Sandia National Laboratories is developing technology to reduce problems caused by bit vibrations by reproducing downhole vibrations in the laboratory.

The Vibration Dilemma

Drillstring dynamic dysfunctions currently limit the use of advanced technology drill bits and related tools for drilling hard rock formations. The bit, formation, hydraulics and bottom hole assembly can interact in a complex way, resulting in a range of vibration modes. Among these are bit bounce, stick-slip and whirl. In harder formations, these vibrations typically cause bit failure that is often accompanied by significant economic losses.

Technology Status

The drilling industry has developed comprehensive test facilities to characterize bit performance for the challenging environments encountered downhole. These facilities have resulted in improved understanding of the physical interaction between the rock, the bit-cutting elements and wellbore hydraulics. These laboratory-based characterizations have given birth to high performance bits that can effectively reduce soft to moderately hard formations. However, most existing laboratory facilities fail to include the critical influence of drillstring vibration, and optimal performance relies upon the bit remaining engaged in the formation at its intended condition.

Analytical models have been developed to predict the forces and motions resulting from drillstring dynamic events. This information is used in component-test facilities to develop more impact-resistant cutting elements. More recently, Sandia has modified its Hard-Rock Drilling Facility to introduce longitudinal and rotational compliance representative of a typical drillstring. This facility has demonstrated the prevalence and consequences of bit vibrations in drilling. It also has emphasized the necessity of eliminating drillstring vibrations in hard rock formations. The drilling industry has used downhole telemetry during field drilling to capture these dynamic events in real time. While the industry is developing vibration suppression tools and methodologies, an advanced capability is needed to support this research and guide the development of new materials, designs and processes.

Simulation of the Drilling Dynamics

The purpose of the drilling dynamics simulator is to represent the dynamic motion of the drillstring in a controlled laboratory setting, in order to more accurately reflect a field-drilling condition, so that the bit response may be monitored, characterized and improved before committing the bit and drilling tools to field drilling operations. Several motivations:

• Support development of instability theories in drilling, and provide guidance for specification of drill bit designs and operating conditions for a given formation relative to the dynamic properties of a drillstring.
  
  
• Identify deficiencies in drill bit material properties and designs as representative impact loadings that occur in the field can be simulated.
  
  
• Validate development of hardware and methodologies to introduce stability to the drilling process to eliminate drillstring dynamic dysfunctions.

Chosen Approach

Sandia’s approach to development is to integrate advanced technology in computational modeling and electronic controls with high-force, fast-acting, servo-hydraulic actuators to represent the properties of a virtual drillstring in the laboratory. The drilling function would be performed by an actual bit in a representative rock sample, yet the bit would behave as though it were attached to a long, flexible drillstring specified by a user-defined configuration.

The facility would consist of two primary subsystems – a drilling simulator and a dynamics simulator. The drilling simulator includes a drill rig gantry with a vertically traversing platform to support the dynamics simulator. The dynamics simulator supports the drill bit and bottom hole-assembly tool in order to represent a virtual drillstring. The dynamic properties of any user-defined drillstring would be virtually reproduced with the actuators that are controlled by a computational model of a drillstring. Sandia already has completed prototype characterizations that validate this approach.

The computational model is the control engine behind the simulator. This model predicts the dynamic compliance to be maintained at the simulation interface to represent the user-specified drillstring. Solution procedures have been developed for these models that can be solved in real-time using today’s desktop computers. Field data also can be used as a simulation event driver to reproduce damaging drillstring vibrations encountered.

Future Benefits

The drilling dynamics simulator will provide a platform for developing a wide range of drilling tool technologies under representative downhole environments that are encountered in today’s hard rock drilling conditions. This capability will result in improved well construction methods and reduced production costs. It will allow development of vibration theories, hardened drill bit materials, stable bit designs, reliable passive and active dampers and other downhole tools, and improved operational methodologies.
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