Rotary closed-loop steering systems are well known in the oil and gas industry and have become the accepted standard for drilling complex 3-D well paths and extended reach wells. These system's continuous drillstring rotation eliminates static friction between the string and the borehole permitting longer horizontal displacements in the reservoir and improving hole cleaning by stirring up the solids in the mud. While a 3-D rotary steering system could also be used to drill the hole's vertical top section, a closer examination reveals that efficiently drilling the top hole section requires a completely different approach. Static friction and solids settlement are not critical issues in vertical hole sections - permitting the design of a Vertical Drilling System engineered to run without string rotation in sliding mode. This approach protects the borehole walls from mechanical damage and leads to improved hole quality. To compensate for the lack of drill string rotation, the system design incorporates a latest generation power section which delivers almost double the torque of a conventional power section of the same length. Also, because a Vertical Drilling System's sensor package typically requires only inclination measurements and does not need azimuthal data, the system's electronic design is simplified. Although there are basic differences between 3-D rotary steering systems and Vertical Drilling Systems, there are also features which can be shared between both technologies - simplifying the service provider's engineering, maintenance, manufacturing and logistic efforts1. Examples of shared components include a steerable stabilizer to keep the BHA exactly on the desired (vertical) wellpath, the downhole power supply, the internal closed-loop information system and bi-directional data transmission. The Vertical Drilling System is designed to keep the top hole vertical without interaction from surface and without compromising critical drilling parameters (flow rate, WOB or bit speed) while maintaining high ROP along the entire section and avoiding time-consuming correction runs. The resulting improvement in hole quality and the precise wellpath allows the use of "lean casing profiles"2,3- reducing the amount of steel, cement, mud and cuttings. Vertical Drilling Systems are also beneficial in reducing the wellhead spacing at surface. All of these benefits result in significant cost savings to any operator. The paper describes the features of a Vertical Drilling System, presents case histories from 8–1/2" to 26" hole sections from various areas around the globe and gives an overview about the fast emerging market of this technology. Development of Rib Steering Systems for Vertical Holes While "rib steering systems" are receiving a great deal of attention in the drilling industry, the majority of the notice is focused on the impressive success rate of 3-D rotary steering systems. However, rib steering technology has also improved efficiency in both coiled tubing operations and vertical drilling applications. Typically, these systems all possess a steering head with three steering pads. It is often mistakenly assumed that there is no real difference between these systems with the exception of the tool size. However, the requirements and expectations for vertical drilling systems and directional drilling systems are as different as their applications - resulting in different designs. Advanced 3-D rotary steering systems are designed to drill complex well paths or long horizontal sections through the reservoir. Conventional motor assemblies are not always suitable for this purpose as directional corrections require stopping drillstring rotation. This lack of string rotation results in excessive static friction between the string and the borehole wall - making it difficult, or even impossible, to apply the necessary weight on the bit. Rotary steering systems overcome this problem by using pads to steer while rotating continuously. Eliminating static friction extends the hole's maximum horizontal displacement significantly. Similarly, string rotation is also very beneficial in removing cuttings from long, horizontal sections and cleaning the hole.
The elastomer and its bonding system has always been the one of the most critical areas in downhole motor design. The elastomer's physical properties change due to temperature or from reactions with certain chemicals commonly used in drilling applications. This limits both the motor's operational temperature range and the chemicals that can be used in the mud system. While drilling with a conventional motor, heat is concentrated in a regular elastomer power section's lobes forming hot spots which contribute to a premature heat aging of the elastomer. Today, however, a new manufacturing process for stator tubes permits the forging of a pre-contoured lobe configuration - significantly reducing the elastomer content in the power section. This new design broadens motor capabilities by allowing heat to radiate faster from the thin elastomer and significantly improving the motor's mechanical and volumetric efficiency. The power output of all sizes of positive displacement downhole motors has more than tripled during the past fifteen years. For example, today's 4–3/4" motors are significantly stronger than the 9–1/2" motors built in the 1980's. The latest step change in motor power was achieved in the manufacturing processes that eliminated 60%–80% of the elastomer used in the power section's stator. These latest generation downhole motors are used as performance motors - delivering over 50% more power output compared to the previous generation motors of the same length and diameter and permitting the operator to select more aggressive bit designs. As a result, operators have experienced Rates of Penetration (ROP) improvements of 100%–300%. In addition, high speed configurations with bit speeds up to 1200 RPM are used in hard and abrasive formations with temperatures up to 160°C (320°F). These configurations outperform previously used turbines where the motors have greater torque capabilities while maintaining full steerability throughout the entire run. Typical performance drilling applications cover re-entry, deepwater and extended reach wells in all parts of the world. This paper describes the technical features of the latest generation downhole motors in detail and documents their capabilities with case histories from various worldwide applications. Introduction Since the commercial introduction of hydrostatic downhole motors to the oil and gas industry in the 1970's, development engineers have worked on the increase of reliability and power output of these systems. Today downhole motors are among the most reliable components in the BHA - providing rotary power to the bit and keeping the well on the desired wellpath. Due to the development of improved manufacturing processes, better materials, and the application of latest design and simulation software, the power output of these downhole motors has increased steadily. Figure 1 shows the development in power output for three typical tool sizes during the past 20 years. Power sections are available asHigh-speed power sections for applications with impregnated bits,High-torque power sections for PDC bit applicationsLow-speed power sections for roller cone bit applications The latest generation of high-performance motors delivers up to twice the power of previous generation motors and was developed specifically for challenging performance drilling applications. The rugged design reduces the number of thread connections and includes strengthened thread connections to prevent tool failures, high load axial and radial bearings for the use of greater weight-on-bit and faster penetration rates and an improved steering head for more precise directional control.
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