WO2014058745A2

WO2014058745A2 - System and method for monitoring fracture treatment using optical fiber sensors in monitor wellbores

Info

Publication number: WO2014058745A2
Application number: PCT/US2013/063609
Authority: WO
Inventors: Michael S. Bahorich
Original assignee: Apache Corporation
Priority date: 2012-10-09
Filing date: 2013-10-07
Publication date: 2014-04-17
Also published as: WO2014058745A3

Abstract

A method for monitoring movement of a hydraulic fracturing fluid through a subsurface formation includes measuring at least one of a pressure and a temperature of a fluid using an optical fiber sensor disposed in at least one wellbore drilled through the subsurface formation at a selected position away from a location wherein the hydraulic fracturing fluid is pumped into the subsurface formation. A position of a fracturing fluid front is inferred using at least one of the measured pressure and measured temperature.

Description

SYSTEM AND METHOD FOR MONITORING FRACTURE TREATMENT USING OPTICAL FIBER SENSORS IN MONITOR WELLBORES

Background

[0001] This disclosure relates generally to the field of hydraulic fracture treatment of subsurface formations through wellbores drilled therethrough. More specifically, this disclosure relates to methods and systems for monitoring movement of fracturing fluid through subsurface formations using optical fiber sensors disposed in one or more monitor wells drilled through the formation being fracture treated.

[0002] Hydraulic fracturing includes pumping fluid under pressure into a subsurface formation so that the fracture pressure of the formation is exceeded and the fluid can create openings (fractures) in the formation. The fluid may include particles of material called "proppant" that causes the fractures to remain open after the fluid pressure is relieved and fluid from the formations is allowed to flow therefrom into the wellbore. Hydraulic fracturing may be used, for example, to increase the effective wellbore radius of low permeability formations, or to hydraulically interconnect naturally occurring fractures that may be present in some subsurface formations so as to create a flow path from such connected fractures to the wellbore. Such flow paths may enable economical production of gas and/or oil from the natural fractures.

[0003] It is desirable during pumping of a hydraulic fracture treatment to determine where the pumped fluid has traveled within the subsurface formations. Various techniques are known in the art for determining movement over time of the "frac front", i.e., the leading edge of the body of fluid pumped into the formation. See, for example, U.S. Patent No. 7,663,970 issued to Duncan et al. which discloses a method for mapping the frac front using passive acoustic energy detection. It is also known in the art to use acoustic sensors such as geophones disposed in wellbores, called "monitor" wellbores drilled through the formation to be treated in a predetermined pattern. Detected acoustic energy may be correlated to movement of the frac front. Summary

[0004] One aspect is a method for monitoring movement of a hydraulic fracturing fluid through a subsurface formation including measuring at least one of a pressure and a temperature of a fluid using an optical fiber sensor disposed in at least one wellbore drilled through the subsurface formation at a selected position away from a location wherein the hydraulic fracturing fluid is pumped into the subsurface formation. A position of a fracturing fluid front is inferred using at least one of the measured pressure and measured temperature.

[0005] Other aspects and advantages of the invention will be apparent from the description and claims which follow.

Brief Description of the Drawings

[0006] FIG. 1 shows an example fracture pumping operation.

[0007] FIG. 2 shows an example arrangement of monitor wellbores used during a fracture pumping operation such as shown in FIG. 1.

[0008] FIG. 3 shows example sensors that may be used in one or more monitor wellbores.

Detailed Description

[0009] In FIG. 1, each of a plurality of seismic sensors, shown generally at 12, may be deployed at a selected position proximate the Earth's surface 14. In marine applications, the seismic sensors may be deployed on the water bottom in a device known as an "ocean bottom cable." The seismic sensors 12 in the present embodiment may be geophones, but may also be accelerometers or any other sensing device known in the art that is responsive to velocity, acceleration or motion of the particles of the Earth proximate the sensor. The seismic sensors 12 may generate electrical or optical signals in response to the particle motion or acceleration, and such signals are ultimately coupled to a recording unit 10 for making a time-indexed recording of the signals from each sensor 12 for later interpretation by any of several methods. In other implementations, the seismic sensors may be disposed at various positions within a wellbore drilled through the subsurface formations. The foregoing disposition of seismic sensors in wellbores will be further explained with reference to FIGS. 2 and 3.

