US3368400A - Method for determining the top of abnormal formation pressures - Google Patents

Description

2 Sheets-Sheet 1 ETAL ERMINING THE TOP OF ABNORMAL J. R. JORDEN. JR. METHOD FOR DET TIME COMPUTER Feb. 13, 1968 Filed July 14, 1964 JL'ZQ RATE OF PENETRATION "d" EXPONENT FIG. 3

INVENTORSI JAMES R. JORDEN, JR.

ORVAL J. SHIRLEY BY: 1% 2? Ma THEIR ATTORNEY DIFFERENTIAL PRESSURE FIG.

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INVENTORSI JAMES R. JORDEN, JR.

ORVAL J. SHIRLEY ep a 412M THEIR ATTORNEY Patented Feb. 13, 19 68 M METHOD FOR DETERMINING THE TOP OF ABNORMAL FORMATION PRESSURI'S James R. Jordel, Jr., and Orval 1. Shirley, Jefferson Parish, La., assipors to Shell Oil Company, New York, N.Y., a corporation of Delaware Filed July 14, 1964, Ser. No. 382,567

6 Claims. (Cl. 7315l.5)

ABSTRACT OF TIIE DISCLOSURE A process for detecting when a borehole enters a geopressured shale section utilizing the penetration rate of the drill bit as the measured variable. The penetration rate for the shale sections is plotted to determine the rate of change inthe penetration rate, and the top of the geopressured section is. detected by locating the depth at which the rate of change in the rate of penetration distinctly changes. t f

,gl -Thjis invention pertains to a method utilizing drilling penetration rate data for determining where a well enters a section in which the fluid pressures exceed the hydrostatic head.

In many well drilling operations, it is desirable to determine the fluid pressure of the various formations pene trated by the borehole. For example, when wells are drilled in formations having fluid pressures that exceed thh'ydrostatic head for the depth of the formation it is necessary to take certain precautions to prevent undue damage from the abnormally high pressure. These formations are referred to asgeopressure formations and are more fully described in a copending application of C. A. Stuart entitled Drilling Geopressured Formations filed April 6, 1964, Ser. No. 357,485. The cost of drilling a 15,000 foot well with 5,000 feet of geopressured section is frequently two to four times the cost of drilling a 15,000 foot well in 'a hydropressured section. This cost could be considerably reduced if it were possible to detect ar'what depth the borehole enters the geopressured sectionf Knowing where the geopressured section is penetrat'ed would permit the use of normal drilling techniques until this penetration. The ability to use normal techniques for part of the well would considerably reduce the over-all cost of drilling the well. The added cost of drilling a geopressure well results from the increased cost of the drilling mud due to the requirement that it berheavier to contain the high pressures. The increased weight of the drilling mud often results in a decreasein theid'rilling rate and more frequent fishing jobs to recover stuclt'dn'll pipe. In addition the frequency of lost circulatio'ir' is increased resulting in an increased number of junked wells. In addition to the increased costs of using heav'ier muds, costs are further increased by the necessit'y fof employing heavier casing and additional strings of'casing than are used in hydropressured formations. use of a larger number of easing strings increases theEpbssibility that the well will not reach its target depth before the casing becomes too small.

Accordingly, it is the principal object of this invention to provide a novel method for detecting where a well penetrates a geopressured formation prior to the penetration of a permeable reservoir within the geopressured formation. I

Another object of this invention is to provide a unique method using drilling performance data to determine where a well penetrates a geopressured formation prior to penetration of a permeable reservoir within the geopressured formation.

The aboveobjects and advantages of this invention are achieved by determining the normalized rate of penetration of the drill bit through the shale formations traversed by the borehole. The measurements of penetration rate 'are made at frequent intervals during the drilling of the well and a plot of the penetration rate with depth is maintained. As the borehole enters the geopressured formations, the rate at which the normalized rate of penetration in the shale sectionsdecreases with each increment of depth becomes less than the rate. exhibited in the hydropressured formations. At this point,

the penetration rate increases with respect to'that which I would be predicted from the rate of decreasein the hydropressured formations. The point at which the normali zed rate of penetration in the shale sections increases indicates the penetration of the geopressured section. Thus, it is possible to drill the portionof the well above the geopressured section using normal drilling tech niques and only the portion of the well. whichactually penetrates the geopressured section will require ,t he high cost special techniques. This will ,result in ari ove'rall reduction of the drilling cost of the complete well.

