Stinger technology aims to optimize pre-salt drilling

Claudio Paschoa

September 1, 2014

Claudio Paschoa discusses Stinger conical diamond technology with João Pedro Tocantins, Senior Product Engineer from Schlumberger’s Smith Bits.

 The CDE, centrally placed in a PDC drill bit cutting structure, increased ROP and stability.Images courtesy of Smith Bits, a Schlumberger company.

Smith Bits’ innovative Stinger conical diamond element increases drilling speed and improves stability, showing good potential for tackling the complex pre-salt formations. This Smith Bits technology is focused on rock destruction efficiency to increase drilling performance. The Stinger element is located at the bit’s center and enables high-point loading to fracture rock more efficiently for increased durability and rate of penetration (ROP).

OE: Could you tell us what led to the development of the central Stinger?

A common challenge to conventional PDC bits is that they are inefficient at removing rock from the center of the borehole, because the rotational velocity of cutters decreases with their proximity to the center of the cutting structure; rock removal by the center-most cutters is much less efficient, especially in hard formations. Because the center cutters bear the highest load, operational and formation changes can cause large variations in depth of cut (DOC), including bit torque fluctuations.

This results in decreased drilling efficiency at the center of the bit’s cutting structure, which can cause low ROPs, destructive lateral vibrations, and cutter damage. To decrease damage and increase performance at the center of a PDC bit’s cutting structure, bit design engineers conceived a center-placed conical diamond element (CDE).

To position the element in the bit, engineers removed the center cutters. The absence of these cutters allows a stress-relieved column of rock to develop while drilling, which continuously fractures and crushes, thereby improving drilling efficiency. The Stinger technology is a new line of PDC drill bits that incorporate the conical diamond elements called Stingers in the center of the bit. This product was intended for applications where impact damage to conventional PDC cutters is prevalent–For example: the pre-salt formation offshore Brazil–and the main limiter for performance.

OE: Are PDC bits still dominant in the drilling market?

Yes, with continuing advances in synthetic diamond cutter technology and improved bit stability, PDC bits have become the dominant force in the worldwide drilling scheme. In 2004, total footage drilled by diamond bits (54%) surpassed that of roller cone bits (46%) and the trend has continued. Due to its reliability the system Turbine + Impregnated bit have been used quite frequently in the pre-salt formation offshore Brazil.

OE: Has thermal damage when drilling in highly abrasive formation been decreased by using the PDC bits with conical diamond Stinger element and why is it less susceptible to this kind of damage?

Yes, this happens because the PDC bits with conical diamond Stinger element fails the rock through crushing action. The load is concentrated at a single point, which significantly reduces the required load to fracture the rock. The effect of this single-point loading is compounded with a Stinger element because the column of rock that the bit allows to develop is isolated and thereby unconstrained, making it easier to destroy. The unique combination of the Stinger geometry and its cutting action reduce frictional heat resulting from improved wear resistance. The enhanced stability provided from the center-placed element will deliver a better load distribution throughout the cutting structure, which will decrease and share the wear evenly among all cutters.

This plot shows typical forces and cutter velocity, from bit center to the gauge. The center. Most cutters experience the highest loads and have the lowest rotational velocity, subjecting them to more stress.

OE: Are the PDC bits with conical diamond Stinger element more resistant to impact damage in non-homogeneous formations and formations with chert and pyrite inclusions?

Yes, once PDC cutting elements are worn or damaged, ROP drops and ring-out on the bit body may occur; a bit trip is therefore necessary. The Stinger element proved to have superior impact resistance when compared to PDC cutters. The CDE placed in the center of the bit increases bit stability and decreases vibration. This will provide a better protection to the cutting structure of the bit and increase durability. This feature will prolong the bit’s cutting structure life; therefore, the bit will drill longer footage and reduce the number of trips.

OE: In the heterogeneous carbonates formations, common to Brazil’s pre-salt, what are the main challenges faced by drill bits?

The main challenges found in drilling throughout the pre-salt carbonates are the low penetration rates, poor bit cutting structure durability and additional problems related to drilling dynamics, such as torsional and lateral vibrations.

OE: How do the ROP of PDC bits with conical diamond Stinger element compare to that of other PDC and impregnated bits when drilling in Carbonate formations?

The ROP is a lot higher compared to an impregnated bit. Comparing to a conventional PDC bit, the ROP will be also higher because the PDC with the Stinger element will deliver a more aggressive cutting mechanism at bit’s center. Also the ability to keep the cutting structure alive and sharp in high weight-on-bit (WOB) applications for a longer period of time will aid in improving the aggressiveness of the bit (improved ROP) and help reach section total depth (TD) in one run.