[0010] In some embodiments, the seismic sensors 12 may be arranged in sub-groups having spacing therebetween less than about one-half the expected wavelength of seismic energy from the Earth's subsurface that is intended to be detected. Signals from all the sensors in one or more of the sub-groups may be added or summed to reduce the effects of noise, e.g., horizontally propagating noise, in the detected signals.

[0011] In other embodiments, the seismic sensors 12 may be placed in one or more wellbores, either permanently for certain long-term monitoring applications, or temporarily, such as by wireline conveyance, slickline conveyance, tubing or pipe conveyance or any other sensor conveyance technique known in the art. As will also be explained with reference to FIG. 3, such wellbores may include optical fiber type pressure and temperature sensors.

[0012] A wellbore 22 is shown drilled through various subsurface Earth formations 16,

18, through a hydrocarbon producing formation 20. A wellbore casing or tubing 24 having perforations 26 formed therein corresponding to the depth of the hydrocarbon producing formation 20 is connected to a valve set known as a wellhead 30 disposed at the Earth's surface. The wellhead may be hydraulically connected to a pump 34 in a fracture (frac) pumping unit 32. The frac pumping unit 32 is used in the process of pumping a fluid, which in some instances includes selected size or size range solid particles, collectively called "proppant", are disposed. Pumping such fluid, whether having proppant or otherwise, is known as hydraulic fracturing. The movement of the fluid is shown schematically at a leading edge thereof or "fluid front" 28 in FIG. 1. In hydraulic fracturing techniques known in the art, the fluid is pumped at a pressure which exceeds the fracture pressure of the particular producing formation 20, causing it to rupture, and form fissures therein. The fracture pressure is generally related to the pressure exerted by the weight of all the formations 16, 18 disposed above the hydrocarbon producing formation 20, and such pressure is generally referred to as the "overburden pressure." In propped fracturing operations, the particles of the proppant move into such fissures and remain therein after the fluid pressure is reduced below the fracture pressure of the formation 20. The proppant, by appropriate selection of particle size distribution and shape, forms one or more high permeability channels in the formation 20 that may extend a great lateral distance away from the tubing 24, and such channels remain permeable after the pumped fluid pressure is relieved. The effect of the proppant filled channels is to increase the effective radius of the wellbore 24 that is in hydraulic communication with the producing formation 20, thus substantially increasing productive capacity of the wellbore 24 to hydrocarbons.

[0013] The fracturing of the formation 20 by the fluid pressure creates seismic energy that may be detected by the seismic sensors 12. The time at which the seismic energy is detected by each of the sensors 12 with respect to the time-dependent position in the subsurface of the formation fracture caused at the fluid front 28 is related to the acoustic velocity of each of the formations 16, 18, 20, and the position of each of the seismic sensors 12.

[0014] Signals from the seismic sensors 12 may be processed, for example, using a method described in U.S. Patent No. 7,663,970 issued to Duncan et al. Such processing may enable mapping of the fluid front 28 using signals detected as a result of fracturing of the formation. In other examples, the foregoing processing may be omitted. In some examples, the recording unit may include a processor or programmable computer, as shown at 10A, to execute the method described in the Duncan et al. '970 patent or other processes as will be further described below

[0015] FIG. 2 shows an example arrangement of monitor wellbores 112 in relation to the location of the wellbore 22 being fracture treated. One or more of the monitor wellbores 112 may include sensors therein as will be explained with reference to FIG. 3. The sensors (FIG. 3) may be in signal communication with the recording unit 10 so as to enable monitoring of movement of the frac front (28 in FIG. 1) with respect to time.

