Abstract:
A formation monitoring system includes a casing. An array of electromagnetic field sensors is positioned in the annular space and configured to communicate with the surface via a fiberoptic cable. A computer coupled to the fiberoptic cable receives measurements from the array and responsively derives the location of any fluid fronts in the vicinity such as an approaching flood front to enable corrective action before breakthrough. A formation monitoring method includes: injecting a first fluid into a reservoir formation; producing a second fluid from the reservoir formation via a casing in a borehole; collecting electromagnetic field measurements with an array of fiberoptic sensors in an annular space, the array communicating measurements to a surface interface via one or more fiberoptic cables; and operating on the measurements to locate a front between the first and second fluids.

Description:
BACKGROUND 
     Oil field operators drill boreholes into subsurface reservoirs to recover oil and other hydrocarbons. If the reservoir has been partially drained or if the oil is particularly viscous, the oil field operators will often inject water or other fluids into the reservoir via secondary wells to encourage the oil to move to the primary (“production”) wells and thence to the surface. 
     This flooding process can be tailored with varying fluid mixtures, flow rates/pressures, and injection sites, but may nevertheless be difficult to control due to inhomogeneity in the structure of the subsurface formations. The interface between the reservoir fluid and the injected fluid, often termed the “flood front”, develops protrusions and irregularities that may reach the production well before the bulk of the residual oil has been flushed from the reservoir. This “breakthrough” of the flood fluid is undesirable, as it typically necessitates increased fluid handling due to the injected fluid&#39;s dilution of the oil and may further reduce the drive pressure on the oil. Continued operation of the well often becomes commercially infeasible. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Accordingly, there are disclosed herein various fiberoptic systems and methods for formation monitoring. In the drawings: 
         FIG. 1  shows an illustrative environment for permanent monitoring. 
         FIGS. 2A-2E  show various illustrative injected-current system configurations. 
         FIGS. 3A-3E  show various illustrative sensing array configurations. 
         FIG. 4  shows yet another illustrative sensing array configuration. 
         FIGS. 5A-5B  show illustrative combined source-sensor cable configurations. 
         FIG. 6  is a function block diagram of an illustrative formation monitoring system. 
         FIGS. 7A-7C  show illustrative multiplexing architectures for distributed electromagnetic (“EM”) field sensing. 
         FIGS. 8A-8C  show various illustrative EM field sensor configurations. 
         FIG. 9  is a signal flow diagram for an illustrative formation monitoring method. 
     
    
    
     It should be understood, however, that the specific embodiments given in the drawings and detailed description below do not limit the disclosure. On the contrary, they provide the foundation for one of ordinary skill to discern the alternative forms, equivalents, and other modifications that are encompassed in the scope of the appended claims. 
     DETAILED DESCRIPTION 
     The following disclosure presents a fiberoptic-based technology suitable for use in permanent downhole monitoring environment to track an approaching fluid front and enable actions to optimize hydrocarbon recovery from a reservoir. One illustrative formation monitoring system has an array of electromagnetic field sensors positioned in an annular space around a well casing, the sensors being coupled to a surface interface via a fiberoptic cable. Each electromagnetic field sensor is a device that produces signals that are a function of external electric or magnetic fields. Illustrative sensors provide signals that are directly or inversely proportional to electric or magnetic field strength, the temporal or spatial derivative of the electric or magnetic fields, or the temporal or spatial integral of the fields. Other illustrative sensors have reception characteristics that measure both electric and magnetic fields. The sensor measurements in response to an injected current or another electromagnetic field source can be used to determine a resistivity distribution around the well, which in turn enables tracking of the flood front. (Although the term “flood front” is generally used herein to refer to the interface between reservoir fluid and injected fluid zones, the teachings of the present disclosure will apply to the interface between any two fluids having different bulk resistivities.) 
     Turning now to the drawings,  FIG. 1  shows an illustrative permanent downhole monitoring environment. A borehole  102  contains a casing string  104  with a fiber optic cable  106  secured to it by bands  108 . Casing  104  is a tubular pipe, usually made of steel, that preserves the integrity of the borehole wall and borehole. Where the cable  106  passes over a casing joint  110 , it may be protected from damage by a cable protector  112 . Electromagnetic (EM) field sensors  114  are integrated into the cable  106  to obtain EM field measurements and communicate those measurements to a surface interface  116  via fiberoptic cable  106 . 
