Method for optimizing the tripping velocity of a drill string

The present invention is directed to a method for optimizing the tripping velocity of a drill string. The method of the present invention comprises determining the hydraulic fluid pressure in the borehole in a plurality of intervals as the drill string is pulled out of or run into the borehole. These actual pressures are compared against a predetermined acceptable pressure range for each interval. The limits for this acceptable pressure range are set by the maximum acceptable surge pressures and the minimum acceptable swab pressures established for the interval. The velocity of the drill string in each such interval is then adjusted in order to maximize the tripping velocity while maintaining the hydraulic fluid pressure within the acceptable range for each interval. In fact, the velocity is preferably adjusted to maintain the fluid pressure near the maximum acceptable surge pressure while the drill string is being run into the borehole and to maintain the fluid pressure near the minimum acceptable swab pressure while the drill string is being pulled out of the borehole.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention generally relates to a method for optimizing the 
tripping velocity of a drill string. The present invention is particularly 
useful for maximizing the efficiency of a drilling operation while 
maintaining the hydraulic pressure of the drilling fluid in the borehole 
within an acceptable range. More particularly, the present invention 
maximizes the velocity at which a drill string may be run into or pulled 
out of a borehole while not producing surge or swab pressures which are 
outside a desired safety range. 
2. Description of the Background 
Rotary drilling operations employ a drill bit for cutting into earth 
formations to produce a borehole. The drill bit is affixed to the end of a 
drill string, comprising a plurality of coupled together pipe joints, each 
about 30 feet in length. Additional pipe joints are added to the drill 
string as drilling progresses. 
During the drilling operation, it is often necessary to remove the drill 
string in order to change the drill bit. It may also be necessary to 
remove the drill string for other reasons, such as to perform desired 
logging operations. After the bit is changed or after such logging 
operations are completed, the drill string must be reconstructed and 
returned to the borehole. The process of pulling out and running in the 
drill string, known as "tripping", is well known in the industry. Tripping 
is a tolerated and necessary evil which must be completed in as short a 
time as possible; there is no progress being made in completing the 
drilling operation during the tripping procedure, resulting in the loss of 
valuable rig time. 
Although pipe joints may be added or removed individually, in order to save 
time and maximize the efficiency of the drilling rig, it is a general 
practice to add or remove pipe sections during tripping operations in 
interconnected lengths of two or three pipe joint sections. These sections 
are known, respectively, as "doubles" or "trebles" and are often 
collectively referred to as "stands." Such interconnected stands are 
normally stored vertically in pipe stands or racks on or near the drilling 
rig. 
In a typical tripping operation, a double or treble pipe stand will be 
pulled out of or run into the borehole through the rotary table of the 
drilling rig by use of a vertically movable traveling block in the rig 
derrick. The double or treble stand is disconnected or connected just 
above the floor of the drilling rig while the drill string remaining in 
the borehole is firmly grasped by slips in the rotary table. This 
operation is continued sequentially until the entire drill string has been 
pulled out of the borehole or until the drill string has been run into the 
borehole so that the drilling bit has been lowered into contact with the 
bottom of the borehole. The operation of pulling out or running in the 
drill string thus occurs in a plurality of pulls or runs wherein the pipe 
is accelerated from a stationery position to a pulling or running speed 
and then decelerated to another stationery position, generally in 60-90 
foot intervals. The running or pulling speed is generally constant over 
substantially the entire length of each of these stepped intervals. 
A drilling fluid, commonly known as drilling mud, is continuously 
circulated down the drill string and up the annulus formed by the drill 
string and the borehole in order to maintain sufficient pressure in the 
borehole to impede the entrance of formation fluids into the borehole in 
order to prevent a well "blowout". It is desirable to maintain the 
hydrostatic pressure of the static drilling fluid column in the well in 
balance with the bottomhole formation pressure. However, while a pipe 
string is being pulled out or run into a borehole, a variety of hydraulic 
effects create pressure differentials in the drilling fluid relevant to 
the hydrostatic borehole pressure. 
Surging or "surge pressure" involves an increase in the total hydraulic 
pressure in the borehole to a pressure greater than the normal hydrostatic 
pressure for the static drilling fluid column in the well. This condition 
often occurs when the drill string is being run into the borehole at an 
excessive speed. Excessive surge pressure may result in damage to the 
surrounding formation, even producing undesirable fractures in the 
formation. Such fractures may result in the loss of drilling fluid, 
sticking of the drill pipe and other undesirable results. 
Another hydraulic effect, swabbing, may also be produced. Swabbing or "swab 
pressure" involves a reduction in the total hydraulic pressure in the 
borehole to a pressure less than the normal pressure for the hydrostatic 
pressure of the static drilling fluid column in the well. This condition 
often occurs when the drill string is being pulled out of the borehole at 
an excessive speed. Such excessive reduction in the hydraulic pressure may 
result in formation fluids entering the borehole in a condition known as 
"kick". Additional reduction of the hydraulic pressure may result in an 
uncontrolled kick or blowout. Further, in soft formations, excessive 
swabbing may result in collapse of the borehole walls. 
