System and method for determining downhole conditions

The present invention provides a system for determining downhole conditions including a time domain reflectometer (172) that is operable to generate a transmission signal and receive a reflected signal. A tubular (192) is positioned downhole in a downhole medium (214, 216, 218, 220, 222) and a waveguide (186), which is in electrical communication with the time domain reflectometer (172), is operably contacting the downhole (214, 216, 218, 220, 222). The waveguide (186) is operable to propagate the transmission signal and operable to propagate the reflected signal that is generated responsive to an electromagnetic property of the downhole medium (214, 216, 218, 220, 222).

TECHNICAL FIELD OF THE INVENTION

This invention relates, in general, to determining downhole conditions in a wellbore that traverse a subterranean hydrocarbon bearing formation and, in particular, to a system and method for real time sampling of downhole conditions during completion and production operations utilizing time domain reflectometry.

BACKGROUND OF THE INVENTION

It is well known in the subterranean well drilling and completion art that relatively fine particulate materials may be produced during the production of hydrocarbons from a well that traverses an unconsolidated or loosely consolidated formation. Numerous problems may occur as a result of the production of such particulates. For example, the particulates cause abrasive wear to components within the well, such as tubing, pumps and valves. In addition, the particulates may partially or fully clog the well creating the need for an expensive workover. Also, if the particulate matter is produced to the surface, it must be removed from the hydrocarbon fluids using surface processing equipment.

One method for preventing the production of such particulate material to the surface is gravel packing the well adjacent the unconsolidated or loosely consolidated production interval. In a typical gravel pack completion, a sand control screen is lowered into the wellbore on a work string to a position proximate the desired production interval. A fluid slurry including a liquid carrier and a relatively coarse particulate material, such as sand, gravel or proppants, which is typically sized and graded and which is referred to herein as gravel, is then pumped down the work string and into the well annulus formed between the sand control screen and the perforated well casing or open hole production zone.

The liquid carrier either flows into the formation or returns to the surface by flowing through a wash pipe or both. In either case, the gravel is deposited around the sand control screen to form the gravel pack, which is highly permeable to the flow of hydrocarbon fluids but blocks the flow of the fine particulate materials carried in the hydrocarbon fluids. As such, gravel packs can successfully prevent the problems associated with the production of these particulate materials from the formation.

It has been found, however, that a complete gravel pack of the desired production interval is difficult to achieve. For example, incomplete packs may result from the premature dehydration of the fluid slurry due to excessive loss of the liquid carrier into highly permeable portions of the production interval causing the gravel to form sand bridges in the annulus. Thereafter, the sand bridges may prevent the slurry from flowing to the remainder of the annulus which, in turn, prevents the placement of sufficient gravel in the remainder of the annulus.

Numerous attempts have been made to improve the quality of the gravel packs. For example, changing fluid slurry parameters including flow rate, viscosity and gravel concentration and providing alternate paths for the fluid slurry delivery provide for a more complete gravel pack in some completion scenarios. Even using these improved techniques, however, a nonuniform distribution of the gravel that results in the presence of localized spaces that are void of gravel within the production interval is typically undetectable. As such, well operators are typically not aware that corrective action is required until after sand production from the well has commenced.

Accordingly, a need has arisen for a system and method for gravel packing a production interval traversed by a wellbore that provide for monitoring downhole conditions during a gravel packing operation. A need has also arisen for such a system and method that generate a real time profile of the downhole conditions surrounding the sand control screen. A need has further arisen for such a system and method that inform well operators that corrective action is required during both the completion and production phases of well operation.

SUMMARY OF THE INVENTION

A system and method are disclosed that are utilized to determine downhole conditions during a variety of wellbore operations such as completion operations including gravel packing, fracture packing, high rate water packing and the like as well as production operations. The system and method of the present invention generate a real time profile of downhole conditions that may be utilized by a well operator to determine the quality of a gravel pack as well as the type of fluid being produced into specific regions of the production interval.

In one aspect, the present invention is directed to a system for determining downhole conditions that includes a time domain reflectometer operable to generate a transmission signal and receive a reflected signal. A tubular is positioned in a downhole medium and a waveguide, which may comprise a plurality of transmission lines, is operalby contacting the downhole medium and is in communication with the time domain reflectometer. The time domain reflectometer transmits pulses, such as electrical or optical pulses, through the waveguide and receives reflections indicative of spatial changes in the electrical properties of the downhole medium. More specifically, the electromagnetic properties of the waveguide are influenced by the electrical properties of the downhole medium and change in response to changes in the medium.

