Natural gas has gained widespread acceptance as a relatively low-carbon fuel source, with ever increasing demand from countries that include Asia and Europe.

The International Energy Agency (IEA) has estimated that, there are now enough technically recoverable gas reserves worldwide for at least 250 years at current production-consumption rates. Oil and gas exploration companies increasingly are looking to capitalize on this in a bid to replace declining production from conventional reserves.

Protocols for evaluating conventional reservoirs using core and core data were established decades ago. Determining accurate gas volumes, content, and deliverability also are critical when attempting to assess the economics of an unconventional gas play.

In the past, wireline technology primarily was used as a means of reducing trip time for thick reservoirs or to bring core to the surface quickly to minimize lost gas in formations such as coal. However, there is an argument that with shale, gas escapes regardless of the speed at which the core is brought to the surface – a fact that has diminished the value of wireline coring.

Quest Coring saw an opportunity not only to deliver longer cores but also cores of a larger diameter (enabling better analytical studies of the core), through the development of Quick Core technology.

The system was launched in January 2010. Today a number of major oil and gas producers across the US and Canada are using it. Wells in France, Columbia, Australia, and Poland have also been secured after just 60 days of marketing the tool in the Eastern Hemisphere through an alliance between Quest Coring and Corpro.

The core is housed in a steel inner tube that holds a thin aluminum sleeve. (Images courtesy of Quest Coring)

Need for innovation

The Quick Core system is designed to enhance the speed of the coring process, while providing both superior core quality and recovery. The Quick Core 76 (QC76) and Quick Core 89 (QC89) coring systems bring a newly developed product to operators looking to maximize information for E&P activities.

In the case of shale gas, operators tend to be concerned primarily with minimizing micro-fractures to the core, which can lead to false readings related to the permeability of a formation. Core is normally housed in an aluminum inner barrel while coring before being transferred from a vertical hanging position during core recovery to a horizontal position for site handling. During this process, there is potential for significant flexing of the aluminum barrels traditionally used to house the core. Flexing can cause the core to crack.

The Quick Core tool offers a different solution. The core is housed in a steel inner tube that holds a thin aluminum sleeve. The steel serves as a stiff tube for lay-down purposes, while the core and sleeve can be extruded for surface handling. This system gives the operator quick access to the core. The aluminum sleeve also serves as a container in which the core can be left in an undisturbed state until samples are taken to the lab for analysis. On the QC89 model, a fluted aluminum inner barrel eliminates the possibility of collapsing inner tubes as a result of pressure spikes. The alternative option of a steel inner barrel with an aluminum liner protects the core from fluid invasion while still enabling gases to slowly vent.

The technology’s ability to deliver longer cores sets it apart. The tool is designed to bridge the gap between 27-m (90-ft) conventional coring and 9-m (30-ft) wireline coring tools while still using a wireline to retrieve the samples. This characteristic not only offers operators significant rig time savings, but also ensures they do not have to sacrifice core quality.

The tool primarily is being used for applications such as shale gas where, due to the thickness and depth of the reservoirs, it is able to deliver better value than conventional technology. In reducing the time and cost required to retrieve core, shale gas becomes a more viable and affordable endeavor.

Safety benefits are key to the technology’s success. Most injuries on a rig are from tripping drill pipe. Because the tool reduces both the number of wireline trips and drillstring trips required, it greatly minimizes the potential for injury.

Another benefit is that the wireline retrievable coring system ensures continuous cores can be cut and retrieved without tripping the drillstring, with the core recoverable through the internal bore of the drillpipe. The tool also has the ability to core and drill without tripping, thereby allowing multiple zones of interest to be cored – a drill insert can be installed and removed via wireline through the bore of the drillpipe, enabling favorable time and, therefore, cost savings.

High torque connections on a CSX 54 pipe enable the system to go to extreme depths while also allowing core barrels to be made up to longer lengths. Both the QC89 and QC76 have a core jamming indication feature – if a jam occurs while coring, the system automatically alerts the coring technician on the surface that there is a potential problem downhole. This reduces the probability that valuable core data will be milled away.

The steel serves as a stiff tube for lay-down purposes, while the core and sleeve can be extruded for surface handling. The aluminum sleeve also serves as a container in which the core can be left in an undisturbed state until samples are taken to the lab for analysis.

A world first

The Quick Core recently facilitated the successful completion of the first ever 38-m (125-ft) wireline run in a shale gas reservoir for major a US operator. The record-setting core is 35% longer than the amount of core cut previously over the same recovery time.

Prior to this, Quest Coring also undertook a successful 27-m (90-ft) wireline run in a shale gas reservoir. The operator’s reasons for using the Quick Core were twofold. It wanted to achieve rig time savings without sacrificing core quality, and, because it was exploring a new reservoir, the probability of being able to cut 27-m (90-ft) cores conventionally was low.

Four cores were taken from a recent well. The data shows the length of the core and the time required to produce it.

A number of cores were taken last summer for this operator with an average 99.25% recovery across the wells. Prior to this well, the longest wireline core ever cut was 18 m (60 ft) long with a 3-in. diameter. The 27-m (90-ft) core cut by Quest Coring – which also was a world first – used a high-torque drill pipe in a 7 7/8-in. well to cut a 28-m (93-ft) core run length with a 3.5-in. diameter.

The larger diameter core, in addition to the speed and efficiency of the 38-m (125-ft) wireline run, presents operators with excellent analytical options. The larger diameter core aids in the recovery process while also improving the quality of core being cut. Most wireline coring tools offer only 3-in. diameter or smaller cores, often considered less than optimal by operators.

Through bit development that accompanies the coring technology, the company has managed to increase the length of core that can be cut, its quality, and the efficiency of the recovery process. Continued improvements in coring depth and diameter will provide operators with solutions that help improve the efficiency and quality of their exploration while leading to better returns on capital spending – the ultimate metric.