Method and apparatus for determining formation pressure

Method and apparatus are provided for determining the formation pressure. The magnitude of formation pressure may be derived as a function of changes in bottomhole pressure following swabbing the borehole to draw formation fluids into the borehole, monitoring the borehole for influx of formation fluids, determining the reduced pressure due to swabbing, repeating the swabbing and monitoring steps until an influx of formation fluids is detected thereby determining the pressure of the formation.

This invention relates to methods and apparatus used while drilling oil and 
gas wells and more particularly relates to a method and apparatus for 
determining the pore pressure of a formation by reducing bottomhole 
pressure thereby to draw formation fluids into the borehole, detecting the 
influx of formation fluids into the borehole, and determining the reduced 
bottomhole pressure which is related to the pore pressure. 
BACKGROUND OF THE INVENTION 
It is well known that oil and gas deposits are contained in subterranean 
earth formations and that boreholes are drilled into these formations for 
the purpose of recovering these petroleum deposits. During the drilling 
operations, it is common to pump a drilling fluid, or drilling mud, into 
the borehole through the drill string to lubricate and cool the bit, to 
maintain hydrostatic pressure head in the borehole to overbalance the 
subterranean formation pressures, and to carry the drill cuttings from the 
bit to the surface of the earth. 
It is also well known that subterranean formation pressures generally 
increase with depth. Low permeability formations, such as shales, exhibit 
a pressure that is a measure of the pressure exerted by fluid trapped 
within non-interconnected interstices or pores of the formation. The 
measure of this pressure is commonly called "formation pore pressure." In 
permeable formations the exhibited pressure is a measure of the fluid 
trapped within the interconnected interstices or pores of the formation, 
and is generally referred to as "formation pressure." Further, it is 
generally known that low-permeability formations, such as shales, commonly 
overlie abnormally high-pressured fluid within the porous formation. 
A problem in all oil and gas well drilling operations is the maintenance of 
sufficient hydrostatic pressure head of drilling mud to overbalance the 
subterranean formatin pressure at the bottom of the borehole. A pressure 
overbalance or "bottomhole pressure differential" must be maintained in 
order to prevent high-pressured fluids within porous formations from being 
released through the borehole to the surface. An uncontrolled release of 
high pressured fluid from within the formation through the borehole is 
commonly referred to as a "blowout". A blowout can cause irreparable 
damage to the borehole and surface equipment and death and injury to 
drilling personnel located near the surface drilling equipment. 
Excessive hydrostatic pressure head, together with additional pressure due 
to friction while circulating the drilling mud or while lowering the drill 
string into the borehole, can cause the formation to be fractured with 
possible resultant loss of mud to the surrounding formation. Thus, 
maintenance of a proper bottomhole pressure differential, i.e., 
overbalance, is important to well safety. However, this is difficult since 
the pressure varies with the drilling mud being used and the formation 
being encountered. Exact knowledge of formation pressure is necessary but 
is not easily obtained. Generally accepted practice requires the removal 
of the drill string and the running of a wireline log to determine the 
bottomhole pressure differential with the resultant loss of time and 
expenditure of money. 
A general object of this invention is to provide an improved system that 
may be used in connection with downhole testing during drillling 
operations, wherein it is possible to measure formation pore pressure 
without removing the drill string from the hole. 
Still another object is to provide an improved system for measuring 
formation pressures with accuracy. 
Yet another object is to provide apparatus for obtaining the pressure 
measurements of subsurface earth formations in connection with surface 
drilling operations wherein a minimum amount of rig time is lost. 
Other objects and features of the invention will become apparent upon 
consideration of the following description thereof when taken in 
connection with the accompanying drawings.

SUMMARY OF THE INVENTION 
According to one aspect of the present invention a method is disclosed for 
determining the pressure of a formation traversed by a borehole including 
the steps of reducing bottomhole pressure of the fluid contained in the 
lower portion of the borehole, and, upon monitoring of formation fluid 
influx, determining the reduced borehole pressure which is indicative of 
the pressure of the formation. 
