If high oil prices weren’t enough to drive increased exploration and production activity in 2005, improved drilling technology was. Some experts now estimate the average footage drilled per rig has increased more than 12% over the past five years. Ironically, this has come despite an industry-wide trend toward drilling deeper and more difficult wells as operators explore smaller and smaller reservoirs in harsher environments. While general rig footage trends are moving higher, total footage drilled appears to be increasing faster, up 10% from 2004 and an estimated 100% increase from a few years ago. Key technical advances in drill bits have sparked much of the increase in activity along with other innovations in the drilling process.
Old Drivers with a New Twist
The art of “Makin’ Hole” isn’t what it used to be due to the industry’s technology advances on many fronts. Drill bits were once considered the weak link in the drilling process but continued innovation has led to major breakthroughs. Higher hydraulics, rotary speed, torque, and weight on bit (WOB) are now the limiting factors to even better performance, even though average footage per bit has increased dramatically in just two years.
The advances in drilling have proved a benefit to both contractors and operators. Contractors are able to charge higher day rates for better technology and operators are willing to pay if they can more quickly and efficiently monetize prospects. Price pressures from high activity levels have put a premium on drill bit performance though. Improved drilling efficiency has become a buffer to help offset the escalating cost of drilling equipment and components in a boom environment.
Despite technology inroads, one thing hasn’t changed. Many of the fundamental drivers guiding improvements in dill bits are still in place. Operators and contractors agree that economics remains the number one criteria for bit selection. Cost-per-foot is still a good overall measure of value, but actual bit price is much less important today. As the cost of exploiting has increased, bit costs as a component of total well costs have moved lower, and they now represent even a smaller percentage of a well’s expenditures. Some estimate these costs at less than 10% onland and almost minuscule offshore. The performance of bits, however, directly impacts total economics and well profitability. A bit’s ability to efficiently chip, crush, grind or shear formations is dependent upon a host of interdependent factors including stability, hydraulics, design, fluid systems, BHA, rig capabilities, WOB, etc. Its ability to deliver economic solutions is measured by many performance dimensions that affect a well’s overall drilling costs and the productivity of the well afterwards.
Rate-of-Penetration (ROP) Saves Time
ROP might be the most important challenge in the drilling process. The faster operators can drill wells, the lower their expenditures and day rate exposure. The faster they can reach target or complete a section, the less time and money is spent on rig costs, maintenance and support services. The faster a bit is able to drill ahead and reach TD, the quicker production comes online and investment costs recovered.
Many factors influence the ROP, including rock properties, borehole and pore pressures, mud properties, bit design, cutter wear, bit hydraulics and other operating parameters. ROP can be reduced by the occurrence of many different drillability problems such as bit or drill string vibrations, bit balling, trouble zones, lost circulation, and underpowered drive systems. To counter this, ROP has been enhanced by numerous bit design and material changes along with other innovations in drilling components and systems. Speed (ROP) is maximized when all these drilling components are properly matched and work in unison with the bit for the application and the formation being drilled.
Durability Means Finishing
Bit durability is an equally important performance dimension. How fast a bit drills doesn’t matter much if it doesn’t last long. In many applications, the critical factor is total drilling footage, which is a factor of both ROP and bit longevity. Higher rates of penetration combined with durability mean there are fewer trips in and out of hole to change bits. This translates into less downtime and more productive drilling. With a durable, long-life bit, sections are often completed in one continuous run. TD is reached without delay. Targets are reached on time. Longevity means the operator gains more footage in a run, again saving rig time and all the associated expenses. ROP and durability are true value components of drilling bits, but there are other emerging parameters now considered in the bit selection process.
Steerability Improves Aim
Drilling has become more precise as operators seek exacting targets or try to pinpoint an overlooked zone for further extraction in a producing field. The ability to control and accurately steer the bit has thus become an important element in the drilling equation. As a result, the industry has seen an evolution in bit design, mechanics and material applications to augment directional control.
Directional drilling is usually required when a wellbore needs to be drilled at some angle other than vertical. These are frequently offshore wells, but a large portion of land wells are now drilled directionally to produce a field more efficiently. Additionally, wells drilled vertically through highly angled rock formations will use directional services just to keep the hole straight.
The advent of rotary steerable technology has had a major impact on directional drilling and related bit requirements for performance. New Rotary Steerable Systems (RSS) are now better able to manage torque control, one of the most important bit failure mechanisms. This allows the bit to drill flatter on bottom thereby reducing vibration and mitigating damage to other drill string components. Better tool face control has also improved steerability. A better understanding of what tools and bit design are required to drill more accurately has improved aim.
Another contributing factor to directional competence has been improved cutting structures and placement. Many of these have been optimized to consider formation strength and downhole motor or system characteristics. Bits are additionally being designed with better gage protection to extend life and improve steerability in demanding applications. The ability of operators to steer a bit precisely is now an important consideration in many applications and another value component considered.
Hole Quality Helps Production
Today, it’s not enough to drill a fast, precise and if necessary, long hole to TD. Hole quality has also become an important factor to not only enhance the drilling process but also for completion and production benefits that accrue in activities like gravel packing or setting casing. Hole quality often reflects bit stability and it is particularly important in the growing number of complex well profiles.
Wellbore or hole quality is generally associated with a directionally drilled borehole that is smooth, in-gage and has minimal spiraling. The benefits of such a wellbore include improved drilling performance with reduced torque and drag, ease of running casing or a completion string, better logging tool response, and highly successful cement jobs. Spiraling is the primary cause of poor hole quality so many recent improvements in drill bits are designed to eliminate it. The elimination of spiraling enhances hole cleaning and helps maintain gage. A non-spiraled, high quality hole will have a drift diameter equal to its gage, presumed to be the nominal bit diameter.
If it exists, the perfect wellbore is a flawless, three-dimensional hollow cylinder with a smooth, frictionless finish so that casing or a completion string, if required, can be easily run to depth within it. The ideal wellbore brings many associated benefits such as virtually zero torque and drag. The elimination of frictional drag benefits extremely high step-out extended reach or very complex three-dimensional wells that could be drilled with minimal mechanical loads. A common view is that a perfect wellbore is associated with excellence in directional drilling. The recent maturing of 3D rotary steerable technology in particular has led to renewed interest in wellbore quality and its benefits. Additionally, the growth in LWD applications requires better hole quality to obtain good log data. Here, vibration control is important to avoid drilling oversize holes that reduce log quality and damage expensive LWD and other downhole equipment. Hole quality affects the well’s economics in many different areas.]
