Reducing Drilling Costs by Drilling Faster
Authors: Bland Ron , Jones Tom , Tibbitts G.A. 
Published: December 2007

A new synthetic-based Penetration Rate Enhancer designed as an environmentally acceptable alternative to terpenes is shown to be an effective treatment for increasing penetration rates by preventing or minimizing bit balling while drilling active shales prone to forming "gumbo". Both laboratory drilling simulator results and field case-histories are presented documenting the effectiveness of this chemistry and efficient techniques for application. The results show that this chemistry is effective with tri-cone bits but is more efficient with polycrystalline diamond cutter (PDC) bits and is particularly efficient when used with advanced design polished cutter bits with super sharp PDC cutters.

Introduction

Stagnant oil & gas prices coupled with escalating rig rates have placed an ever increasing premium on reducing drilling costs. The most significant component of the total drilling cost is typically the rig operating cost which makes drilling rate one of the key variable in determining a project's economic viability. One of the most significant developments toward maximizing penetration rates over the last 2 decades, particularly drilling shales, has been the introduction of PDC bits. These bits have set new penetration rate benchmarks when drilling shales, especially when used with oil and synthetic-based emulsion drilling fluids. PDC bits used with water-based drilling fluids have not performed nearly as well, especially in drilling deeper shales. The problem has proven to be so persistent that the Drilling Engineering Association formed cooperative research project DEA #90 in 1995 to investigate the causes. The initial findings of DEA 90, as released in Test Module #1, duplicated slow ROP scenarios seen in actual drilling operations. Low penetration rates with PDC bits in plastic shales, drilled at high bottom hole pressures with water-based drilling fluids were ascertained, along with mechanisms causing the slow penetration rates.

PDC Bits and Shale

PDC bits have traditionally been suited for drilling in shales.

These bits become less effective, and possibly uneconomical, as certain shale types become stressed from increase in depth, mud weight, and cutter induced loading.

Normally, PDC cutters easily shear shale as they drill. Shearing generates formation chips, which can be removed from under the bit face using conventional hydraulics. As the pressure stresses increase at depth, the formation becomes more plastic and requires different bit cutting mechanics to cut efficiently. This is accelerated as fluid density increases.

Shale, and other ductile formations, tend to flow more easily at stress and thus conform to, and adhere more strongly, to surfaces they contact. In effect, the shear stress necessary to displace the cuttings from the face of a cutting element increases significantly under these conditions. It is believed that the shear stress required to displace a cutting from the face of cutter may be higher than the stresses necessary to shear the formation in front of the cutter. This phenomenon has been documented in the literature as a "built up edge". As the built up edge remains relatively stationary on the face of the cutting element, it becomes a tough, solidified agglomeration of formation, through shear enhanced compassion and hydration.

The built up edge forms a very dull geometry, which hinders the performance of the cutting structure and results in unacceptable penetration rates. The concept of generating lower volume chips (thinner) and lower frictional forces on the cutter face, has proven to be a combination of technologies which significantly improves the drilling rate when drilling shales that typically result in low penetration rates.

By polishing the surface of the diamond cutting element, the shear stress to displace the formation chip is drastically lowered. In addition, the rock strengthening effect that often occurs in front of a cutting element is avoided, allowing the formation to be failed at lower bit loads. It is also advantageous to concentrate the highest cutting stresses where the formation stress is initiated.

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