Rotary Drilling Techniques

Rotary drilling includes many separate techniques or practices which have been developed through field experience and/or analytical appraisals. In this chapter we will discuss three such operations, namely:

  • 1) Control of hole deviation in essentially vertical wells: vertical drilling
  • 2) Control of hole deviation in wells which are intentionally aimed at horizontally displaced bottom hole targets: directional drilling
  • 3) Retrieving undesirable objects from the hole; in particular, portions of the drill string and/or bit: fishing

As we shall see, the first two items are variations of the same thing. Fishing is essentially a separate topic.

9.1 Vertical Drilling

In oil well drilling there is no such thing as a truly vertical»liole; however, wells which aim at a target directly below their surface location are considered to be vertical wells. That is to say, their deviation from vertical is held to small angles.* The compass direction of deviation is of secondary importance (and generally is not even measured); principal consideration is given to the angle between the hole and the vertical.

Hole crookedness was considered a serious disadvantage to the early use of rotary tools. Contrary to much popular belief, cable tool holes may also be crooked.1'2 Geologists, who rely on depth measurements for subsurface mapping, found contour mapping almost impossible in some instances. The plan view of the 14 wells shown in Figure 9.1 illustrates such problems. Depths obtained from drill pipe measure-

*In common field language, such holes are called straight, which is something of a misnomer.

ments in these wells would hardly allow accurate subsurface mapping. Wells have also been known to run into each other during drilling. In the early days at Seminole, Oklahoma, two wells 660 ft apart at the surface ran together at 1900 ft. Two California wells 2000 ft apart at the surface ran together at 6115 ft.3 These and other similar occurrences spurred the development of down hole surveying instruments which could measure hole deviation from the vertical. One of the earliest of these was the acid bottle, in which hydrofluoric acid etched a horizontal line on the inside of a partially filled glass bottle. Other instruments were designed around the plumb-bob or pendulum principle. One of these later types and its operation is shown in Figure 9.2. These instruments measure only the vertical deviation and not its compass direction. Similar instruments which incorporate compass readings are also available and will be mentioned under directional drilling.

The extreme crookedness of early wells caused the industry to become quite straight hole conscious. As soon as reliable surveying instruments became available, severe restrictions were imposed on hole deviation by the producing companies. Drilling contracts commonly specified 3 to 5° as the maximum acceptable deviations in vertical holes. As a result, lighter bit weights had to be used, and penetration rate was consequently reduced. Drilling personnel then began to look for other means of minimizing crooked hole problems. The use of longer sections of drill collars, which furnished all of the bit weight, helped to a great extent. Various types of stabilizers were used with indifferent success, probably because no one knew for sure where to place them. Other approaches such as bit alterations and numerous changes in operating techniques were tried. Despite the empirical knowledge gained from such experimenting^

a-

r

oJ

A

EB»

A:

át

s

Y

1

]

S

(

Ota

\

Fig. 9.1. Plan view of 14 vertical wells drilled to 6000 ft.

After Suman,3 courtesy AIME.

Scale l=

Fig. 9.1. Plan view of 14 vertical wells drilled to 6000 ft.

After Suman,3 courtesy AIME.

no approach was widely successful and no complete agreement existed as to the basic causes of hole deviation.

It was not until 1950 when Arthur Lubinski published his analytical treatment of drill string buckling4 that a sound basis for solving hole deviation problems became available. This work, however, as well as a subsequent one,6 was based upon a simplifying assumption of perfect hole verticality. This assumption was quickly removed6-8 and has resulted in charts and tables9 for universal use. Field experience has proved the theoretical findings and standard operating procedures have been based on them.