The time domain reflectometer includes a signal generator and a signal receiver. In one embodiment, time domain reflectometer includes a step generator and an oscilloscope. In another embodiment, the time domain reflectometer includes a signal generator and sampler, a datalogger and a data interpreter. The time domain reflectometer may generate a transmission signal having a short rise time and may sample and digitize the reflected signal. The downhole medium in which the waveguide is positioned may include constituents such as water, gas, sand, gravel, proppants, oil and the like. In operation, once the reflected signal is received by the time domain reflectometer, a profile of the downhole medium may be generated based upon the amplitude and phase of the reflected signal, by comparing the reflected signal to a control waveform or by comparing the reflected signal to the transmitted signal. The electromagnetic profile of the downhole medium is created due to variations in the electromagnetic properties of the downhole medium such as impedance, resistance, inductance or capacitance.

In another aspect, the present invention is directed to a method for determining downhole conditions. The method comprises the steps of generating a transmission signal, propagating the transmission signal through a transmission line operably contacting a downhole medium, reflecting the transmission signal in response to an electromagnetic property of the downhole medium, receiving the reflected signal and analyzing the reflected signal to determine at least one downhole condition.

In a further aspect, the present invention is directed to an apparatus for determining downhole conditions that includes a tubular positioned in a downhole medium and a waveguide operably contacting the downhole medium. The waveguide may include one or more transmission lines. In an embodiment having three transmission lines, one of the transmission lines is positioned between the other two transmission lines such that the transmission lines are approximately equidistant from one another. The transmission lines of the waveguide are operable to propagate a transmission signal received from a time domain reflectometer and propagate a reflected signal generated responsive to an electromagnetic property of the downhole medium. In one embodiment, the transmission lines of the waveguide may form U-shaped patterns on the tubular, a helical pattern about the tubular or traverse the tubular a plurality of times. In addition, the electrical characteristics of the distal end of one or more of the transmission lines may be altered to alter characteristics of the reflected signal from the distal end.

In yet another aspect, the present invention is directed to a system for determining downhole conditions that includes a time domain reflectometer which generates a short rise time electromagnetic pulse transmission signal and samples a reflected signal. A sand control screen assembly is positioned in a downhole medium with a waveguide operably contacting the downhole medium and in communication with the time domain reflectometer. The waveguide is operable to propagate the transmission signal and operable to propagate the reflected signal generated responsive to an electromagnetic property of the downhole medium.

DETAILED DESCRIPTION OF THE INVENTION

Referring initially toFIG. 1, an offshore oil and gas platform during a gravel packing operation wherein a system for monitoring downhole conditions is being utilized according to the present invention is schematically illustrated and generally designated10. A semi-submersible platform12is centered over a submerged oil and gas formation14located below sea floor16. A subsea conduit18extends from deck20of platform12to wellhead installation22including blowout preventers24. Platform12has a hoisting apparatus26and a derrick28for raising and lowering pipe strings such as work string30.

A wellbore32extends through the various earth strata including formation14. A casing34is cemented within wellbore32by cement36. Work string30includes various tools including a cross-over assembly38, a sand control screen assembly40and packers44,46which define an annular region48. When it is desired to gravel pack annular region48, work string30is lowered through casing34until sand control screen assembly40is positioned adjacent to formation14including perforations50. Thereafter, a fluid slurry including a liquid carrier and a particulate material such as gravel is pumped down work string30.

During this process, the fluid slurry exits work string30through cross-over assembly38such that the fluid slurry enters annular region48. Once in annular region48, the gravel portion of the fluid slurry is deposited therein. Some of the liquid carrier may enter formation14through perforations50while the remainder of the fluid carrier can travel through sand control screen assembly40and cross-over assembly38to the surface in a known manner, such as through a wash pipe and into the annulus52above packer44. The fluid slurry is pumped down work string30through cross-over assembly38until annular section48surrounding sand control screen assembly40is filled with gravel.

