Sampling and Analysis of Drilled Cuttings (Rotary Drilling and Cuttings Generation)

Rotary Drilling and Cuttings Generation

Rotary Drilling and the Generation of Well Cuttings

While the environment and purpose of drilling will control the type, design, and capabilities of the rig used, the essential components used to "make hole" remain the same. All rotary rigs require hoisting, rotating, and circulating systems in order to locate and power the drilling tool on the bottom of the borehole.

• Rotary drilling is accomplished by the rotation of a drill bit at the end of a rigid drillstem. The cooling of the drill bit and removal of rock cuttings is performed by the circulation of drilling mud down through the hollow drillstem and back to the surface. It is important for the present discussion to realize that although drilling fluid is commonly called "mud," it has, in fact, a complex and carefully controlled composition. The drilling fluid is required to:
  
• have sufficient density to control subsurface pressures and prevent borehole collapse;
  
• provide the carrying capacity to remove rock cuttings from the hole;
  
• form a gel when circulation is halted in order to suspend the cuttings in place;
  
• cool and lubricate the drill bit and drillstem;
  
• line the borehole wall with an impermeable clay filter cake to protect exposed formations from contamination;
  
• release the rock cuttings and debris readily at surface;
  
• provide buoyancy to help support the weight of the drillstem when it is being lifted from the hole;
  
• cause minimum pollution damage to the drilled formations and the surface environment;
  
• minimize corrosion of and abrasion damage to the drilling equipment caused by downhole formation fluids.
  

In general, the drill bit is the most important component of the drillstem to the geologist not simply because it is the means of penetrating and exposing fresh formations, but because the process of penetration-the interaction between the drill bit and rock-yields important geological information. An understanding of this interaction allows the geologist to correlate the drill time, or rate-of-penetration log, with the cuttings recovered at surface to construct a lithological section with true boundaries and relationships.

 

 

 

Bit Type and Its Influence on Penetration

Drag Bit

The earliest type of rotary drill bit was the drag, or fishtail, bit ( Figure 1 , Drag bit courtesy of Petroleum Extension Service, University of Texas, Austin ). Its simple scraping action is efficient at high rotational speeds in soft and plastic formations, and it is still sometimes used for this task, However, harder or more abrasive rocks produce rapid blade wear, even when the blades are hardfaced with tungsten carbide. Excessive rotary torque may subject such blades to "twist-off" failures; they also have a tendency to deviate from the vertical by following structural weaknesses, such as steeply dipping bedding planes or fault lines.

 

Tricone Bit

In 1934, the tricone rock bit was introduced; with subsequent refinements, it has remained the standard drilling tool. The tricone bit ( Figure 2 , A soft formation tricone bit), with its intermeshing rotating teeth, has a self-cleaning action that is an important contribution to drilling efficiency.

The cutting process of the tricone bit is the result of the crushing action of the teeth as they roll across the formation. Beyond a certain threshold weight per inch of bit diameter required to initiate rock failure, increasing the weight on bit will produce increased rate of penetration. However, a limiting weight on bit exists, beyond which further increase cannot induce increased crushing and may result in the bit becoming embedded and clogged in compacted debris.

The rate of penetration will also increase with increasing rotary speed up to a limiting maximum beyond which the buildup of crushed debris will again prevent further penetration. Additionally, both the use of excessive weight on the bit or of excessive rotary speed will result in accelerated wear and decreased bit life.

The key to optimum bit performance is to use suitable drilling fluid viscosity and flow rate so that cuttings can be removed effectively as they are created. Without this, increasing either the weight on the bit or the rotary speed will not only be ineffective in increasing rate of penetration, but will shorten the lifetime of and therefore the total footage drilled by the bit.

In order to maximize bit life, the teeth on bits designed for hard formations are short, broad, and closely spaced. This gives maximum tooth impact per bit revolution and reduces tooth wear and breakage. With this type of bit, rate of penetration will increase as rock strength decreases. In general, this indicates increasing porosity and decreasing cementation and rock cohesion. However, where rock matrix strength decreases, e.g., in clays, the soft material will clog the short, closely spaced teeth and prevent penetration. Thus in very soft formations, the hard formation bit will drill slowest!

By contrast, the teeth of soft formation bits are more widely spaced and are longer and more slender to allow better fluid circulation and removal of soft debris. Each tooth penetrates more deeply into the formation with a gouging action that removes a greater volume of formation per impact. Such bits will drill very quickly in soft lithologies, but the long teeth are easily broken when encountering a hard rock matrix or strong cementation. In this case, rates of penetration will decrease rapidly and the bit will soon be useless.

