Time-lapse Electromagnetic Reservoir Monitoring
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Solution Overview
Problem
Current methods for determining the 3D distribution of hydrocarbon fluids in subsurface reservoirs during hydrocarbon production are inefficient due to low sensitivity of seismic methods, limited penetration of well logging signals, and the presence of electrically conductive casing, which hinders effective monitoring of reservoir fluid resistivity and saturation.
Innovation Solution
A method using controlled-source electromagnetic surveys to measure time-dependent changes in vertical and horizontal resistivities by solving Maxwell's electromagnetic field equations with data from horizontal electric or magnetic dipole sources, accounting for resistivity anisotropy and imaging hydrocarbon saturation changes using 3D inversion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If seismic methods are used for time-lapse monitoring of reservoir fluids, then 3D distribution of hydrocarbons can be monitored over time, but the sensitivity to hydrocarbon saturation changes is low and the cost is high
Solution Approach 1:
The patent replaces seismic (acoustic) methods with electromagnetic methods for reservoir monitoring. Instead of using acoustic waves to detect hydrocarbon saturation changes, the invention employs electromagnetic fields that are more sensitive to resistivity changes caused by hydrocarbon-water transitions, thereby achieving higher measurement precision without relying on the less sensitive seismic response
Solution Approach 2:
The invention changes the physical parameter used for monitoring from acoustic properties (seismic) to electrical resistivity (electromagnetic). By measuring resistivity changes at different frequencies and positions, the system can detect hydrocarbon saturation changes more effectively, as resistivity is more sensitive to fluid composition changes than acoustic properties
2Measurement precision
If well logging methods are used to measure reservoir fluid resistivity, then accurate measurements can be obtained near the wellbore, but the penetration distance is limited and cannot effectively monitor areas between wells
Solution Approach 1:
The patent introduces electromagnetic fields as an intermediary medium to transmit information about reservoir fluid resistivity over long distances. Instead of relying on direct contact measurements near the wellbore, electromagnetic waves penetrate through the reservoir rock and seawater to carry resistivity information from distant locations, enabling monitoring between wells without requiring physical proximity
Solution Approach 2:
The invention transitions from one-dimensional well-to-well monitoring to three-dimensional spatial monitoring using electromagnetic fields. By measuring resistivity at multiple positions and depths using electromagnetic sources and receivers, the system creates a volumetric view of reservoir saturation, overcoming the linear limitation of well logging penetration
3Area of stationary object
If cross-borehole electromagnetic methods are used to monitor reservoir resistivity, then measurements can be obtained between wells, but the conductive casing in wells severely limits the probing signal and requires expensive well shutdowns
Solution Approach 1:
The patent extracts the electromagnetic measurement system from the wellbore environment entirely. Instead of placing sources and receivers inside wells where casing causes shielding, the invention positions all electromagnetic equipment on the sea surface, eliminating the harmful shielding effect of conductive casing while maintaining the ability to monitor reservoir resistivity between wells
Solution Approach 2:
The invention creates a surface-based copy of the cross-borehole measurement concept. Rather than physically placing electrodes in wells, electromagnetic fields generated at the surface simulate the effect of downhole sources, providing equivalent monitoring capability without the need for well access or casing penetration
4Measurement precision
If mathematical simulation with history matching is used to estimate fluid saturation, then saturation distribution can be inferred, but many simplifications and assumptions about rock properties are required
Solution Approach 1:
The patent enables the electromagnetic measurement system to directly provide saturation information without requiring complex external modeling. By measuring resistivity at multiple frequencies and positions, the system self-determines saturation distribution through direct physical relationships between resistivity and fluid composition, eliminating the need for simplified rock property assumptions required in mathematical simulation
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Enables accurate remote estimation and imaging of hydrocarbon saturation throughout the reservoir, accounting for resistivity anisotropy, allowing for effective monitoring of hydrocarbon production and reservoir changes over time.
Implementation Method 1
controlled-source electromagnetic surveys typically in offshore environments, in which a controlled electromagnetic transmitter is towed above or positioned between electromagnetic receivers on the sea floor
Implementation Method 2
solving Maxwell's electromagnetic field equations for horizontal resistivity and vertical resistivity at a plurality of (x,y,z) positions in the subsurface reservoir
Data Source
Figure 1
Figure 2
Figure 3A~3B
AI summary
Method for determining time-dependent changes [73] in the earth vertical and horizontal electrical resistivity and fluid saturations from offshore electromagnetic survey measurements. The method requires both online and offline data, which should include at least one electromagnetic field component sensitive at least predominantly to vertical resistivity and another component sensitive at least predominately to horizontal resistivity [62]. Using a horizontal electric dipole source, online Ez and offline Hz measurements are preferred. For a horizontal magnetic dipole source, online H2 and offline E2 data are preferred. Magnetotelluric data may be substituted for controlled source data sensitive at least predominantly to horizontal resistivity. Maxwell's equations are solved by forward modeling [64,65] or by inversion [66,67], using resistivity models of the subsurface that are either isotropic [64,66] or anisotropic [65,67]. Fluid saturation is determined from the vertical and horizontal resistivities using empirical relations or rock physics models [70].