US3750766A

US3750766A - Controlling subsurface pressures while drilling with oil base muds

Info

Publication number: US3750766A
Application number: US00193322A
Authority: US
Inventors: G Thompson; L Carlton
Original assignee: Exxon Production Research Co
Current assignee: ExxonMobil Upstream Research Co
Priority date: 1971-10-28
Filing date: 1971-10-28
Publication date: 1973-08-07
Anticipated expiration: 1990-08-07

Abstract

The water percents by volume of input and output circulating oil muds are measured and compared during well drilling operations. When the water percent measurements are made at the surface, the output measurements lag the input measurements so that the same portions of drilling mud are compared. The measurements are made continually (at frequent intervals) or continuously. Accurate measurement of the liquid volume of the water phase emulsified in the oil mud, positive or negative differentials of in and out water percents and overall changes in water percents will determine whether the formation pore pressure is exceeding the hydrostatic pressure imposed by the drilling mud. When formation pressure exceeds the hydrostatic pressure, water from the wet subsurface formations being penetrated enters the well bore and is imbibed (mixed and probably emulsified) in the oil mud. Control of the well is maintained by adjusting drilling mud weight.

Description

Thompson et al.

CONTROLLING SUBSURFACE PRESSURES WHILE DRILLING WITH OIL BASE MUDS Filed:

Inventors: Gene D. Thompson, Louis A.

Carlton, both of New Orleans, La.

Assignee: Esao Production Research Company, Houston, Tex.

Oct. 28, 1971 Appl. No.: 193,322

Primary Examiner-Marvin A. Champion Assistant Examiner-Richard E. Favreau Attorney-Thomas B. McCulloch et al.

[ 57] ABSTRACT The water percents by volume of input and output circulating oil muds are measured and compared during well drilling operations. When the water percent measurements are made at the surface, the output measurements lag the input measurements so that the same portions of drilling mud are compared. The measurements are made continually (at frequent intervals) or continuously. Accurate measurement of the liquid volume of the water phase emulsif ed in the oil mud, positive or negative differentials of in and out water percents and overall changes in water percents will determine whether the formation pore pressure is exceeding the hydrostatic pressure imposed by the drilling mud. When formation pressure exceeds the hydrostatic pressure, water from the wet subsurface formations being penetrated enters the well bore and is imbibed (mixed and probably emulsified) in the oil mud. Control of the well is maintained by adjusting drilling mud weight.

.17 Claims, 1 Drawing Figure our PUT Aug. 7, 1973 PATENFEU 7W5 3.750.766

INPUT OUTPUT 1 l k)\\\\\\\\\ l GENE D. THOMPSON, LOUIS A.CARLTON,

5 INVENTORS. n

Z Y BY JOHN S. SCHNEIDER ATTORNEY.

CONTROLLING SUBSURFACE PRESSURES WHILE DRILLING WITH OIL BASE MUDS BACKGROUND OF THE INVENTION Early detection and evaluation of abnormally pressured formations minimizes or avoids potential or actual blowouts. When abnormally pressurized formations are detected, drilling mud weight is increased to balance the increased pressure of the abnormally pressured formations. Many techniques, such as measurement of shale densities, formation conductivity, formation resistivity, formation acoustic travel times, drilling rate, mud gas and chloride contents of drilling mud, have been used heretofore to detect and evaluate formation pore pressures.

In U. S. Pat. application Ser. No. 145,833 filed May 2l, 1971, by Louis A. Carlton et al. entitled DETER- MINATION F SUBSURFACE FORMATION PRES- SURES, a technique is described and claimed in which small subtle increases in salinity of the drilling mud are measured. Both the input and output drilling mud salinities are measured at frequent intervals or continuously and when compared trends and changes in the salinity of the drilling mud are readily recognized. An increasing or fluctuating trend in salinity of the drilling mud indicates a flow of salt water from subsurface formations open to the well bore. Such flow is evidence that formation pore pressure is exceeding the hydrostatic pressure imposed by the drilling mud column. Increasing the drilling mud weight until the influx of formation fluids is stopped minimizes the problems associated with taking formation fluid kicks.

