High-Power/High-Frequency Acoustic Stimulation: A Novel and Effective Wellbore Stimulation Technology

Abstract

Summary Local impairment of formation permeability often causes reduced production because of the interaction of the reservoir with drilling and completion fluids. The problem may be further compounded by impairment caused by fines migration during production. High-frequency sonic and ultrasonic waves have been used in many industrial applications to remove contaminants, such as dirt, oil, and grease from parts that are immersed in fluids. An obvious extension of this application is the removal of wellbore impairment by exposing it to high-frequency acoustic waves. The influence of high-frequency waves is limited to the near-wellbore environment because of high attenuation. Dedicated experiments under realistic downhole conditions have been carried out in both linear and radial configurations. We have examined the acoustic power needed to remove near-wellbore formation damage caused by fines and particles plugging pores as well as drilling-induced damage. Specific issues related to well completion and the envelope of acoustic stimulation are presented. The laboratory results have led to the design and construction of a slim, high-power, and high-frequency (above 10 kHz) downhole acoustic tool for field deployment. This paper outlines the concept and presents key experimental results to support the claim. Key features of a prototype downhole tool are also described. Introduction Acoustic waves are traditionally used in the oil industry for exploration and appraisal during seismic and logging surveys. Recently, with the advent of 4D technology, repeated seismic surveys are carried out to 1 monitor the production behavior of a field. Surprisingly, acoustic waves can also be used for production enhancement. The two main potential applications are near-wellbore cleaning and enhanced oil recovery. This paper highlights the key development and understanding in the wellbore cleaning area, specifically regarding high-frequency acoustic stimulation. Formation damage can arise from many well activities during drilling, completion, and production. The associated damage mechanisms are numerous. One of the most pervasive mechanisms is the plugging of pores by solid particles. This may be caused by external sources such as drilling mud and drilled solid invasion, or it may originate in the porous medium itself, for example when in-situ clay fines are mobilized during production. It is not always possible to prevent formation damage completely, and well stimulation techniques to remove or mitigate the impact of formation damage have been used in the industry for more than half a century. Although conventional well stimulation techniques - both matrix and fracturing stimulation - have been applied very successfully, they do suffer from some severe limitations. Acoustic cleaning, however, presents a promising new well-stimulation technology in the combat against formation damage. It complements the existing stimulation technologies and enlarges the range of options available for cost-effective well stimulation. It uses high-frequency sound waves to shake loose damaging particles and to facilitate their removal by flowing the well. The following discussion substantially enhances and complements earlier discussions by Wong et al.1 First, we outline the motivations and potential applications of acoustic stimulation. Then, experimental data are presented to illustrate cleaning efficiency of acoustic stimulation. Finally, key issues surrounding the design and testing of a prototype downhole tool are expounded. Acoustic stimulation field trials are being planned, but the details are left to future publications. Conventional Well Stimulation Both matrix stimulation and hydraulic-fracture treatments involve the pumping of specialized fluids. Therefore, these techniques are "invasive," and two critical issues immediately become paramount:Compatibility between injected fluid and in-situ rock/fluid, tubing, and even-surface equipment.Fluid placement, diversion, and penetration into the rock. In matrix stimulation of sandstone reservoirs, acid/rock interaction often produces unwanted precipitates that further reduce formation permeability. Numerous new acid formulations and procedures have been proposed to eliminate or minimize this negative impact, but lithological heterogeneities in sandstone do not always make the task easy. Moreover, many of these techniques require the pumping of multiple stages of different fluids or acids. For example, in mud-acid stimulation, carefully designed acid preflush and post-acid displacement stages are needed. Straight away, fluid placement and diversion techniques involving multistages of different fluids become critical. Therefore, it is not surprising that mud-acid stimulation in a long, horizontal openhole section is still prohibitive, even with the use of coiled tubing. The corrosive nature of acid is yet another drawback that must be considered; the use of corrosion inhibitors adds to the cost of acid stimulation. In tight reservoirs, massive fracture stimulation aims to increase drainage area, but variations of the technique known as "skin frac" and "frac pack" can be applied to bypass near-wellbore formation damage and provide sand control in medium- and high-permeability reservoirs. Although less critical than matrix stimulation, successful application of this versatile technology also needs to consider compatibility issues between frac fluid and in-situ rock/fluid. Placement of multiple fractures presents significant challenges - appropriate isolation techniques are required. Frac fluid has to be viscosified to carry proppant, which is important for maintaining a conductive fracture. The proppant-loaded frac fluid is erosive; we need a surface blender, appropriate pumping capacity, and related equipment for successful fracture treatment. Therefore, a hydraulic-fracturing stimulation operation is relatively expensive.

Field Testing of a Novel Drilling Technique to Expand Well Diameters at Depth in Unconsolidated Formations

“…Use of vibrations was thus aimed at stimulating the entire borehole wall, including clogged zones. The main difference with previous studies (Champion et al 2004 ; Wong et al 2004 ) is that we used frequencies in the acoustic (200 Hz) instead of ultrasonic range (10,000 Hz) to increase the penetration depth. Tests performed in a formation with similar diameter grainsize had demonstrated that penetration depth increases from several centimeters for ultrasonic stimulation (Bunnik 2004 ) to 10 m for acoustic stimulation (van der Schans et al 2014 ), thus more than enough to reach the borehole wall with a downhole apparatus.…”

Analysis of capillary interaction and oil recovery under ultrasonic waves

“…• Micro-emulsification of oil in the presence of natural or introduced surfactants (Abismail et al 1999), • Coalescence and dispersion of oil drops due to the Bjerknes forces (Bjerknes 1906;Metting et al 1997), • Increase in rock permeability and porosity due to removal of fines and clays, paraffin wax and asphaltenes (Roberts et al 2000;Champion et al 2004;Wong et al 2004;Poesio et al 2004), • Oscillation and excitation of capillary trapped oil drops due to pressure perturbations generated by cavitating bubbles and mechanical vibrations of rock and fluid Higdon 2000a, b, 2002a, b).…”

“…They attributed these findings to the fact that vibrations provide the necessary mechanical force to overcome capillary entrapment, thereby mobilizing oil drops within pores. High power ultrasound may also be applied to reduce formation damage caused by fines and drilling mud solids, and to help mobilize clays in rocks (Venkitaraman et al 1995;Wong et al 2003Wong et al , 2004Roberts et al 2000;Champion et al 2004;Poesio et al 2004), thereby increasing rock permeability and porosity. Venkitaraman et al (1995) conducted a series of laboratory experiments to investigate the feasibility of using ultrasound to remove formation damage caused by water-based drilling mud and fines.…”

The effect of NaCl and HPAM solution concentration on HPAM gel structure degradation by ultrasonic waves

“…[30][31][32] In contrast, acoustic systems have been shown to be beneficial for removing inorganic plugs, reducing the viscosity of heavy crude oil, and degrading polymer gel structures. [30,[33][34][35][36] Acoustic wave technology is known for its efficient, reliable, convenient, adaptable, and environmentally friendly operations. [37,38] With this method, production engineers can monitor real-time effects and optimize sonication parameters and exposure times.…”