Slow Drilling

Slow drilling refers to the rate of penetration (ROP) which is not in a 

desired level. ROP is defined as the speed at which the drill bit can break 

the rock under it and thus deepen the wellbore. This speed is usually 

reported in units of feet per hour (ft/hr) or meters per hour (Schlumberger 

glossary). ROP is one of the indicators and operational parameters for 

evaluating drilling performance. Slow drilling is the result of this perfor￾mance. In addition, drilling efficiency will have the desired effects on costs 

when all critical operational parameters are identified and analyzed. These 

parameters are referred to as performance qualifiers (PQs). PQs include 

footage drilled per bottomhole assembly (BHA), downhole tool life, vibra￾tions control, durability, steering efficiency, directional responsiveness, 

ROP, borehole quality, etc.

Most of the factors that affect ROP have influencing effects on other 

PQs. These factors can be grouped into three categories: i) planning, ii) 

environment, and iii) execution. The planning group includes hole size, 

well profile, casing depths, drive mechanism, bits, BHA, drilling fluid (i.e., 

type, and rheological properties), flow rate, hydraulic horsepower, and hole 

cleaning, etc. In environmental, factors such as lithology types, formation 

drillability (i.e., hardness, abrasiveness, etc.), pressure conditions (i.e., dif￾ferential, and hydrostatic) and deviation tendencies are included. Weight 

on bit (WOB), RPM and drilling dynamics belong to the execution cat￾egory. ROP can be categorized into two main types: i) instantaneous, and 

ii) average. Instantaneous ROP is measured over a finite time or distance, 

while drilling is still in progress. It gives a snapshot perspective of how a 

particular formation is being drilled or how the drilling system is function￾ing under specific operational conditions. Average ROP is measured over 

the total interval drilled by a respective BHA from trip-in-hole (TIH) to 

pull-out-of-hole (POOH).

It has long been known that drilling fluid properties can dramatically 

impact drilling rate. This fact was established early in the drilling litera￾ture, and confirmed by numerous laboratory studies. Several early stud￾ies focused directly on mud properties, clearly demonstrating the effect of 

kinematic viscosity at bit conditions on drilling rate. In laboratory condi￾tions, penetration rates can be affected by as much as a factor of three by

altering fluid viscosity. It can be concluded from the early literature that

drilling rate is not directly dependent on the type or amount of solids in

the fluid, but on the impact of those solids on fluid properties, particularly

on the viscosity of the fluid as it flows through bit nozzles. This conclusion

indicates that ROP should be directly correlated to fluid properties which

reflect the viscosity of the fluid at bit shear rate conditions, such as the

plastic viscosity. Secondary fluid properties reflecting solids content in the

fluid should also provide a means of correlating to rate of penetration, as

the solids will impact the viscosity of the fluid.

2.1.7.1 Factors Affecting Rate of Penetration

Factors that affect the ROP are numerous and perhaps important variables.

These variables are not recognized well up to-date. A rigorous analysis of

ROP is complicated by the difficulty of completely isolating the variables

under study. For example, the interpretation of field data may involve

uncertainties due to the possibility of undetected changes in rock prop￾erties. Studies of drilling fluid effects are always plagued by difficulty of

preparing two muds having all properties identical except one which is

under observation. While it is generally desirable to increase penetration

rate, such gains must not be made at the expense of overcompensating det￾rimental effects. The fastest ROP 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 the cost.

The factors that have an effect on ROP are listed under two general

classifications such as environmental and controllable. Table 2.2 shows the

list of parameters based on these two categories. Environmental factors 

such as formation properties and drilling fluids requirements are not con￾trollable. Controllable factors such as weight on bit, rotary speed, and bit

hydraulics on the other hand are the factors that can be instantly changed.

Drilling fluid is considered to be an environmental factor because a certain

amount of density is required in order to obtain a specific objective to have

enough overpressure so that it can avoid flow of formation fluids. Another

important factor is the effect of overall hydraulics to the whole drilling

operation. This operation is influenced by many factors such as lithology,

type of the bit, downhole pressure and temperature conditions, drilling

parameters and mainly the rheological properties of the drilling fluid. ROP

performance is a function of the controllable and environmental factors.

