Factors Affecting Penetration Rate

8.1 Introduction

Contract drilling prices have remained essentially constant over the last ten or fifteen years despite the fact that the greatest inflationary period in United States history has occurred during the same time. This unique price stability has been due, largely, to the highly competitive and resourceful nature of the drilling industry in general, and to the rig floor, desk, and laboratory thinking and experimentation which has resulted in improved techniques and equipment for making hole faster.

The factors which affect rate of penetration are exceedingly numerous and certainly are not completely understood at this time. Undoubtedly, influential variables exist which are as yet unrecognized. A rigorous analysis of drilling rate is complicated by the difficulty of completely isolating the variable under study. For example, interpretation of field data may involve uncertainties due to the possibility of undetected changes in rock properties. Studies of drilling fluid effects are always plagued by the difficulty of preparing two muds having all properties identical except the one under observation. These and other complexities will become more apparent in later sections.

While it is generally desirable to increase penetration rate, such gains must not be made at the expense of over-compensating, detrimental effects. The fastest on-bottom drilling rate does not necessarily result in the lowest cost per foot of drilled hole; other factors such as accelerated bit wear, equipment failure, etc., may raise cost. These restrictions should be kept in mind during the following discussion, which deals largely with on-bottom drilling rate.

Some of the more recognizable variables which affect penetration rate are the following:

1. Personnel efficiency a. Competence

  • 1) experience
  • 2) special training b. Psychological factors
  • 1) company-employee relations
  • 2) pride in job
  • 3) chance for advancement
  1. Rig efficiency a. State of repair, preventive maintenance b. Proper size c. Ease of operation, degree of automaticity, and power equipment
  2. Formation characteristics a. Compressive strength b. Hardness and/or abrasiveness c. State of underground stress (overburden pressure, etc.")
  3. Elasticity — brittle or plastic e. Stickiness or balling tendency f. Permeability g. Fluid content and interstitial pressure h. Porosity i. Temperature
  4. Mechanical factors a. Weight on bit b. Rotating speed c. Bit type
  5. Mud properties a. Density b. Solid content c. Flow properties d. Fluid loss

INTRODUCTION

2,000

1,200

FORCE, POUNDS

FORCE, POUNDS

FORCE, POUNDS

1,200

2,000

1,200

2,000

FORCE, POUNDS

FORCE, POUNDS

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FORCE, POUNDS

1,200

FORCE, POUNDS

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  1. 8.1. Tracings of typical force waveforms for various heights of drop with an 0.03-in. bit and weight of 15 lb for single blows on Indiana limestone. After Pennington,1 courtesy API.
  2. Oil content f. Surface tension—wettability
  3. Hydraulic factors—essentially bottom hole cleaning
  4. 2 Fundamentals of Rock Failure

For drilling purposes, rocks may be classified into three general types, namely:

  • 1) Soft rocks: soft clays and shales, unconsolidated to moderately cemented sands
  • 2) Medium rocks: some shales, porous limestones and dolomites, consolidated sands, gypsum
  • 3) Hard rocks: dense limestones and dolomites, highly cemented sands, quartzite, and chert.

As mentioned in our earlier discussion of bits, soft rocks may be drilled by the scraping-cutting action of dragtype bits, or by the combined grinding-scraping action of offset cone-angle, rolling cutter bits. The harder formations are drilled mostly by the crushing penetration of the bit teeth. Since it is in the latter type of formation that penetration rates are lowest, let us further consider the failure mechanism of these rocks.

Experiments on the failure of elastic rocks conducted by Battelle Memorial Institute for Drilling Research, Inc. are of considerable fundamental interest.1-3 In part of these studies a drop tester, consisting of a weighted bit on a rod, was dropped from various heights, striking the rock specimen below. Strain gauges close to the bit allowed the force waveforms to be recorded by an oscilloscope camera. Four such patterns for single blows at various drop heights are shown in Figure 8.1. Note that the first force peak is 2000 lb for all cases, with the magnitude of the second peak increasing with drop height. Figure 8.2 shows the first energy peak with an expanded time scale. The curve between 0 and A corresponds to the bit crushing small surface irregularities on the rock. Between A and B, the constant steep slope denotes that elastic deformation is being undergone by the rock. If the impact force is not above point B, the rock does not fail and only a

3000

2500

2000

1500

1000

f> o rebrPc

0-A — Surface crushing A-B — Elastic deformation wilh some subsurface damage B-C — Crushing and compacting