     The remaining annular space may be filled with cement  118  to secure the casing  104  in place and prevent fluid flows in the annular space. Fluid enters the uncemented portion of the well (or alternatively, fluid may enter through perforated portions of the well casing) and reaches the surface through the interior of the casing. Note that this well configuration is merely illustrative and not limiting on the scope of the disclosure. Many production wells are provided with multiple production zones that can be individually controlled. Similarly, many injection wells are provided with multiple injection zones that can be individually controlled. 
     Surface interface  116  includes an optical port for coupling the optical fiber(s) in cable  106  to a light source and a detector. The light source transmits pulses of light along the fiber optic cable, including any sensors  114 . The sensors  114  modify the light pulses to provide measurements of field strength, field gradient, or time derivative for electrical fields and/or magnetic fields. The modifications may affect amplitude, phase, or frequency content of the light pulses, enabling the detector to responsively produce an electrical output signal indicative of the sensor measurements. Some systems may employ multiple fibers, in which case an additional light source and detector can be employed for each fiber, or the existing source and detector may be switched periodically between the fibers. Some system embodiments may alternatively employ continuous wave (CW) light rather than light pulses. 
       FIG. 1  further shows a power source  120  coupled between the casing  104  and a remote earth electrode  122 . Because the casing  104  is an electrically conductive material (e.g., steel), it acts as a source electrode for current flow into the formations surrounding the borehole  102 . The magnitude and distribution of the current flow will vary in accordance with the source voltage and the formation&#39;s resistivity profile. The EM field measurements by sensors  114  will thus be representative of the resistivity profile. This resistivity profile in turn is indicative of the fluids in the formation pores, enabling the flood front to be located and tracked over time. 
     The surface interface  116  may be coupled to a computer that acts as a data acquisition system and possibly as a data processing system that analyzes the measurements to derive subsurface parameters and track the location of a fluid front. In some contemplated system embodiments, the computer may further control production parameters to reduce risk of breakthrough or to otherwise optimize production based on the information derived from the measurements. Production parameters may include the flow rate/pressure permitted from selected production zones, flow rate/pressure in selected injection zones, and the composition of the injection fluid, each of which can be controlled via computer controlled valves and pumps. 
     Generally, any such computer would be equipped with a user interface that enables a user to interact with the software via input devices such as keyboards, pointer devices, and touchscreens, and via output devices such as printers, monitors, and touchscreens. The software can reside in computer memory and on nontransient information storage media. The computer may be implemented in different forms including, e.g., an embedded computer permanently installed as part of the surface interface  116 , a portable computer that is plugged into the surface interface  116  as desired to collect data, a remote desktop computer coupled to the surface interface  116  via a wireless link and/or a wired computer network, a mobile phone/PDA, or indeed any electronic device having a programmable processor and an interface for I/O. 
       FIG. 2A  is a schematic representation of the system configuration in  FIG. 1 . It shows a borehole  102  having a casing  104  and a fiberoptic cable  106  (with an integrated sensor array) in the annular space. An injected current  202  flows along casing  104  and disperses into the surrounding formations as indicated by the arrows. Two formations are shown, labeled with their respective resistivities R1 and R2. The heavier arrows in the lower formation represent a larger current flow, indicating that resistivity R2 is lower than resistivity R1. Due to divergence pattern of the currents away from the casing, depth of investigation is typically around 5-15 feet. 
       FIG. 2B  shows an alternative system configuration, in which the fiberoptic cable  106  is replaced by an alternative fiberoptic cable  206  having a conductor or a conductive layer to transport an injected current  212  along the cable. The conductor may be a protective metal tube within which the fiberoptic cable is placed. Alternatively, the conductor may be a wire (e.g., a strength member) embedded in the fiberoptic cable. As another alternative, a metal coating may be manufactured on the cable to serve as the current carrier. Parts of the cable may be covered with an insulator  205  to focus the current dispersal in areas of interest. The optical fiber in cable  212  may act as a distributed sensor or, as in previous embodiments, localized sensors may be integrated into the cable. Because conductive layers can significantly attenuate certain types of electromagnetic fields, the sensors are designed to be operable despite the presence of the conductive layer, e.g., magnetic field sensors, and/or apertures are formed in the conductive layer to permit the EM fields to reach the sensors. 
       FIG. 2C  shows another alternative system configuration. A conductor or conductive layer of fiberoptic cable  206  is electrically coupled to casing  104  to share the same electrical potential and contribute to the dispersal of current into the formation. Parts of the cable  206  and/or casing  104  may be covered with an insulator  205  to focus the current dispersal in areas of interest. 