Accordingly, it is necessary to minimize the swab and surge pressures 
associated with pulling out the drill string or with running in the drill 
string during the tripping operation. However, because the tripping 
operation is non-productive, it is also desirable to maximize the speed 
with which these operations are performed so that the down time of the 
drilling rig is minimized and the efficiency of the rig is maximized. 
Commonly, a tripping schedule or listing providing suggested optimal 
tripping velocities per stand is available to the driller. The driller 
then attempts to pull out or run in the stands of the pipe string at the 
suggested uniform velocity by noting the total time required for moving 
each stand of pipe. However, these suggested tripping velocities do not 
provide maximized velocities per stand in order to minimize the total 
tripping time while maintaining the drilling fluid pressure within 
acceptable limits. Accurate and maximized tripping velocities are actually 
affected by many factors. The maximum, safe velocity may be effected by 
the amount of moving pipe in a hole, by the depth to which the pipe 
extends and by the characteristics and pressures of the formations through 
which the borehole has progressed. 
Others have attempted to solve this problem by merely establishing maximum 
tripping velocities which should not be exceeded and by monitoring the 
actual tripping velocity to sound a warning signal if the established 
maximum velocity is exceeded. See the velocity measurement and warning 
system disclosed by Smith in U.S. Pat. Nos. 3,866,468 and 3,942,594. 
Others have attacked this problem by determining the volume, and hence the 
weight or mass, of drilling fluid required to fill the borehole as pipe is 
pulled out or of drilling fluid displaced from the borehole as pipe is run 
in. See the comparison system of Leonard disclosed in U.S. Pat. Nos. 
3,646,808 and 3,729,986. 
However, these efforts have not resulted in a solution to the long felt but 
unresolved need for a method for optimizing the tripping velocity of a 
drill string by maximizing the tripping velocity while maintaining the 
hydraulic pressure of the drilling fluid within an acceptable range. 
SUMMARY OF THE INVENTION 
The present invention provides a new and improved method for optimizing the 
tripping velocity of a drill string. This method is particularly useful in 
maximizing the tripping velocity of the drill string while maintaining the 
hydraulic pressure of the drilling fluid in the borehole within an 
acceptable range. The present method permits the tripping velocity to be 
maximized while monitoring the hydraulic pressure of the drilling fluid in 
the borehole to maintain the well pressure within an acceptable range. 
In the present method, the fluid pressure near the drill bit is determined 
at a plurality of locations in the borehole during the tripping operation. 
Additionally, the velocity of the drill string in a plurality of 
corresponding intervals in the borehole is determined during the tripping 
operation. An acceptable fluid pressure range having a minimum and maximum 
acceptable fluid pressure is established for each of these locations 
within the borehole. The actual, determined fluid pressure at each of 
these locations is compared with the corresponding acceptable fluid 
pressure range. During subsequent tripping operations, the tripping 
velocity in each interval is adjusted to maximize the tripping velocity 
while maintaining the determined pressure within the acceptable fluid 
pressure range. This method may employ determination and comparison of 
surge pressures, swab pressures or both. This method may employ pressure 
determinations made while the drill string is being pulled out of the 
borehole, run into the borehold, or during both procedures of the tripping 
operation. This method may employ pressure determinations made during the 
movement of each stand of pipe or it may employ determinations made during 
any lesser number of connections. Furthermore, those skilled in the art 
will appreciate that such a method, when computer controlled, could 
continuously monitor pressure while the drill string is both moving and 
stationary to provide continuous adjustment. 
The present invention provides a method to optimize or maximize the 
tripping velocity of a drill string while maintaining the hydraulic 
pressure of the drilling fluid in the borehole within an acceptable range 
in order to maximize the efficiency of the drilling operation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The present invention comprises a method for optimizing the velocity of a 
drill string during the tripping operation. The present invention is 
directed to maximizing the incremental velocities attained by the drill 
string over the plurality of intervals during which it is separately 
accelerated during tripping while maintaining the hydraulic pressure of 
the drilling fluid near the drill bit within an acceptable range. This 
acceptable range of hydraulic pressure is established in order to prevent 
excessively high surge pressures and dangerously low swab pressures. As 
explained in the background, excessively high surge pressures may result 
in a variety of problems, including unwanted fracturing of the formation 
and loss of drilling fluid. Further, excessively low swab pressures may 
result in a variety of other problems, including collapse of the borehole, 
invasion of formation fluid and ultimately blowout conditions. 
