MWD/LWD services have been used since the 1970s to provide petrophysical measurements for evaluation of the reservoir and to monitor depletion of reservoirs as they mature. Initially, wireline services provided the same measurements as those of LWD services but with higher quality and accuracy.

However, after more than 30 years of intensive development, LWD services can now provide the same quality logs as wireline in real time to aid geosteering and early reservoir evaluation. These services are deployed along with MWD data to help mitigate drilling risks and reach farther into previously inaccessible reservoirs. Additionally, the trajectories of some wells prohibit wireline conveyance, increasing the need for MWD/LWD services, especially in high-cost operations such as deepwater drilling.

One of the main challenges that operators face is risk mitigation while optimizing returns on the reservoir. Considering the difficult market conditions, this article will particularly focus on MWD/LWD technologies that offer potential significant cost savings and risk mitigation when drilling in harsh HP/HT environments and when wellbore stability is more challenging in formations such as shale.

HP/HT technology advancements

As hydrocarbon reserves become scarcer, technology development efforts are focused on the capability of drilling in hostile downhole environments, particularly in extreme temperatures. HT wireline logging tools have existed for many years and have mostly used heat flask technology to insulate the tool’s electronics from the high reservoir temperatures. The short duration of most wireline runs makes this a practical solution, but MWD/LWD tools operate downhole for longer periods of time, negating the effectiveness of this technology.

Without an HT tool, current drilling practice for these HT wells has been to drill until standard MWD/LWD tools "temped out,” and then to pick up a dumb iron assembly to drill to total depth (TD). This method of drilling results in real-time logging data loss and borehole positioning uncertainty, forcing the operator to drill the well blind.

Another common practice is to extend tool runs as long as possible by employing temperature mitigation practices such as reducing rpm, staging to cool the tools and using mud chillers, but these practices increase the amount of rig time and add significant costs to drill the reservoir section.

In 2013 and 2014 Sperry Drilling launched its HT MWD/LWD drilling service, the 4¾-in. and 6¾-in. Quasar Pulse systems, which offer real-time gamma, vibration, directional and pressure measurements in environments up to 200 C (392 F) and 25,000 psi. The system contains hermetically sealed electronics designed to operate in extreme temperatures, helping the operator reach final TD with potentially just one run.

Quasar Pulse service case study

In a recent well offshore Southeast Asia, where bottomhole temperatures are among the highest in the world, an operator wanted to reduce costs and achieve logging objectives in HT wells drilled from a jackup rig in 74 m (243 ft) of water.

To help the operator prevent the costly measures associated with HT conditions, Sperry Drilling recommended using the Quasar Pulse MWD/LWD service to operate in the anticipated bottomhole static temperature of 195 C (383 F). Sperry Drilling ran the Quasar Pulse MWD/LWD service with the adjustable gauge stabilizer in five wells and successfully drilled and logged in each section.

In one single-run performance where circulating temperature reached 187 C (369 F), the service drilled and logged 2,284 m (7,493 ft) to TD for an average ROP of 31 m/hr (101.4 ft/hr).

The operator calculated that using the service from Sperry Drilling saved it about $1 million and more than 100 hours of rig time by reducing the number of wireline runs, eliminating trips for temped-out tools and avoiding temperature mitigation practices to achieve TD.

Sonic technology advancements

There are many challenges when drilling a well, but optimizing reservoir production and decreasing drilling risks are among operators’ top priorities. Advances in sonic sensors have made it possible to provide solutions to these challenges. Sonic sensors can use multipole sources to measure compressional and shear velocity data to calculate mechanical rock properties and to help optimize the hydraulic fracturing process and completion design.

In addition, sonic measurements can help determine lithology, porosity and pore pressure to calibrate the geomechanical model and optimize or define a safe mud weight operating window.

Without this information there is a risk that the formation could fracture if the mud weight is too high, or the wellbore could collapse or suffer pressure kicks if the mud weight is too low. Both could result in increased costs, potentially up to two days of rig time and safety risks for the rig personnel.

Since 2013 Sperry Drilling has released the XBAT sonic service with a full suite of tool sizes, covering 4¾ in., 6¾ in., 8 in. and 9½ in. The XBAT LWD service, the third generation of Sperry Drilling sonic services, delivers accurate acoustic measurements in a wide range of formations via sensors and electronics that are much less sensitive to drilling noise and have a wider frequency response than earlier designs. The result is a greater signal-to-noise ratio that enables better measurements even in noisy drilling environments and poor hole conditions.

The XBAT tool combines multi-array azimuthal sonic velocity measurements with multi-axis ultrasonic stand-off measurements to improve the operator’s ability to determine rock properties that have a direct impact on the geomechanical model.

Sonic service case study

An operator in Nigeria drilling in a shale formation in 1,666 m (3,827 ft) of water encountered issues in determining the optimum mud weight needed to maintain wellbore stability. In a previous well in the same field the operator had various challenges associated with wellbore stability. The operator had to make a clean-out run because the 9⅝-in. casing could not be run to setting depth and had to be pulled out of the hole. The clean-out bottomhole assembly got stuck at the same depth at which the casing was held up, requiring a sidetrack.

To avoid repeating these problems, Sperry Drilling recommended the XBAT service to provide real-time shear and compressional data while drilling the 12¼-in. hole, helping to determine the appropriate mud weight. As a result, the operator was able to successfully complete the well while avoiding any wellbore stability issues and high-cost potential sidetracks.

Future technologies

Future developments will likely include further increases in the temperature and pressure specifications of MWD/LWD tools, allowing operators to exploit reserves in even more hostile conditions. Further enhancement of downhole rock mechanics measurements will improve operators’ understanding of the drilling environment. These technology advancements will help reduce nonproductive time, minimize overall costs and maximize return on investment.

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