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[[file:AlsaudKatterbauerFigure3.jpg|thumb|300px|{{figure number|3}}Cross-well EM System simple design.<ref name=Wildetal_1995>M. Wilt, H. Morrison, A. Becker, H.-W. Tseng, K. Lee, C. Torres-Verdin and D. Alumbaugh, "Crosshole electromagnetic tomography: A new technology for oil field characterization," The Leading Edge, vol. 14, no. 3, p. 147–222.]]

[[file:AlsaudKatterbauerFigure3.jpg|thumb|300px|{{figure number|3}}Cross-well EM System simple design.<ref name=Wiltetal_1995>M. Wilt, H. Morrison, A. Becker, H.-W. Tseng, K. Lee, C. Torres-Verdin and D. Alumbaugh, "Crosshole electromagnetic tomography: A new technology for oil field characterization," The Leading Edge, vol. 14, no. 3, p. 147–222.</ref>]]

==Instrument Design of the EM System==

==Instrument Design of the EM System==

In the early 1970s, the Lawrence Livermore National Laboratory (LLNL) developed the first-ever high-frequency cross-well electromagnetic system using electric dipole sensors. These sensors were intended primarily for hard rock, either igneous or metamorphic rocks, and tunnel detection. Due to the initial EM system high frequency and high conductivity regions in the reservoirs, the signals can only propagate a few inches into the formation (small depth of investigation). Thus, to measure the resistivity of the formation by using this technology, a low-frequency EM system is needed. However, the emitting signals are highly diffusive so, it is difficult to map the data. Based on this situation, the LLNL, together with the University of California-Berkley (UC-Berkley), extended the work on the induction logging tools to create the cross-well EM tomography [4].  

In the early 1970s, the Lawrence Livermore National Laboratory (LLNL) developed the first-ever high-frequency cross-well electromagnetic system using electric dipole sensors. These sensors were intended primarily for hard rock, either igneous or metamorphic rocks, and tunnel detection. Due to the initial EM system high frequency and high conductivity regions in the reservoirs, the signals can only propagate a few inches into the formation (small depth of investigation). Thus, to measure the resistivity of the formation by using this technology, a low-frequency EM system is needed. However, the emitting signals are highly diffusive so, it is difficult to map the data. Based on this situation, the LLNL, together with the University of California-Berkley (UC-Berkley), extended the work on the induction logging tools to create the cross-well EM tomography.<ref name=Wiltetal_1995 />

The LLNL and UC-Berkley team tested this technology by using very simplistic tools (see [[:file:AlsaudKatterbauerFigure3.jpg|Figure 3]]). The system worked by sending a high alternating current from the source on the surface down the logging cable to the transmitter. The transmitter is made of a magnetic core wrapped with a wire, and it is tuned with a capacitor. To achieve the optimum single operating frequency suitable for the borehole condition, the coil is tuned by either replacing the capacitor or/and increasing/decreasing the number of turns around the magnetic core. The research team tested different frequencies to reach the optimum resolution and range. The antenna detects the emitted signal from the transmitter in the receiver located in the other borehole. The signal is then sent uphole through the logging cable to the lock-in detector to decode the signal. The received signal is coupled with the source signal with an isolated wireline for configuration. The transmitter and receiver are two isolated structures, where each one has its generator. The only way to connect these coils is by a surface isolated cable. This kind of separation is vital for generating high-quality data and minimizing the earth loops' noise. In the borehole, the logged information is between an interval of depths, so to log the data, multiple receivers stay at a separated interval while the transmitter travels upward at a constant velocity. Following this technique, a complete cross-well tomography is generated [4] [3].  