       FIG. 2D  shows yet another alternative system configuration. Rather than providing an injected current  202  from the surface as in  FIG. 2A , the configuration of  FIG. 2D  provides an injected current  222  from an intermediate point along the casing  104 . Such a current may be generated with an insulated electrical cable passing through the interior of casing  104  from a power source  120  ( FIG. 1 ) to a tool that makes electrical contact at the intermediate point, e.g., via extendible arms. (An alternative approach employs a toroid around casing  104  at the intermediate point to induce current flow along the casing. The toroid provides an electric dipole radiation pattern rather than the illustrated monopole radiation pattern.) 
       FIG. 2E  shows still another alternative system configuration having a first borehole  102  and second borehole  102 ′. Casing  104  in the first borehole  102  carries an injected current from the surface or an intermediate point and disperses it into the surrounding formations. The second borehole  102 ′ has a casing  104 ′ for producing hydrocarbons and further includes a fiberoptic cable  106 ′ with an integrated EM sensor array in the annular space around casing  104 ′. The EM sensors provide measurements of the fields resulting from the currents dispersed in the formations. 
     The sensor array may employ multiple fiberoptic cables  106  as indicated in  FIG. 3A . With cables  106  positioned in parallel or at least in an overlapping axial range, the azimuthal arrangement of sensors  114  enables a multi-dimensional mapping of the electromagnetic fields. In some embodiments, the sensors are mounted to the casing  104  or suspended on fins or spacers to space them away from the body of casing  104 . If actual contact with the formation is desired, the sensors  114  may be mounted on swellable packers  302  as indicated in  FIG. 3B . Such packers  302  expand when exposed to downhole conditions, pressing the sensors  114  into contact with the borehole wall.  FIG. 3C  shows the use of bow-spring centralizers  304  which also operate to press the sensors  114  into contact with the borehole walls. To minimize insertion difficulties, a restraining mechanism may hold the spring arms  304  against the casing  104  until the casing has been inserted in the borehole. Thereafter, exposure to downhole conditions or a circulated fluid (e.g., an acid) degrades the restraining mechanism and enables the spring arms to extend the sensors against the borehole wall. If made of conductive material, the spring arms may further serve as current injection electrodes, concentrating the measurable fields in the vicinity of the sensors. To further concentrate the fields, the spring arms outside the zone of interest may be insulated. 
     Other extension mechanisms are known in the oilfield and may be suitable for placing the sensors  114  in contact with the borehole wall or into some other desired arrangements such as those illustrated in  FIGS. 3D and 3E . In  FIG. 3D , the sensors are positioned near the radial midpoint of the annular region. In  FIG. 3E , the sensors are placed in a spatial distribution having axial, azimuthal, and radial variation. Balloons, hydraulic arms, and projectiles are other contemplated mechanisms for positioning the sensors. 
       FIG. 4  shows an illustrative fixed positioning mechanism for sensors  114 . The cage  402  includes two clamps  403 A,  403 B joined by six ribs  404 . The fiberoptic cable(s)  106  can be run along the ribs or, as shown in  FIG. 4 , they can be wound helically around the cage. In either case, the ribs provide each fiberoptic cable  106  some radial spacing from the casing  104 . Cable ties  406  can be used to hold the cable in place until cementing has been completed. The ribs can be made of insulating material to avoid distortion of the electromagnetic fields around the sensors. 
     In addition to providing support and communications for sensors  114 , the fiberoptic cable  106  may support electrodes or antennas for generating electromagnetic fields in the absence of current injection via casing  104 .  FIG. 5A  shows two electrodes  502  on cable  106 . A voltage is generated between the two electrodes  502  to create an electric dipole radiation pattern. The response of the electromagnetic sensors  114  can then be used to derive formation parameters. 
     Similarly,  FIG. 5B  shows a solenoid antenna  504  on cable  106 . A current is supplied to the solenoid coil to create a magnetic dipole radiation pattern. The response of the electromagnetic sensors  114  can then be used to derive formation parameters. In both cases the sensors are shown to one side of the source, but this is not a requirement. The source may be positioned between sensors  114  and/or one or more of the sensors may be positioned between multiple sources. The sensors  114  may even be positioned between the electrodes of a electric dipole source. Moreover, it is possible to tilt the sources and/or the sensors to provide improved directional sensitivity. 
