Despite the fact that large fields are getting harder to find, and, when found, seem to be located in increasingly difficult places to drill, the relentless demand to reduce costs persists unabated. Typically claiming as much as 40% of the total cost of constructing and completing a well, drilling costs come under the great scrutiny in operators' efforts to maximize profitability.

Today's operators face a myriad of tough problems whose solutions will have a direct impact on cost reduction. According to Chris Kuyken, team leader, well engineering technology at Shell, "Drilling the limit is not just a slogan. It encourages our well delivery teams to execute each well with optimum designs, avoiding not only non-productive time (NPT), but also what we call 'invisible lost time,'" he said. Kuyken went on to define invisible lost time by way of an example. "We reduce invisible lost time when we are able to drill shoe-to-shoe in a single trip," he said. Eliminating an unnecessary pipe trip delivers huge savings, particularly on deep, tough holes.

Shell also likes to focus on "problems worth solving." Elegant in its simplicity, but powerful in its implications, this phrase is used to drive and prioritize the company's research and development efforts. It keeps engineers focused on practical goals that have real impact on costs, through more efficient operations, improved safety or elimination of costly procedures and techniques by finding better ways to do things.

In well construction, three such problems worth solving are drillstring integrity, hydraulics management and wellbore integrity. Each has a huge impact on NPT and invisible lost time, thereby having a major effect on well construction costs.

Seeking a solution

Several service companies are attacking the problems by offering detailed analyses to facilitate informed decisions. Their philosophy seems to embrace the idea that the best way to affect results is by "doing their homework." For this, they gather, collate and integrate every bit of data they have from seismic surveys, offset wells, drilling records, logs and cores. They employ a risk-based approach to develop a well design with the highest probability of success. The optimized well plan is complemented by relevant data obtained in real time as the well is drilled, allowing extensive dynamic optimization to take place. Often, plans are reviewed by regional experts and specialists to ensure local perspective is included. Solution sets are complex integrations of multiple skills and techniques, bundled under a service name. Though not identical in their scope or content, three similar solutions are Oasis from Baker Hughes, Perform from Schlumberger and ADT Optimization from Halliburton. In the simplest sense, each represents a proactive approach to drilling optimization. Another similar service is DrillWorks from Knowledge Systems, which is an office-based consultancy that does pore pressure studies augmented by expert wellsite monitoring services.

An example of a service solution that has been developed to solve the three key problems is Halliburton's ADT Optimization, which focuses on specialist monitoring and analysis of wellsite drilling problems. According to the company it applies unique measurements to determine actual conditions that contribute to, or detract from, drilling optimization. ADT Optimization solutions are designed, fit-for-purpose and customized for each specific well being drilled. They focus on wellsite operations to reduce uncertainty of real-time decisions, reduce unexpected costs and improve drilling performance.

Drillstring integrity

Destructive mechanical downhole forces are the root cause of problems that manifest themselves in poor hole quality, reduced penetration rates and damage to the bottomhole assembly (BHA) and drillstring. Returning to Shell's limit philosophy, one can see how each has direct or indirect effects on NPT and/or invisible lost time. Poor hole quality affects borehole cleaning efficiency and dynamic torque and drag during drilling. Subsequently, casing, cementing and completion operations can be negatively affected. These, along with reduced penetration rates, all qualify as invisible lost time because the added cost is rarely traced back to the root cause. More to the point, direct NPT results when destructive vibration damages the bit or BHA requiring a pipe trip.

By performing real-time 3-D vibration measurements at the bit and analyzing surface drilling parameters, the destructive mechanical forces can be detected and mitigated. Often the solution is simply altering weight-on-bit or rotation speed. Although effective, this solution can adversely affect penetration rate, so it must be considered in context with overall performance goals. Taking a more proactive stance, since vibration results from the bit's interaction with the formation, part of the customized approach is a rigorous pre-spud analysis that can help drillers choose the right bit and BHA design for each section of the hole, thus minimizing the likelihood or intensity of BHA vibration.

Hydraulics management

Drilling efficiency, hole quality and safety benefit from the application of dynamic hydraulic management. Starting with an engineered drilling plan that specifies the drilling fluid design, engineers use real-time pressure-while-drilling and flowrate measurements to optimize borehole cleaning and maintain equivalent circulation density (ECD) within known margins. Good hydraulics management is the key to maintaining wellbore integrity.

Wellbore integrity

Using all available pre-drilling data, Halliburton's Sperry Drilling Services engineers use a comprehensive analysis suite to calculate the pore pressure and fracture pressure. Additional geomechanical modeling developed by its partners at GMI, Inc. is employed to calculate borehole collapse pressure and fracture initiation, link up and growth pressures, defining the upper and lower pressure limits at every point of the well bore. Then they monitor actual ECD and use it together with logging-while-drilling (LWD) and surface measurements in real-time as drilling progresses to recalculate the pore pressure and geomechanical models. By comparing these actual measurements with the geomechanical well plan, a dynamic wellbore stability solution is generated. The ultimate objective is to eliminate drilling surprises that result in NPT, but an additional benefit is optimization of the drilling curve, or elimination of invisible lost time.

Another dynamic measurement that can pay big dividends is vibration. Not only does vibration have the potential to damage the drillstring, particularly instrumented BHAs, but it accelerates bit wear, leading to more invisible lost time. Energy wasted on vibration is energy taken away from making hole so penetration rate suffers. In addition, vibration is one of the major causes of poor borehole quality which affects hole cleaning as well as post-drilling activities such as running and cementing casing. To be definitive, vibration sensors must measure triaxial acceleration at the bit as well as drillstring torsional vibration. By monitoring vibration forces in real time, drillers can make adjustments on the fly that minimize vibration consistent with maintaining a high rate of penetration.

To a certain extent, drilling optimization is a three-step process. Together with customized pre-drill planning and real-time measurements while drilling is a thorough post-drilling analysis that closes the loop and can provide iterative fine-tuning that is beneficial on subsequent projects. But the greatest value comes from using dynamic real-time measurements while drilling to reduce uncertainty, prevent cost overruns and optimize drilling performance.

Comments