The LLNL and UC-Berkley team tested this technology by using very simplistic tools (see [[:file:AlsaudKatterbauerFigure3.jpg|Figure 3]]). The system worked by sending a high alternating current from the source on the surface down the logging cable to the transmitter. The transmitter is made of a magnetic core wrapped with a wire, and it is tuned with a capacitor. To achieve the optimum single operating frequency suitable for the borehole condition, the coil is tuned by either replacing the capacitor or/and increasing/decreasing the number of turns around the magnetic core. The research team tested different frequencies to reach the optimum resolution and range. The antenna detects the emitted signal from the transmitter in the receiver located in the other borehole. The signal is then sent uphole through the logging cable to the lock-in detector to decode the signal. The received signal is coupled with the source signal with an isolated wireline for configuration. The transmitter and receiver are two isolated structures, where each one has its generator. The only way to connect these coils is by a surface isolated cable. This kind of separation is vital for generating high-quality data and minimizing the earth loops' noise. In the borehole, the logged information is between an interval of depths, so to log the data, multiple receivers stay at a separated interval while the transmitter travels upward at a constant velocity. Following this technique, a complete cross-well tomography is generated.<ref name=WiltAlumbaughetal_1995>M. Wilt, D. L. Alumbaugh, H. F. Morrison, A. Becker, K. H. Lee and M. Deszcz-Pan, "Crosswell electromagnetic tomography: System design considerations and field results," Geophysics, vol. 60, no. 3, pp. 629-929.</ref> <ref name=Wiltetal_1995 />

The team did two field trials in Richmond, California, where the distance between the two wells is 50 m, the other in Devine, Texas, where the wells are 100 m apart. The Richmond test targeted a shallower depth compared to the Devine trial, and the EM system was used to monitor the injected saltwater [4] [3].  

The team did two field trials in Richmond, California, where the distance between the two wells is 50 m, the other in Devine, Texas, where the wells are 100 m apart. The Richmond test targeted a shallower depth compared to the Devine trial, and the EM system was used to monitor the injected saltwater.<ref name=WiltAlumbaughetal_1995 /> <ref name=Wiltetal_1995 />

The design standards of the cross-well EM system are mainly described by the following:  

The design standards of the cross-well EM system are mainly described by the following:  

Equation (4) S_wⁿ = (aR_w)/(ϕ^m R_t )

Equation (4) S_wⁿ = (aR_w)/(ϕ^m R_t )

Where n is the saturation exponent, m is the cementation factor, a is the tortuosity, Rt is the measured formation resistivity in ohm-m, and S_w is the water saturation [6]. The rock and fluid properties are taking from cores or/and logs.  

Where n is the saturation exponent, m is the cementation factor, a is the tortuosity, Rt is the measured formation resistivity in ohm-m, and S_w is the water saturation.<ref name=Ellissinger_2007>D. V. Ellis and J. M. Singer, Well Logging for Earth Scientists, 2nd ed., Dordrecht: Springer, 2007.</ref> The rock and fluid properties are taking from cores or/and logs.  

From the cross-sectional measurements of the inter well EM data (see [[:file:AlsaudKatterbauerFigure5.jpg|Figure 5]] and [[:file:AlsaudKatterbauerFigure6.jpg|Figure 6]]) taking together with the porosity from logs and cores, the water saturation can be interpolated by using Archie’s equation.

From the cross-sectional measurements of the inter well EM data (see [[:file:AlsaudKatterbauerFigure5.jpg|Figure 5]] and [[:file:AlsaudKatterbauerFigure6.jpg|Figure 6]]) taking together with the porosity from logs and cores, the water saturation can be interpolated by using Archie’s equation.