Large fracture occurs in the neighborhood of C

Time, 0.00002 second per dot

Fig. 8.2. Expanded first force pulse of a force waveform for a single blow with an 0.03-in. bit on Indiana limestone. After Pennington,1 courtesy API.

surface mark is left. Between points B and C, the narrow wedge of rock beneath the bit is crushed, and the blow energy is transmitted to the rock around the wedge, and chips are formed. Beyond point C, the energy level drops until the bit once again contacts solid rock. If sufficient energy is left, another peak occurs, etc. It was found that the amount of rock removed was linearly related to the blow energy. Other experiments with static rather than impact loading gave similar results, except that a lower energy level was required to fracture a given amount of rock. The results in Figures 8.1 and 8.2 are exactly analogous to cable tool drilling (or any percussive device) in brittle rocks.

The sequence of pictures in Figure 8.3 shows the drilling" action of a rolling cutter bit in sandstone at atmospheric conditions.4 Cuttings are removed by air circulation. In the first picture, a force is applied to the center tooth as the cone rolls. The rock does not fail until the fourth picture, when the applied load finally exceeds the rock's strength. The failure is a small explosion, with chips flying in all directions. This continues as the tooth sinks deeper and is complete, in the eleventh picture. It may thus be surmised that the rotary drilling mechanism is not too basically different from the straight percussive (cable tool) method.

8.3 Rock Characteristics

The rock properties which govern drilling rate are not completely understood. Furthermore, correlation is lacking between strength and elastic properties as measured at laboratory conditions, and those which exist at the depths of interest to the oil industry. Considerable data and an extensive bibliography on rock properties has been published by Wuerker.6 Other investigations concerned with the effect of pressure on rock drillability are also available.6'7

In general, penetration rate varies inversely with the compressive strength of the rock being drilled. The related property of hardness or abrasiveness enters the picture because of its effect on bit life. The hardest and also the strongest sedimentary rocks are chert (a form of quartz, Si02) and various quartzites which offer severe drilling problems when encountered.

The elastic properties of various formations are greatly influenced by the state of stress at which they exist. The behavior of most shales is typical of this effect, because they become increasingly difficult to drill at greater depths. For explanation of this behavior, consider Figure 8.4 which shows a thin impermeable element of formation directly below the bit. If the hole is filled with liquid, the upper surface of the element is subjected to a pressure which is dependent on mud density and depth. Practically speaking, this pressure tends to prevent removal of the element much as though the rock's strength were increased by the applied pressure. Therefore it may be expected that the effect of such superimposed stresses will be more pronounced in weak (soft, relatively compressible) rocks than in stronger, more competent beds [see Figure 8.5(A)]. Note that drilling rate reaches an essentially minimum value at some confining pressure, with little change occurring at subsequent higher pressures. This is particularly evident for the soft shale shown in Figure 8.5(C). Also note that the effect of confining pressure on drilling rate is greater at higher bit weights, as is shown in Figure 8.6.

Payne and Chippendale8 have reported test results which lend further insight into the mechanism of rock failure at high vs. atmospheric pressures. In these experiments, crushing loads were applied to rocks with a round (5 mm diameter) tungsten carbide penetrator, the size and type of indentation being the object of study. Typical results are shown in Figure 8.7. The letters A and H denote imposed hydrostatic pressures of atmospheric and 5000 psi; the numbers refer to the applied load in pounds. In the atmospheric tests visible cracks appeared, and the chips around the indentation were separate, easily removable flakes. Under 5000 psi, however, the failure was more plastic, as evidenced by the extruded material which piled up around the depression and Could not easily be removed. Hence, the sandstone was brittle at low pressure and plastic at high pressure. Not all rocks behaved in this manner. Quartzite, granite, and dolomite experienced the same type of failure at the higher pressure; however, the

Rock Bit Compressive Failure

Fig. 8.3. Elastic rock failure as caused by rolling cutter bit. Conditions are atmospheric pressure with cuttings being blown away by air. Starting in picture 1, a tooth starts to apply pressure to the rock as the cone rolls forward. The rock does not break immediately. Then in picture-4, the pressure of the tooth exceeds rock strength and failure starts. The failure is practically an explosion of rock. Chips continue to fly as the tooth sinks deeper Finally, by picture 10 particle discharge diminishes and then ceases. Filmed by Hughes Tool Company with a 9f-in. O.S.C. bit cutting Berea sandstone. After Murray and Mac-Kay,4 courtesy Hughes Tool Company.