The technique disclosed and claimed in U. S. Pat. application Ser. No. 145,833 is not applicable to oil base or invert oil muds (referred to herein by the term oil muds) because of the oil external phase. Thus, for deep wells which require oil muds there is a need for a technique to determine subsurface pressures.

During conventional drilling operations, the oil industry has learned to determine chlorides of the water phase internal emulsion in an oil mud by taking a sample, adding strong emulsion breakers, and then determining the parts per million chlorides in the water by conventional titration. The water percent by volume in oil muds is determined by a retort process which prohibits direct chloride measurements. Furthermore, the salinity (sodium chloride-NaCl and calcium chloride- CaCl content of the water phase in an oil mud is often varied for the purpose of shale stability. Consequently, this batch" process doe not permit continuous salinity measurements as conducted in the aforementioned U. S. Pat. application Ser. No. 145,833.

A significant improvement of the present invention over past practices resides in measuring water percent by volume in the oil mud continuously or at frequent intervals to achieve the same end result with approximately the same accuracy using oil muds as achieved with water base muds. The differential (increase or decrease) in water percent of the oil mud system before and after circulating down the drill pipe and back up the annulus opposite the new formations just drilled is determined. Rather than measuring and comparing the salinity of the input and output circulating drilling mud while drilling as when drilling with water base muds, the percent water in an oil mud is measured. The output measurement of percent water is properly lagged and compared with the water percent of the same fluid as it occurred at the input conditions. Metering or other techniques may be used to measure the liquid volume of the water phase emulsified in the oil mud, positive or negative differential and overall changes in water percents determine whether the fonnation pore pressure is exceeding the hydrostatic pressure imposed by the drilling mud. When the formation pressure exceeds the hydrostatic pressure, water from the wet sub surface formations being penetrated will enter the well bore and be imbibed (mixed and probably emulsified) in the oil mud.

SUMMARY OF THE INVENTION.

Briefly, the present invention involves a method for determining subsurface formation pressures which comprise measuring and comparing the water percents of the input and output oil muds being circulated during drilling of a well. The output measurement follows the input measurement by a lag time period which corresponds to the time period required for the drilling mud to circulate from the input measurement point to the output measurement point. The water percent measurements are made at frequent intervals or continuously.

BRIEF DESCRIPTION OF THE DRAWINGS The sole FIGURE illustrates one mode for carrying out the method of the invention.

DETAILED DESCRIPTION OF THE INVENTION Referring to the figure there is shown a borehole 9 being drilled through subsurface formations indicated at 10 by a drill bit 11 connected to the lower end of a drill string 12 which extends upwardly to the rotary table 13 through casing pipe 15, wellhead 16 and blowout preventer equipment 17. A returns or output flowline 22 is connected to the annulus surrounding drill pipe 12 above the blowout preventer to return the circulating drilling mud to a mud pit 23. An input suction line 24 is located in mud pit 23 and is connected to a circulating pump 25 which, in turn, is connected to an input flowline 26 which, as indicated by the dotted lines 27, connects to the upper end of drill pipe 12. A water percent detector unit 30 is connected into flowline 22 and a similar detector unit 31 is connected into mud pit 23 near the suction end of line 24. These detector units are shown connected to

suitable recorders

35 and 36 as indicated by

lines

32 and 33, respectively.

As illustrative of the operation of the apparatus of the figure, drilling mud is circulated by means of pump 25 from mud pit 23 through suction line 24 and discharge line 26 into drill pipe 12. Drilling mud is circulated through drill bit 11 and up the annulus between drill pipe 12 and the wall of borehole 9 and the annulus between drill pipe 12 and easing pipe 15 and through wellhead l6 and blowout preventer l7 and into return line 22 and mud pit 23.

