Feasibility Study of Ultrasound for Oilwell Stimulation Based on Wave Properties Considerations

Abstract

Abstract Ultrasound or high frequency (20 kHz-100kHz) pressure wave has been used in diagnose and treatments in different areas, such as: medicine, dentistry, civil engineering, and many other industrial applications. In Oil industry there are applications such as pipeline inspections, fluid velocity measurements, etc, but to the present its application in formation stimulation has been incipient, and only few lab and field test experiences have been reported. Stimulation with ultrasound is not a common operation offered by oil service companies. In order to visualize the real potential of ultrasound in oil well stimulation, it is necessary to understand the wave phenomenon, its properties, the parameters that define its behavior, and its interaction with the propagation media. This basic knowledge together with the understanding of the different formation damage mechanisms are the keys to comprehend the real potential and application window of the ultrasound in oil well stimulation. The present paper presents the theoretical basis of ultrasound and wave phenomena that must be considered when thinking about stimulation with ultrasound. Finally some suggestions about the application window of this technology are given. Introduction Ultrasound has been applied in many areas such as: diagnosis, quality control, inspections, cleaning, etc. Industrial cleaning is achieved flaking out the particles by mechanical action of the pressure waves (Fig 1). Usually the piece is submerged in fluids inside a container whose walls are fixed ultrasonic sources. Clearly, there is a great difference with an application for oil well stimulation, where the source is running inside the hole and the cleaning area is around the source. Each application has a particular frequency and power associated according to the sample dimensions and the purpose: for example, the power and frequency used for control echography in pregnant mothers are different than the used in muscular therapeutic treatments. In the first case is enough to detect an echo with high resolution (higher frequencies). In the second case it is required to transfer energy to the tissue, but high resolution is not required (lower frequencies). It is clear that the purpose and the propagation media affect the ultrasound parameters, highlighting the importance to understand which are the damage mechanisms where ultrasound could be applied and viceversa. Figure 1. Piece before and after Industrial ultrasound cleaning. The advantage of applying ultrasound comparing with conventional stimulation is that no invasion or external fluids are required, avoiding fluid/rock interaction analysis, the placement and the associated equipment and risky operation of handling high pressures at the wellhead. Additionally, ultrasound would allow under balance treatments without shut-in the well. Ultrasound cleaning is not a common tool offered by service companies in the field. Just some field tests in China and Russia have been reported with more qualitative than quantitative information making these tests not conclusive. Recent references about lab experiences and tool prototypes suggest the potential of this technology. However, ultrasonic stimulation has low understanding of the phenomena that are taking place in the porous media, how the waves are interacting with the matrix and the trapped particles. The parameters for suitable cleaning with ultrasonic treatment are not well defined and how these parameters change while the wave is propagating in the porous media is not clear either.