Fig. 8.3. Elastic rock failure as caused by rolling cutter bit. Conditions are atmospheric pressure with cuttings being blown away by air. Starting in picture 1, a tooth starts to apply pressure to the rock as the cone rolls forward. The rock does not break immediately. Then in picture-4, the pressure of the tooth exceeds rock strength and failure starts. The failure is practically an explosion of rock. Chips continue to fly as the tooth sinks deeper Finally, by picture 10 particle discharge diminishes and then ceases. Filmed by Hughes Tool Company with a 9f-in. O.S.C. bit cutting Berea sandstone. After Murray and Mac-Kay,4 courtesy Hughes Tool Company.

loads necessary to produce the fracture were 50 to 100% higher than at atmospheric conditions.

The balling tendency of a formation is primarily dependent on its mineral composition. Hydratable clays, bentonite in particular, form a sticky, pasty mixture with water which becomes imbedded between bit teeth and surrounds the cones and the entire bit. This reduces tooth penetration and, consequently, drilling rate.

Permeability affects the drillability of rocks through its effect on the relief of imposed pressures. Consider again the rock element shown in Figure 8.4. If this rock were sufficiently permeable to the drilling fluid, no ap-

Free Clipping Mask Frames

8.3 (cont.). Failure is explosive when bit pressure finally exceeds strength of rock.

preciable pressure differential would exist across a thin element; hence the pressure effect would be minimized. This is the conclusion of Bredthauer,9 and Murray and Cunningham,7 and is in agreement with their experimental evidence, i.e., that permeable rocks which allowed pressure equalization ahead of the bit showed no appreciable change in drillability with pressure. From these data, it may be concluded that any other factors facilitating more rapid pressure relief will also decrease the effect of pressure on drilling rate. A rock completely saturated with incompressible fluids (water, oil) should be less sensitive to borehole pressure effects than one containing a gas, since in the former a small quantity of mud filtrate is sufficient to equalize pressure for a substantial distance.

A porous zone drills faster than a dense section of the same rock. This fact has long been used by drillers and geologists for detecting the presence of such zones. A major portion of this effect is probably due to the lower compressive strength of porous zones.

The effect of temperature (in the range of our interest) on rock properties is not generally considered. However, rock failure becomes more plastic as temperature

Earth surface

Earth surface

Rock Bit Elements
Fig. 8.4. Element of formation beneath rock bit. After Murray and Cunningham,7 courtesy AIME.

confining pressure (x 10* psi)

confining pressure (x 10* psi)

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Fig. 8.5. Penetration rate vs. confining pressure for various rock types. After Murray and

Cunningham,7 courtesy AIME.

increases; such effects may be of interest in the future, as drilling goes deeper.

No mention has been made of such properties as grain size, Poisson's ratio, modulus of elasticity, modulus of resilience, or other more involved measurements of strength and elastic behavior.5 These properties are, in general, unknown for sedimentary rocks at the conditions of interest in this field. A knowledge of these factors could lead to a more thorough knowledge of the mechanics of rock failure.

It might seem to the reader that too much space has been allotted to rock properties, which are beyond human control. It should be realized, however, that it is these rock properties which completely govern drilling practices in different geologic areas. Hole size, bit type, drilling fluid selection, and general operating procedures are all dictated by the nature of the rocks to be drilled. Consequently, rock characteristics are of prime importance, with operational procedures being secondary.

8.4 Mechanical Factors

In all areas, penetration rate is governed by the weight on the bit and/or the rotary speed which may be applied. Normally, these limits are imposed by either crooked hole, equipment, and/or hydraulic considerations. Let us postpone our discussion of crooked hole problems until Chapter 9, and deal with the latter factors in this chapter. Other variables will be considered constant or of no consequence, except as noted.

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Fig. 8.6. Drilling rate vs. confining pressure, variable bit load. After Murray and Cunningham,7 courtesy AIME.

Fig. 8.6. Drilling rate vs. confining pressure, variable bit load. After Murray and Cunningham,7 courtesy AIME.