Detector units

30 and 31 measure properties of the drilling muds which enable determination of the percent water in the drilling muds. The water percent measurements on the chart of recorder 35 may be adjusted for lag time byphysically shifting this chart relative to the chart on recorder 36. Lag time is readily calculable from the pumping rate, the depth and diameter of the hole being drilled and the size of the drill pipe. Of course, periodic adjustments may be made to increase the lag time as the borehole being drilled increases in depth. Lag time adjustment and plotting or recording of the water percents of the input and output drilling muds may be performed automatically. One way to compensate for lag time automatically would be to use a circular recorder chart and two pins, one for recording output mud water percent and the other for recording input mud water percent. The angle between the recording pins would represent lag time when a constant pump rate is used. When the pump rate is changed or the lag time is changed, the angle between the pins is adjusted to reflect such change. A differential water percent log may be automatically plotted by electronically subtracting the input water percent measurements from the output water percent measurements prior to recording these measurements. Also, a computer or other memory type device might be used to store the input water percent data for the lag time period prior to substracting the input from the output measurement.

However, plotting of the water percent data may be performed manually. An operator may take readings of the water percent devices at frequent intervals and plot the results. In making these plots the operator would take into account lag time.

Oil mud water percents may be measured downhole, preferably near the formation being drilled, to detect changes in the water percents in the input and output drilling mud as the formation is being drilled. These data may be transmitted continuously or at frequent intervals to suitable service recording devices. The procedure is the same as when making surface measurements except no lag time adjustments are necessary. The devices for measuring water percents of the input and output drilling muds would be preferably mounted on the interior and exterior of drill collars or special subs located adjacent the drill bit. Suitably modified conventional well logging tools might be employed in conducting these operations.

The liquid phase of conventional oil muds may vary in water percents from approximately 4 to 40 percent water. Any influx of subsurface water from a porous formation during drilling will be detected by the water percent measuring devices and such data will be utilized by the operators as necessary to increase the mud weight (hydrostatic pressure) to control the well and prevent a blowout. The process is repeated during the entire drilling operation in which the mud weight is raised each time a differential increase in percent water occurs until the well reaches total depth.

Measuring the water percents of the input and output drilling muds at frequent intervals or continuously avoids drastic changes in mud flow properties by the detection of small formation fluid influxes and adjusting mud weights before enough contamination has entered the mud system to thicken the mud substantially. Use of the technique of this invention to control mud weights will substantially reduce the occurrence of lost returns caused by excessive mud density. In addition to the time spent circulating and conditioning mud due to contamination will be reduced. When a drilling mud has thickened substantially, operators are frequently forced to stop drilling and thin the mud in order to avoid lost circulation. The lost time required to condition the mud may total several days in deep high pressure wells and may substantially increase costs for rig time and mud materials.

Meters for measuring the water percent accurately (0.1 to L percent) may be of various types. Any of the various kinds of sensing devices capable of sensing the percentage of a component in a fluid mixture and which produce a signal having a quantitative response indicative of that percentage may be used. For example, the device may be a combined nuclear density gauge and amplifier which provides a sensitive and accurate measure of the specific gravity and instantaneous percent water of the oil mud. Another type of sensing device which may be used includes a circuit containing a capacitance cell to sense the dielectric constant of the oil mud passing through that cell. The capacitance cell may be contacted with the oil mud by means of a probe inserted in the oil mud. The capacitance of the cell is directly proportional to the dielectric constant of the oil mud flowing through the cell and the dielectric constant changes relative to changes in the percent water of the oil mud. The percent water by volume in the oil mud may also be determined by known techniques such as the retort technique in which the water is separated from the oil mud by distillation and measured.

Changes and'modifications may be made in the illustrative embodiments shown and described herein without departing from the scope of the invention as defined in the appended claims.

Having fully described the nature, objects, operation and advantages of our invention, we claim:

1. A method for determining subsurface formation pressures during the drilling of a well comprising the steps of:

measuring the percent water of the input and output oil muds being circulated in a well during drilling thereof, said output drilling mud water percent measurement following said input drilling mud water percent measurement by a lag time measurement which corresponds to the time period required to circulate said drilling mud from the input measurement point to the output measurement point; and

comparing said input and output drilling mud water percent measurements, variations in said input and output water percent measurements providing indications of said formation pressures.

2. A method as recited in claim 1 in which said water percent measurements are made continually at frequent intervals.

3. A method as recited in claim 1 in which said water percent measurements are made continuously.

4. A method as recited in claim 3 in which said water percent measurements are made by measuring electrical properties of said drilling muds.