The inventin comprises apparatus for determining the pore pressure of a 
formation traversed by a borehole and includes a drill string for 
insertion into the borehole, means for detecting influx of fluids from the 
formation into the borehole, means for reducing the pressure in the 
borehole, and a pressure measurement means responsive to the pressure 
reducing means. 
DETAILED DESCRIPTION OF THE INVENTION 
Referring now to FIG. 1, a typical borehole 12 is shown traversing a 
subsurface formation 11. Suspended in the borehole 12 is the drilling 
apparatus conventionally employed in the drilling operation. In 
particular, drilling rig 13 is shown in place over borehole 12, drill 
string 14, pressure measurement sub 15, drill collar 16 and drill bit 18 
within borehole 12 with casing 20 set to a preselected depth. Borehole 12 
is shown in cross section as it penetrates a normally pressured shale 
formation 22 and a higher pressure layer 24 of the shale formation. 
Formation 24 overlies an abnormally high-pressured permeable formation 26. 
Drilling mud 32 is drawn from mud circulating pit 34 through a mud intake 
pipe 36 to a mud pump 38. Mud weight detector 40 on the pipe 36 measures 
the weight in lbs/gal. of the mud flowing into the mud pump 38. The pump 
pressure of pump 38 can be varied and the operating pressure of pump 38 is 
indicated by meter 42. Drilling mud 32 is then pumped through a pump 
discharge pipe 44 where the mud flow rate is measured by a flow rate 
detector 46. 
Flexible housing 47 conducts mud 32 from the pump discharge pipe 44 through 
drill string 14 and drill collar 16 to drill bit 18 where it is discharged 
past cutting heads and circulated upwardly through the annulus 50 between 
drill string 14 and collar 16 and the borehole 12, and through annulus 52 
between drill string 14 and casing 20 in the direction as shown by the 
arrows. Mud 32 is then forced sequentially through the borehole discharge 
pipe sections 53, mud weight detector 56 and adjustable choke 54 to 
thereafter be discharged into mud pit 34 for reuse. Detector 56 measures 
and indicates the weight in lbs/gal. of the mud flow out of the borehole 
12. The choke device 54 is a radially compressive sleeve that can be 
opened or closed to vary the rate of mud flow out of the borehole. As the 
sleeve is closed, the flow is "choked" and back pressure is exerted on the 
mud circulating in the borehole which in turn increases the downhole 
pressure. 
With reference now to FIG. 2, the measurement sub 15 in addition to 
component parts which are not shown, includes an influx detector 60 and 
pressure measurement means 62 coupled serially together. In actual 
practice. the configuration may approach that shown in FIG. 3 where influx 
and pressure subs are separated by short collars 16, 17. 
The pressure gauge 62 and influx detector 60 are coupled to a cable or a 
downhole computing and telemetry system, not shown. The cable in turn 
includes electric conductors for transmitting the output signals from the 
pressure gauge 62 and influx detector 60 to apparatus at the earth's 
surface. The downhole computing system continuously monitors the influx 
detector and the bottomhole pressure gauge. The computing system 
continuously transmits the measurements to the surface for analysis. 
The function of the influx detector 60 is to determine the displacement of 
the drilling mud, which normally occupies the immediate vicinity of the 
detector 60, by formation fluids drawn from the formation by the effective 
swabbing action to be described. One such detector is a fluid resistivity 
detector which may consist of a separate tubular member screw-threaded to 
sub 15, an electrically conducting annular electrode and an insulator of 
rubber or other non-conducting material for separating and electrically 
insulating the tubular member from the electrode. The electrode is 
electrically connected to a conductor by means of a connector which is 
electrically insulated from the annular member. The electrical conductor 
is connected to suitable resistance measuring apparatus which electrical 
measuring apparatus is also connected to the drill string 14 so as to 
measure the electrical resistance of the fluid between the electrode and 
the drill string 14. 
Other suitable types of influx detectors 60 include pressure transducers 
illustrated in U.S. Pat. No. 4,297,880, acoustic wave measurement devices 
illustrated in U.S. Pat. No. 3,776,032 and gamma ray detectors, these 
patents being incorporated herein by reference. 
