RIASSUNTO
Summary. This paper discusses the constructive use of resonant power transmission down the axial length of a stuck oilfield tubular for the purpose of extracting the tubular from inadvertent (e.g., stuck drillpipe or tubing) or purposeful (e.g., gravel-packed liners) confinement. Resonant and nonresonant oilfield systems are examined with appropriate field measurements of resonant parameters.
Introduction
Resonance has been used for many years in the electrical field for transmission and application of high power. But in many fields, effort has centered on the avoidance of resonance and its destructive effectse.g., flutter in aircraft and chatter in machine tools. Buildings are designed to avoid frequencies resonant with earthquake vibrations. On the other hand, nonresonant vibration has been used extensively in many mechanical, chemical and material transport applications. The extent of achievement in these applications is frequently limited to the ability to apply sufficient power to the system. Oilfield jars are a perfect example of nonresonant vibratory or high-energy dissipative-type systems because of their inherent use of broadband high-frequency impacts. An eccentric-weight oscillator tuned to the resonant frequency of a system is an example of a highly efficient resonant circuit. Both resonant and nonresonant systems are examined, with major emphasis on an oilfield-applicable resonant system.
Nonresonant Vibratory Systems-Jarring Operations
Stuck-tubular extraction services have been available to the oil industry for quite some time. Drilling or fishing jars are routinely run with the bottomhole assembly (BHA), during drilling operations. These jars must be strategically placed along the drillstring to optimize their performance. This jarring technique is currently used to unstick all types of oilfield tubulars e.g., sanded-up tubing strings or the extraction of gravel-packed liners during workover operations. Although the amount of impact near the stuck point may reach 150 g, the pulse duration is on the order of 8 milliseconds. Some mechanical jars can be easily cocked and tripped up to three times per minute, whereas the waiting time between blows typically ranges from 1.5 to 2.5 minutes for hydraulic jars. Because the number of blows delivered to free a stuck tubular can vary from 10 to much more than 1,000 cycles, the amount of time involved can reach 50 hours, assuming continuous jarring. Fishing operations involving jarring usually last days, sometimes weeks, resulting in considerable rig time. Another consideration during jarring, especially in drilling, is that the jar must be located above the neutral point in the BHA (above the point of zero axial stress). This is necessary to keep the jar in tension to avoid accidental jarring and to maximize the life of the tool. In some cases, the stuck points end up above the jar location, negating its use. If this occurs, a controlled backoff at a preferred rotary shouldered connection above the stuck point is done with a known quantity of detonating charge in a ""string shot"" inside the drillstring. After the drillstring is backed off at a known location, a fishing assembly that uses a more powerful ""fishing"" jar and additional collars is run and made up with the fish in the hole. This entire fishing operation also takes up considerable rig time, often resulting in sidetracking operations when jarring becomes unsuccessful.
Resonant Vibratory System Eccentric Weight Oscillator
The apparatus used in this study to transmit sonic energy into such oilfield tubulars as a drillpipe or tubing string was performed by an eccentric-weight oscillator, schematically depicted in Fig. 1. This particular type of oscillator uses a pair of eccentric weights rotated in opposite directions by geared shafts so that the weights cancel each other for sideways forces but are added together during vertical oscillation. This gives a pure, simple harmonic vertical-force component that is transmitted down the pipe column as a series of alternate tension and compression waves, as shown in Fig. 2. This causes a rapid series of large percussive forces at the stuck point. These types of machines are available to the oilfield market, and a photograph of one of these is shown in Fig. 3. This entire assembly hangs from elevators on the traveling block and clamps onto the exposed tubular above the rotary table. The wave pattern shown on the left in Fig. 2 is the conventionalized form with a lateral pattern to illustrate alternate compression and tension being transmitted longitudinally down the pipe.
SPEDE
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