Production of oil and gas wells is often reduced due to damage in the near-wellbore region related to the drilling process. In many instances, the damage is caused by the interaction of the reservoir formation with drilling and completion fluids. This damage can increase during the life-time of the well due to influx of formation damage (fines, scale, asphalthenes, etc). An example is formation fines plugging a slotted liner or screen.
High frequency sonic waves have been used in many industrial applications to remove contaminants like dirt, oil, and grease from parts immersed in fluids. An obvious extension of this application is the removal of wellbore impairment by exposing it to high frequency acoustic waves. Although the concept is old, successful large-scale application of acoustic well stimulation is not yet common. During the last decade the manufacturing of high-power piezo-electrical elements developed fast and opened the potential to design a downhole acoustic cleaning tool.
The advantages of an acoustic cleaning tool is the selectivity of the treatment compared with conventional matrix acidizing treatment. Also the need for large quantities of chemicals (acid, corrosion inhibitor etc.) can be eliminated.
In previous publications the potential of acoustic cleaning has been reported using experimental results obtained in a laboratory set-up using linear acoustic waves. In this paper the results will be presented on the cleaning of mud damage using a prototype acoustic tool in a radial geometry.
Acoustic waves are traditionally used in the oil industry for exploration and appraisal during seismic and logging surveys. Acoustic waves also have been used for stimulation of oil reservoirs. Beresnev and Johnson1 presented an excellent overview of past activities, claimed successes and failures in this area. The two main applications are near wellbore cleaning and enhanced oil recovery. This paper reports on developments in the near wellbore-cleaning area, specifically, high frequency acoustic stimulation and as such is an extension of earlier papers by Wong et al.2,3
Near wellbore formation damage can arise from many activities during drilling, completion and production. 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/or drilled solid invasion, or may originate in the formation itself, for example when clay fines are mobilized during production. It is not always possible to prevent formation damage, 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 particularly in long horizontal wells or in wells with multiple branches.
Acoustic cleaning is a promising new well stimulation technology to combat the effects of 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 can be substantially enhanced by maintaining an underbalance pressure in the well.
This paper is intended to bridge the gap between the linear configuration that was used in the experiments described by Wong et al.2,3, and the radial configuration that is required for field scale application of the technique. The main purpose of the paper is to demonstrate the feasibility of acoustic stimulation in a radial geometry under laboratory conditions. The ultimate proof of the feasibility of course requires field trials.