The 'above objects and advantages of this invention will be more easily understood from the following detailed description of a preferred embodiment of the method when taken in conjunction with the attached drawing, in which:

FIGURE 1 is a schematic plot of the pressure gradient versus depth for the formations of the Texas-Louisiana Gulf Coast region of the United States, and a schematic plot of the pressure gradient versus depth of the drilling fluid in the borehole of 'a typical well drilled in this geographical location;

FIGURE 2 is a schematic plot of the ditferential'pres sure between the formations and the drilling fluid in the wellbore, and is derived from FIGURE 1;

FIGURE 3 is a schematic plot of the normalized" rate of penetration in shale formations with depth for a complete well showing the change in penetration rate 'as the borehole penetrates the geopressured section;

FIGURE 4 is a drawing of a system capable of performing the method of this invention and FIGURE 5 is a nomogram for normalizing drilling penetration rates.

Geopressured sections occur in many different geographical locations where it is desired to drill oil wells. One particular geographic location is the Texas-Ipuisi'ana Gulf Coast region of the United States that has tertiary shale formations of great thickness. These shale forrnations are usually deep water marine shales that contain few sand formations and are subjected essentially to a uniaxial compaction as a result of the compressive stress of the overburden.

In order for a shale formation to compact the fluids contained in the formation must be removed. The fluids can only be removed by flowing into sand formations or other permeable avenues of escape. Since the thick shale formations that occur in the Gulf Coast area have very few sand formations to act as avenues of escape the fluids are removed 'at a much slower rate than from thinner shale formations sandwiched between sand formations. This inability of the fluids to escape from the shale formations results in the creation of pressures within the formation that exceed the hydrostatic pressure for the depth of the formation. These excessive pressures are referred to as geopressures and the formations as geopressured formations.

The creation of geopressures within a shale formatio can be more easily understood by considering the follow ing conception of a shale model. The shale model is formed from perforated metal'plates which are separated by springs and water with the complete structure being enclosed within a cylindrical tube. The springs simulate the communication between the clay particles while the plates themselves simulate the clay particles. Upon the application of pressure to the uppermost plate the height of the springs between the plates remains unchanged as long as no water escapes from the system. Thus, in the initial stage the applied pressure is supported entirely by the equal and opposite pressure of the water. As the water escapes from the system through the perforations in the plate the uppermost plate will move downward slightly and the springs will carry part of the applied load. As more water escapes the springs will carry additional load until finally the complete axial load will be borne by the springs and the system will reach 'a state of equilibrium.

The clay particles forming a shale formation undergo a similar movement to that described above for the model when subjected to a uniaxial compaction due to the overburden. All formations are subject to an axial compaction but more permeable formations reach equilibrium much faster than shale formations. The inability of shale formations to reach equilibrium results in the occurrence of geopressured shales and geopressures in the fluids contained in permeable rocks which are enclosed within such shales. It can then easily be appreciated that the differential pressure between a column of drilling fluid of given density and the formation will be less in this case of geopressured formations than in the case of hydropressured formations.

Many mechanical factors affect the rate of penetration of the drill bit through shale formations including but not limited to, weight on the drill bit, rotary speed, drill bit diameter and design characteristics, drill bit wear, and drilling fluid circulation rate. The rate of penetration is also affected by the strength characteristics of the shale formations and the differential pressure between the column of drilling fluid and the shale formations. It is generally agreed within the petroleum industry that rate of penetration increases as the differential pressure decreases.

In order to utilize rate of penetration data to identify changes in differential pressure, all mechanical variables which influence rate of penetration should be equated to a common base, that is, normalized rate of penetration data should be used. The preferred method to normalize rate of penetration data is to hold all mechanical drilling variables and the mud density constant while operating the drill bit. With mechanical variables held constant, the rate of penetration is controlled primarily by the strength characteristics of the formation and the pressure difference between the mud column and the formation being drilled. In normally pressured shale formations, the strength increases with depth so that a gradual decrease in penetration rate will occur as a well is deepened. With constant mud density the pressure difference between the mud column and formation also increases as shown by FIGURES 1 and 2 causing additional decrease in penetration rate. When a geopressured shale is penetrated the pressure difference decreases sharply causing an increased rate of penetration which identifies the top of the geopressured section.

If it is not possible to hold mechanical drilling variables constant, an approximate method to normalize some of the more important variables is available by use of the equation wherein R=rate of penetration, feet per hour N=rotary speed, revolutions per minute W- -'weight'on drill bit, pounds D 'driIlbit diameter, inches d=empirical exponent required to equate left and right sides of equation Referring now to FIGURE 3 there is shown a sche matic plot of the normalized penetration rate in shale formations versus depth for a well drilled into geopressured formations. The section of the curve 7 represents the portion of the well drilled in a hydropressured section.

