This originally appeared in World Oil Magazine OCTOBER 2019 issue, used with permission.
TRANSITIONAL DRILLING CHALLENGE
One of the biggest challenges facing the drilling industry is interfacial severity, or transitional drilling. This is broadly defined as layers of rock with different hardness that require the bit to drill through the transition, putting cutting forces on different parts of the profile. The imbalance of force concentration can induce vibration or even cause cutter overload, both of which lead to severe cutting structure damage.
Historically, bit companies would drill transition zones using heavy set bits with secondary features to control and limit the amount of force going through the cutting structure. They would also reduce drilling parameters to lessen force and “protect the bit”; a practice that has subsequently been proven to cause even more damage, because reduced weight or rotary speed can reduce the stability of the entire drilling system.
To solve the problem, an engineering team at Ulterra took a different approach. Bit designers developed an active cutting structure, called RipSaw™, which regulates cutting forces, redirecting and absorbing force to increase stability, Fig. 3. This is achieved without resorting to secondary features or high cutter densities, both of which can limit performance in other parts of the section. RipSaw’s patent-pending technology works like a shock absorber, allowing movement and high engagement, but preventing damaging shock and impact. Ulterra’s bit designers achieve this balance by using highly-engineered cutter positioning on different parts of the cutting structure. Depth of cut is regulated, but because all the cutters are fully engaged with the formation, work is optimized, and the bit continues to drill ahead efficiently.
West Texas Case Study
Ulterra first applied RipSaw in the extremely challenging 12¼-in. intermediate section in West Texas. The hole section is well-known as a definitive transitional application. The upper part of this section is relatively easy to drill, where the bit is pushed hard, and ROP is high. However, in the lower part of the section, the bit encounters hard and interbedded lithology, and the strategy switches to survival, trying to keep the bit on bottom as long as possible.
RipSaw bits have drilled almost 2 million ft of hole, on approximately 450 separate runs in this application, as part of an extensive testing program. Ulterra engineers ran a wide-ranging, macro-level field study of these bit runs and compared them to approximately 2,800 offsets. Examining this type of large data set eliminates the effect of outliers, both good and bad, while providing an objective review of performance and reliability over an extended period (six months).
Some of the most meaningful statistics to come from this study related to the first bit run on each section. This first run is crucial, since the deeper the first bit drills, the less work the next bit must perform to finish the section, increasing the chance of a more economical section cost. From the start, RipSaw technology increased the chance of scoring a shoe-to-TD run 28%, compared to other-type products.
A unique analysis method was used to create statistical risk profiles, which enable field engineers to model the probability of performance, based on the available data set. In this case, a risk profile was created for the first bit in every section, and how deep that bit drilled before it was pulled. Using this method, Ulterra determined that RipSaw bits had a 50% chance of hitting 7,000 ft on the first run out of the shoe. This compared favorably to other-type bits that had less than a 20% chance of hitting the same depth. RipSaw’s inter-quartile range was 2,000 ft deeper than other-type bits, and the median average run for RipSaw would have been an upper quartile run for any other product.
In other analyses, the industry-standard methodology was used to calculate bit run economics. Looking at all the runs combined, it was evident that RipSaw delivered exceptional value for the money, with a 39% improvement in cost/ ft compared to other bit types. This was achieved by drilling further and deeper and maintaining ROP compared to the average. Applying cost/ft for other-type bits to the total footage drilled by RipSaw theoretically saved operators a combined $74 million over this six-month period.