Residual Oil Reservoir Recovery With Seismic Vibrations
- V.N. Nikolaevskiy (Inst. of Physics of the Earth, Russian Academy of Sciences) | G.P. Lopukhov (Krylov All-Russian Scientific Oil-Gas Inst.) | Liao Yizhu (Texas A&M U.) | M.J. Economides (Texas A&M U.)
- Document ID
- Society of Petroleum Engineers
- SPE Production & Facilities
- Publication Date
- May 1996
- Document Type
- Journal Paper
- 89 - 94
- 1996. Society of Petroleum Engineers
- 5.3.4 Integration of geomechanics in models, 5.1.1 Exploration, Development, Structural Geology, 2.5.1 Fracture design and containment, 5.6.9 Production Forecasting, 5.3.4 Reduction of Residual Oil Saturation, 5.4.1 Waterflooding, 5.2 Reservoir Fluid Dynamics, 5.2.1 Phase Behavior and PVT Measurements, 5.6.1 Open hole/cased hole log analysis, 4.1.2 Separation and Treating, 3 Production and Well Operations, 2.2.2 Perforating, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 4.1.5 Processing Equipment
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Evidence suggests that certain vibrations, generated either by natural seismic events or by artificial explosions, have altered the production behavior of oil wells at distances as much as 200 km from the epicenter. These changes have affected the produced water/oil ratio: the water production rate increased from a formation that was at approximately the interstitial water saturation, while the oil rate increased in watered-out reservoirs that were near the residual oil saturation.
Theoretical and field investigations of the phenomena suggest that vibrations may influence substantially the water or oil relative permeability that appears to be partially reconstituted at saturations that ordinarily would prohibit the flow of a particular phase. The key role is played by ultrasound oscillations, generated by seismic waves within the stratum, and it has been confirmed by in-situ measurements during the vibrostimulation of reservoirs.
This paper provides an interpretation of the process and describes wave requirements, wave generation, and propagation in oil-bearing porous media supported by laboratory experiments and field cases of vibrostimulation of oil production from water-flooded reservoirs.
Although stimulation, as practiced today, may increase well productivity, it has limited potential for enhanced oil recovery. The amount of oil reserves within the radius of influence, if anything, may decrease because of water intrusion from water-flooding or from water influx from adjoining aquifers.1 In particular, the latter would be the case for hydraulic fracturing, where undesirable fracture height migration may result in rapid water influx. Often, hydraulic fracturing is justified only by the acceleration of oil recovery (net present value, NPV).2 The ultimate oil recovery may be less than that from an unfractured well.
Recently, the notion was introduced that vibrations can be used successfully in oil recovery.3 This paper describes this novel idea and how it can be applied to oil reservoirs. Particular emphasis will be given to water-flooded fields.
Displacement of oil droplets in water-saturated porous media was studied in the laboratory under simultaneous gravitational and vibration action.4 These experiments were based on the idea that vibrations could accelerate oil and gas separation in developed reservoirs at the macroscale level. It was found that high amplitudes of vibrations were needed. However, generating such high amplitude vibrations in situ is quite unrealistic because they can occur only in the case of earthquakes or high-impact explosions and only for short periods of time.
It has also been observed that during standard acoustic well logging, oil production increased in certain cases.5 In these occasions, ultrasound frequencies were used. It was evident that these treatments were cleaning the near-well zone. Near-wellbore ultrasound treatments using in-situ devices is a relatively well-known operation with several inventions already presented.6-9 However, ultrasound vibrations, generated at the well bottomhole, cannot penetrate porous rocks deeper than 1 m,10 and frequently, only a few centimeters.
A possibility to generate vibrations at an oil-producing layer is to put a bell-shape generator in the well that uses the energy of fluid flow. The method was developed and used in Siberia. Even though the use of mechanically or electrically generated sound is possible,11 the energy level is limited by the size of the well cross section. Again, this restriction implies that vibrations at the bottom of a vertical borehole can change the fluid phases in the porous medium only in the near-well zone. Thus, such methods of generating vibrations for reservoir stimulation, although potentially useful, cannot influence a large radius in the reservoir.
Studies of wave propagation in porous or fractured rocks revealed certain unusual features that cannot be explained by conventional linear elastic or viscoelastic theories.
In field experiments, it has been shown that fluid-bearing sands can change the frequencies of seismic waves. The energy of seismic waves converts to the energy of dominant frequency waves. This dominant frequency depends on the size and the compactness of grains and the fluid saturation. The dominant frequency is independent of the source of energy12 and the spectrum of frequencies emanating at the source.
In a series of experiments, sands were saturated with water from 0% to 100% of the pore space. These experiments were the key in understanding that the observed seismic waves under partial saturation of sands correspond to Frenkel-Biot longitudinal waves of the second type.13 Therefore, the dominant frequency wave characterizes the slow-moving wave, and the ultrasound waves characterize the fast-moving wave. For sands, the dominant frequency has been found to be 25 Hz, for clays 40 Hz, for gravels 10Hz, and for eroded granites 100 Hz. If this frequency coincides with the stratification resonance frequency, such oscillations last considerably longer.
It is also clear that seismic noise can be generated in rock masses deformed by solid tides and other tectonic or technogenous events.14 In addition, vibrator signals can be observed definitely at very large distances.15
To ascertain the technology, special field experiments were conducted in which vibrators were placed at the ground surface above the reservoirs. Refs. 3, 15, 16, and 17 summarize the results and interpretation of these field tests. An abbreviated form of this interpretation follows.
Early Field Experiments
The first test was to study the influence of earthquakes on oil wells at distances of 70 to 200 km from the earthquake epicenters.18 Fig. 1 shows changes of oil and water production of two wells during the earthquake episodes. The top illustration shows clearly that an earthquake swarm can decrease the water/oil ratio (WOR) if the WOR had an initially large value. The bottom illustration shows that the WOR can increase if it was very small initially.19 These were limiting observations. In other wells with intermediate WOR values, the results were inconclusive. However, the changes in the WOR were unmistakable in the extreme cases of the WOR range, as shown in Fig. 1.
The second test was a field vibration test. This test was conducted at the Abuzy reservoir in the Krasnodar region of the North Caucasus. The reservoir is a sandstone at 1400 m depth and has been developed since 1938, mostly under waterflood. A seismic vibrator was used with an active weight of 20 tons. The total well production from each well was 2 to 3 tons/d [16 to 24 bbl/D]. Producing WOR was high (Table 1). In this test, vibrations were continuous for 20 min/hr, and the interval of operations was 15 to 20 hr/D. The test lasted for 37 days, and the increased producing WOR was maintained for 17 days after the operation.
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