At the point 8, the penetration rate changes and starts to increase. Point 8 then is the top of the geopressured section. The penetration rate. then increases over an interval 9.

From the information plotted in FIGURE 3 it is easily appreciated that the point 8 which is indicated by an increase in the normalized penetration rate in shale formations denotes the point at which the borehole entered the geopressured section. Thus, it would be possible to drill the well using normal techniques until the depth indicated by the point 8 was reached. At this point it would be necessary, of course, to adapt the techniques used for drilling the geopressured wells.

The data shown in FIGURE 3 is for a single geological age, for example, the Miocene. Other geological ages, such as the Oligocene or the Eocene will have similar characteristics but slightly different values. These values can easily be determined by utilizing the method of this invention.

Referring now to FIGURE 4 there is shown a well that penetrates a hydropressured section 1. and enters a geopressured section 11 while the interface between the two sections is shown at 12. Of course it should be remembered that there is no sharp definition between a hydropressured and a geopressured formation, but rather a change from one to the other over a few feet. The well is shown as having a surface casing 13 installed for the first few hundred feet of the well and then a protective liner 1 that is installed so that it completely cases off the hydropressured formation 10 and extends into the geopressured formation 11. The drilling of a well and setting of surface casing and protective liner are more fully described in the above-referenced copending application of C. A. Stuart. The well is drilled by means of the conventional rotary bit 15 that is rotated by means of a drill string 16. The drill string extends to the surface where it passes through the conventional well head 21 that includes the blowout preventers and other equipment well known to those skilled in the art of rotary well drilling. The drill string 16 is rotated by means of a rotary table 16. The upper end of the drill string is coupled to swivel joints 17 that has a U-shaped bail member 19 attached thereto. A portion of the weight of the drill string is supported by the hook 20 which is moved by the drilling rig not shown in FIGURE 4.

The depth of the well of course is related to the length of drill string extending into the well and may be determined by means of a calibrated wheel 22 which is rotated by the travel of the drill string 16 into or out of the well. The calibrated wheel 22 may be coupled to a conventional selsyn unit in order that the depth of the well can be converted to a related electrical signal. Similarly, the weight on the drill bit 15 is determined by a transducer assembly 23 which determines the hook load on the drilling rig 20. Thehook load can be related to the weight on the drill bit IS by subtracting the hook load as determined by the-transducer from the total weight of the drill string. Conventional equipment is available for converting the hook load to an electrical signal W of the well bore as determined by the-calibrated wheel- 22 is supplied toatime depth computer 24 which also receives an electrical signal 25 that represents time. The signal representirig time can be obtained from a suitable clock mechanism" or may be obtained directly from commercially available 60-cycle AC power. ,The time depth computer 24 then divides the depth by time to obtain a signal R representing the rate of penetration of the drill bit. The signal R is supplied to an operational amplifier 30 that also receives a signal N by means of a lead 27, wherein N represents the speed of the rotary table 16. The-.operationalamplifier 30 divides R by N and supplies the result to an operational amplifier 31. The operational amplifier 31 adjusts the product by a factor K wherein K represents 1 divided by 60. The operational amplifier 31 supplies the adjusted product to a logarithm converting unit 32 thatconverts the product to a related logarithm. In a sintilar manner a signal W representing the weight on the drill bit is suppliedto an operational amplifie'r 33 that alsoreceives a signal D representing the diameter of the drill bit. The signal D may be represented by a variable potential that is manually adjusted for changes in the'drill bit. The operational amplifier 33 divides W by D and supplies the result to an operational amplifier 34. The operational amplifier 34 adjusts the product by a factor K, wherein K, represents 12 divided by 1.. The operational amplifier 34 supplies the adjusted product to a logarithm converting unit 35 that converts the product to a related logarithm. Both of the related logarithms are supplied to an operational amplifier. 36 that divides the two logarithms and supplies a signal related to d. The operational amplifier 36 is coupled to a.chart recorder 38 by a lead 37 in order that the value of d maybe recorded. The chart recorder is driven by a signal related to the depth of the well and thus the record is related to depth. The record 39 illustrated in FIGURE 4 is related to the well shown and thus indicates; an increase in the penetration rate at a point 40. The point 4. corresponds to the top 12 of the geopressure formation 11.

