Measurement-while-drilling (MWD) and logging-while-drilling (LWD) systems have proven themselves in successful, record-setting high-angle, high-pressure (HP) and

Figure 1. Electromagnetic (EM) data transmission gives the option of using positive-pulse transmission, allowing for data transmission entirely independent of rig activity and mud properties. (Graphic courtesy Weatherford)
high-temperature (HT) wells. Both address drilling demands that have been evolving from today’s activities in both brownfield and frontier environments. Reentries and short-radius horizontal recompletions are being used increasingly to revive mature fields. Deep-well conditions and deepwater operations now are becoming development opportunities with the new while-drilling technology.

Tool for high angle, HP/HT
Weatherford’s new-generation, combined MWD/LWD system incorporates technological advances specifically developed to meet the drilling challenges encountered in brownfield and frontier environments, specifically high-angle, short-radius wells; deep, hostile environments and harsh, ultradeep water. Entirely new generation while-drilling tools were developed and designed to fundamental specifications, rather than including enhancements or add-ons to existing MWD/LWD technology.

Combining the two systems — MWD and LWD — forms a system that measures wellbore position and formation properties, providing triple-combo gamma ray, resistivity and neutron/density porosity data. Both MWD and LWD systems incorporate electromagnetic (EM) data transmission with the option of using positive-pulse transmission, allowing for data transmission entirely independent of rig activity and mud properties (Figure 1). Using EM data transmission means that data can be made available while tripping as well as during extended-reach directional, underbalanced and lost-circulation drilling operations.
Higher reliability was built into the LWD and MWD systems through rigorous and extreme environmental testing runs, including combinations of thermal cycling, vibration, flow-loop and pressure trials.

Another major design goal was to achieve high data accuracy in an LWD triple combo system, at logging speeds much faster than maximum capability at the time. To achieve this, a 400-ft (122 m)/hr logging speed was specified — double the generally accepted upper limit for mature triple combo LWD systems.

Higher angles
These while-drilling systems have aided extended-reach performance, permitting operators to tap into previously unreachable resources, from development drilling in deep-well and deepwater frontier environments, to redevelopment in brownfields. For example, the MWD/LWD system was used for an ambitious, high-angle, short-radius re-entry drilling program in the Middle East to revive a mature field. The purpose of these short-radius recompletions was to restore oil production in wells that had watered out because of water breakthrough from the field’s ongoing waterflood operations implemented years ago.

In this mature Middle East field, short-radius horizontal wells were planned in vertical re-entries with the objective of exploiting hydrocarbons not recovered by existing wellbore configurations and bypassed by ongoing secondary recovery operations. Horizontal wells can be used to both reduce water coning rates and drain thin zones at economic production volumes. Short-radius wells help avoid potential problem formations and ensure the target formation is reached and well placement is maintained. Moreover, short-radius wells enter the formation target much closer to the vertical well bore, with some formations allowing the kickoff and lateral to take place within the pay zone.

The operator wanted to drill build sections that approached those of extreme short-radius wells. In the past, available RSS and MWD technologies permitted such aggressive build sections, but LWD capabilities had been the limiting factor in achieving them. Advances made in the company’s LWD technology have specifications defined to perform aggressive drilling operations such as extreme short-radius profiles.

Key to this project’s success was to use LWD data to the full extent to direct the build-well profiles specified by the operator. Using the LWD system achieved total directional well curvature, or maximum dogleg severity, ranging from about 49°/100 ft (30.5 m) to over 61°/100 ft, for the nine short-radius wells drilled in this field over a 6-month period in 2006. The lateral extensions drilled out of the short-radius build curves ranged from just over 1,000 ft (305 m) to almost 3,000 ft (915 m).

Sidetracking plugged wells and re-completing the short-radius horizontal sections enabled the development and production of reservoir portions that were either untapped, upswept, or both. Additional reserves were produced, ultimate recoveries were increased and productive life was extended in this field.

Higher pressures
Most real-life HP field situations are not also HT situations, and vice versa. Most operational settings can be classified as HP applications or HT applications, rather than both.

As wells and water depths get increasingly deeper, there is a greater need for higher-pressure capabilities of while-drilling measurement systems. Using higher strength materials, the MWD/LWD system offers a hydrostatic pressure rating of 30,000 psi.
In late 2005, this system was used in a 34,189-ft (10,427-m) offshore well, the deepest one drilled to date in the Gulf of Mexico, according to the US Minerals Management Service. The record-setting depth broke the previous 32,727-ft (9,981-m) record set by a different operator, but with the same while-drilling measurement system. The MWD/LWD system transmitted real-time and recorded triple-combo log data during drilling operations under extremely hostile downhole conditions, with temperatures reaching 280°F (138°C) and pressures exceeding 30,000 psi (206.8 MPa). These record setters provide proof that these while-drilling systems significantly expand industry pressure capabilities and thus open development opportunities in the Gulf of Mexico and beyond.

Higher temperatures
The MWD/LWD system was run in 2007 on two deep HT directional wells in one of the most challenging drilling environments in offshore India. Previous well experience had proven that these wells would be difficult to drill, log, case and test in a field where high temperatures had been by far the biggest challenge. While drilling these wells, the temperature rose continually. After drilling, temperatures continued to rise in the absence of circulation. Despite the high temperatures, the system successfully provided directional inclination and azimuth, gamma ray, resistivity, borehole pressure and temperature for both wells.

One of the wells encountered lost circulation problems. After it was brought back under control and normal circulation resumed, the MWD/LWD system was still functioning normally, recording a maximum temperature of 349°F (176°C). Upon pulling out of hole, the LWD tool memory was downloaded and the temperature and pressure data retrieved for the well control operations period. These data allowed the operator to profile and better understand the downhole events occurring during lost circulation. The data are now being used to develop a detailed knowledge base for future wells in the area.

The real-time data enabled timely decision-making regarding offset correlation, core point selection, casing point identification and determinations with respect to wireline sections. Also, the operator used the data for monitoring hole cleaning, optimizing mud weight and detecting packoffs. Using an MWD/LWD system that could perform in this HT environment saved the operator substantial time and money.

Conclusion
Today, operators are drilling in deep-well and deepwater frontier environments that place demands on MWD/LWD systems that did not exist when they were designed initially. Operators are also reviving brownfields with aggressive drilling practices that were unavailable until recently. The MWD/LWD system is designed specifically to meet the requirements needed for directional, extended-reach and extreme-short radius performance in addition to deeper and hotter drilling operations. Providing services from the routine to the extreme, these systems represent a significant step-change in directional and formation evaluation capabilities, serving as technological enablers in the development of complex reservoirs, hostile environments and ultra-deep reservoirs in the Gulf of Mexico and worldwide.