Fundamental Principles

The mathematical treatment of hole deviation is quite long and involved and will not be presented here. There are, however, certain basic concepts which must be understood. Consider Figure 9.3, which shows the lower portion of the drill string in a straight* but inclined hole whose angle of inclination with respect to the vertical is a. It is assumed that the drill string lies on the low side of the hole and contacts the wall at the point of tangency T. The force with which the bit acts on the formation (frictional and rotational effects are ignored) is FB, applied at an angle <f> with the vertical. The force FB may be resolved into two components, namely the longitudinal force F1 in the direction of the axis of the hole, and the lateral force F 2 perpendicular to the axis of the hole. F2 may either act on the low side of the hole [Figure 9.3(A)], or be nil [Figure 9.3(B)], or act on the high side of the hole [Figure 9.3(C)]. Whenever F2 acts on the low side of the hole [Figure 9.3(A)], the hole deviation with respect to vertical will decrease. Conversely, whenever F2 acts on the high side of the hole [Figure 9.3(C)], the hole deviation will increase. Finally, if F2 is nil [Figure 9.3(B)], i.e., if 0 and a are equal, then a stable condition occurs, and drilling will proceed in the prolongation of the axis of the hole, which means that hole deviation a will be maintained. (Actually, as will be explained further in this section, the above statements are valid for isotropic formations only.) In the event that the hole deviation is decreasing [Figure 9.3(A)], the lateral force F2 will become smaller and smaller until it is nil. Thereafter, a stable condition is reached for a smaller value of a. Similarly, if the hole deviation is increasing, a stable condition will be reached for some larger value of a.

Drift Indicator Disc

DRIFT INDICATOR DISC Punch marks show 3 1/2° Inclination i i ii!ii

DRIFT INDICATOR DISC Punch marks show 3 1/2° Inclination

Fig. 9.2. Drift indicator for measuring hole deviation angle. Instrument is positioned at desired depth; pre-set timing mechanism allows plumb-bob to pierce target, recording deviation angle. Chart rotates 180° and second measurement is obtained as a check (note sample chart). Courtesy Eastman Oil Well Survey Company.

*The word straight is used here in its geometrical meaning and not its common oil-field meaning, nearly vertical.

Let Fp denote the buoyant weight of the section of drill collars below the point of tangency. Fp is applied at the center of gravity of that section. In the case of Figure 9.3(A), an increase of Fv results in an increase of F2. In the case of Figure 9.3(C), an increase of Fp results in a decrease of F2, which may even become negative, i.e., the case of Figure 9.3(C) may become the case of Figure 9.3(A). From the above, it is clear that an increase of Fp results in a smaller equilibrium angle a.

Fig. 9.3. Idealized sketch of forces affecting hole deviation angle.

Fig. 9.3. Idealized sketch of forces affecting hole deviation angle.

Therefore, Fp has a beneficial effect which is often called the pendulum effect.

In isotropic formations, the value of the equilibrium angle o is dependent on three variables:

  • 1) Weight on bit
  • 2) Drill collar size
  • 3) Hole size

Let us qualitatively consider the separate effects of these factors.

Weight on Bit

An increase in the weight on bit increases the bending of the unsupported portion of the collar string above the bitf which moves T closer to the bit and decreases the weight Fp. Therefore, it is apparent that increased weight on bit results in increased hole deviation.

Drill Collar and Hole Size

These two factors are interrelated through their mutual effect on clearance, which is the difference between hole and drill collar diameters. First, consider the effect of drill collar size with a constant clearance.

For the same weight on bit, large drill collars, being stiffer, are less subject to bending. In other words, for large collars, the point T is located higher. Therefore, the length of the portion of the string below T is greater. Both the fact that this length is greater and the fact that the weight per unit length is greater result in a greater Fp, thereby reducing the equilibrium angle.