.lWhile an analog system has been describedabove for normalizing the penetration rates obviously other systems such as digital systems could be used. Likewise a range of; values for the various terms could be plotted in a logarithm form on a monogram and value of d obtained. A suitable monogram is shown in FIGURE 5 with a representative solution for one set of data. To use the monogram there must be available the rate of penetration, the rotary-speed, bit weight and bit diameter. These factors are either known or can readily be determined by equipment that is normally available in the rotary drilling art. Assuming that thefrate of penetration is feet per hourand the rotary speed is 100 r.p.m., these values are plotted on the two left hand scales in FIGURE 5 and a line drawn between the two values. This line is extended until it crosses the third scale where the value for R/60N is 'ob'tained. Next the bit weight and bit diameter are plotted on the two right hand scales and a line drawn between the plotted values. When this line is extended the value' of l2W/10'D is obtained; a line connecting the value of R/60N and IZW/IO'D provides the value of the "exponent d or 1.65 for the example shown. The monogram of FIGURE 5 permits the rapid obtaining of normalized values of the drilling penetration rate that may be plotted as shown in FIGURE 3. As explained above when the penetration rate increases it indicates that the borehole has penetrated the geopressured formation.

Normally the drilling rate is slow enough to permit the use of the monogram of FIGURE 5 and hand plotting of the values of d to detect the penetration of geopressured formation. Thus, the expense of a complete computer systeirrto calculate the value of the exponent d can be avoided.

fweclaim as our invention:

LA methodfor detecting the depth at which a bore hole enters geopressured shale sections, said method con1- prising: 7

measuring a rate of penetration property of only shale 5 sections penetrated by the borehole;

recording said measured rate of penetration property of thefshale sections with relation to depth;

determining from said recording the rate of change in said rate of penetration property and detecting the m top of the geopressured shale section by locating the depth at which said am of change distinctly changes.

2. A method for detecting the depth at which a borehole enters geopressured shale sections, said method com prising:

normalizing mechanical drilling variables including, but not limited to, weight on bit, rotary speed, bit size circulation rate and mud density of maintaining all such variables constant;

recording the resultant rate of penetration in shale formations with relation to depth of the drill bit; and detecting the top of the geopressured shale sections by locating on the recording the depth at which the rate of decrease of said rate of penetration distinctly changes. 3. A method for detecting the depth at which a bore hole enters geopressured shale sections said method coinprising:

normalizing mechanical drilling variables including but not limited to weight on bit, rotary speed, bit size and circulation rate in a manner that establishes a normalized rate ofpenetration in which the effects of mechanical variables are essentially removed;

recording the result 5normalized" rate of penetration in shale formations with relation to the depth of the drill bit; and

detecting the top of the geopressured shale sections by locating on the recording the depth at which the rate of penetration distinctly changes.

4. An improvement in the process of drilling a well by rotating a drill bit, circulating a drilling fluid, and casing the walls of the borehole, said improvement comprising:

initiating the drilling employing a drilling fluid and a casing program designed for fluid pressure substantialthe depth of the borehole;

measuring a rate of penetration property of the shale formations penetrated by 'the borehole;

determining the rate of variation with depth of the rate of penetration property of the shale formations encountered by the well;

determining the top of the geopressured shale section by locating the depth at which said determined rate of variation undergoes a change in the direction of an anomalous lack of decrease in penetration rate with depth; and resuming the drilling employing a casing program and a drilling fluid designed for fluid pressures significantly greater than the hydrostatic head corresponding to the depth of the borehole.

S. A process for detecting the depth at which a bore hole enters geopressured shale sections, said process comprising:

measuring the weight on the drill bit, rotary speed, hit

size; measuringthe rate of penetration property of only the shale sections penetrated by the borehole;

correcting the measured rate of penetration property of the shale sections for variations in the measured weight on the drill bit, rotary speed and bit size;

plotting the corrected rate of penetration property of the shale sections with relation to depth; and

detecting the top of the geopressured shale sections by locating on the plotted record the depth at which the rate of change in the corrected rate of penetra= tion distinctly changes.

ly equaling the hydrostatic head corresponding to 7 8 v 6. The process of claim 5 wherein said measured rate 2,539,758 1/1951 Silverman et al 73-1515 of penetration is corrected by converting it to a d ex- 2,550,420 4/ 1951 McNatt 73-1515 ponent, wherein said d exponent is obtained from a nomo- OTHER REFERENCES gram having penetration rate and rotary speed plotted to a log scale at one side and bit weight and bit size 5 McCray, A. W., et al.: Oil Well Drilling Technology. plotted to a log scale at the opposite side, said d ex- Norman, Oklahoma. University of Oklahoma Press. 1959. ponent being plotted to a log scale at the center of the 1st edition. Pages 69 and 283.

nomogl'am.

R ferences Cl RICHARD C. QUEISSER, Primary Examiner.

UNITED STATESPATENTS 10 JAMES GILL, Examiner. 2,357,051 8/1944 McLaine 73-l5l.5 X J. W. MYRACLE, Assistant Examiner.

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