Near Wellbore Stimulation by Acoustic Waves

Abstract

Abstract Reduced well productivity or injectivity is often caused by near-wellbore formation damage due to the interaction of reservoir formation with drilling and completion fluids. The problem can be further compounded by production induced formation damage. A prime example is fines migration and fines plugging of rock pores or gravel packs. High frequency sonic waves, especially 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. Although the concept is old, successful large-scale application of acoustic well stimulation is not common. Greater understanding of the technology's applicability and limitations are essential in order to design effective downhole acoustic tool and guide successful field implementation. To this end, we have embarked on a dedicated project to mature the technology, which includes dedicated experimentation and tool design. In this paper, we focus on some key experimental results and discuss potential applications in production engineering. Introduction Reduction in productivity of hydrocarbon producing wells is often associated with near-wellbore formation damage. This is sometimes known as damage ‘skin’. The prediction, evaluation, prevention and mitigation of formation damage have been the subjects of intense study and discussion. The present conference is yet another testament to the importance of these aspects of formation damage in production engineering. 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. There are two main classes of well stimulation techniques - matrix stimulation and hydraulic fracturing. The first involves the use of appropriate remediation fluids, usually acids, to dissolve and remove the damage. The second is essentially the parting of formation rock by high rate pumping of fluid, creating a narrow but conductive path from the wellbore into the rock formation. This flow path is typically ‘propped’ open by proppant and it bypasses the near wellbore damaged zone. Although appropriate applications of matrix and fracturing stimulation technologies have been very successful, they do suffer from some severe limitations. Acoustic Waves cleaning is a promising new well stimulation technology in the combat against formation damage. It uses high frequency sound waves to shake loose damaging particles and facilitate their removal by flowing the well. We do not for one moment, think that Waves stimulation will completely replace the conventional stimulation techniques, but surely, it will enlarge the range of options available for cost effective well stimulation. The main aim of this paper is to stir the interest and present the vision of a novel and promising well stimulation methodology. We first outline the limitations of conventional stimulation techniques. Then, the motivations and potential applications of acoustic stimulation are discussed. Key experimental data are presented to support the claims, but details of downhole tool design and technology field trials are left to future publications. Conventional Well Stimulation The first thing to note is that 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 equipments, andfluid placement, diversion and penetration into the rock.