As will be explained in further detail hereinbelow, in order to monitor downhole conditions and, in particular, the integrity of the gravel pack, a plurality of transmission lines are associated with sand control screen assembly40. Each of the transmission lines is in electrical communication with a time domain reflectometer, which is preferably disposed at the surface. The time domain reflectometer generates a transmission signal which travels through the transmission lines and the downhole medium surrounding the transmission lines at sand control screen assembly40. The downhole medium, whether drilling mud, gas, water, sand, gravel, proppants, oil or the like, has electrical properties that effect a reflected signal which is returned to the time domain reflectometer. The electrical properties of the downhole medium may be analyzed or graphically represented in order to describe and monitor the electromagnetic profile of the constituents of the downhole medium about sand control screen assembly40.

Even thoughFIG. 1depicts a vertical well, it should be noted by one skilled in the art that the apparatus for monitoring downhole conditions of the present invention is equally well-suited for use in deviated wells, inclined wells or horizontal wells. Also, even thoughFIG. 1depicts an offshore operation, it should be noted by one skilled in the art that the apparatus for monitoring downhole conditions of the present invention is equally well-suited for use in onshore operations or other types of offshore operations, such as those involving jackup rigs.

In addition, it should be apparent to those skilled in the art that the use of directional terms such as above, below, upper, lower, upward, downward and the like are used in relation to the illustrative embodiments as they are depicted in the figures, the upward direction being toward the top of the corresponding figure and the downward direction being toward the bottom of the corresponding figure.

Referring now toFIG. 2, therein is depicted a sand control screen according to the teachings of the present invention that is generally designated60. Sand control screen60includes a base pipe62that has a plurality of openings64which allow the flow of production fluids into the production tubing. The exact number, size and shape of openings64are not critical to the present invention, so long as sufficient area is provided for fluid production and the integrity of base pipe74is maintained.

Spaced around base pipe62is a plurality of ribs66. Ribs66are generally symmetrically distributed about the axis of base pipe62. Ribs66are depicted as having a cylindrical cross section, however, it should be understood by one skilled in the art that the ribs may alternatively have a rectangular or triangular cross section or other suitable geometry. Additionally, it should be understood by one skilled in the art that the exact number of ribs will be dependant upon the diameter of base pipe62as well as other design characteristics that are well known in the art.

Wrapped around ribs66is a screen wire68. Screen wire68forms a plurality of turns, such as turn70, turn72and turn74. Between each of the turns is a gap through which formation fluids flow. The number of turns and the gap between the turns are determined based upon the characteristics of the formation from which fluid is being produced and the size of the gravel to be used during the gravel packing operation. Together, ribs66and screen wire68may form a sand control screen jacket76which is attached to base pipe62at welds78,80or by other suitable technique.

Transmission lines may be utilized in association with sand control screen jacket60to monitor the downhole conditions therearound. As illustrated, transmission lines82,84,86are being employed in conjunction with sand control screen60to monitor, for example, the integrity of a gravel pack during both completion and production phases of well operations. As illustrated, transmission lines82,84,86form three U-shapes. AlthoughFIG. 2depicts transmission lines82,84,86as being positioned exteriorly of sand control screen jacket76, it should be understood by those skilled in the art that the transmission lines used to determine downhole conditions of the present invention may alternatively be positioned in other locations relative to a downhole tubular such as between a filter medium and an outer tubular or to the exterior of an outer shroud of a sand control screen. Likewise, the transmission lines of the present invention may be used in conjunction with other types of tubular members such as tubing, casing, drill pipe, line pipe, mandrels or other types of pipe as well as other non tubular downhole equipment. Further, it should be appreciated that although three transmission lines are illustrated inFIG. 2, the present invention may be practiced with any number of transmission lines. Also, it should be noted that the transmission lines may be wire such as copper or stainless steel wire, control lines such as hydraulic fluid control lines, optic fiber or other conductor suitable for transmission of electromagnetic signals.

Referring now toFIG. 3, therein is depicted a sand control screen according to the teachings of the present invention that is generally designated110. Sand control screen110includes an outer shroud112having openings114. It should be noted by those skilled in the art that even thoughFIG. 3has depicted openings114as being circular, other shaped openings may alternatively be used without departing from the principles of the present invention. In addition, the exact number, size and shape of openings114are not critical to the present invention, so long as sufficient area is provided for fluid production therethrough and the integrity of outer shroud112is maintained. Positioned within an outer shroud112is a filter medium such as a fluid-porous, particulate restricting, filter medium formed from a plurality of layers of a wire mesh that are sintered or diffusion bonded together to form a porous wire mesh screen designed to allow fluid flow therethrough but prevent the flow of particulate materials of a predetermined size from passing therethrough. The filter medium is positioned around a perforated base pipe116. Outer shroud112is attached to base pipe116at welds118,120.