The method of the present invention begins with determining the hydraulic 
pressure of the drilling fluid near the drill bit at a plurality of 
locations in the borehole. These determinations may be made during 
tripping, during drilling or while pipe stands are being connected or 
disconnected to the drill string. These determinations are made with any 
conventional pressure measurement device useful in a measurement while 
drilling environment. These determinations may be made by direct or 
indirect measurement of pressure. Either a single measurement or a 
plurality of measurements may be made at each location. It is presently 
preferred to make only a single measurement at each location or during 
each interval. However, in an alternative embodiment, it is contemplated 
that a plurality of measurements, e.g., five, may be made in each interval 
and at different velocities in order to provide more data for more 
accurate and better optimization. In fact, it is also contemplated that 
where a plurality of determinations are made in any interval, the data may 
be processed downhole and only the minimum and maximum pressures recorded 
or transmitted for comparison. These determinations may be made at any 
desired interval as the drill string is being pulled out of or run into 
the borehole. In fact, with appropriate measurement equipment and computer 
control, it is contemplated that these determinations may be continuously 
made if desired. These determinations may be made during either the 
pulling out or the running in of the drill pipe or during both phases of 
the tripping operation. Further, these determinations may be made during 
every tripping operation to most efficiently maximize the correction and 
tripping velocities or these determinations may be made during only 
certain tripping operations as desired by the operator. The measured fluid 
pressures or other data may be transmitted to the surface for recording by 
an operator or storage in a computer or other data handling device. 
Alternatively, the pressures or other data measured during each tripping 
operation may be stored in an appropriate storage device located at or 
near the pressure sensing device in the drill string and the data 
recovered for analysis and comparison when the sub containing this storage 
device is tripped to the surface. 
The method of the present invention further includes the step of 
determining the velocity of the drill string in a plurality of intervals 
corresponding to the pressure determination intervals in the borehole 
during the tripping operation. The tripping velocities are most 
conveniently determined at the surface where pipe movement can be readily 
observed and the data easily recorded or stored for analysis. Means for 
measuring the tripping velocity in these intervals are well known to the 
art. Any appropriate means, such as that disclosed by Smith in U.S. Pat. 
Nos. 3,866,468 and 3,942,594 may be employed to determine the 
instantaneous and average tripping velocities during the measured 
intervals. 
The method further comprises the step of determining and establishing 
maximum and minimum acceptable hydraulic fluid pressures for each of said 
intervals in the borehole. These fluid pressures will establish an 
acceptable hydraulic fluid pressure range. It is desirable that the surge 
pressure not exceed a maximum acceptable hydraulic fluid pressure in order 
to prevent damage to the formation and other problems associated with 
excessively high pressures. Furthermore, it is desirable that the swab 
pressure not decrease below a minimum acceptable level to ensure that 
formation fluids do not invade the borehole and more importantly to assure 
that blowout conditions do not develop. The acceptable hydraulic fluid 
pressure range for each interval may be established based upon prior 
knowledge developed or obtained by standard logging techniques concerning 
the formation, such as knowledge concerning the lithology and other 
characteristics of the formation near these intervals. 
The method of the present invention further comprises the step of comparing 
the actually determined, hydraulic fluid pressure for each interval with 
the acceptable fluid pressure range for that interval. This comparison may 
be performed manually or visually by an operator, although it is preferred 
that this comparison be performed automatically by an appropriate digital 
or analog computer or other means. By comparing this pressure data for the 
determined velocities and knowing that as the tripping velocity increases 
the surge pressure increases and the swab pressure decreases, the operator 
can maximize the tripping velocity while maintaining the fluid pressure 
within the acceptable range. 
Finally, the present invention comprises the step of adjusting for 
subsequent trips, the tripping velocity in each of the measured intervals 
in order to maximize the tripping velocity while maintaining the actual, 
determined pressure within the acceptable hydraulic fluid pressure range 
for each interval. In fact, it is most desirable to maintain the tripping 
velocity at its maximum level while maintaining the surge pressure near 
its maximum acceptable level while running the drill string into the 
borehole. Further, it is most desirable to maintain the tripping velocity 
at its maximum level while maintaining the swab pressure near its minimum 
acceptable level while pulling the drill string out of the borehole. 
Although these operations may be performed manually, it is preferred that 
the comparison of pressures and adjustment of tripping velocities be 
performed automatically, such as by a computer or other device. 
The foregoing description of the invention has been directed in primary 
part to a particular preferred method in accordance with the requirements 
of the patent statutes and for purposes of explanation and illustration. 
It will be apparent, however, to those skilled in the art that many 
modifications and changes in the specifically described method may be made 
without departing from the scope and spirit of the invention. For example, 
the pressure may be determined either directly or indirectly by any 
appropriate means known to those skilled in the art. Therefore, the 
invention is not restricted to the particular method described, but covers 
all modifications which may fall within the scope of the following claims. 
It is Applicants' intention in the following claims to cover such 
modifications and variations as fall within the true spirit and scope of 
the invention.