As exploration and development ventures into frontier drilling environments and new rig equipment, BHAs and operator capabilities push technology limits, drilling bits face a continual challenge to meet performance expectations in different applications and formations.
According to Gary Weaver, Manager, Technical Marketing for Security DBS, “The drill bit market definitions are changing as customers increasingly demand not products, but solutions to their drilling challenges. We refer to these challenges many times as the customer’s ‘Pain Points’, which literally translates into significant costs savings opportunities during the construction of the well.”
More and more operators are willing to pay premium prices for these technical solutions. The solution, however, requires the right balance of bit ROP, durability, steerability, and hole quality in the right application. Keeping these key bit performance factors in balance requires continuous improvements that keep pace or lead advancing capabilities in drilling technology. Three key future challenges for bits are Directional Drilling, Hard Rock, and Emerging Applications.
Directional Drilling Escalates
Industry data indicates Directional Drilling has shown a 17% compound annual growth rate. Rotary Steerable Systems (RSS) have added to the directional services market, and now drill about 14% of the directional footage representing close to $1billion in sales. It has been estimated the typical Gulf of Mexico well has seen drilling days fall by 50% due to the use of RSS. With no change in days required to complete the well, RSS may have reduced days over the hole by about 25%.
Steve Berkman, Marketing Manager ReedHycalog states, “Rotary steerable systems have provided a significant challenge to drill bit manufacturers. From just a handful of systems which utilize a push-the-bit configuration, today over a dozen companies are marketing RSS with a variety of push-the bit, point-the-bit, and push-to-point geometries. These unique methodologies require bits that match the rotary steerable system utilized to optimize drilling and directional performance.”
RSS systems present a challenge to bit technology because they place greater wear loads on the side of the bit requiring design and cutter adjustments. Complex drilling applications also require high durability so it’s a continual “catch-up” loop for bit manufacturers. More rotary steerable, horizontal and directional drilling demands that bits are more controllable and “assembly friendly” to reduce torque responses, etc. With more side loading, these advanced systems also require bearing improvements on roller cones and gage modifications on diamond bits. In net, bit technology must continually respond to the equipment upgrades, especially with costly drilling operations that place increasingly high premiums on speed, reducing risk and minimizing non-productive time.
Hard Rock is “Two-Faced”
Another unique challenge has been defined as “Hard Rock”, not just high compressive strength formations, but also denoting sections or sequences that are problematic to drill. In many drilling budgets, 20% of the footage requires 80% of the cost because of problem areas such as tough drilling at the bottom of a well, or abrasive, inter-bedded, or hard rock formations.
High rock abrasivity requires special cutters with high resistance to wear and heat without losing resistance to impact damage. The industry’s current focus on drilling for natural gas symbolizes this challenge. High pressure, deeper depths and harder, more abrasive rock raises drilling costs so bit performance is even more important. These applications often require “hard rock” bits “matched” with RSS targeted at reducing non-productive time (NPT). If a system is matched appropriately, the savings over an entire interval can be great enough to overshadow the cost of an individual bit.
Previously, hard- and/or abrasive-rock drilling was reserved for impregnated and roller-cone bits. R&D efforts still focus on these bit types to improve ROP and durability in this environment. One of the biggest challenges today, however, is the extension of PDC bits into this heretofore reserved geologic domain. Heat and impact damage along with abrasive wear has historically limited PDC performance. Recently, there has been more focus on better understanding the cutter/formation interaction, cutter performance, bit dynamics and BHA dynamics. As a result, the performance of PDC bits in these applications has become more effective and highly consistent. In many applications around the world, PDC bits continue to replace impregnated and roller-cone bits simply because they are drilling faster and longer.
The Emerging Contests
There are a growing number of applications that continue to present drilling bit challenges, especially as operators push for accelerated times to completion. As mentioned previously, steerability is one of them. For optimum performance, specific bits must be compatible with the drilling environment, formation and system capabilities. There are other emerging drilling applications that present opportunities for bit technology advancements.