Three transmission lines122,124,126are coupled to outer shroud112and form U-shaped patterns thereon. In the illustrated embodiment, transmission line124is positioned between transmission line122and transmission line126. Preferably, transmission line124is positioned approximately equidistance from transmission line122and transmission line126. As one skilled in the art will appreciate, the symmetrical and even spacing between transmission lines122and124and between transmission lines124and126enables the transmission lines122,124,126to better detect impedance mismatches in the constituent materials that define the downhole conditions being measured by time domain reflectometry.

In general, time domain reflectometry involves feeding an impulse of energy into the system under test, e.g., the downhole environment surrounding outer shroud112, and observing the reflected energy at the point of insertion. When the fast-rise input pulse meets with a discontinuity or other electromagnetic mismatch, the resultant reflections appearing at the feed point are compared in phase and amplitude with the original pulse. By analyzing the magnitude, deviation and shape of the reflected signal, the nature of the electromagnetic variation in the system under test can be determined. Additionally, since distance is related to time and the amplitude of the reflected signal is directly related to impedance, the analysis yields the distance to the electromagnetic variation as well as the nature of the fault.

More specifically, electromagnetic waves traveling through the transmission lines are reflected at locations where changes in an electromagnetic characteristic, such as impedance, exist. By way of example, lengths X1, X2and X3are characterized by impedances Z1, Z2and Z3, respectively. In operation, any electromagnetic wave moving from the length of line X1to the length of line X2will be reflected at the interface of X1and X2. The reflection coefficient, ρ, of this reflection can be expressed as follows:
ρ=(Z2−Z1)/(Z2+Z1)
The transmission coefficient, τ, for a wave traveling from section X1to section X2is provided by the following equation:
τ=2Z2/(Z2+Z1)
If the incident wave has an amplitude, Ai, the reflected and transmitted waves have the following amplitudes:
Ar=ρAiand At=τAi
Arand Atare the amplitudes of the reflected and transmitted waves, respectively. Those skilled in the art will appreciate that similar equations may be derived for the interface of X2and X3. Further, it will be understood that the impedances Z1, Z2and Z3change in response to the varying composition and, in particular, oil, water and gas composition, within lengths X1, X2and X3.

In addition to the amplitudes of the reflected and transmitted waves, the propagation velocity of the electromagnetic wave that travels through the downhole medium as it propagates through transmission lines122,124,126is of interest in time domain reflectometry. Continuing with the example illustrated inFIG. 3, transmission lines122,124,126will contact physical discontinuities at the interface of X1and X2as well as at the interface of X2and X3, such that the physical discontinuities are separated by a distance X2. The time that a reflection from the discontinuity at interface X1−X2arrives at the time domain reflectometer may be designated T1and the time that the reflection from the discontinuity at interface X2−X3arrives at the time domain reflectometer may be designated T2, such that the propagation velocity, V, may be expressed as:
V=2X2/(T2−T1)
By normalizing the propagation velocity to the speed of light, c, the apparent dielectric constant, Ka, of the downhole medium surrounding transmission lines122,124,126over distance X2may be expressed as follows:
Ka=(c/V)2
The apparent dielectric constant of the downhole medium is related to the amount of oil, water, sand, gas, gravel and proppants, for example, present in the downhole medium. In one implementation, an expert system based upon empirical data may be utilized to determine the constituent materials of a downhole medium corresponding to a measured apparent dielectric constant.

These equations or similar equations are utilized to determined the downhole conditions when the transmission signal is generated at a time domain reflectometer and propagated through transmission lines122,124,126associated with the tubular that is positioned in the downhole medium. Transmission lines122,124,126may be utilized independently in different configurations to propagate the signal. Transmission lines122,124,126and outer shroud112assist the propagation of the signal by forming a waveguide that effectuates the characteristics of a coaxial cable. In one implementation, the outer transmission lines122,126, provide shielding and the central transmission line124provides a central conductor. In another implementation, outer shroud112provides the shielding and one or more of transmission lines122,124,126provide a central conductor. It should appreciated that transmission lines122,124,126that are not being used as either conductors or shielding may be disconnected to reduce noise interference. In any of the above-mentioned implementations, the transmission signal is reflected in response to the electromagnetic profile of the downhole medium and, in particular, in response to an impedance change in the downhole medium caused by a change in the electromagnetic profile of the constituents of the downhole medium. The reflected signals are received at the time domain reflectometer and analyzed using the equations discussed hereinabove to determine the downhole conditions.

