An essential part of the drilling process is adding force to the drill bit in order to successfully break the rock. Weight on the Bit, or WOB, is the amount of downward force exerted on the drill bit provided by thick-walled tubular pieces in the drilling assembly that are known as drill collars. The downward force of gravity on these steel tubes provide force for the drill bit in order to effectively break the rock.
The weight of the drilling assembly is controlled and measured while the drill bit is just off the bottom of the wellbore. Then, the drill string is slowly and carefully lowered until it reaches the bottom. As the driller continues to lower the top of the drill string, more of the weight of the assembly is being applied to the bit and harmoniously less weight is hanging at the surface.
Weight on bit is an essential part of drilling optimization to ensure that the well deepens as drilling moves forward. Finding the right amount of WOB per application is crucial to drilling operations. If the WOB is greater than the optimum value, the drill bit has a higher chance of wear or damage and there is even a chance for the drill string to buckle. On the contrary, if the WOB is less than optimal, the Rate of Penetration (ROP) slows down and drilling performance is subpar. The ROP is the speed at which a drill bit breaks the rock or sediment; ROP is typically measured either in feet or meters per hour.
It is important to maximize the rate of penetration to reduce rig time and cost. In order to optimize penetration, drilling operators must pay close attention to Weight on bit and alter it as necessary. Finding the optimum WOB is determined by the design and parameters of the drill bit, as well as external factors such as mud weight, BHA, and the rock being drilled. There is no standard range of weight that should be applied to the bit. It can be anywhere between 1,000 lbs. to 100,000 lbs. depending on the size and type of bit, the rock being drilled, and the application. At Ulterra, recommended values for WOB to the customer are based purely on local knowledge and experience of the application.
FINDING OPTIMUM WOB & ROTARY SPEED
It is important to select the best bit weight and rotary speed to optimize the drilling operation, minimize cost, and increase bit life. The drilling environment, such as the lithology of the rock and drilling dysfunction, impacts the drilling conditions and can have a negative effect on drilling efficiency. Rotary speed and weight on bit can control vibration and ROP. It is important to be in control of drilling vibration in order to keep the bit in smooth contact with the rock, prevent damage and maximize efficiency by reducing wasted energy.
A minimum WOB must be achieved in order to get the drilling started, which is considered the threshold weight. There are average values that have been determined for drilling weights, but proper weight can be determined for each application by increasing the bit weight in steps of 1,000-2,000 lbs., with an optimized rotary speed. Optimum weight has been reached when additional weight is not providing further penetration and the bit starts to founder.
Ulterra assembled a team of material specialists, design engineers, and performance optimization experts to create a PDC bit platform that was superior to both traditional matrix and steel PDC bodies and that would allow the cutters to get deep into the formation to increase ROP. This team of experts ended up creating the FastBack™ series of bits, which are designed to drill faster with lower WOB. FastBack is designed to get the bit body out of the way so that the drilling is focused on the sharp, diamond edge of the PDC cutter. The energy provided by the PDC cutting structure in these designs requires less WOB while still providing a greater ROP than traditional bits.
FASTBACK™ TECHNOLOGY BY ULTERRA
Tungsten carbide, diamond, unique alloys, high tensile treated steel. PDC bits are made up of some of the most extreme materials on the planet. However, radical properties often require big trade-offs in design and manufacturing that affect performance.
Ulterra decided to change the game, and set themselves the challenge of creating a PDC platform that was superior to both traditional matrix and steel bodied PDC bits. Instead of limiting PDC bits by the weakness of the materials, FastBack aligns their strengths with each part of the construction enhancing the others.
FastBack bits utilize superior materials and engineering principles, we can design the bit in a way that focuses more of the drilling energy on the PDC cutters themselves. The most obvious visual effect of this is the tall, thin, sculpted blades ofa FastBack bit, a feature that is only possible through the use of the most advanced materials technology in the business. Traditional PDC blade geometry uses wide blades which engage with the formation at high ROP and high WOB, leading the ROP to founder. With FastBack we get the blade out of the way and bury the cutters deep into the formation.
Traditional PDC blade geometry uses wide blades which engage with the formation at high ROP and high WOB, leading the ROP to founder. With FastBack we get the blade out of the way and bury the cutters deep into the formation.
- Mechanically supports the PDC cutters in a minimalist fashion.
- All energy focused on sharp, diamond-cutting surfaces.
- No energy wasted pushing flat blade surfaces into the formation.
FOCUSED DRILLING ENERGY
Today’s drilling environment is focused on higher parameters and increased efficiency by eliminating factors which get in the way of increased performance. FastBack has the ability to drill even faster in the higher WOB ranges, using world leading materials and engineering technology Ulterra has designed a drill bit that has the ability to extend the limit of performance. In addition the faster we drill, the more cuttings need to be evacuated from the bit face, if not then performance starts to founder from cuttings build up. Since FastBack technology allows thinner blades with a higher blade standoff, the open face volume of the bit is much higher allowing for greater hydraulic efficiency and better evacuation of cuttings.
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