Abstract

Reduced production is often caused by local impairment of the formation permeability due to 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 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. The influence of high frequency waves is limited to the near wellbore environment due to high attenuation. Dedicated experiments under realistic downhole conditions have been carried out in both linear as well as radial configurations. We have examined the acoustic power needed to remove near wellbore formation damage due to fines and particles plugging and drilling induced damage. Specific issues related to well completion and 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 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 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, 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 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 since more than half a century ago. Although conventional well stimulation techniques - both matrix and fracturing stimulation - have been applied very successful, they do suffer from some severe limitations.

Acoustic cleaning 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 facilitate their removal by flowing the well.

The following discussion substantially enhances and complements that given by Wong et al1. We first 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 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:

  1. compatibility between injected fluid and in-situ rock/fluid, tubing and even surface equipment, and

  2. fluid placement, diversion and penetration into the rock.

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