[0016] FIG. 3 shows an example fiber optic pressure and temperature sensing system that may be used in any one or more of the monitor wellbores (112) shown in FIG. 2, and/or may be used in the fracture treating wellbore shown at 22 in FIG. 1. The example sensors shown in FIG. 3 may be those described in detail in U.S. Patent No. 6,351,987 issued to Winston et al, however, the configuration of the sensors is not intended to limit the scope of the present disclosure.

[0017] A pressure and temperature sensor system 110 is shown in FIG. 3 being used to measure the pressure and temperature of a wellbore fluid 111 (e.g., the fracture fluid) moving through tubing or by the wellbore 112, having production tubing 112a with particularly formed walls as will be explained further. The production tubing 112a may in turn be enclosed in a casing 113. The casing 113, which has substantial walls, is indicated merely by a dashed line. The wellbore fluid 111 has a direction of flow indicated by an arrow 114 if allowed to escape from the monitor wellbore 112, or may not move vertically if the upper portion of the monitor wellbore 112 is hydraulically closed.

[0018] A pressure and temperature measuring optical sensor system 110 may be affixed to the production tubing 112a, inside the casing 113, and may include a first optical sensor 110a and a second optical sensor 110b. The two optical sensors 110a, 110b each include a sensing material 116a, 116b disposed within a respective mandrel 117a, 117b. The sensing materials 116a, 116b are different in each optical sensor 110a, 110b, especially in how the velocity of sound in each sensing material depends on the temperature and pressure of the sensing material. Examples of such sensing materials may include, for example, different compositions of oil. With the exception that the two sensing materials 116a, 116b are different, the first 110a and second 110b optical sensors 110a, 110b may be substantially identical. It is important to understand that the sensing material can be a substance different than oil, and can even be a gas. It is only necessary that sound travel at a rate that depends on the pressure and temperature of the sensing material in the second optical sensor 110b in a measurably different way than it depends on the pressure and temperature of the sensing material in the first optical sensor 110a, as will be explained in more detail below.

[0019] The fluid 111 in the tubing 112a may be in contact with a thin diaphragm or bellows 115a, 115b of both the first optical sensor 110a and the second optical sensor 110b, respectively of the pressure and temperature optical sensor system 110. The pressure and also the temperature of the wellbore fluid 111 may be communicated through each thin diaphragm or bellows 115a, 115b to the sensing material 116a, 116b within the corresponding optical sensor 110a, 110b. In the present example the sensing materials 116a, 116b within each optical sensor 110a, 110b may be at the same pressure and temperature as the wellbore fluid 111, but it is only necessary that there be some known correlation between the pressure and temperature of the wellbore fluid 111 and that of each of sensing material in each optical sensor 110a, 110b.

[0020] The pressure and temperature optical sensor system 110 will now be described specifically with respect to the first optical sensor 110a, with the understanding that a similar description may apply to the second optical sensor 110b. The first optical sensor 110a includes a mandrel 117a. The mandrel 117a may have wound thereon optical fibers in at least two different known, spaced apart locations, preferably three locations but sometimes more than three locations. The resulting optical fiber windings 118a, 118b, 118c are used to convert a change in diameter of the mandrel 117a into an optical signal. The mandrel 117a may be enclosed in a mandrel housing 122 that protects the mandrel 117a and its array of optical fiber windings 118a, 118b, 118c from any backpressure that might be exerted by any material in the region 121 outside of the mandrel 117a, between the production tubing walls 112a and the casing 113 for the production tubing. Region 121 inside the mandrel housing 122 may include a low acoustic impedance fluid that acoustically isolates the mandrel 117a from the surrounding environment and provides a benign environment for the optical fibers. Preferably, the fluid in region 121 is an inert gas at low pressures, e.g., at surface atmospheric pressure.