It has been observed that ROP generally increases with decreased equiva￾lent circulating density (ECD).

Another important term controlling the ROP is the cuttings transport.

Ozbayoglu et al. (2004) conducted extensive sensitivity analysis on cut￾tings transport for the effects of major drilling parameters, while drilling

for horizontal and highly inclined wells. It was concluded that the average

annular fluid velocity is the dominating parameter on cuttings transport,

the higher the flow rate the lesser the cuttings bed development. ROP and

wellbore inclinations beyond 70° did not have any effect on the thickness

of the cuttings bed development. Drilling fluid density have moderate

effects on cuttings bed development with a reduction in bed removal with

increased viscosities. Increased eccentricity positively affected cuttings

bed removal. The smaller the cuttings the more difficult it is to remove

the cuttings bed. It is clear that turbulent flow is better for bed develop￾ment prevention. However, in any engineering study of rotary drilling it

is convenient to divide the factors that affect the ROP into the following

list: i) personal efficiency; ii) rig efficiency; iii) formation characteristics

(e.g., strength, hardness and/or abrasiveness, state of underground for￾mations stress, elasticity, stickiness or balling tendency, fluid content and

interstitial pressure, porosity and permeability etc.); iv) mechanical factors

(e.g., bit operating conditions – a) bit type, and b) rotary speed, and c)

weight on bit); v) hydraulic factor (e.g., jet velocity, bottom-hole cleaning);

vi) drilling fluid properties (e.g., mud weight, viscosity, filtrate loss and

solid content); and vii) bit tooth wear, and depth. However, for horizontal

and inclined well bores, hole cleaning is also a major factor influencing the

ROP. The basic interactive effects between these variables were determined

by design experiments. Variable interaction exists when the simultaneous

increase of two or more variables does not produce an additive effect as

compared with the individual effects. The rate of penetration achieved with

the bit as well as the rate of bit wear, has an obvious and direct bearing on

the cost per foot drilled. The most important variables that affect the ROP

are: i) bit type, ii) formation characteristics, iii) bit operating conditions

(i.e., bit type, bit weight, and rotary speed), iv) bit hydraulics, v) drilling

fluid properties, and vi) bit toot wear.

1. Personal Efficiency: The manpower skills, and experiences are referred

to as personal efficiency. Given equal conditions during drilling/comple￾tion operations, personnel are the key to the success or failure of those

operations and ROP is one of them. Overall well costs as a result of any

drilling/completion problem can be extremely high. Therefore, continuing

education and training for personnel is essential to have a successful ROP

and drilling/completion practices.

2. Rig Efficiency: The integrity of drilling rig and its equipment, and main￾tenance are major factors in ROP and to minimizing drilling problems.

Proper rig hydraulics (e.g., pump power) for efficient bottom and annu￾lar hole cleaning, proper hoisting power for efficient tripping out, proper

derrick design loads, drilling line tension load to allow safe overpull in

case of a sticking problem, and well-control systems (e.g., ram preventers,

annular preventers, internal preventers) that allow kick control under any

kick situation are all necessary for reducing drilling problems and opti￾mization of ROP. Proper monitoring and recording systems that monitor

trend changes in all drilling parameters are very important to rig efficiency.

These systems can retrieve drilling data at a later date. Proper tubular hard￾ware specifically suited to accommodate all anticipated drilling conditions,

and effective mud-handling and maintenance equipment will ensure that

the mud properties are designed for their intended functions.

3. Formation Characteristics: The formation characteristics are some

of the most important parameters that influence the ROP. The following

formation characteristics affect the ROP: i) elasticity i.e., elastic limit, ii)

ultimate strength, iii) hardness and/or abrasiveness, iv) state of under￾ground formations stress, v) stickiness or balling tendency, vi) fluid con￾tent and interstitial pressure, and vii) porosity and permeability. Among

these parameters, the most important formation characteristics that affect

the ROP are the elastic limit and ultimate strength of the formation. The

shear strength predicted by the Mohr failure criteria sometimes is used to

characterize the strength of the formation.