[[file:AlsaudKatterbauerFigure4.jpg|thumb|300px|{{figure number|4}}Wells location: five spot patterns (3 observers, 1 injector, 1 producer) [7]]

[[file:AlsaudKatterbauerFigure4.jpg|thumb|300px|{{figure number|4}}Wells location: five spot patterns (3 observers, 1 injector, 1 producer)<ref name=Bhattietal_2008>Z. N. Bhatti, Y. J. Al Mansoori, S. M. El Sembawy, V. Vahrenkamp, N. Clerc, M. Wilt and S. L. Reeder, "Tracking Interwell Water Saturation in Pattern Flood Pilots in a Giant Gulf Oil field," in Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi.</ref>]]

==Pattern Water Injection Pilot==

==Pattern Water Injection Pilot==

In 2008, a pilot was done on the lower cretaceous heterogeneous carbonate reservoir in the United Arab Emirates. The pilot targeted both the Kharaib and Shuaiba formations in the Thamama series. With the continuation of oil production, pressure will deplete. Therefore, a continuous water injection job is needed to maintain the reservoir pressure and enhance recoverable oil. Due to the altered carbonate texture of the formation, which affects both the porosity and permeability, the movement of the interface between the injected water and the oil is uneven. Also, the formation resistivity of each layer changes, and with water injection, the resistivity changes with time. As a result, a sophisticated flood front monitoring tool is essential to improve the oil recovery from the reservoir, monitor the patterned water flooding job, and determine water breakthrough [7].  

In 2008, a pilot was done on the lower cretaceous heterogeneous carbonate reservoir in the United Arab Emirates. The pilot targeted both the Kharaib and Shuaiba formations in the Thamama series. With the continuation of oil production, pressure will deplete. Therefore, a continuous water injection job is needed to maintain the reservoir pressure and enhance recoverable oil. Due to the altered carbonate texture of the formation, which affects both the porosity and permeability, the movement of the interface between the injected water and the oil is uneven. Also, the formation resistivity of each layer changes, and with water injection, the resistivity changes with time. As a result, a sophisticated flood front monitoring tool is essential to improve the oil recovery from the reservoir, monitor the patterned water flooding job, and determine water breakthrough.<ref name=Bhattietal_2008 />

[[file:AlsaudKatterbauerFigure5.jpg|thumb|300px|{{figure number|5}}Time lapse EM resistivity image between OBS1 and OBS2. (A) before water injection (B) After four months of water injection [8].]]

[[file:AlsaudKatterbauerFigure5.jpg|thumb|300px|{{figure number|5}}Time lapse EM resistivity image between OBS1 and OBS2. (A) before water injection (B) After four months of water injection.<ref name=Alalietal_2009>M. Al Ali, V. Vahrenkamp, S. Elsembawy, Z. Bhatti, S. L. Reeder, N. Clerc and M. Wilt, "Constraining Interwell Water Flood Imaging with Geology and Petrophysics: An Example from the Middle East," in SPE Middle East Oil & Gas Show and Conference, Manama.</ref>]]

A cross-well EM monitoring technique was suggested to utilize the inverted five-spot water injection pattern. The project was done using three vertical observers, one horizontal water injector, and an existing oil producer. The drilled observer’ wells were cased with chrome to demolish the effect of the magnetic casing on the EM measurements. The triangular configuration of the observers provided a reasonably good volumetric coverage of the region (see [[:file:AlsaudKatterbauerFigure4.jpg|Figure 4]]) [7].

A cross-well EM monitoring technique was suggested to utilize the inverted five-spot water injection pattern. The project was done using three vertical observers, one horizontal water injector, and an existing oil producer. The drilled observer’ wells were cased with chrome to demolish the effect of the magnetic casing on the EM measurements. The triangular configuration of the observers provided a reasonably good volumetric coverage of the region (see [[:file:AlsaudKatterbauerFigure4.jpg|Figure 4]]).<ref name=Bhattietal_2008 />

[[file:AlsaudKatterbauerFigure6.jpg|thumb|300px|{{figure number|6}}Unit 3,4, and 5 time-lapse cross-sectional EM resistivity image of the observers and injector (Baseline) before water injection (Time lapse survey) After six months of water injection [8].]]