[0021] In the present example, each optical fiber winding 118a, 118b, 118c may include a fiber Bragg grating 119a (FBG) at one end of the optical fiber winding 118a, 118b, 118c and another FBG 119b at the other end of the optical fiber winding 118a, 118b, 118c. Each FBG 119a, 119b is designed to reflect light at or near a particular selected wavelength. When light from a light source, which may be a narrow band light source forming part of part of a combined narrowband light source and signal processor 130, is introduced into one end of the optical fiber winding 118a, some of the light is reflected by each FBG 119a, 119b, and the distance between the two fiber Bragg gratings can be determined interferometrically. The total optical distance between the two FBGs 119a, 119b in a single fiber winding 118a, 118b, 118c may vary according to the pressure variations in the sensing material 116a. Because of this sensitivity to the pressure variations in the sensing material, the array of windings 118a, 118b, 118c can be used to determine the speed of sound in the sensing material 116a, 116b. The speed of sound in the sensing material 116a, 116b is one piece of information needed to determine the pressure and temperature of the sensing material 116a, 116b, and so also that of the fluid 111, since as explained above the optical sensor system 110 may be configured so that the sensing material 116a, 116b is at substantially the same pressure and temperature as the wellbore fluid 111 , or is at a pressure and temperature that can be readily correlated with the pressure and temperature of the wellbore fluid 111.

[0022] It is important to understand that unless some independent source of information on either the wellbore fluid 111 temperature or the wellbore fluid 111 pressure is available, the pressure and temperature optical sensor system 110 includes both a first optical sensor 110a and a second optical sensor 110b. Having a first optical sensor 110a and a second optical sensor 110b in combination providing a pressure and temperature optical sensor system 110 may be used because neither of the individual optical sensors 110a, 110b may provide information that in any way distinguishes whether the speed of sound in the sensing materials 116a, 116b changes because of a change in pressure of the sensing materials or because of change in temperature of the sensing materials or both. If it were possible to distinguish between a change in the speed of sound in the sensing materials 116a, 116b on account of a change in pressure compared to temperature, then only a single optical sensor 110a or 110b would be needed, because then an equation of state could relate the pressure to the temperature.

[0023] In the present example system, two optical sensors 110a, 110b may be used in the following way. In both the first optical sensor 110a and in the second optical sensor 110b, the speed of sound is measured by determining a time difference between changes in length of each of the optical fiber windings 118a, 118b, 118c as explained above. Essentially, the array of optical fiber windings 118a, 118b, 118c detects a pressure wave associated with the propagation of sound within the first optical sensor 110a, and the narrowband light source and signal processing module 130 determines the speed of sound in the sensing material 116a, 116b on the basis of when each optical fiber winding 118a, 118b, 118c detected the pressure wave. The narrowband light source and signal processing module 130 may use acoustic spatial array signal processing techniques. By having two optical sensors each with a different relationship between speed of sound, pressure and temperature, the apparent speed of sound in the two sensors may be used to resolve both the pressure and temperature of the wellbore fluid. The light source and signal processing module 130 may be disposed in the recording unit (10 in FIG. 1) or other convenient location.

[0024] The sound itself may be provided as a natural by-product of the flow of the wellbore fluid 111 through the production tubing 112a, thermally and acoustically coupled into the pressure and measurement optical sensor system 110 through the thin diaphragms or bellows 115a 115b, but can also be forced and tailored by providing vortex shedding sites 131 in the production tubing 112a or other means of acoustic excitation (such as an external noise source, a speaker, or a shaker). It is also possible to customize the acoustic resonant characteristics of the production tubing 112a or the mandrels 117a, 117b, or by other means not necessarily requiring an active sound source.

[0025] Other types of pressure and temperature sensors using optical fibers are known in the art and may be substituted for the one explained above with reference to FIG. 3. One example of such a sensor is described in U.S. Patent No. 7,654,562 issued to Choi. Another optical fiber sensor that may be used to measure pressure and/or temperature in a wellbore is described in U.S. Patent No. 7,245,382 issued to Ronnekleiv.

