Abstract

Steel body bits have dominated the soft formation PDC market since their introduction in the 70's, primarily because of the advantages of the higher yield point of steel. Using steel body bits affords design benefits that include high blade standoff for increased face volume, and thinner blades for greater junk slot area. However, steel body bits are susceptible to erosion under conditions where high flow rates and hydraulic horsepower per square inch (HSI) are required.

Matrix, the industry's "second choice" of material for bit manufacture, offers ideal erosion resistance, but is very brittle. Matrix bits with a high blade standoff and insufficient mass (blade thickness) can result in premature failure by loss of a blade downhole, with associated round trips and junk fishing. High blade standoff with greater blade thickness, on the other hand, reduces junk slot area and face volume, inhibiting cuttings removal and reducing rates of penetration. And too low a blade standoff reduces face volume, causing poor cleaning that may result in bit balling due to formation packing.

In Western Venezuela, conventional PDC bits historically have shown little likelihood of replacing roller cone products in soft formation applications, where high flow rates and HSI are required to optimize well bore and bit cleaning. The primary challenge to effective use of PDC bits in these formations is balancing hole cleaning with erosion resistance.

A case history comparison of offset well performance and results obtained with a "Novel Matrix" drill bit design reveals how incorporating a steel "paddle" within the bit blade compensates for limitations associated with matrix as explained above. The "novel" bit is shown to provide an excellent alternative to steel bits that are susceptible to extreme erosion in high flow rate applications.

Additionally, other major design features of the unique bit are discussed, including hydraulic nozzle configuration and variable cutter backrake. These features have been identified as key contributions to the successful introduction of the "Novel Matrix" bit into a soft formation application which previously required three to four roller cone bits to drill.

Background

Smith, et. al., revealed that approximately 60 to 80% of the formation drilled in the oil and gas industry is shale or shaly type formations which are mechanically weak and easily penetrated. Challenges to drilling efficiency arise from the capacity of shale to absorb water and swell, causing stickiness that binds cuttings to the drill bit. This phenomenon, known specifically as "bit balling," reduces the rate of penetration (ROP) of the bit, allowing formation cuttings to accumulate and/or packoff, and reducing drilling efficiency.

Koskie, et. al., presented that conventional PDC bit designs have severe problems with balling in these formations, which make their use impossible unless inhibited by drilling fluids, such as oil base muds, are employed. The use of oil base muds; however, has its attendant problems. These muds are economically higher in cost and maintenance. Additionally, they require increased environment safeguards for their use, storage and disposal.

Introduction

Development of the first cemented tungsten carbides date to the 1915 (Stellite), which used a cobalt binder and tungsten carbide mix. Succeeding decades saw progressive advances in the development of cemented tungsten carbide as a result of a greater understanding of such factors as grain size and distribution, binder content, additives, manufacturing processes, and coatings.

To the drilling industry, the advantage of cemented tungsten carbide is its wear resistance and its hardness which is surpassed only by diamond and ceramics.

The Composite Structure

Matrix (a mold in which something is cast or shaped) is composed of a cemented tungsten carbide powder and binder material, thus creating a composite structure. The binder, as implied, binds the tungsten carbide grains by filling in the voids.

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