The elastic limit and ultimate strength of the formation are the most

important formation properties affecting penetration rate. It is men￾tioned that the crater volume produced beneath a single tooth is inversely

proportional to both the compressive strength of the rock and the shear

strength of the rock. The permeability of the formation also has a signifi￾cant effect on the penetration rate. In permeable rocks, the drilling fluid

filtrate can move into the rock ahead of the bit and equalize the pressure

differential acting on the chips formed beneath each tooth. It can also be

argued that the nature of the fluid contained in the pore space of the rock

also affects this mechanism since more filtrate volume would be required

to equalize the pressure in a rock containing gas than in a rock contain￾ing liquid. The mineral composition of the rock also has some effect on

penetration rate.

To determine the shear strength from a single compression test, an aver￾age angle of internal friction varies from about 30 to 40° from the most

rock. The following model has been used for a standard compression test:

The threshold force or bit weight (W/d)

t

required to initiate drilling was

obtained by plotting drilling rate as a function of bit weight per bit diam￾eter and then extrapolating back to a zero drilling rate. The laboratory cor￾relation obtained in this manner is shown in Figure 2.17.

The other factors such as permeability of the formation have a signifi￾cant effect on the ROP. In permeable rocks, the drilling fluid filtrate can

move into the rock ahead of the bit and equalize the pressure differential

acting on the chips formed beneath each tooth. Formation as nearly an

independent or uncontrollable variable is influenced to a certain extent by

hydrostatic pressure. Laboratory experiments indicate that in some forma￾tions increased hydrostatic pressure increases the formation hardness or

reduces its drill-ability. The mineral composition of the rock also has some

effect on ROP. Rocks containing hard, abrasive minerals can cause rapid

dulling of the bit teeth. Rocks containing gummy clay minerals can cause

the bit to ball up and drill in a very inefficient manner.

4. Mechanical Factors: The mechanical factors are also sometimes

described as bit operating conditions. The following mechanical factors

affect the ROP: i) bit type, ii) rotary speed, and iii) weight on bit.

Bit Type: The bit type selection has a significant effect on rate of penetra￾tion. For rolling cutter bits, the initial penetration rates for shallow depths 

are often highest when using bits with long teeth and a large cone off set 

angle. However, these bits are practical only in soft formations because 

of rapid tooth wear and sudden decline in penetration rate in harder for￾mations. The lowest cost per foot drilled usually is obtained when using 

the longest tooth bit that will give a tooth life consistent with the bearing 

life at optimum bit operating conditions. The diamond and PDC bits are 

designed for a given penetration per revolution by the selection of the size 

and number of diamonds or PDC blanks. The width and number of cut￾ters can be used to compute the effective number of blades. The length of 

the cutters projecting from the face of the bit (less the bottom clearance) 

can limit the depth of the cut. The PDC bits perform best in soft, firm, and 

medium-hard, nonabrasive formations that are not gummy. Therefore, the 

bit type selection must be considered, i.e., whether a drag bit, diamond bit, 

or roller cutter bit must be used, and the various tooth structures affect to 

some extent the drilling rate obtainable in a given formation.

Figure 2.18 shows the characteristic shape of a typical plot of ROP vs. 