The operation of the apparatus described above is as follows: 
The drilling bit 18 penetrates a subsurface stratum of which the formation 
pressure is desired or advisable. The mud pump 38 is turned off thereby 
ceasing circulation of mud 32 down the drill string 14 and up the annulus 
50, 52. 
During drilling operations the bottomhole pressure is determined by factors 
including the hydrostatic head of drilling mud in the borehole 12, 
frictional pressure losses in the mud due to the borehole walls and the 
drill string 14, the weight of the drilling mud being used and the back 
pressure of the choke 54. Under static conditions bottomhole pressure is 
simply the head of drilling mud. To bracket the formation pressure, a 
pressure drop due to swabbing with the drill string is created. The drill 
bit function is similar to a swabbing section in that it forms a 
constricted region about the drill string which drives fluids up the 
annulus thereby reducing the borehole pressure below the drill bit. 
Swabbing causes a pressure drop which reduces the bottomhole pressure to a 
pressure which may be at, above or below the formation pressure. If the 
reduced pressure is below the formation pressure, formation fluids will 
migrate into the borehole where the fluids mix with the borehole fluids, 
i.e. the drilling mud. The influx of formation fluids may be detected 
using the methods listed below and the detection of influx indicates that 
the borehole pressure, at its reduced level, is below the formation 
pressure. The reduced borehole pressure for different swabbing rates may 
be calculated knowing the drill string velocity and the initial bottomhole 
pressure. The required pressure drop due to swabbing must exceed the 
pressure difference between the mud hydrostatic pressure and the formation 
pressure. The difference is normally about 250 psi. 
The desired pressure drop is inserted in equation (2) described below and 
the swabbing velocity required to produce the desired pressure drop is 
determined for the equipment and drilling mud in use. 
In the effective swabbing action, the drill string 14 is moved upwardly at 
the predetermined velocity, thereby drawing drilling mud from the lower 
end of the borehole 12 up the annulus 50, 52 toward the surface thereby 
reducing presure. The velocity required to achieve a required swab 
pressure may be calculated using the method described in an article 
entitled "An Improved Method for Calculating Swab/Surge and Circulating 
Pressures in a Drilling Well"; SPE paper 4521, June 28, 1974, this article 
being incorporated herein by reference. In calculating the required 
velocity of the fluid resulting from drill string movement, the swab 
pressure is given by 
##EQU1## 
Solving for V.sub.sw, the required velocity, 
##EQU2## 
where P=swab pressure 
f=laminar friction factor 
.rho.=mud density 
L=length of the section 
V.sub.sw =velocity of the drill string 
d=diameter of the borehole 
.alpha.=ratio of diameter of drill string (collars) to diameter of borehole 
If the influx of formation fluids is not detected, the pressure reducing 
step is repeated to further reduce the bottomhole pressure. The velocity 
of the withdrawing drill string 14 is increased so that the pressure drop 
is increased and a lower reduced bottomhole pressure is achieved. 
Following each successive pressure reduction step, monitoring of the 
borehole fluids is performed to detect any influx of formation fluids. 
Several pressure reducing steps may be necessary. In due course, if the 
swab pressure exceeds the overbalance pressure, formation fluids will move 
into the borehole and past the influx detector 60 which will indicate 
their presence. The monitoring step includes mixing the fluids contained 
in the lower portion of the borehole by rotating the drill string. 
The mixing can also be accomplished by circulating drilling fluids down the 
drill string 14, out the drill bit 18, and into the borehole 12 below the 
bit. The influx detectors 60 are preferably located on the exterior of the 
drill pipe about 15 to 30 feet above the drill bit. 
Numerous variations and modifications may obviously be made in the 
apparatus herein described without departing from the present invention. 
Accordingly, it should be clearly understood that the forms of the 
invention described herein and shown in the figures of the accompanying 
drawings are illustrative only and are not intended to limit the scope of 
the invention.