimental vs. CFD The various frequency ranges, configurations, sizes, and patterns were made to suite the application. The proposed work involved CFD analysis, prototyping, and experimental correlation for the PD. A rigorous CFD analysis has been developed for the PD, which can be used for similar types of devices for development. It was observed that the new design is better in several aspects. CFD enabled the service company to yield following benefits: Fig. 9: Experimental Test Setup For different inlet pressure, a close correlation for mass flow rate and oscillation frequency between experimental measurement and CFD prediction was observed. CFD prediction of results accelerated product development time.

5 The new design imparted a lower oscillation frequency for deeper penetration into the formations. The new design provided higher energy output over narrower frequency ranges compared to a conventional device. The new design has no moving parts or seals in its robust design. This new-generation device has proven to be effective at increasing the efficiency of wells 4. Apparatus or Creating Pulsating Fuid Flow. United States Patent US B1, 20-Dec-05. Acknowledgement The authors thank the management of Halliburton for their support and permission to publish this paper. They also express gratitude to all of the team members who contributed to the job design, preparation, and execution of the operation to achieve the results presented in this paper. References 1. Earl D. Webb, R.L. Schultz, R.G. Howard and J.C. Tucker, Next Generation Fluidic Oscillator. Paper SPE presented at the SPE/ICoTA Coiled Tubing Conference and Exhibition, The Woodlands, TX U.S.A. 4 5 April Kritsana Kritsanapkak and Salah Tirichine, Effectively Enhancing the Near-Wellbore Stimulation Treatment using Fluidic Oscillation Technique. Paper SPE presented at the SPE Production and Operations Conference and Exhibition, Tunis, Tunisia, 8 10 June R. P. Roger and S. C. Chan, Numerical Study of Fluidic Bistable Amplifiers. Paper AIAA presented at 33 rd AIAA Fluid Dynamics Conference, Orlando, FL., June Earl D. Webb, James C. Tucker, Mardy L. Meadows, and Robert L. Pipkin,