WOB obtained experimentally where all other drilling variables remain 

constant. No significant penetration rate is obtained until the threshold 

bit weight is exceeded (point a). ROP increases gradually and linearly with 

increasing values of bit weight for low-to-moderate values of bit weight 

(segment ab). A linear sharp increase curve is again observed at the high 

bit weight (segment bc). Although the ROP vs. WOB correlations for the 

discussed segments (ab and bc) are both positive, segment bc has a much 

steeper slope, representing increased drilling efficiency. Point b is the tran￾sition point where the rock failure mode changes from scraping or grinding 

to shearing. Beyond point c, subsequent increases in bit weight cause only 

slight improvements in ROP (segment cd). In some cases, a decrease in ROP 

is observed at extremely high values of bit weight (segment de). This type 

of behavior sometimes is called bit floundering (point d – bit floundering 

point). The poor response of ROP at high WOB values is usually attributed 

to less-efficient hole cleaning because of a higher rate of cuttings genera￾tion, or because of a complete penetration of a bit’s cutting elements into 

the formation being drilling, without room or clearance for fluid bypass.

ii) Rotary Speed: Figure 2.19 shows a characteristic shape typical response 

of ROP vs. rotary speed obtained experimentally where all other drilling 

variables remain constant. Penetration rate usually increases linearly with 

increasing values of rotary speed (N) at low values of rotary speed (seg￾ment ab). At higher values of rotary speed (after point b, segment b to c), 

the rate of increase in ROP diminishes. The poor response of penetration 

rate at high values of rotary speed usually is also attributed to less effi￾cient bottom-hole cleaning. Here, the bit floundering is due to less efficient 

bottom-hole cleaning of the drill cuttings.

Maurer (1962) developed a theoretical equation for rolling cutter bits 

relating ROP to bit weight, rotary speed, bit size, and rock strength. The 

equation was derived from the following observations made in single-insert 

iii) Weight on Bit: The significance of WOB can be shown as explained by 

Figure 2.18. The figure shows that no significant penetration rate is obtained 

until the threshold bit weight (Wt

) is applied (Segment oa, i.e., up to point 

a). The penetration rate then increases rapidly with increasing values of bit 

weight (Segment ab). Then a constant rate in increase (linear) in ROP is 

observed at moderate bit weight (Segment bc). Beyond this point (c), only 

a slight improvement in the ROP (segment cd) is observed. In some cases, a 

decrease in penetration rate is observed at extremely high values of bit weight 

(Segment de). This behavior is called bit floundering. It is due to less efficient 

bottom-hole cleaning (because the rate of cutting generation has increased).

5. Drilling Fluid Properties: The properties of the drilling fluid reported 

to affect the penetration rate include: i) density, ii) rheological flow prop￾erties, iii) filtration characteristics, iv) solids content and size distribution, 

and v) chemical composition. ROP tends to decrease with increasing fluid 

density, viscosity, and solids content. It tends to increase with increasing 

filtration rate. The density, solids content, and filtration characteristics of 

the mud control the pressure differential across the zone of crushed rock 

beneath the bit. The fluid viscosity controls the parasitic frictional losses 

in the drillstring and, thus, the hydraulic energy available at the bit jets 

for cleaning. There is also experimental evidence that increasing viscosity 

reduces penetration rate even when the bit is perfectly clean. The chemi￾cal composition of the fluid has an effect on penetration rate, such that 

the hydration rate and bit balling tendency of some clays are affected by 

the chemical composition of the fluid. An increase in drilling fluid den￾sity causes a decrease in penetration rate for rolling cutter bit. An increase 

in drilling fluid density causes an increase in the bottom hole pressure 

beneath the bit and, thus, an increase in the pressure differential between 

the borehole pressure and the formation fluid pressure.

6. Bit Tooth Wear: Most bits tend to drill slower as the drilling time elapses 

because of tooth wear. The tooth length of milled tooth rolling cutter bits 

is reduced continually by abrasion and chipping. The teeth are altered by

hard facing or by case-hardening process to promote a self-sharpening

type of tooth wear. However, while this tends to keep the tooth pointed, it

does not compensate for the reduced tooth length. The teeth of tungsten

carbide insert-type rolling cutter bits and PDC bits fail by breaking rather

than by abrasion. Often, the entire tooth is lost when breakage occurs.

Reductions in penetration rate due to bit wear usually are not as severe for

insert bits as for milled tooth bits unless a large number of teeth are broken

during the bit run.

