George A. Cooper

Wear and failure mechanisms of polycrystalline diamond compact bits

Abstract Four mechanisms of polycrystalline diamond compact cutter failure are identified in a detailed study of cutters damaged in both laboratory rock-cutting tests and field operation. One mechanism is smooth wear. This occurs when individual diamond grains are polished away by a combination of high mechanical and thermal loads. A second mechanism is microchipping of the polycrystalline diamond table. This is caused mainly by the action of the bit cutting forces. A third mechanism is gross fracturing. This is caused by the application of excessive normal force to the bit. A fourth mechanism is delamination at the interface between the diamond table and the cemented tungsten carbide substrate. This results both from impact loading in the bit normal-force direction and thermal stress build-up at the interface between the diamond layer and carbide substrate due to the mismatch in the thermal expansion coefficients of these materials. All these processes lead to the formation of a wear flat beneath the cutter. Once this flat forms, repeated frictional heating and cooling cause thermal fatigue which results in the commonly observed heat checking on this surface.

Fatigue test on polycrystalline diamond compacts

Abstract We have carried out compressional fatigue tests on notched polycrystalline diamond compacts (PDC). Fatigue cracks were seen to grow in both the sintered tungsten carbide and the polycrystalline diamond parts of the compact. The cracks in the cemented carbide were very fine and sharp, with an unstressed opening of about 1 μm. That the cracks propagated under the far-field compression was mainly a result of the induced residual tensile stresses that arose upon unloading from the compressive load. These cracks grew at a decreasing rate away from the starting notch, no doubt because compressive load was transferred directly across the crack faces by crack closure during the (compressive) loading part of the cycle. The fact that the fatigue crack was propagating into a region of greatly reduced stress may also have contributed to the deceleration in crack propagation. The cracks in the polycrystalline diamond, however, were relatively wide. At the start of crack growth, we believe that failure of the diamond occurred under mainly uniaxial loading, as happens in the case of borehole breakouts. This is characterized by the formation of spalled flakes whose long axes are nearly parallel with the direction of the applied load. As the crack deepened, the size of the fracture zone decreased to an approximately constant width of about 30 μm. Failure at the root of the crack was then more probably in multiaxial compression, in which the diamond grains were crushed and disintegrated over a certain volume, resulting in the detachment of fragments of diamond, and the maintenance of a wide crack.

Bit Balling Reduction by Electro-Osmosis While Drilling Shale Using a Model BHA (Bottom Hole Assembly)

In this paper we present further work on the use of electro-osmosis to prevent bit-balling. Indentation tests in wet clay have indicated that the process of electro-osmosis (EO) becomes more efficient in clays with lower water content. This fundamental behavior of wet clay correlated well with coefficient of friction measurements under EO, between a rotating metallic cylinder and a stationary shale sample for different types of shales. Test results have demonstrated that shales with a higher water content (>10%) respond to EO to a smaller degree compared to shales with lower water content (<10%). Furthermore, in clay/sand mixtures it was clearly seen that the process of EO became active when the clay content exceeded a threshold value of about 10-12% by weight. This finding indicates that EO may be effective in sand/clay mixtures whereas until the present it has been believed to be effective only in clays. We have also conducted drilling studies using a model BHA (Bottom Hole Assembly) in which a stabilizer above the bit acts as the anode while the bit is cathodic. This self contained arrangement is preferred for practical application of the technology in deep wells. Drilling tests in Pierre shale using this equipment have shown increases in ROP (rate of penetration) of 30% in roller cone bits and up to 158% in the case of PDC bits when the bit was made negative, with substantially less cuttings adhering to the bit. The greatest increases in ROP were found at the higher weights on bit. The effects of EO increased progressively as the current density to the bit was increased.

An Interactive Drilling Simulator for Teaching and Research

An interactive program has been constructed that allows a student or engineer to simulate the drilling of an oil well, and to optimize the drilling process by comparing different drilling plans. The program operates in a very user-friendly way, with emphasis on menu and button-driven commands. The simulator may be run either as a training program, with exercises that illustrate various features of the drilling process, as a game, in which a student is set a challenge to drill a well with minimum cost or time under constraints set by an instructor, or as a simulator of a real situation to investigate the merit of different drilling strategies. It has three main parts, a Lithology Editor, a Settings Editor and the simulation program itself. The Lithology Editor allows the student, instructor or engineer to build a real or imaginary sequence of rock layers, each characterized by its mineralogy, drilling and log responses. The Settings Editor allows the definition of all the operational parameters, ranging from the drilling and wear rates of particular bits in specified rocks to the costs of different procedures. The simulator itself contains an algorithm that determines rate of penetration and rate of wear of the bit as drilling continues. It also determines whether the well kicks or fractures, and assigns various other accident conditions. During operation, a depth vs. time curve is displayed, together with a mud log showing the rock layers penetrated. If desired, the well may be logged, casings may be set and pore and fracture pressure gradients may be displayed. During drilling, the total time and cost are shown, together with cost per foot in total and for the current bit run. A demonstration version of the program is available on the World Wide Web at the Berkeley Petroleum Engineering Page. Its current address is : msel6.mse.berkeley.edu

Evaluation of a New Thermally Stable Polycrystalline Diamond Material in Soft and Hard Formations

Comparative wear tests were carried out on a new Thermally Stable Polycrystalline Diamond (TSP) composite and Tungsten Carbide (6% Co) (WC) cutters, by machining rock cylinders in an instrumented lathe. Cutting tests were performed on Berea Sandstone and Sierra Granite rocks. It was observed that the new TSP material removes 1.4 times more Berea Sandstone rock than WC and 13 times more Sierra Granite than WC. In addition its volumetric wear is 25 times less in Berea Sandstone and 10 times less in Sierra Granite than WC. WC was also found to have much higher coefficient of friction, which can limit its use in abrasion resistant applications.Copyright

In-situ Casing Consolidation by Electrokinetic and Electrochemical Methods

Companies drilling in the unconsolidated sea-floor mud and shallow over-pressurized zones of the Gulf of Mexico are encountering cement wash-out problems while setting the initial structural casing (0-300 ft depth). Jet-drilled casings can fail due to insufficient frictional bearing capacity. This paper presents a method of increasing the bearing capacity of a jet-drilled (or pile-driven) casing in-situ by applying a potential difference such that the casing is anodic compared to a remote cathode. It has been shown experimentally that clayey formations will swell and stick to a simulated anodic casing by the combined electrokinetic processes of electroosmosis and electrophoresis. Any cavities around the casing are eliminated and the formation is flush against the metal surface, increasing bearing capacity. The formation around the casing dries out due to electroosmotic migration of water away from the anode and shear strengh is further increased. Increases of 50% to 1000% in shear strength and bearing capacity were achieved in a matter of hours in the laboratory, depending on the type and water content of the surrounding soil formation. In addition, it was found that there exists an optimum level of electrical treatment for each formation type, beyond which the shear strength begins to degrade. The process has a modest power requirement and is not expected to cause safety or environmental problems.