Downhole Harmonic Vibration Oil-Displacement System: A New IOR Tool

Abstract

Abstract Excessively large amplitude excitations resulting from earthquakes, explosions, or mechanically vibrated systems are found to generate macroscopic and microscopic mechanisms that have beneficial effects on the production of wellbores. A downhole harmonic vibration oil-displacement system (DHVOS) that generates a sudden large amplitude excitation to the liquid in the wellbore was developed by researchers at CNPC (China National Petroleum Company). Compared with other downhole artificial vibration systems, the DHVOS system is a heavy-duty unit that can provide long period operations and large amplitude excitations. This system has been implemented in CNPC's oil fields such as Liaohe, Huabei, Shengli and Zhongyuan. The DHVOS system provides a harmonic vibration frequency of 88 shocks per minute with each shock having 5-ton strength. More than 100 oil-producing wells have been stimulated with the downhole vibration system since 1997. The field results demonstrated that periodic application of large amplitude excitation to oil reservoirs led to increased oil production rates and decreased produced fluid water content. The observed oil production-rate increments ranged from 27.5% to over 400% with an average increment of 47.6% compared to the pre-vibration production rates. The reduction in water oil ratio (WOR) varied 10% to 75% with an average of 48.4% in WOR reduction. Introduction Declining oil recovery is of major concern in the oil industry. Searching for new methods to enhance oil recovery is currently an urgent focus to oil industry. Vibration stimulation is a possible method for improving oil production and increasing recovery in situations where high water-cut has led to abandonment. Interest in the effect of seismic stimulation on in-situ fluids in petroleum reservoirs was dated back to 1950s with observations made on the correlation between water-well levels and seismic excitation produced from cultural noise and earthquakes. Similar effects were observed in producing oil fields where distant earthquakes caused increase in production, and wells close to operating machinery, highways, or railroads appeared to produce more oil than wells in quieter areas[1]. These observations led to the concept ofc seismic excitation as a flow enhancement method for fluids in porous media[2]. Seismic excitation involves applying large amplitude excitations, either at the surface above the reservoir, or down hole within the formation. In surface excitation, due to the amplitude attenuation with depth, frictional energy dissipation and geometric spreading, very high-energy surface sources are required to provide sufficient energy to the reservoir. In downhole methods, it is challenging to induce large amplitude excitations without casing damage, and the restrictive casing diameter intensifies the difficulties[2]. In a review paper by Beresnev and Johnson[1], the methods and results of enhanced oil recovery achieved by application of relatively weak elastic waves were well presented. Documented observations include effects such as changes in water level in wells and oil production changes in response to various seismic sources. However, most of these observations are of little use in quantitative analyses due to the lack of enough data and the intricacy of the phenomena involved. For example, the effects of an earthquake on an oil reservoir might have been caused not only by elastic vibrations, but also by fracturing, or compression of surrounding rocks. The data reported are also often inconsistent and contradictory[3].