Referring now toFIG. 4, therein is depicted a sand control screen of the present invention that is generally designated130. Sand control screen130has an outer shroud132having openings134that is positioned around a filter medium (not pictured) both of which are mounted to a perforated base pipe136. Two transmission lines140,142are coupled to outer shroud132to form a substantially helical pattern which includes turns144,146,148. It should be appreciated that the transmission lines140,142may be coupled to a conventional sand control screen without negatively impacting the functions of the sand control screen. Like transmission lines122,124,126ofFIG. 3, preferably transmission lines140,142maintain approximately a uniform distance from one another to provide a two-wire waveguide. The distance between turns is appropriately greater than the distance between transmission lines in order to avoid crosstalk or other electromagnetic phenomena between adjacent helical loops which may adversely affect the time domain reflectometry measurements. Further, the helical pattern, which may include more or less turns, provides 360° coverage around sand control screen130, thereby providing complete visibility into the conditions which surround sand control screen130. In particular, the present invention provides great resolution and depth penetration while simultaneously offering high measurement precision. Moreover, the electromagnetic waves traverse the transmission lines quickly to provide real time resolution of the downhole conditions.

Referring now toFIG. 5, therein is depicted a sand control screen of the present invention that is generally designated150. Sand control screen150has an outer shroud152having openings154that is positioned around a filter medium (not pictured) both of which are mounted to a perforated base pipe156. Two transmission lines160,162are coupled to outer shroud152and traverse outer shroud152multiple times. For example, transmission lines160,162traverse outer shroud152forming a plurality of loops that provide real time resolution of downhole conditions and 360° coverage around outer shroud152. The distance between loops is greater than the distance between transmission lines160,162. It should be appreciated, that although particular implementations of the transmission lines have been depicted inFIGS. 3–5, other implementations are within the teachings of the present invention. Moreover, the transmission lines described herein may be utilized during completion operations, production operations and the like. For example, the transmission lines may be utilized during a completion operation to ensure a complete gravel pack having no voids. By way of another example, the transmission lines may be utilized during a production operation to enhance production by determining the location of water production such that certain production intervals or regions within a production interval may be shut off.

FIG. 6depicts a system for monitoring downhole conditions according to the present invention that is generally designated170. System170includes a time domain reflectometer172that generates electromagnetic pulses or signals, such as electrical, optical or other signal types within the electromagnetic spectrum, and receives reflections of the electromagnetic signals. Time domain reflectometer172may be temporarily or permanently positioned at a surface location, a downhole location or other remote location such that a one time survey or series of surveys may be performed to determine downhole conditions in a system under test, which is a downhole environment174inFIG. 6. In a preferred embodiment, time domain reflectometer172includes a signal generator and sampler176, a datalogger178and a data interpreter180. For example, the time domain reflectometer172may comprise a step generator and an oscilloscope. In one embodiment, signal generator and sampler176is a digital device that generates a very short rise time electromagnetic pulse that is applied to a coaxial conveyance182which is coupled to a multiplexer184which increases the number of transmission lines that may be employed with time domain reflectometer172. It should be appreciated, however, that time domain reflectometer172may comprise any combination of hardware, software and firmware.

As illustrated, transmission line sets186and188are coupled to multiplexer184, which in a presently preferred exemplary embodiment, may comprise a time domain multiplexer. Although only two sets of transmission lines are depicted connected to multiplexer184, it should be understood that any number of sets of transmission lines may be coupled to multiplexer184depending upon the number of independent downhole surveys desired. Also, it should be understood that multiple sets of independent transmission lines may be associated with the same system under test174through multiplexer184such that results from the independent systems can be compared to one another. Use of such independent transmission lines is one way to make alterations in end point characteristics of the transmission lines as will be discussed in greater detail in association withFIGS. 7 and 8below. In the illustrated embodiment, after sending the pulse which may be input into either end of a transmission line, signal generator and sampler176samples and digitizes the reflected signals which are stored by datalogger178and analyzed by data interpreter180. Preferably, data interpreter180is operable to produce graphical representations, such as graph190, of the data collected and interpreted. As previously discussed, the elapsed travel time and pulse reflection amplitude contain information used by an engineer, a computer system or an expert system, for example, to quickly and accurately determine the water content, bulk electrical conductivity or other user-specific, time domain measurement.