7. Bit Hydraulics: Significant improvements in penetration rate could be

achieved by a proper jetting action at the bit. The improved jetting action

promoted better cleaning of the bit face as well as the hole bottom. There

exists an uncertainty on selection of the best proper hydraulic objective

function to be used in characterizing the effect of hydraulics on penetra￾tion rate. Bit hydraulic horsepower, jet impact force, Reynolds number,

etc., are commonly used objective functions for describing the influence of

bit hydraulics on ROP.

8. Directional and Horizontal Well Drilling: Since the 1980s, when the

horizontal well technology was ‘perfected’, the majority of the wells in the

developed world use horizontal wells. This is also accompanied by inclined

and directional wells that had already gained usefulness in offshore drill￾ing. Common field of applications for directional and horizontal drilling

are in offshore and onshore, where vertical wells are impractical to drill or

much higher return for investment is assured with horizontal wells. Over

the last three decades, there has been a major shift from vertical to hori￾zontal wells. The use of horizontal wells has allowed for greater formation

access. As more and more horizontal wells are drilled, the cost of hori￾zontal well drilling declines. As IEA report (2016) indicates, over the past

decades, lateral lengths have increased from 2,500 feet to nearly 7,000 feet

and, at the same time, we have seen nearly a threefold increase in drilling

rates (feet/day). This is shown in Figure 2.20. Even though such an increase

in efficiency in horizontal well has driven the drilling cost down, the tech￾nology has not caught on in the developing countries, where horizontal

wells are still deemed prohibitively expensive.

The major applications of directional drilling are to i) develop the fields

which are located under population centers, ii) drill wells where the reser￾voir is beneath a major natural obstruction, iii) sidetrack out of an exist￾ing well bore, and iv) elongate reservoir contact and thereby enhance well

productivity (Hossain and Al-Majed, 2015).

9. Improve ROP in Field Operations: Time spent to drill ahead is usually

a significant portion of total well cost. Rotating time usually accounts for

10% to 30% of well cost in typical wells. This means that the penetration rate

achieved by the drill bit has considerable impact on reduction on drilling

cost. A method has been developed to identify which factors are control￾ling ROP in a particular group of bit runs. The method uses foot-based mud

logging data, geological information, and drill bit characteristics to produce

numerical correlations between ROP and applied drilling parameters or

other attributes of drilling conditions. These correlations are then used to

generate recommendations for maximizing ROP in drilling operations. The

objective of this method is to quantify the effects of operationally control￾lable variables on ROP. To reveal the effects of these variables, data sets must

be constructed so as to minimize variation in environmental conditions. The

first step is therefore to select a group of bit runs made with the same bit

size through similar formations. Next, intervals of consistent lithology are

identified with a preference for formations exhibiting lateral homogeneity.

Formations such as shale and limestone are in general more suitable than

variable lithologies such as sandstone. Rock property logs can be used to ver￾ify comparability. Further sorting can be made depending on the objectives

of each specific analysis to separate bit runs in different mud types with dif￾ferent classes of bit or to separate intervals drilled with sharp bits versus those

in worn condition. Each step helps to further expose the effects on ROP of

bit design, and mechanical or hydraulic drilling parameters. Once intervals

have been selected and sorted, numerical averages of the variables of interest

are obtained. This is critical because many sources of error exist in drilling

parameter measurements, and improvement in data quality. Averaging to

raise sample size is the most obvious method to minimize the effects of error.

Figure 2.21 shows a log, for which data have been extracted and aver￾aged from an interval of shale early in the bit run, prior to a drop in ROP 

related to bit wear in a sandstone. This process would then be repeated 

for other bit runs made through the same stratigraphic interval, yielding a 

data set suitable for analysis. For example, BP Exploration customized pet￾rophysical software which is used to automate the extraction and averaging 

of drilling data though manual processing from paper logs. Once data are 

prepared, correlation analysis is performed in conventional spreadsheets. 

Cross plots are used to seek visible correlations between ROP and the inde￾pendent variables, and statistics functions are used to establish the degree 

of correlation and to build models for prediction of ROP.