The effect of hole size will be considered with drill collar size held constant. This is equivalent to considering hole-drill collar clearance. A larger clearance requires a larger lateral deflection before the hole wall is contacted. Hence, the point T moves up the hole as bit size is increased. This results in a greater force Fp, which should reduce the equilibrium angle, a. However, another factor acting in the opposite direction must be considered. A large deflection results in a greater angle between the axis of the bit and the vertical. This, in turn, results in a greater angle <j> between the force FB and the vertical; this has a tendency to increase the equilibrium angle, a. For all except small clearances, the second factor dominates and hole deviation is

Petroleum Engineering Chart

6 7 8 9 collar size- inches

Fig. 9.4. Effect of collar size on allowable bit weight for 3° hole deviation and 1-in. clearance. After Woods and Lubinski,7 courtesy API.

6 7 8 9 collar size- inches

Fig. 9.4. Effect of collar size on allowable bit weight for 3° hole deviation and 1-in. clearance. After Woods and Lubinski,7 courtesy API.

FORMATION e

FORMATION e

Fig. 9.5. Effect of clearance on allowable bit weight for 3° hole deviation. After Woods and

Lubinski,7 courtesy API.

Fig. 9.5. Effect of clearance on allowable bit weight for 3° hole deviation. After Woods and

Lubinski,7 courtesy API.

increased. The effect of collar size and hole clearance is demonstrated in Figures 9.4 and 9.5.

Thus far we have considered only isotropic formations, that is, formations having identical properties in all directions. In other words, we have not considered the possibility of formation characteristics influencing the magnitude of hole deviation. In general, the bit tends to drill updip, which implies that formations are drilled more easily perpendicular to the bedding planes than parallel to them. As a result, the direction of drilling is no longer that of the force FB with which the bit acts on the formation. In an anisotropic formation, the direction of drilling under equilibrium conditions is more inclined with respect to the vertical than the direction of FB■ Figure 9.6 serves to clarify this concept. Therefore, some force Fit directed as in Figure 9.3(A), exists for equilibrium conditions.

It has been explained above that, for isotropic formations, the equilibrium angle a is dependent on the weight on bit, hole size, and drill collar size. For anisotropic formations, a depends also on additional factors. These were resolved by LUbinski and Woods6 in terms of dip and anisotropic index. The greater the dip and/or the anisotropic index, the more crooked is the formation. The letters /, i, I, o, and u in Figures 9.4, 9.5, and 9.7 denote the following variations of formation crookedness, which may be due to various combinations of dips and anisotropic indices.

The foregoing discussions have been somewhat simplified. Those desiring a more rigorous and complete treatment should study the original references.

Weight

Capprox. lb) Weight

Crookedness Designation to maintain 3° (approx. lb)

of of with 6\-in. collars per in. of formation formation and 1-in. clearance hole diameter

Very Severe / 6,500 900

Severe i 12,000 1,600

Moderate I 20,000 3,000

Mild o 30,000 4,000.

Very Mild u 50,000 7,000

Problems in Hole Deviation

Figure 9.7 allows the rapid solution to practical hole deviation problems in terms of the five variables previously mentioned. This chart is based on a mud density of 10 lb/gal and a drill collar with inside-to-outside diameter ratio of 0.375. Corrections for other mud weights may be ignored; the same holds true for other diameter ratios, except for the very smallest collars (errors may be appreciable for 4 in. OD drill collars with a large bore). Figures 9.8(A) and (B) show or indicate the solution to the following example problems. These have been taken directly from the original Woods and Lubinski paper.7

Problems similar to the following may be solved by Figure 9.7. In a given formation 10° hole inclination is maintained by carrying 4,000 lb with 5-in. drill collars in a 9-in. hole. Formation dip is 45°. What weight may be carried with 11-in. drill collars in a 12-in. hole, if formation dip does not change and the same angle of 10° is maintained?

This and any other problem solvable by Figure 9.7 must contain the following elements:

  1. Established data: These are the numerical values of drill collar OD, weight on bit, hole size, and formation dip that resulted in a given hole inclination. They are obtained from past drilling experience in the formation under consideration, and establish its degree of crookedness.
  2. Problem data: These concern the hole to be drilled. They are the numerical values for all but one of the quantities listed under established data. At least one of the quantities is different in the problem data than in the established data.
  3. Unknown: The numerical value of the quantity not given in the problem data.