       FIG. 6  provides a function block representation of an illustrative fiberoptic-based permanent monitoring system. The sensors  114  include electrodes, antennas, or other transducers  602  that convert a property of the surrounding electromagnetic field into a signal that can be sensed via an optical fiber. (Specific examples are provided further below.) An energy source  606  may be provided in the form of a pair of conductors conveying power from the surface or in the form of a powerful downhole battery that contains enough energy to make the device operate for the full life span. It is possible to use an energy saving scheme to turn on or off the device periodically. It is also possible to adjust the power level based on inputs from the fiber optic cable, or based on the sensor inputs. 
     A controller  604  provides power to the transducers  602  and controls the data acquisition and communication operations and may contain a microprocessor and a random access memory. Transmission and reception can be time activated, or may be based on a signal provided through the optic cable or casing. A single sensor module may contain multiple antennas/electrodes that can be activated sequentially or in parallel. After the controller  604  obtains the signal data, it communicates the signal to the fiberoptic interface  608 . The interface  608  is an element that produces new optical signals in fiberoptic cable  610  or modifies existing optical signals in the cable  610 . For example, optical signal generation can be achieved by the use of LEDs or any other type of optical source. As another example, optical signals that are generated at the surface can be modified by thermal or strain effects on the optical fiber in cable  610 . Thermal effects can be produced by a heat source or sink, whereas strain effects can be achieved by a piezoelectric device or a downhole electrical motor. 
     Modification can occur via extrinsic effects (i.e., outside the fiber) or intrinsic effects (i.e., inside the fiber). An example of the former technique is a Fabry Pérot sensor, while an example of the latter technique is a Fiber Bragg Grating. For optimum communication performance, the signal in the optical transmission phase may be modulated, converted to digital form, or digitally encoded. The cable is coupled to a receiver or transceiver  612  that converts the received light signals into digital data. Stacking of sequential measurements may be used to improve signal to noise ratio. The system can be based on either narrowband (frequency type) sensing or ultra wideband (transient pulse) sensing. Narrowband sensing often enables the use of reduced-complexity receivers, whereas wideband sensing may provide more information due to the presence of a wider frequency band. 
     Optionally, a power source  614  transmits power via an electrical conductor  616  to a downhole source controller  618 . The source controller  618  operates an EM field source  620  such as an electric or magnetic dipole. Multiple such sources may be provided and operated in sequence or in parallel at such times and frequencies as may be determined by controller  618 . 
     Multiple sensors  114  may be positioned along a given optical fiber. Time and/or frequency multiplexing is used to separate the measurements associated with each sensor. In  FIG. 7A , a light source  702  emits light in a continuous beam. A circulator  704  directs the light along fiberoptic cable  106 . The light travels along the cable  106 , interacting with a series of sensors  114 , before reflecting off the end of the cable and returning to circulator  704  via sensors  114 . The circulator directs the reflected light to a light detector  708 . The light detector  708  includes electronics that separate the measurements associated with different sensors  114  via frequency multiplexing. That is, each sensor  114  affects only a narrow frequency band of the light beam, and each sensor is designed to affect a different frequency band. 
     In  FIG. 7B , light source  702  emits light in short pulses. Each sensor  114  is coupled to the main optical fiber via a splitter  706 . The splitters direct a small fraction of the light from the optical fiber to the sensor, e.g., 1% to 4%. The sensor  114  interacts with the light and reflects it back to the detector  708  via the splitter, the main fiber, and the circulator. Due to the different travel distances, each pulse of light from source  702  results in a sequence of return pulses, with the first pulse arriving from the nearest sensor  114 , the second pulse arriving from the second nearest sensor, etc. This arrangement enables the detector to separate the sensor measurements on a time multiplexed basis. 
     The arrangements of  FIGS. 7A and 7B  are both reflective arrangements in which the light reflects from a fiber termination point. They can each be converted to a transmissive arrangement in which the termination point is replaced by a return fiber that communicates the light back to the surface.  FIG. 7C  shows an example of such an arrangement for the configuration of  FIG. 7B . A return fiber is coupled to each of the sensors via a splitter to collect the light from the sensors  114  and direct it to a light detector  708 . 
     Other arrangement variations also exist. For example, multiple sensors may be coupled in series on each branch of the  FIG. 7B ,  7 C arrangements. A combination of time division, wavelength-division and/or frequency division multiplexing could be used to separate the individual sensor measurements. 