OE: What are the cutting mechanisms found in PDC bits with conical diamond Stinger element, which allow them to continue cutting when encountering nodules impregnated with hard silicate inclusions?

Crushing (Stinger) and Shearing (PDC) action. Stinger and PDC cutting elements can provide independently complete bottom-hole coverage. The PDC bit with a Stinger element will deliver a more stable cutting structure design that will be less vulnerable to lateral and torsional shocks and vibrations. The Stinger element is expected to absorb or share the impact loads while encountering very hard layers or inclusions and protect the PDC cutters from catastrophic failure.

OE: How do the hydraulics work and what are its advantages?

Placing the Stinger element in the center of the cutting structure makes it essential that nozzle orientation and the resulting flow field are optimized. This will ensure the Stinger element is efficiently cleaned and cooled. A detailed hydraulic analysis was undertaken using advanced computational fluid dynamic (CFD) software to accurately simulate the flow around the Stinger element (cross flow). In the course of each new bit development, nozzle positions are adjusted and fine-tuned to maximize cleaning of the Stinger element and the bottom hole around the bit’s center.

OE: Could you tell us about your experiences with the bits in different Brazilian pre-salt formations?

The primary reason behind using the PDC bits with conical diamond Stinger element technology was to face the hostile environment from pre-salt reservoir rock, whose composition of organic microbiolites carbonates and other sediments could possibly offer different heterogeneous characteristics such as the presence of hard silicate nodules and low porosity layers. We have tested one unit so far in Campos basin offshore Brazil. Overall result was very promising showing a huge potential for this technology. The Central Stinger design drilled 234m until section TD throughout hard limestone with an average ROP of 15.4m/h setting the highest ROP and the lowest cost per meter in the field. We have learned a lot throughout this run and have been modifying latest designs applying lessons learned in order to optimize cutting structures and improve performance. Further tests into the pre-salt environment, this time offshore Espírito Santo basin, are ongoing at this very moment.

PowerDrive was used for CDE runs.  Image courtesy of Schlumberger.

OE: Can you tell us about the different drive systems and drilling parameters used with the central Stinger in Brazil’s pre-salt?

The PDC bits with conical diamond Stinger element design drilled the second part of the 8 ½-in. section with excellent performance, presenting minimal wear after drilling 234m at an average ROP of 15.4m/h. Vibration levels were kept within an acceptable range throughout the run. The changes in lithology encountered were effectively drilled at high rates, since bit cutting structure was kept sharp until the end of the run. The PDC bits with conical diamond Stinger element design was used with a rotary steerable BHA and the following parameters were used:

OE: What were the lessons learned from drilling with the bits in Brazil and how have operators reacted to the results?

The main lessons were related to hydraulics, bit design, cuttings quality, stability and vibration (S&V) and the need for more impact resistant cutters. The most important knowledge gained from drilling runs can be summarized as:

Hydraulics: Maximize HSI (recommendation: HSI > 2.5hp/in2) to ensure good bottom hole cleaning, better cutter refrigeration and cutting structure durability.

Bit design: The center-placed Stinger element has proven to increase stability and reduce vibration levels; The CDE also has proven to improve drilling efficiency at bit center, increasing the ROP; All versions tested presented minimum wear in the cone area.

Cuttings: Good cuttings size generation confirming that the conical diamond element was crushing the rock into larger-than-normal fragments (desirable feature for geology evaluation).

S&V: The PDC bits with conical diamond Stinger element design clearly showed better dynamical behavior than conventional PDC bits.

Cutters: Impact damage was the main failure mode, therefore bigger bevels and cutters with more resistance to frontal impact load should be considered.

OE: What future developments do you foresee in PDC bits with conical diamond Stinger element technology and its uses?

Currently there is an ongoing project to investigate different uses of Stinger elements in new concepts of drill bit cutting structures. We are also investigating the use of different shapes in Stinger insert to further improve side impact resistance over conical Stinger inserts while maintaining aggressiveness and also the use of high frequency downhole measurements for the next iterations. Reducing the development cycle and identifying optimum parameters and procedures are also necessary along with working to improve the durability on Nose/Shoulder area.

João Pedro Tocantins
is a Senior Product Engineer for Smith Bits, a Schlumberger company, located in Brazil.  His focus includes continuous product development for Brazil, drilling optimization and technical support for all bit types. He joined Schlumberger in 2006.  João Pedro Tocantins earned his Bachelors in Civil Engineering from Pontifícia Universidade Católica do Rio de Janeiro (PUC-Rio) and his Post Graduate Diploma in Petroleum Engineering from Pontifícia Universidade Católica do Rio de Janeiro (PUC-Rio).