In system under test174, a sand control screen assembly192is disposed in a wellbore194proximate formation196. Wellbore194includes a casing198having perforations200that provide for fluid communication between formation196and production tubing (not illustrated) which is associated with sand control screen assembly192. As illustrated, an annulus202is defined between casing198and sand control screen192. The completion of wellbore194includes a gravel pack204that prevents the production of particulates from formation196. As illustrated, transmission line set186is positioned within annulus202and in direct contact with the downhole medium of gravel pack204. Alternatively, transmission line set186could be located within the outer shroud or even within the filter medium of sand control screen assembly192in which case transmission line set186may not directly contact gravel pack204but will nonetheless be influenced by the electromagnetic properties of the downhole medium, and will accordingly be considered to operably contact the downhole medium. In the illustrated embodiment, gravel pack204has irregularities, however, including a region having a void206. In addition, formation196includes regions that are producing different fluids. Specifically, formation196has a region208producing gas G, a region210producing oil O and a region212producing water W. Due to the production profile of the formation196, the downhole environment surrounds sand control screen192has a variety of conditions.

In the illustrated embodiment, the uppermost region214within annulus202has a combination of gravel pack204and gas G. The next lower region216is a gas G only environment as the gravel pack in region216has failed. Another gravel pack204and gas G environment is found in region218. The next region220within annulus202is a gravel pack204and oil O environment. The lower most region222is a gravel pack204and water W environment. The electromagnetic properties within the various regions214,216,218,220,222will be determined by the specific constituents that define the environments therein. In addition, boundaries or interphase regions exist between the various regions214,216,218,220,222. Specifically, interphase region224exists between regions214,216, interphase region226exists between regions216,218, interphase region228exists between regions218,220and interphase region230exists between regions220,222.

As previously discussed, a transmission signal is propagated through transmission lines186and reflected in response to the electromagnetic profile of the downhole medium surrounding sand controls screen192. In particular, the transmission signal is reflected in response to the impedance changes in the downhole medium in regions214,216,218,220,222and at interfaces224,226,228,230. The reflected signals are received at time domain reflectometer172, stored in data logger178and analyzed using data interpreter180to produce graphical representation190. The analysis may involve comparing the reflected signal to a control waveform or comparing the reflected signal to the transmission signal. Further, to improve the signal to noise ratio, the analysis may involve averaging the measurements provided by several reflected signals.

In instances where an incomplete gravel pack is present, the electromagnetic profile of the downhole medium may include a change in an electromagnetic property such as impedance, resistance, inductance or capacitance that corresponds to the location of the discontinuity in the gravel pack. Accordingly, the present invention provides a system and method for monitoring downhole conditions during a gravel packing operation to enhance the uniformity of gravel placement. The real time graphical representation190provides the necessary information to engineers or well operators so that appropriate corrective action may be taken. For example, if voids are detected during a gravel packing operation, then gravel packing parameters such as flow rate, viscosity and proppant concentration may be altered to alleviate the voids. By way of another example, if an undesirable condition such as water production or sand production is detected during a production operation, valves may be closed to isolate that region of the production interval.

FIG. 7depicts a pulse input and measurement circuit250associated with a time domain reflectometer, such as reflectometer172ofFIG. 6. The circuit250includes a signal generator and sampler252and a transmission line254having an end point256. A switch258is located at end point256that is operable to alter the electrical characteristics of end point256. Specifically, switch258provides three positions; namely a finite load position260, an infinite load or open circuit position262and a ground or closed circuit position264. By propagating signals, preferably short rise time pulses in the 10–10,000 nanosecond range, through transmission line254and monitoring the reflection of the signals from the end point256, characteristics of transmission line254and its surroundings can be determined. For example, the velocity of the signals propagating through transmission line254can be used to determine electromagnetic characteristics in the medium surrounding transmission line254. The velocity of the signals can be determined by monitoring the reflections from end point256of transmission line254with signal generator and sampler252when switch258is in its various positions260,262,264.