     Thus each production well may be equipped with a permanent array of sensors distributed along axial, azimuthal and radial directions outside the casing. The sensors may be positioned inside the cement or at the boundary between cement and the formation. Each sensor is either on or in the vicinity of a fiber optic cable that serves as the communication link with the surface. Sensor transducers can directly interact with the fiber optic cables or, in some contemplated embodiments, may produce electrical signals that in turn induce thermal, mechanical (strain), acoustic or electromagnetic effects on the fiber. Each fiber optic cable may be associated with multiple EM sensors, while each sensor may produce a signal in multiple fiber optic or fiber optic cables. Even though the figures show uniformly-spaced arrays, the sensor positioning can be optimized based on geology or made randomly. In any configuration, the sensor positions can often be precisely located by monitoring the light signal travel times in the fiber. 
     Cement composition may be designed to enhance the sensing capability of the system. For example, configurations employing the casing as a current source electrode can employ a cement having a resistivity equal to or smaller than the formation resistivity. 
     The sensors  114  referenced above preferably employ fully optical means to measure EM fields and EM field gradients and transfer the measurement information through optical fibers to the surface for processing to extract the measurement information. The sensors will preferably operate passively, though in many cases sensors with minimal power requirements can be powered from small batteries. The minimization of electronics or downhole power sources provides a big reliability advantage. Because multiple sensors can share a single fiber, the use of multiple wires with associated connectors and/or multiplexers can also be avoided, further enhancing reliability while also reducing costs. 
     Several illustrative fiberoptic sensor configurations are shown in  FIGS. 8A-8C .  FIG. 8A  shows an atomic magnetometer configuration in which light from an input fiber  802  passes through a depolarizer  804  (to remove any polarization biases imposed by the fiber) and a polarizing filter  806  to produce polarized light. A gradient index (GRIN) lens  808  collimates the polarized light before it passes through an alkali vapor cell  812 . A quarter-wave plate  810  enhances optical coupling into the cell. A second GRIN lens  814  directs light exiting the cell into an output fiber  816 . The light passing through the cell consists of a pump pulse to polarize the alkali atoms, followed by a probe pulse to measure the spin relaxation rate. The attenuation of the probe pulse is directly related to the magnetic field strength. 
       FIG. 8B  shows a sensor having a support structure  820  separating two electrodes  822 ,  824 . A center electrode  826  is supported on a flexible arm  828 . The center electrode  826  is provided with a set charge that experiences a force in the presence of an electrical field between electrodes  822 ,  824 . The force causes displacement of the center electrode  826  until a restoring force of the compliant arm  828  balances the force from the electrical field. Electrodes  824  and  826  are at least partially transparent, creating a resonant cavity  830  in the space between. The wavelengths of light that are transmitted and suppressed by the cavity  830  will vary based on displacement of center electrode  826 . Thus the resonant cavity shapes the spectrum of light from input electrode  802 , which effect can be seen in the light exiting from output fiber  816 . The electrodes  822 ,  824  may be electrically coupled to a pair of spaced-apart electrodes (for electric field sensing) or to the terminals of a magnetic dipole antenna (for magnetic field sensing). 
       FIG. 8C  shows a sensor having a support structure  840  with a flexible arm  842  that supports a mirror  846  above a window  844  to define a cavity  848 . The arm further includes a magnet  850  or other magnetically responsive material that experiences a displacing force in response to a magnetic field from a coil  852 . The coil&#39;s terminals  854  are coupled to spaced-apart electrodes (for electric field sensing) or another coil (for magnetic field sensing). Light entering the cavity  848  from fiber  840  reflects from mirror  846  and returns along fiber  840  to the surface. Displacement of the arm  842  alters the travel time and phase of the light passing along fiber  840 . 
     The foregoing sensors are merely illustrative examples and not limiting on the sensors that can be employed in the disclosed systems and methods. An interrogation light pulse is sent from the surface through the fiber and, when the pulse reaches a sensor, it passes through the sensor and the light is modified by the sensor in accordance with the measured electromagnetic field characteristic. The measurement information is encoded in the output light and travels through the fiber to a processing unit located at the surface. In the processing unit the measurement information is extracted. 
       FIG. 9  provides an overview of illustrative formation monitoring methods. A controlled electromagnetic field source generates a subsurface electromagnetic field. While it is possible for this field to be a fixed (DC) field, it is expected that better measurements will be achievable with an alternating current (AC) field having a frequency in the range of 1-1000 Hz. (In applications where shallow detection is desired, higher frequencies such as 1 kHz to 1 GHz can be used.) In block  902 , each of the sensors convert the selected characteristic of the electromagnetic field into a sensed voltage V i , where i is the sensor number. For energy efficiency, sensors can be activated and measurements can be taken periodically. This enables long-term monitoring applications (such as water-flood movements), as well as applications where only small number of measurements are required (fracturing). For further efficiency, different sets of sensors may be activated in different periods. 