Jet Bit

The introduction of the jet bit ( Figure 3 , Drilling fluid path in the conventional and jet tricone bit courtesy of Petroleum Extension Service, University of Texas, Austin ) greatly increased drilling efficiency and rate of penetration. Instead of flowing freely between the cones, drilling fluid is forced through three narrow jet ports in the face of the bit. This helps penetration by improving the removal of cuttings and soft debris from the bottom of the hole while simultaneously cleaning the face of the bit. In very soft formations, jetting alone may result in penetration, even without the bit touching the bottom of the hole,

Tungsten carbide inserts for tricone bits provide little or no improvement over the performance of an equivalent steel tooth bit. They do, however, extend bit life substantially.

Observing the performance of a tricone rock bit can be useful to the geologist. The bit will provide small but identifiable cuttings at the shale shaker. Its efficiency naturally depends upon the formation strength and porosity; thus its rate-of-penetration log will show sharp changes at lithological boundaries and relatively uniform, characteristic rates of penetration for each lithological type. A note of caution must be observed, however. While a bit will show increasing rate of penetration with decreasing rock strength (increasing porosity) for the range of formations it is designed to drill, outside of that range its performance may be anomalous. For example, a short-toothed bit designed for hard formations will become clogged with debris when drilling a soft formation and cease to drill.

Diamond Bit

The cutting action of the diamond bit consists of a continuous crushing and scraping process as the diamonds move over the surface of the bottom of the hole. Because the diamonds are small ( Figure 4 , Diamond drill bit courtesy of NL Hycalog ), they do not individually penetrate very deep; therefore, only sufficient weight on bit to cause compressive failure of the rock is required. Extra weight cannot give further penetration or cuttings removal. Because of their construction, diamond bits are vulnerable to damage caused by jolts or metal debris. On the other hand, rate of penetration is a direct function of rotation, and this relationship holds up to very high rotary speeds. It is common to operate diamond bits in combination with down hole motors to give maximum rates of penetration. Such rates do not commonly exceed those possible with tricone bits, but the strength and simplicity of the cutting structure allows very long bit life. Diamond bits may drill for thousands of feet and hundreds of hours without appreciable wear or reduction in rate of penetration.

For the geologist, however, the diamond bit is a far from ideal drilling tool. The shallow penetrating, scraping action of the bit makes it insensitive to rock strength and porosity variations. Unlike a tricone bit, the diamond bit will drill all formations, hard or soft, at relatively uniform rates, making the recognition of formation boundaries difficult. The grinding action of the bit produces, at best, very fine cuttings, sometimes little better than unidentifiable rock flour. At high rotary speeds, with downhole motors, frictional heating on bottom further degrades the cuttings, yielding debris burnt to the point that it can be mistaken for metamorphosed sediments.

PDC Bit

The polycrystalline diamond compact (PDC) bit combines high rates of penetration with wear resistance and long bit life, adding, as positive side effects, lithologically responsive rates of penetration and undamaged, identifiable well cuttings samples.

The PDC bit ( Figure 5 , Polycrysalline diamond compact (PDC) drill bit courtesy of Smith Tool ) cutters consist of a disk of cemented tungsten carbide faced with a layer of synthetic diamond. This "drill blank" is as wear-resistant as diamond, but less susceptible to shock-induced breakage. The cutters do not crush rock, like a tricone or diamond bit, but scrape material from the bottom of the hole in the same manner as a drag bit. With this action, the rock fails by shear, not compression. This allows a much greater volume of formation to be removed with each rotation of the bit, especially in formations that exhibit plastic deformation. PDC bits, like tricone bits, have jets to ensure efficient bit cleaning and cuttings removal, even at the high rotary speeds obtained using down hole motors.

Although comparable in price to diamond bits, PDC bits offer a reduction in cost per foot because of their increased rate of penetration; in addition, they extend bit life beyond that expected for a tricone bit. Although cuttings from a PDC bit are usually smaller than those normally seen from a tricone bit, they are usually large enough to show lithological character and, even in the case where downhole motors are used, they are never pulverized or burnt like those from a diamond bit.

Uses of Cores and Cuttings

At the wellsite, the first use of well cuttings is for the preparation of a geological sample log. The more crucial study of cuttings and core samples, however, begins after the well is drilled. Such study examines:

paleontology and palynology for geological dating and stratigraphic correlation;

geochemistry of rock mineralogy, oil, and source kerogen composition;

measurement of porosity, permeability, and other petrophysical properties;

sedimentological studies of grain texture and sedimentary structures for regional geology.

The results of such sample examination become important for:

facies analysis and reservoir characterization;

reservoir engineering, well stimulation, and production programs;

reference material for partners, government agencies, and trade.

The value of all of these studies is based entirely upon the quality of the original sample. Every effort should thus be made to adopt drilling practices that provide best sampling conditions. However, the practical demands of drilling often dictate the use of methods, tools, or materials that may produce suboptimal well cuttings.