Rotary steerable systems creating niche in extended reach drilling

Horizontal drilling techniques have historically been utilized primarily in older fields with the expectation of recovering hydrocarbons left behind from traditional drainage methods.

New technology developed to improve this technique such as short radius motors, coiled tubing, and multi-lateral technology have helped make horizontal drilling a very viable alternative for enhancing reservoir recovery.

Traditional directional steering tools, adjustable steerable motors, and adjustable

stabilizers have recently been utilized to extend wellbore lateral displacement. Extended reach drilling objectives by operators have taken existing technology to the limit. Alternatives

for drilling these extended reaches are needed.

History

The problem of "running out of drill weight" in directional steering operations has always been a problem and often was the reason for premature ceasing of drilling operations. Numerous times in the industry, total depth has been reached not by

plan or decision, but by the simple fact that drilling could not continue due to lack of drill weight.

The engineering explanation is that the resultant weight of the drillstring at the bit was not enough to overcome the friction force acting on the drillstring. Common knowledge dictated that rotating the assembly helped reduce these friction effects and allowed drilling to continue further. Monitoring drilling fluid parameters and optimizing the lubricating properties of the drilling fluid also helped. But these methods were only good for a certain amount of time. Rotary steerable tools were introduced to further reduce these friction affects resulting from longer wells.

Early versions of the current rotary steerable drilling systems, sometimes referred to as a SRD system, were conceptualized in mid-1950. By the end of the 1950s, more than one rotating sleeve/housing tool was available. Interest in the technology waned about the same time that a new technology called a "positive displacement motor," or PDM, was being developed. The early 1960s saw this technology take off and dominate the directional drilling business over two decades.

During these two decades, directional drilling objectives were being achieved with the use of PDM motors and bent subs - with limited advances in rotary drilling techniques being the result. Traditional rotary bottomhole assemblies continued to be used as tangent drilling and direction maintaining assemblies.

PDM evolution

The PDM motor eventually evolved into the adjustable steerable motor we know today. As the "boom" days faded into history, the industry became more cost conscious, and began looking for cheaper alternatives to the steerable motor. Budget constraints and some less challenging well plans created an opportunity to experiment with variations in stabilizer placement as the mechanism of directional control.

As a result, the traditional pendulum assembly began to evolve further. This assembly, when modified slightly via various stabilizer placement and blade OD, could be utilized in a angle-building or dropping situation. This functionality, used with the natural turning characteristics of specific bit types, gave drillers three dimensional, rotary, directional control possibilities.

The sometimes inability to successfully drill these assemblies as desired, resulted in an adjustable stabilizer design being developed and utilized. The ability to change a given bottomhole assembly from a short pendulum to long pendulum assembly, or from an aggressive, angle-dropping pendulum to a maintaining assembly without having to trip the assembly to the surface was of great use to a creative directional driller.

These tools and techniques, though effective for their time, eventually reached their limit. Operators continually faced the issues of increased drillstring friction due to aggressive directional plans, extended the length of wells being drilled, hole tortuosity, differential sticking, equivalent circiulating density (ECD) fluctuations, and buckling/lockup. An alternative was needed. These problems and the past success of adjustable stabilizers and steerable motors resulted in renewed interest in the 1960s SRD technology.

The mid-1990s saw the introduction of newly design ed SRD systems. New engineering and creative well planning have made these new tools an instant success. New improvements and modifications are currently being added to the systems further enhancing tool effectiveness.

SRD fundamentals

The fundamental operating concept of the original SRD systems has remained intact. However, updated machining, metal, and electronics technology has given service providers the materials to greatly expand on the original SRD concept. Current engineering designs vary across the industry, but all provide similar directional control characteristics.

The two distinct areas of classifying SRD systems are orientation method, and method of mechanical directional control. Automated or manual orientation methods are utilized. The new design tools utilize electronics for monitoring and dynamic control of the SRD system. Manual orientation is similar to the techniques used for steering motor orientation. It is not dynamic, therefore requiring rig time to orient at each drilling connection.

Mechanical deflection methods in rotary drilling assemblies vary with three alternatives available. They are indirect side force, direct side force, and bit tilt without side force. These are basically an extension of the 3-point concept utilized in directional drilling planning applications. Utilizing stabilizers to create fulcrum points at various locations in the bottomhole assembly allows hole trajectory to be more easily directed.

Using the above methods, different industry alternatives have been developed.

• Two companies are utilizing the indirect side force method. Resultant bit orientation from this method is in line with the wellbore.
• Another alternative utilizes a flexible joint design resulting in direct bit alignment with the wellbore axis.

The majority of service providers are developing systems predominantly utilizing a direct side force mechanism of deflection, resulting in a bit orientation at an angle with the wellbore.

Most of the systems utilize the same operating principle with the differences in tools being primarily in the pressure distribution within the respective tools. Mud flow is diverted via valves or ports into an outer rotating (or non-rotating with some tools) sleeve where the resulting pressure differential forces pistons outward from the sleeve body. Some of the designs continually alternate diverted flow to sequentially activate and deactivate the pistons arranged around the sleeve circumference. Overall operational pressure loss across these tools is small.

Advances in electrical components led to the development of the automated dynamic systems. Powered by mud flow via a turbine/alternator assembly in the tools, integrated tool electronics monitor pre-programmed commands against realtime directional and geological information. This information and using downlink techniques to send realtime commands from the surface, allows for constant directional control of the drilling assembly.

Current tools can provide at-the-bit inclination, induction resistivity, and focused gamma ray and resistivity measurements. Full geosteering control can be enhanced with link to a conventional MWD/LWD system to provide sonic measurements, and in the future, bit seismic information.

Future development

As these drilling systems evolve and become more efficient, drill bit side-cutting ability while maintaining gage integrity will be a key issue. Development of smaller systems for use in slimhole applications may soon follow.

Changes in traditional well planning are sure to come too. Operators with the opportunity to drill further in shorter amounts of time may have to modify their field development decision-making processes.

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