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The upper plunger has no pathway for fluid travel and remains in its barrel to act as a seal. The tubing, damper barrel, and plunger create a dampening system to decelerate the upward velocity that the system experiences on firing. More importantly it maintains a minimum 2,500-lb load on the pumping unit to prevent zero loading.
The operator can monitor the tool operation with a dynamometer card, a direct measurement of the load on the pumping unit. This load relates to the loads the tool creates during its compression and release cycles.
The shape of the dynocard tells ASR precisely how the tool is operating downhole.
Periodically, perhaps twice during the tool's lifespan, ASR will request the operator to adjust the setting of the polished-rod clamp to modify tool behavior. This is the only maintenance required on the tool.
The pumping unit sizes range from Lufkin 456 to 912, having stroke lengths from 120 to 168 in., and set speeds of 5 spm.
The load placed on the pumping unit is 10,000 lb above rod weight (at the top of stroke). The unit runs 12 hr on and 12 hr off to give the reservoir a relaxation period that enhances oil droplet mobilization.
Seismic stimulation
Reports of low-frequency, high-energy elastic waves mobilizing oil date from the early 1950s when earthquakes were shown to increase oil production by as much as 45%. The first man-made low-frequency, high-energy source used to mobilize oil was a Russian surface vibroseis in the early 1980s.
Detailed Russian field studies using surface vibroseis proved oil could be successfully mobilized and more importantly the mobilized oil came from the virgin reservoir area; thus, the application of low-frequency, high-energy elastic waves is an enhanced oil recovery method.
In 1998, Los Alamos National Laboratories (LANL) showed only a small pore pressure disturbance, less than 70 Pa (0.01 psi), could increase the permeability of a saturated porous medium and mobilize trapped fluids; hence, the process does not need elastic waves with large magnitudes to improve oil production.
In real terms, seismic stimulation has improved oil production and oil cut in wells as far away as 1.4 mile from the well in which the tool was installed.
The mechanisms describing the process of mobilizing oil under low-frequency, high-energy elastic wave fields center on two main phenomena:
1. Dislodging oil droplets from pore walls.
2. Coalescence of two or more droplets or thin oil films into one of higher mobility.
During the past decade, ASR has performed dozens of field studies and has proven the tool's effectiveness in diatomites with microdarcy permeability and unconsolidated sandstones with darcy permeability. The studies included oil gravities from 11 to more than 40, depths from 700 to 8,000 ft, and GORs from the low 100s to more than 3,000 scf/stock-tank bbl.
In general, permeability and oil gravity (viscosity) govern the time for observing a response. Mobilization of oil with a gravity below 14 is unlikely.
Gas content above 3,000 scf/stock-tank bbl tends to limit areal coverage; however, vibroseis stimulations of gas fields have succeeded. ASR did one successful gas field stimulation in the Austin chalk.
ASR believes, but has yet to pilot, seismic stimulation for improving recovery in unconventional reservoirs such as oil and gas shales and methane from coalbeds. The technology also may have an application in carbon-dioxide floods, alkaline-surfactant-polymer floods, and steam-assisted gravity drainage projects.
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