     In block  904 , the voltage (or electric field or magnetic field or electric/magnetic field gradient) is applied to modify some characteristic of light passing through an optical fiber, e.g., travel time, frequency, phase, amplitude. In block  906 , the surface receiver extracts the represented voltage measurements and associates them with a sensor position d i . The measurements are repeated and collected as a function of time in block  908 . In addition, measurements at different times can be subtracted from each other to obtain time-lapse measurements. Multiple time-lapse measurements with different lapse durations can be made to achieve different time resolutions for time-lapse measurements. In block  910 , a data processing system filters and processes the measurements to calibrate them and improve signal to noise ratio. Suitable operations include filtering in time to reduce noise; averaging multiple sensor data to reduce noise; taking the difference or the ratio of multiple voltages to remove unwanted effects such as a common voltage drift due to temperature; other temperature correction schemes such as a temperature correction table; calibration to known/expected resistivity values from an existing well log; and array processing (software focusing) of the data to achieve different depth of detection or vertical resolution. 
     In block  912 , the processed signals are stored for use as inputs to a numerical inversion process in block  914 . Other inputs to the inversion process are existing logs (block  916 ) such as formation resistivity logs, porosity logs, etc., and a library of calculated signals  918  or a forward model  920  of the system that generates predicted signals in response to model parameters, e.g., a two- or three-dimensional distribution of resistivity. All resistivity, electric permittivity (dielectric constant) or magnetic permeability properties of the formation can be measured and modeled as a function of time and frequency. The parameterized model can involve isotropic or anisotropic electrical (resistivity, dielectric, permeability) properties. They can also include layered formation models where each layer is homogeneous in resistivity. Resistivity variations in one or more dimensions can be included. More complex models can be employed so long as sufficient numbers of sensor types, positions, orientations, and frequencies are employed. The inversion process searches a model parameter space to find the best match between measured signals  912  and generated signals. In block  922  the parameters are stored and used as a starting point for iterations at subsequent times. 
     Effects due to presence of tubing, casing, mud and cement can be corrected by using a-priori information on these parameters, or by solving for some or all of them during the inversion process. Since all of these effects are mainly additive and they remain the same in time, a time-lapse measurement can remove them. Multiplicative (scaling) portion of the effects can be removed in the process of calibration to an existing log. All additive, multiplicative and any other non-linear effect can be solved for by including them in the inversion process as a parameter. 
     The fluid front position can be derived from the parameters and it is used as the basis for modifying the flood and/or production profile in block  924 . Production from a well is a dynamic process and each production zone&#39;s characteristics may change over time. For example, in the case of water flood injection from a second well, water front may reach some of the perforations and replace the existing oil production. Since flow of water in formations is not very predictable, stopping the flow before such a breakthrough event requires frequent monitoring of the formations. 
     Profile parameters such as flow rate/pressure in selected production zones, flow rate/pressure in selected injection zones, and the composition of the injection fluid, can each be varied. For example, injection from a secondary well can be stopped or slowed down when an approaching water flood is detected near the production well. In the production well, production from a set of perforations that produce water or that are predicted to produce water in relatively short time can be stopped or slowed down. 
     We note here that the time lapse signal derived from the receiver signals is expected to be proportional to the contrast between formation parameters. Hence, it is possible to enhance the signal created by an approaching flood front by enhancing the electromagnetic contrast of the flood fluid relative to the connate fluid. For example, a high magnetic permeability, or electrical permittivity or conductivity fluid can be used in the injection process in the place of or in conjunction with water. It is also possible to achieve a similar effect by injecting a contrast fluid from the wellbore in which monitoring is taking place, but this time changing the initial condition of the formation. 
     The disclosed systems and methods may offer a number of advantages. They may enable continuous time-lapse monitoring of formations including a water flood volume. They may further enable optimization of hydrocarbon production by enabling the operator to track flows associated with each perforation and selectively block water influxes. Precise localization of the sensors is not required during placement since that information can be derived afterwards via the fiber optic cable. Casing source embodiments do not require separate downhole EM sources, significantly decreasing the system cost and increasing reliability. 
     Numerous other variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. For example, this sensing system can be used for cross well tomography with EM transmitters are placed in one well and EM fields being measured in surrounding wells which can be drilled at an optimized distance with respect to each other and cover the volume of the reservoir from multiple sides for optimal imaging. It is intended that the following claims be interpreted to embrace all such variations and modifications where applicable.