• Hole enlargement where the customer challenge is to increase recovery factors. There is an accelerating need for further development of eccentric bits and hole enlarging tools to increase hole size and flow rates in deep (and deepwater) wells. This is especially true where Equivalent Circulation Density (ECD) control is important to prevent fluid (and hole) loss. Other hole enlargement applications include the need to add clearances when running liners, casing drilling, and adaptations to rotary steerable systems. A major operator in Norway is enlarging 75% of the footage on current wells, and expects this to go to 100% when monobore expandable technology becomes proven. Ream-while-drilling tools for hole enlargement are also growing in popularity. There is strong development moving forward with a concentric hole opening market and there is expanded demand for bi-center bits to be used in hole opening solutions and to drill-out casing. It’s estimated hole enlargement activities could now account for 10% of the total footage drilled, making this niche market ripe for more drilling bit innovations.
• Hot temperature/hot pressure (HT/HP) holes that require constant vigilance and the use of thermally stable materials for best performance. In particular, this relates to offshore deepwater applications where HT/HP concerns are commonplace. Deep drilling and extreme drilling continue to escalate and present unique challenges. PDC bits typically drill several times faster than tricone bits but still have some shortcomings when drilling in extreme environments.
• The greater use of high powered motors. Over the last 15 years, the horsepower and torque available from Positive Displacement Motors (PDMs) has tripled in some cases. New “super” motors now permit greater WOB and RPM, further pressing the limits of material strength and bearing load carrying capabilities on roller cone bits. Turbine drilling and other high-speed motor and directional applications for specific environments also test bit endurance on a continual basis.
• The increased use of top-drives. These offer a further advantage from a drilling standpoint with accompanying demands on drilling bits. With a wider range of RPM, top drives produce greater torque and the capability to make three joint connections at a time. Both of these attributes speed the drilling process. Top drives are now almost always used on offshore rigs and a growing percentage are used onland (10-20% estimated).
• Hole instability, lost circulation, getting casing to total depth, and drilling into trouble zones. These are all endemic challenges drill bit technology must address. Sometimes the problems are actively controlled by drilling the liner and casing down through the trouble zone, either in one operation or after the hole has been drilled...another growing application.
• The need for bits to drill longer intervals with varying geology. Whether it’s softer rock with hard stringers or harder rock with soft stringers, varying lithologies present a unique challenge in bit design, materials and cutter placement and several bit manufacturers have specifically designed bits for these circumstances.
Mother Nature Still in Charge
The IADC publishes six principal formation categories and within each category are myriad formations that vary in compressive strength, abrasion and drillability. The objective of any drilling programs is to match the bit to the formation. Unfortunately, there is very little consistency in drilling across the world and meeting the different demands. By definition, Mother Nature is quixotic with such vagaries as formation layering and inter-bedding and the dreaded hard stringer. Very seldom does a bit encounter only the formation anticipated. In the past, it was not atypical to put a new bit in the well every time the formation changed. Now bits must drill thru multiple lithologies and work well on the drilling and mud system used. Performance of the bit is different in different rock types and failure is not always the bit’s fault. Operators and bit manufactures are working diligently to develop worldwide databases that better align drilling systems, bits and formations everywhere.
External Factors Also Impact Performance
There are other external factors also affecting drilling performance not directly related to the formation or the bit and its design, cutters or materials. One of these today is the operator’s lack of experienced personnel. There are many reasons for this including the aging workforce, supply and demand in a boom period and repercussions from prior down cycles that saw a decline in petroleum engineer enrollment. The fact is there’s not enough qualified engineers and drilling staff to keep pace with activity levels. Operators are relying more for this expertise from service companies who often find themselves in the same quandary.
Another limiting performance factor is non optimum drilling parameters used in many locations, especially onland where the quality of rigs needs to be upgraded. Typical rigs are underpowered from a rotary and hydraulic standpoint. The flow rates or pressures needed for torque are simply not available and many bits are under utilized as a result.