Weight on Bit

The general effect of this variable on rate of penetration is shown in Figure 8.8.10 Note that there are two distinct drilling regions:

  • 1) drilling at bit loads below the compressive strength of the rock — out to approximately 30,000 lb, for the pink quartzite.
  • 2) drilling at bit loads above this critical weight. Obviously bit weights in region (2) are desirable and are applied if possible. This principle is well illustrated by Figure 8.9, which shows the increase in penetration rate and footage when sufficient bit weight to overcome the rock's compressive strength was applied.

A/600

A 22O o

Fig. 8.7. Rock failure under different imposed pressures. Letters A, H denote atmospheric and 5000 psi pressure, respectively. Numbers indicate applied load, lbs. After Payne and Chippendale,8 courtesy Hughes Tool Company. **

A 22O o

Fig. 8.7. Rock failure under different imposed pressures. Letters A, H denote atmospheric and 5000 psi pressure, respectively. Numbers indicate applied load, lbs. After Payne and Chippendale,8 courtesy Hughes Tool Company. **

From Figure 8.8 it is evident that for each rock type the points above the critical load lie on approximately straight lines:11

where Rp = instantaneous or on bottom penetration rate, ft/hr

W = weight on bit, lb a, b — intercept and slope respectively, which are dependent on rock properties, bit size and type, drilling fluid properties, etc.

Other data, both field12-14 and laboratory 16>16 tend to substantiate the linearity of Rp versus W over reasonable ranges of bit loading. This relation will hold true only if other factors are constant. In particular, adequate cleaning and cutting removal must be maintained. This does not necessarily mean constant circulation rate or constant nozzle velocity, since at higher values of W more and probably larger cuttings are being generated. Consequently, circulation inadequacies will affect this relationship, unless the flow rate is high enough to perform its function equally well at all TPs of interest. This will be further emphasized in a later section devoted to hydraulic factors.

Rotational Speed,

The effect of this variable on penetration rate is not too well established. In general, on bottom drilling rate increases with increased rotary speed. Figures 8.10 and 8.11 are typical of the rotary speed effect and suggest that the following relationship exists:

Typical Sandstone Compressibility

Fig. 8.8. The effect of bit loading on penetration rate for various rocks. Laboratory data — atmospheric pressure. Courtesy Hughes Tool Company.10

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  • Caoimhe Craig
    What factors affect rock drillability?
    8 years ago
  • milly
    Why permeability of rock affects the rate of penetration?
    8 years ago
  • sini
    Why does the permeability of rocks affect the rate of penetration?
    8 years ago
  • uwe
    Why permeability of rock affect the rate penetration?
    8 years ago
  • Semere
    Why is the factors that affect penetration in petrolem?
    8 years ago
  • ricky
    How saturation and ball tendency affect penetration rate of drilling bit?
    7 years ago
  • ulrich
    How saturation and ball tendency affects penetration rate?
    7 years ago
  • Dehab Aziz
    How does saturation and balling tendency affect penetration rate?
    7 years ago
  • Milly
    What are the factors affecting the rate of penetration?
    7 years ago
  • Jonas Berg
    What are the factors that affect penetration rate of drill bit?
    4 years ago
  • MARIA TERESA
    How does mud density affect rate of penetration?
    4 years ago
  • orlando
    What are the factor affecting penetration rate?
    3 years ago
  • dustin
    What factor that are affecting of drillimg?
    3 years ago
  • Estella
    How do rock characteristics affect drilling cost?
    3 years ago
  • Fred
    What are the intrinsic environmental factors which affect the rate of rock penetration?
    3 years ago
  • ruth
    What are the most important variables that affects penetration rate?
    3 years ago
  • gennaro
    Why ambient pressure has little influence on the rate of penetration?
    3 years ago
  • ashlyn
    What is penetration rate in drilling engineering?
    3 years ago
  • samuel
    How does rotary speed affect rate of penetration?
    3 years ago
  • yorda
    What are the factors effects the drlling equipments?
    2 years ago
  • karin
    What change has to occur in increase penetration rate in rock drillability?
    2 years ago
  • Jackson
    What are the factors affecting drillability of a rock?
    2 years ago
  • Arthur
    Which factors effect rop while drilling?
    2 years ago
  • mario b
    How do rock properties affect drilling?
    2 years ago
  • maximilian
    How do rock characteristic affect the drilling cost?
    2 years ago
  • mentha
    What is the most important factor effects on the rate of penetration?
    2 years ago
  • john
    What are the factors affecting internet penetration?
    11 months ago
  • TEIJA LARIVAARA
    How to overcome factors that affect penetration rate?
    2 months ago

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