[[file:AlsaudKatterbauerFigure6.jpg|thumb|300px|{{figure number|6}}Unit 3,4, and 5 time-lapse cross-sectional EM resistivity image of the observers and injector (Baseline) before water injection (Time lapse survey) After six months of water injection.<ref name=Alalietal_2009 />]]

The time-lapse EM resistivity was created, as seen in [[:file:AlsaudKatterbauerFigure5.jpg|Figure 5]] and [[:file:AlsaudKatterbauerFigure6.jpg|Figure 6]], where the cross-well EM data were processed and interpolated. [[:file:AlsaudKatterbauerFigure5.jpg|Figure 5]] shows an example of all the layers (units) between the observer OBS1 and OBS2.  A similar image can be constructed between OBS 1-OBS3, and OBS2-OBS3. [[:file:AlsaudKatterbauerFigure6.jpg|Figure 6]] shows a zoomed-in version of the EM tomography focusing on units 3,4 and 5. As seen in both figures, the resistivity decreases around the injector in a horizonal ellipsoidal manner. Between OBS3-1 the water is contained in unit 5, whereas OBS1-2 the injected water slightly affects the resistivity of unit 4. This shows that there is a possibility of a fracture connecting layer 5 with 4. Hence, the water migrates vertically causing a decrease in resistivity. All in all, the EM monitor system demonstrated an effective water flooding monitoring tool for a future recovery monitoring mechanism in complex reservoirs [8].

The time-lapse EM resistivity was created, as seen in [[:file:AlsaudKatterbauerFigure5.jpg|Figure 5]] and [[:file:AlsaudKatterbauerFigure6.jpg|Figure 6]], where the cross-well EM data were processed and interpolated. [[:file:AlsaudKatterbauerFigure5.jpg|Figure 5]] shows an example of all the layers (units) between the observer OBS1 and OBS2.  A similar image can be constructed between OBS 1-OBS3, and OBS2-OBS3. [[:file:AlsaudKatterbauerFigure6.jpg|Figure 6]] shows a zoomed-in version of the EM tomography focusing on units 3,4 and 5. As seen in both figures, the resistivity decreases around the injector in a horizonal ellipsoidal manner. Between OBS3-1 the water is contained in unit 5, whereas OBS1-2 the injected water slightly affects the resistivity of unit 4. This shows that there is a possibility of a fracture connecting layer 5 with 4. Hence, the water migrates vertically causing a decrease in resistivity. All in all, the EM monitor system demonstrated an effective water flooding monitoring tool for a future recovery monitoring mechanism in complex reservoirs.<ref name=Alalietal_2009 />

Cross-well electromagnetic has played a critical role in mapping saturation in reservoirs and improve fluid front tracking. Additionally, it has played a crucial role in optimizing reservoir recovery via the optimization of injection strategies and the location of new wells. This by itself, plays a quintessential role in enhancing sustainability of reservoir operations via reducing the overall carbon footprint as well as minimize water consumption for reservoir recovery. While direct measurements assist, they have to be combined with novel artificial intelligence technologies in order to enhance interpretation and forecasting based on available data. As emphasized before that data is the new oil, utilization of available data in order to improve reservoir understanding will be key to enhance sustainability in operations and achieve higher productivity and prosperity.

Cross-well electromagnetic has played a critical role in mapping saturation in reservoirs and improve fluid front tracking. Additionally, it has played a crucial role in optimizing reservoir recovery via the optimization of injection strategies and the location of new wells. This by itself, plays a quintessential role in enhancing sustainability of reservoir operations via reducing the overall carbon footprint as well as minimize water consumption for reservoir recovery. While direct measurements assist, they have to be combined with novel artificial intelligence technologies in order to enhance interpretation and forecasting based on available data. As emphasized before that data is the new oil, utilization of available data in order to improve reservoir understanding will be key to enhance sustainability in operations and achieve higher productivity and prosperity.

* [[Structural restoration]]

* [[Structural restoration]]

* [[Condensate banking effect]]

* [[Condensate banking effect]]

==References==

==References==

{{reflist}}

{{reflist}}