FIG. 8depicts a plot270of voltage versus time that is associated with the circuit250ofFIG. 7. In the plot, voltage is expressed in volts (v) and time is expressed in nanoseconds (ns). Waveform272is associated with open position262of switch258of circuit250. Similarly, waveform274is associated with finite load position260and waveform276is associated with closed position264. Point278on waveforms272,274,276represents the time at which the signals enter transmission line254. Point280on the waveforms272,274,276represents the time at which the signals reach end point256. More specifically, due to the difference in end point characteristics created by switch258, an end point disturbance is created that differentiates the reflections received from end point256. Due to the differences in the reflected waveforms, the location of end point256can be identified which in turn allows for the identification of other parameters such as the velocity of the signals.

FIG. 9depicts a plot280of voltage versus time wherein downhole conditions within a gravel pack completion are graphically represented. In the plot, voltage is expressed in volts (v) and time is expressed in nanoseconds (ns). The time domain reflectometry teachings of the present invention were utilized to produce a control waveform282and a waveform284. Control waveform282represents a control signal produced by previous empirical testing that is representative of the gravel pack under known or previous conditions. In one embodiment, waveform282is created by subtracting a waveform generated by propagating a signal through a transmission line disposed in the downhole gravel pack medium with an end point having a closed circuit from a waveform generated by a signal propagated through the transmission line disposed in the downhole gravel pack medium with an end point having an open circuit. Use of such a subtraction waveform aids in illustrating the principles of the present invention. In the illustrated embodiment, waveform284, also an open circuit minus closed circuit subtraction waveform, represents the electromagnetic profile of the system under test (SUT) at a time after the control condition, for example, after production has commenced into the gravel pack completion. As depicted, a phase shift is present between waveform284and the control waveform282. Specifically, waveform284is delayed in time, Δt, which indicates the velocity of the signal propagating through the transmission line in the downhole medium after production is less than the velocity of the signal in the downhole medium before production. Under this one particular set of downhole conditions, the illustrated phase shift is indicative of the presence of oil production through the gravel pack at a particular location in the downhole medium. This determination can be made using software tools such as expert systems or neural networks, for example.

FIG. 10depicts another embodiment of a plot290of voltage versus time wherein downhole conditions are graphically represented. Specifically, waveform294represents two reflected signals sampled by a time domain reflectometer of the present invention processed using the subtraction technique described above. Waveform294has two instances of increased amplitude when compared to control waveform292. Under one particular set of downhole conditions, the increased amplitudes are indicative of the presence of voids in the gravel pack at two particular locations in the downhole medium. Further, it should be appreciated that the distance to the electromagnetic variations or discontinuities may be determined based on the location of the discontinuity on the time axis of plot290since the propagation velocity may be approximated as discussed hereinabove.

FIG. 11depicts a further embodiment of a plot300of voltage versus time wherein downhole conditions are graphically represented. Similar to the previous plots ofFIGS. 9 and 10, waveform304represents two reflected signals sampled by the time domain reflectometer of the present invention and processed using the subtraction technique described above and control waveform302represents a control. As illustrated, when compared to control waveform302, waveform304is phase shifted. Additionally, waveform304includes a reduced magnitude. Under one set of downhole conditions, the phase shifting and reduced magnitude may be indicative of the presence of water in the gravel pack around the sand control screen apparatus.

FIG. 12depicts another embodiment of a plot310of voltage versus time wherein downhole conditions of a gravel pack are graphically represented. When compared to control waveform312, a waveform314produced by the time domain reflectometry teachings of the present invention includes a region316of increased amplitude, a region318of phase delay and a region320of reduced amplitude and phase delay. Referring also now toFIG. 6, region316of increased amplitude corresponds to region216ofFIG. 6wherein a void has developed in gravel pack204and the fluid being produced therethrough is gas G. Similarly, region318of plot310corresponds to region220ofFIG. 6wherein oil O is being produced through gravel pack204. Likewise, region320of plot310corresponds to region222ofFIG. 6wherein water W is being produced through gravel pack204. Hence, the present invention provides a well operator real time information regarding the distribution of various constituent materials along a tubular, such as sand control screen192, that translates into knowledge about the effectiveness of a gravel pack as well as the production profile of formation196.