[0026] The one or more monitor wellbores (112 in FIG. 2) may also include one or more acoustic or seismic sensors 150 disposed therein. The seismic sensors 150 may be in signal communication with the recording system (10 in FIG. 1). An example seismic sensor for wellbore emplacement is described in U.S. Patent No. 4,715,469 issued to Yasuda et al. [0027] During pumping of the fracture treatment as explained with reference to FIG. 1, the position of the fluid front may be determined by measurement of fluid pressure and/or temperature in one or more monitor wellbores (FIG. 2) using optical fiber sensors as explained with reference to FIG. 3 and/or the U.S. Patents referred to above. In some examples, seismic sensors may be used to detect the passing of the fluid front through any one or more of the monitor wellbores (112 in FIG. 1).

[0028] While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.

Claims

Claims What is claimed is:

1. A method for monitoring movement of a hydraulic fracturing fluid through a subsurface formation, comprising:

measuring at least one of a pressure and a temperature of a fluid using an optical fiber sensor disposed in at least one wellbore drilled through the subsurface formation at a selected position away from a location wherein the hydraulic fracturing fluid is pumped into the subsurface formation; and

inferring a position of a fracturing fluid front using at least one of the measured pressure and measured temperature.

2. The method of claim 1 further comprising inferring a position of the fluid front by monitoring seismic signals detected at a plurality positions above the subsurface formation.

3. The method of claim 1 further comprising detecting acoustic signals in the at least one wellbore and using the detected acoustic signals in combination with the at least one or measured pressure and measured temperature to infer the position of the fracturing fluid front.

4. The method of claim 1 wherein the optical fiber sensor comprises a sensor system including at least two separate optical sensors, each optical sensor including a plurality of optical fibers wound around a mandrel at known, spaced apart positions along the mandrel, an interior of each mandrel in fluid communication with an interior of the wellbore, and means for detecting a change in length of each of the optical fibers.

5. The method of claim 4 wherein each mandrel is filled with a material having a different relationship between acoustic velocity and pressure and temperature.

6. The method of claim 5 wherein a pressure and temperature of the fluid in the wellbore is measured by determining acoustic velocity at each of the two optical sensors by measuring a time difference between change of length of each of the optical fibers on each sensor in response to acoustic energy in the wellbore, and using the different relationship between acoustic velocity and pressure and the material in each of the separate optical sensors to determine pressure and temperature,

7. A system for monitoring movement of a hydraulic fracturing fluid through a subsurface formation, comprising:

at least one optical fiber sensor disposed in at least one wellbore drilled through the subsurface formation at a selected distance from a position at which the hydraulic fracturing fluid is pumped into the wellbore, the at least one optical fiber sensor responsive to at least one of pressure and temperature of fluid in the wellbore; and a processor in signal communication with the at least one optical sensor, the processor comprising a light source and means for determining pressure and temperature change resulting from response of the at least one optical sensor.

8. The system of claim 8 wherein the at least one optical fiber sensor comprises a Bragg grating.

9. The system of claim 8 wherein the at least one optical fiber sensor comprises a sensor system including at least two separate optical sensors, each optical sensor including a plurality of optical fibers wound around a mandrel at known, spaced apart positions along the mandrel, an interior of each mandrel in f uid communication with an interior of the wellbore, and means for detecting a change in length of each of the optical fibers resulting from change in diameter of the mandrels.

10. The system of claim 9 wherein each mandrel is filled with a material having a different relationship between acoustic velocity and pressure and temperature.

11. The system of claim 10 wherein each material comprises a corresponding composition of oil.

12. The system of claim 7 further comprising at least one acoustic sensor disposed in the wellbore.

13. The system of claim 7 further comprising a plurality of wellbores disposed at spaced apart locations, each of the plurality of wellbores including at least one optical fiber sensor therein responsive to at least one of temperature and pressure of fluid in the wellbore.

14. The system of claim 7 further comprising a plurality of seismic sensors disposed above a portion of the Earth's subsurface to be monitored.

15. The system of claim 14 further comprising a recording unit in signal communication with the plurality of seismic sensors, the recording unit comprising a processor having instructions thereon for determining a position of a fracture fluid front from seismic energy detected by the plurality of seismic sensors.