5. A method for detennining subsurface formation pressures and controlling a well during the drilling thereof comprising the steps of:

circulating an oil mud down a drill string and up the annulus surrounding the drill string to the earth's surface;

measuring water percents of the input and output oil muds, said output oil mud water percent measurement following said input oil mud water percent measurement by a lag time period which corresponds to the time period required to circulate said oil mud from said input measurement point to said output measurement point;

comparing said input oil mud and said output oil mud water percent measurements; and

adjusting weight of said oil mud in response to said water percent measurements of said drilling muds.

6. A method as recited in claim 5 in which said water percent measurements are made continually at frequent intervals.

7. A method as recited in claim 5 in which said water percent measurements are made continuously.

8. A method as recited in claim 5 in which said water percent measurements are made by measuring electrical properties of said drilling muds.

9. A method as recited in claim 5 in which said measured water percents in the input and output oil muds are separately plotted.

10. A method for determining subsurface formation pressures during the drilling of a well comprising the steps of:

measuring the water percent of the input drilling muds being circulated in a well during drilling thereof;

plotting in a first curve the water percent of said input drilling muds versus depth of said wells; measuring the water percent of the output drilling being circulated in said well during drilling thereof; plotting in a second curve the water percent of said output drilling mud versus depth of said well; and comparing said first and second curves, variations therein providing indications of said formation pressures.

1 l. A method as recited in claim in which the output drilling mud measurements are compared with the input drilling measurements by a lag time period which corresponds to the time period required to circulate said drilling mud from the input measurement point to the output measurement point.

12. A method as recited in claim 11 in which said water percent measurements plotted in said first and second curves are measured by measuring electrical properties of said drilling mud.

13. A method for determining subsurface formation pressures duringthe drilling of a well comprising the steps of:

simultaneously measuring water percents of the input and output oil muds being circulated in a well duringdrilling thereof; and

comparing the input and output oil mud water percent measurements of the same portions of oil mud, variations in said input and output water percent measurements providing indications of said formation pressures.

14. A method as recited in claim 13 in which said water percent measurements are made continuously.

15. A method as recited in claim 14 in which said water percent measurements are made by measuring electrical properties of said oil muds.

16. A method as recited in claim 14 in which said water percent measurements are made continually at frequent intervals.

17. A method as recited in claim 16 in which said water percent measurements are made by measuring electrical properties of said oil muds.

Claims

1. A method for determining subsurface formation pressures during the drilling of a well comprising the steps of: measuring the percent water of the input and output oil muds being circulated in a well during drilling thereof, said output drilling mud water percent measurement following said input drilling mud water percent measurement by a lag time measurement which corresponds to the time period required to circulate said drilling mud from the input measurement point to the output measurement point; and comparing said input and output drilling mud water percent measurements, variations in said input and output water percent measurements providing indications of said formation pressures.

5. A method for determining subsurface formation pressures and controlling a well during the drilling thereof comprising the steps of: circulating an oil mud down a drill string and up the annulus surrounding the drill string to the earth''s surface; measuring water percents of the input and output oil muds, said output oil mud water percent measurement following said input oil mud water percent measurement by a lag time period which corresponds to the time period required to circulate said oil mud from said input measurement point to said output measurement point; comparing said input oil mud and said output oil mud water percent measurements; and adjusting weight of said oil mud in response to said water percent measurements of said drilling muds.

10. A method for determining subsurface formation pressures during the drilling of a well comprising the steps of: measuring the water percent of the input drilling muds being circulated in a well during drilling thereof; plotting in a first curve the water percent of said input drilling muds versus depth of said wells; measuring the water percent of the output drilling being circulated in said well during drilling thereof; plotting in a second curve the water percent of said output drilling mud versus depth of said well; and comparing said first and second curves, variations therein providing indications of said formation pressures.

11. A method as recited in claim 10 in which the output drilling mud measurements are compared with the input drilling measurements by a lag time period which corresponds to the time period required to circulate said drilling mud from the input measurement point to the output measurement point.

13. A method for determining subsurface formation pressures during the drilling of a well comprising the steps of: simultaneously measuring water percents of the input and output oil muds being circulated in a well during drilling thereof; and comparing the input and output oil mud water percent measurements of the same portions of oil mud, variations in said input and output water percent measurements providing indications of said formation pressures.