Problems Involving Neither Change in Hole Inclination Nor Formation Dip

If there is neither change in formation dip nor in hole inclination between the established data and the problem data, the problem may be solved without the actual knowledge of dip and the two righthand sections of the chart are not needed.

For clarity, the data of this and the subsequent problems will be tabulated as shown below.

Example 9.1

Established data Problem, data

Hole inclination, ° 10 10

Formation dip, ° Same in both data

Solution:

Using the established data, locate points and draw lines as follows [see Figure 9.8(A)]: Line MiNi (5-in. collars); point Ai (4,000 lb); point Bx (9-in. hole); point Ci (10° hole inclination). Complete rectangle A1B1C1D1.

Using problem data, proceed as follows: M^N2 (11-in. collars); B% (12-in. hole); C2 (10° hole inclination); D2 (point located on the same curve as D i). Complete rectangle A 2B2P2D2. Read unknown weight at A^: 22,000 lb.

Example 9.2

Established data Problem data

Weight, lb 4,000 22,000

Hole inclination, ° 10 10

Formation dip, ° Same in both data

Solution:

A construction of two rectangles similar to that in Example 9.1 may be used. Read unknown hole size at B2: 12 in.

[Chap. 9 Example 9.3

Established data Problem data

Weight, lb 4,000 22,000

Hole inclination, 0 10 10

Formation dip, 0 Same in both data

Solution:

This problem cannot be directly solved. It is necessary to try a few collar sizes and proceed for each of them as in Example 9.1, until a collar size is found that satisfies the other problem data.

Problems Involving a Change in Either Hole Inclination or Formation Dip

When the problem involves a change in either hole inclination or formation dip, we must use formation dip in both established data and problem data. Consider the following:

Example 9.4

Established data Problem data

Collar OD, in.

7

11

Weight, lb

13,500

?

Hole size, in.

9

12

Hole inclination, 0

5

10

Formation dip, 0

45

30

The solution is indicated in Figure 9.8(B). The first part of

the problem is the same as before: viz., from the established the problem is the same as before: viz., from the established

Fig. 9.6. The effect of formation characteristics on hole deviation, y is the dip angle. After Lubinski and Woods,6 courtesy API.
High Temperature Psychrometric Chart

Fig. 9.7. Chart for the solution of bore hole inclination problems. After Woods and

Lubinski,7 courtesy API.

Fig. 9.7. Chart for the solution of bore hole inclination problems. After Woods and

Lubinski,7 courtesy API.

data, exclusive of formation dip, construct the rectangle A1B1C1D1 as in Example 9.1. In Example 9.1, for which hole inclination and formation dip were constant, Z)2 was located on the same curve as Z>i. On the other hand, if either hole inclination or formation dip change, Z>2 is not necessarily located on the same curve as Di. Therefore, we must establish the curve on which D2 is located. Using established data for formation dip and hole inclination, locate points and draw lines as follows [see Figure 9.8(B)]: Ei (intersection of the curve on which £>i is located with the vertical reference line); Fl (45° dip and 5° hole inclination); Gi (intersection of lines through E i and F i).

Using problem data, proceed as follows: F2 (30° dip and 10° hole inclination); G2 (located on the same curve as GO; E2 (located on the reference line); MjSh (11-in. drill collars); B2 (12-in. hole); C2 (10° hole inclination); D2 (point located on the same curve as i?2). Complete rectangle AJiiCiD<>. Read unknown weight at A2: 90,000 lb.

Example 9.5

Established data Problem data

Weight, lb 13,500 90,000

Hole inclination, 0 5 10

Solution:

This problem may be solved by a construction similar to the one used in Example 9.4. Read unknown hole size at B2: 12-in.

drill collar o.d.-inches ic 9876543

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