Underbalanced drilling offers significant advantages in terms of increased rate of penetration (ROP), less formation damage, reduced lost circulation material, decreased cost of cuttings disposal, and increased production. Underbalanced drilling injects gas into a mud column to lower the overall equivalent mud weight to create a drilling environment where the pressure in the wellbore is kept lower than the fluid pressure in the formation being drilled. Air is the ultimate underbalanced fluid, but diminishes the efficiencies of mud motors, and prevents the use of mud pulse telemetry MWD tools due to the lack of an incompressible fluid. With air drilling, the only fluid injected into the well is a small amount of oil needed to prevent corrosion. Downhole mechanical forces are usually more violent due to the lack of a fluid column for dampening as well as the higher air volumes going through the bottom hole assembly (BHA) for cuttings flow. Common drilling technologies to address air drilling include Electromagnetic Telemetry (EM), mud motors, and downhole air hammers, but reliability issues are particularly prevalent, especially for the EM MWD tools and downhole mud motors. Air drilling has become popular especially in the Marcellus and Utica shale reservoirs in the Northeast United States because of higher ROP and less formation damage. As an example, of the 111 rigs drilling in the Marcellus Shale, 27 rigs are drilling underbalanced and 23 are being drilled with air. A unique drilling system incorporating the use of downhole mud motors, EM MWD, and air hammers has been specifically designed and ruggedized to address downhole shock and vibration encountered in air drilling. Use of this system has resulted in significant reduction of non-productive time (NPT) while drilling with air. This paper will describe how air drilling is being successfully utilized in the unconventional reservoir of the Marcellus shale in the Northeast United States. Drilling fluids and their affect on various pressure regimes will be discussed. The new drilling system will be described and drilling parameters highlighting the differences between mud and air drilling will be provided. Modifications to the BHA to increase reliability will be discussed, and success metrics presented.

Certain formations in the Rocky Mountains preclude the use of electromagnetic (EM) telemetry due to their inherent resistivities. A new system has been developed that extends the use of EM telemetry systems where they were previously not usable. The system involves installing a special downhole antenna when surface or intermediate casing is run to improve EM signal to noise ratio. The casing antenna system not only allows operators to take advantage of the benefits of EM loggingwhile-drilling (LWD) systems but also extends the drilling depth range where data can be provided.Drilling wells in the Powder River Basin often requires heavy mud weights and the extensive use of lost circulation material (LCM). Unfortunately, these types of muds play havoc on standard mud pulse telemetry systems. Washed equipment and downhole failures related to plugged pulsers often result in multiple trips for failures and excessive nonproductive time (NPT). This new EM LWD system, by contrast, has no moving parts and a through-bore, which allows for higher concentrations of LCM and heavy weighting materials to pass easily through the bottom hole assembly (BHA). The system is combinable with a variety of LWD sensors so the telemetry type proves especially useful in areas that experience high LCM incidents. A key aspect of this system is that it also extends the depth range of where EM LWD systems can operate.Along with a compensated resistivity tool, a unique LWD spectral azimuthal gamma ray (SAGR) sensor, which provides accurate formation evaluation data, high quality gamma ray images and spectral elemental analysis, was used to help geosteer the well to stay in the zone of interest in the reservoir. This paper will examine a case study where the new system was used in an area where drilling with EM was not previously possible. It will describe the basic system design, key operating parameters, provide insight on the drilling problems, and address formation evaluation challenges and solutions.

M/LWD (Measurement or Logging While Drilling) data transmission from downhole to surface can be accomplished using several different technologies, but the most popular methods include mud pulse telemetry and EM (electromagnetic) telemetry. Mud pulse uses a downhole valve to restrict fluid flow and create a pressure pulse through which data is sent to the surface via the mud column. EM telemetry uses a downhole transmitter and surface receiver to transmit data through the formation using electromagnetic waves. EM telemetry is often the preferred telemetry method when drilling on land or in an underbalanced environment. Advantages of EM telemetry over mud pulse include: reliability (no moving parts downhole), speed (can often be configured to exceed six bits per second and to communicate independent of flow rates), and tolerance to lost-circulation-materials (not dependent on drilling fluid characteristics). A key disadvantage is depth limitation due to high formation resistivity and signal attenuation. A new technique has been developed that reduces signal attenuation and enhances the signal-to-noise ratio to increase the operating depth of an EM telemetry system. This method employs an insulated wire that is externally attached to a standard casing string; a borehole receiver typically located downhole and connected to the casing, and a surface transceiver. The borehole receiver picks up the EM signal at the casing connection terminal and transmits it via the external signal wire to the surface transceiver, which decodes the EM signal. The wire exits the casing near the surface and passes through a wellhead modified to accept the cable pass-through. There is negligible attenuation within the signal wire. This technique extends EM-M/LWD applications into well profiles, including long laterals, where high resistivity and attenuation has previously prevented its use. Of particular interest is pad drilling, where one insulated wire can be installed on the casing string of an anchor well and serve as a receiving antenna for the other satellite wells drilled on multi-well pads. This paper will describe the engineering concepts underlying the new technique and how it improves EM drilling technology. It also documents testing and summarizes actual field results of multiple lateral wells successfully drilled in highly resistive and attenuative formations with the new technique.

Drilling through shallow gas zones can be a major hazard, especially in offshore operations. An innovative technique using tender-assisted coiled tubing (CT) drilling is presented, which allows the determination of hazardous shallow gas presence with minimum risk, prior to mobilization of a large drilling barge. One advantage of the use of CT drilling is in increased safety because there are no personnel on the platform while drilling as the barge is kept 1 50 ft. away from the platform. Should a shallow gas flow occur, the gas is diverted with only minimal equipment exposed to damage and no danger to personnel.The technique of tender-assisted coiled tubing drilling has been used to drill five 3-7/8 in. slim hole wells in Lake Maracaibo, Venezuela. Equipment, techniques, and safety merit to perform the drilling operation are described. The successful multiwell campaign has provided References and i!:ustrations at end of paper a drastic improvement in safety while significantly reducing top-hole drilling costs. With the learning curve progress allowed by a multiwell program, it is demonstrated that drilling rates with CT can exceed those of conventional rotary drilling. Applications:This technique is applicable to many shallow gas problems both on land and offshore.Technical Contributions: 1. A unique, safe solution to the long-standing problem of drilling in a shallow gas environment.2. The first coiled tubing drilling in South America is described.3. Results of the multiwell campaign show that the learning curve requires several wells to obtain a realistic estimate of the efficiency of drilling with CT as compared to conventional rotary drilling.