The benefits of using wireline instruments to log oil and gas wells can diminish quickly due to excessive rig costs, costly fishing operations and/or lost downhole tools, if the logging string becomes stuck in the well bore while tripping.

As the risk-weighted cost of interrupted logging jobs or the total loss of the well has increased with deeper, more complex well designs and more ambitious downhole evaluation procedures, new, more robust components have been developed for wireline logging strings to help ensure that logging services can be delivered successfully.

For instance, stronger grades of wireline have been introduced in the past few years, with breaking strengths up dramatically to 28,700 pounds (lbs). Coupled with a recently developed releasable wireline cable head, maximum wireline pulling capabilities in the past year alone have increased more than 30 percent to more than 14,000 lbs, at the safe load of 50 percent of the wireline maximum rated breaking strength.

Stronger wireline and wireline connectors mean operators can pull harder from the surface to free logging tools stuck downhole, significantly improving the odds of completing logging trips and reducing the possibility of costly delays or lost tool strings.

For situations in which stronger wireline and wireline components alone are not enough to dislodge stuck logging strings, technologies that have proven useful in other phases of well construction also are being adapted to reduce unproductive rig time and improve the reliability and effectiveness of wireline logging. For example, a new, proprietary wireline jar technology--derived from jar technology used for many decades in the oil and gas industry to free stuck drilling strings--is racking up an impressive string of successes in early commercial applications in the Gulf of Mexico.

Wireline logging jars
Wireline jars traditionally have been mechanically activated, although single conductor hydraulic jars have been run occasionally in logging strings for more than a quarter century. Mechanical jars for cased-hole logging applications were introduced in the late 1990s, allowing multiple jar activations on a logging run without returning to the surface to be redressed. While the use of mechanical jars has been embraced by the industry, such devices have proven to be extremely maintenance intensive, in part because when inserted into the well bore their working parts are exposed to the downhole environment.

Other issues also arise relating to the impact and impulse of mechanical logging jars, the two principal forces created when jars of any design are activated. Impact is the initial, instantaneous force generated when a jar is activated; impulse is the residual force of the impact, essentially reverberations occurring in the milliseconds following an activation.

The fundamental objective in activating a jar is to create the strongest impact possible and the strongest and longest impulse. The primary energy generated when a mechanical jar is activated is supplied by the wireline, which can also act as a spring by recoiling above the jar assembly. The mechanical jar primarily creates an impact with little impulse.

Another limiting factor on a mechanical jar is that the impact is preselected before running in the hole. By setting the tension at which the jar will activate while the tool is on surface, the operator decides before running in the hole what impact value will be utilized. Once this is set, it cannot be changed unless the tools are retrieved to surface. Once this predetermined value is exceeded at the top of the jar by the tension pulled on the wireline, the jars will be activated immediately. So there is no chance to pull harder without activating the jars.

Hydraulic wireline jars
A single-conductor electric wireline jar (SCEJ) was introduced about three years ago for cased-hole logging applications that overcomes most of the limitations of mechanical jars.

The SCEJ, which has proven extremely effective for through-tubing perforating and setting bridge plugs, is hydraulically actuated and is equipped with a mechanical lock that can be manually adjusted during shop assembly to trigger activation with a pull from surface of 500 to 1,000 lbs. A 

hydraulic time delay mechanism allows the operator to pull the logging string through a tight spot without activating the jar, as long as the wireline tension is released before the time delay expires.

In a typical wireline logging string, the SCEJ is placed immediately above the logging or perforating tools, can be activated multiple times on every run, and requires very little maintenance.

The SCEJ is pressure balanced to guard against collapse in the well bore, and internal parts--including the jar mechanism and the conductive path--are completely sealed and segregated from the well bore, preventing exposure to downhole fluids, reducing wear and increasing reliability. The hydraulic time delay is not temperature- or pressure-sensitive, and the SCEJ unit is designed to function in reservoir temperatures of up to 400 degrees Fahrenheit and pressures as great as 25,000 psi.

The reliability of the SCEJ has been excellent, with no downhole failures reported in more than three years of commercial availability.

Open-hole wireline jar
Building on the success of the SCEJ, within the past few months, a multi-conductor electric wireline jar (MCEJ) designed by Evans Engineering and Manufacturing has been commercially deployed by the wireline logging business unit of Halliburton.

The MCEJ shares many design characteristics with the SCEJ, including mechanical locking, hydraulic time delay, pressure balanced hydraulic system, and sealed internal parts and contact areas to prevent exposure to the wellbore. The MCEJ is designed to function reliably in reservoir temperatures as great as 400 degrees Fahrenheit and pressures up to 22,500 psi, but these specifications can be easily increased as the tool is pressure balanced.

In a typical open hole logging string, the MCEJ is placed immediately above the logging or formation testing tools, where it is mated with an enhancer to augment the force used when the weight is thrown up hole after the jar is activated. The MCEJ assembly, itself, from the top down consists of a cable head, the enhancer, cable mode and telemetry subassemblies, and lastly the jar, creating as large a mass as possible to help jar loose stuck logging tools. Both the enhancer and jar store energy in belleville springs, which propel a hammer into an anvil upon activation to generate the impact and impulse.

Unlike the mechanical jar, learning to use the MCEJ is quick and easy; logging crews can be trained to use the tool in minutes, so a jar service technician is not required on location. The MCEJ arrives at the well site ready to run, and the lock setting can be adjusted at the well site to begin metering the jar with a pull from surface of 1,700 to 4,000 lbs.

It has been demonstrated in the lab that the MCEJ's impulse is more than twice as powerful with up to five times more duration when the enhancer was added to the jar.

Wireline hydraulic jar benefits
Although they are placed as close as possible to the logging tools, wireline hydraulic jars do not impair logging capabilities or data quality. They have proven cost-effective when run as insurance against sticking--especially when high-cost equipment spreads or costly, high-tech tools are involved--and may allow the operator to free-fall wireline in regions where persistent sticking problems have dictated the need for drill-pipe conveyed logging operations.

In a recent logging run on a well in the Gulf of Mexico--the main region where it has been commercially deployed so far--the MCEJ was activated a total of 20 times in six different intervals, enabling the logging operation to be completed in a field where logging tools had become stuck and fishing operations were consequently required in every previous well.

The cost of including the MCEJ in a logging string is infinitesimal compared to the high cost of operating delays, fishing jobs, or irretrievable logging tools. In the application above, the operator was charged only an additional fee for each zone where the MCEJ was activated. Multiple activations in a given zone resulted in only one activation charge per interval, so the service company in essence shared the risk with the operator. If the jars are not activated there is only a small depth charge.

Operators who have seen the MCEJ in action on their own wells since its commercial introduction earlier this year consistently report the groundbreaking technology routinely frees stuck logging strings many times over in individual wells. Because of its demonstrated reliability and cost-effectiveness, many not only plan to run the MCEJ on upcoming wells but also recommend it to others.

Although MCEJ service presently is being offered commercially by only one oilfield service company, wireline logging professionals who are familiar with it expect the technology to become an industry standard quickly in areas where fishing jobs are required frequently or where expensive spread costs at the well site drive both the risk and cost of excessive delays or failure to unacceptably high levels.