Small-Diameter Sonic Logging Tool Opens Access To Challenging Wells

?High-resolution real-time information can be hard to obtain with conventional logging tools, which are too large to maneuver in complex well geometries such as slimholes and high dogleg-severity wells. In addition, wireline – the only conveyance option for these tools – is prone to bridging events that require hole-conditioning trips, adding significant time and cost to the job.

Weatherford International Ltd.’s Compact suite of logging tools was designed to navigate around these challenges. The tools can acquire the full spectrum of formation evaluation data – including resistivity, porosity, gamma ray, and formation imaging – in packages that are up to half the size of conventional tools. Their 21?4-in. diameter profiles allow them to more easily bypass restrictions or well sections with high angular build rates.

These tools are not limited to wireline conveyance and can be deployed by methods that provide the flexibility of logging data in memory mode. This further reduces acquisition risks and extends the tools’ operating range into formations that previously were considered inaccessible due to logistical or cost hurdles.

Other conveyance options include:

• Well shuttle, which conveys tools inside the safety of a drillpipe garage;

• Through-drillpipe logging, a technique used when well restrictions caused by sloughing formations or ledges pose challenges for openhole wireline conveyance. The technique conveys open-ended drillpipe into the well below the zone of restriction;

• Coiled tubing, which is employed to convey small-diameter logging tools into difficult-to-access wells;

• Slickline, in which small-diameter tools are attached directly to the end of the slickline for deployment in memory mode;

• Wireline tractor, a conveyance technique where logging tools are deployed into highly deviated or horizontal wells on wireline with the aid of tractors powered by electricity transmitted through the wireline from the surface;

• Through-the-bit, which conveys tools through specially designed drill bits with removable central inserts, allowing logging to begin as quickly as possible after drilling or coring; and

• Continuous rod (co-rod), which employs specially designed strings of continuous sucker rods to convey logging tools in memory mode into highly deviated and horizontal wells. Log data are acquired as the co-rod is pulled from the well.

There are several options to deploy logging tools in a wider range of hole sizes. (Images courtesy of Weatherford International Ltd.)

Sonic logging sharpens the view

The CXD tool comprises four main sections: transmitter, isolator, receiver, and memory.

Geologists and geophysicists increasingly have relied on sonic logs to make more informed exploration, appraisal, and production decisions. The compressional and fast/slow shear velocities obtained from cross-dipole sonic logs have proven valuable in applications including well placement, wellbore stability,

formation damage control, and completion and production optimization. But as with other conventional logging tools, deployment of cross-dipole sonic tools into challenging wells is limited by their size, physical durability, and dependence on wireline.

In response, a small-diameter, wireline-style sonic logging technology called the Compact Cross-Dipole Sonic (CXD) tool has been developed. The tool retains the 21?4-in. diameter of the other Compact logging tools and is made up of four sections with a total combined length of 26 ft (7.9 m).

The first section consists of three transmitters – one monopole and two directional high-output, wideband dipole transmitters – which create flexural waves in the X and Y directions that propagate at close-to-the-formation shear velocity.

The second section houses the isolators. Acoustic isolators attenuate energy traveling between the transmitters and receivers along the tool body. These isolators were developed with a new design based on a sprung-mass principle to ensure near-perfect acoustic isolation with minimal loss of mechanical strength.

A receiver section follows, consisting of an array of eight stations, each comprising four gain-matched, highly sensitive piezoelectric hydrophones aligned with the dipole transmitters to provide 64 dipole waveforms plus 32 additional waveforms associated with the monopole transmitter.

The range of mineral analysis and rock mechanical properties obtained by running the Compact CXD and triple-combo tools in tandem helped the operator complete its openhole well in less time and at a lower cost than using conventional wireline deployment.

An enlarged borehole interval example also shows slow shear velocity.

The final component is a memory section capable of storing 16,500 ft (5,000 m) of uncompressed waveform data, which is recorded independently from the data sent via the wireline when present. This delivers data assurance during wireline operations and enables standalone memory logging.

The design innovations incorporated in the flexural wave transmitter, acoustic attenuator, and receiver assemblies allow compressional and shear wave velocities, as well as fast shear azimuth, to be determined from a small package that is not constrained by wireline data transmission rates. As a result, the tool can be combined with a microresistivity imager and other high-data density tools to provide complementary evaluation techniques without compromising logging speed.

Data always are recorded raw, which means the four constituent single-sided waveforms per receiver station always are included – unlike with some tools that combine the waveforms into X and Y prior to transmission. Access to raw data provides flexibility in data reprocessing, even if the original logging objective called only for single-axis shear.

Additional deployment benefits of the small-diameter sonic logging tool versus conventional-sized tools include its ability to be run through the tubing into existing cased or barefoot completion wells, which saves the costs of pulling the tubing. The tool can be run in small internal diameter cemented casing, which is risky for conventional cross-dipole acoustic tools due to size constraints.

Field experience

The small-diameter cross-dipole sonic logging tool underwent an extensive field test program in 2009 in North America and Europe. Three prototype tools were used to log five test wells and 40 commercial wells using deployment options that included two through-drillpipe conveyance operations, six wireline operations in 41?2-in. casing, one tractor conveyance operation in 41?2-in. casing, and a shuttle conveyance operation in a horizontal test well.

One of the through-drillpipe operations on a 77?8-in. openhole well in Texas required the Compact triple-combo and cross-dipole tool to be run together and acquire high-resolution formation data that included lithology, porosity, water saturation, and mechanical rock properties. The tools successfully logged 4,200 ft (1,280 m) of the 11,300-ft (3,444-m) total depth well, and the through-drillpipe deployment saved the client more than 16 hours of rig time per trip by eliminating drillpipe hole conditioning associated with failed wireline trips. The data enabled the operator to make quality decisions on the completion based on the multimineral and rock mechanical properties analysis.

In other field trials, the tool characterized difficult-to-evaluate areas such as unconsolidated or shallow sediments, washouts, and surface boreholes. During a trial in Alberta, Canada, the tool characterized a formation with shear slowness of 670 ms/ft, even though the hole was rugged and washed out to 15 in.

The Compact CXD tool now has successfully completed more than 200 commercial operations worldwide in a range of borehole environments using the full scope of conveyance options.