Drilling Simulation Is Method To Evaluate Incremental Options, Reduce Risk

As computing power has become more affordable and the E&P industry’s understanding of downhole drilling dynamics has improved, sophisticated simulation software has been developed to optimize drilling.

Most drilling engineering analysis software has been based on using static analysis assuming steady-state conditions. Although useful, the end result was little more than a snapshot of anticipated forces. Advanced sensors in the bottomhole assembly (BHA) and improved surface sensors now provide digital measurements of weight on bit (WOB), rate of penetration (ROP), torque, and vibration as a function of time. As a result, it is possible to develop and validate more-complex and -accurate drilling models. Drilling simulation can now provide access to any one of the thousands of nodes presented by means of mesh software rather than by means of static analysis. The i-DRILL drilling simulation from Smith Technologies is an example of this functionality. The opportunity for detailed scrutiny of data leads to understanding normal, tangential, and axial loads that affect system behavior. Since the modeling is based upon the mechanical properties and dimensional data that represent the drillstring and the compressive strength and physical characteristics of the formations drilled, the results are specific to the application and are detailed in terms of what is required to improve performance.

Within the virtual environment, the bit type/style, the individual BHA elements and their relative placement, the drillstring, the operating parameters, and the drive and/or steering mechanism (e.g., positive-displacement motor, rotary-steerable tool, turbodrill) can be varied to identify the optimum combination. Bit durability (dull condition), ROP, and stability are considered and weighted. For example, if in a deep well, long trip time is to be avoided at the expense of all other areas of investigation (ROP, vibration, or stability), these may become less important in determining the optimum system. Typically, problems previously encountered while drilling offset wells and other operator experience in the area lead the engineering team to the most important areas of investigation first. Bits and BHAs are methodically tested and retested through the virtual simulation regime, eliminating the poorer performers until the best match and highest normalized score are found. Once these elements are identified, the targets are quantified and weighted according to the economic and strategic (reservoir) project drivers.

Fig. 1—Smooth-running drilling assemblies drill faster and cause less wear and tear on expensive downhole tools. The simulation on the left shows an assembly experiencing high torque (blue line). The optimized assembly on the right runs smoothly with lower torque.

Once the “best” system is selected from all the options analyzed, dynamic simulations are prepared to visually compare the best vs. worst performance across a variety of drilling dynamics including torque, contact forces, and bending moments. This process works with optimization-matrix and weighting-factor methodology by eliminating guesswork, providing organized engineering data and calculations upon which results and interpretations are based, and providing the sensitivity plots that describe the relationships between ROP, lateral acceleration, and the surface-controlled WOB and rotational rate.

As these are the only parameters that can be changed once the system is tripped into the hole, it is best to have all of the issues addressed and a procedure for optimum performance at hand. Typically, the driller on tower and the office-based engineering supervisor are briefed during the prespud planning meetings so that the ranges of parameters are clearly understood. Finally, a series of video clips are generated so that engineers can see the system in action and to confirm the choices made.

As the subject well is drilled, additional downhole and surface information may be gathered to support a post-well analysis and capture any additional lessons. Fine tuning the data-acquisition process, often through data sampling at specific rates/intervals, increases the focus on transient behavior, further increasing the ability of the model to mimic dynamic response accurately. Comprehensive data collection offers both the optimization engineer and the operator the opportunity to compare modeled behavior to actual performance and to better understand where and why they differ. The model itself may need to be adjusted to reflect differences between actual conditions and original assumptions. For example, it may be found that the lithology drilled differed slightly from the model input; however, there is additional understanding gained from this post-well analysis, and greater performance gains can be expected for subsequent wells.

Fig. 2—In the drilling assembly on the left, the red lines projecting from the drillstring show severe contact with the hole wall, while the assembly on the right experiences much lower contact forces.

Case Study

In a challenging offshore Gulf of Mexico application, engineers were asked to perform an analysis for a shallow-water development well. This operator was experiencing very high stick/slip and severe lateral-vibration problems, leading to multiple tool failures of measurement-while-drilling, logging-while-drilling, and rotary-steerable components. In the offset well, three different BHAs were required to drill the well to total depth.

The objectives of the project included identifying the causes of the vibration, designing an optimum BHA, selecting the best-performing drill bit, and specifying the optimum drilling parameters. Using information from the initial well (supplied by the operator), engineers modeled several scenarios. The operator was able to drill the target well in 8 days compared with 21 days on the offset well and was able to drill the well to total depth in one bit run as opposed to 11 bits for the offset well. The well was drilled with significant reductions in stick/slip and lateral vibrations compared to the offset well.

Information provided by Smith Intl.