ES Xplore’s electroseismic technology directly detects ultimately producible hydrocarbons at seismic resolution. Conventional exploration technologies only convey structural information about rock formations or low-resolution information about the rock fluids. Additionally, conventional methods, specifically seismic exploration, use man-made energy sources that can be cumbersome and environmentally intrusive.
Conversion between electromagnetic (EM) and seismic energies is the key mechanism underlying ES Xplore’s direct hydrocarbon technology. Energy conversion is enhanced by the presence of electrically resistive hydrocarbons and permeability in reservoir rock. Such conversions have been studied in the laboratory and the field for decades and are beginning to see practical use in field applications due to technology advances. To date, ES Xplore has been awarded 11 patents on its disruptive technology.
Technology overview
The Earth’s atmosphere is a source of EM pulses generated by electrical activity such as lightning discharges. Some of the horizontally polarized energy that crosses the boundary between the Earth’s surface and the atmosphere is rotated into a vertical electric field by inhomogeneity in the subsurface. This process is known as “mode rotation,” and the resulting vertical field travels both up and down and is detected at the surface by an EM sensor. When a horizontal current encounters resistive inhomogeneity, it is redirected into a vertical direction, seeking the path of least resistance. Horizontal current is attenuated by the high horizontal electrical conductivity, while the vertical current passes around higher resistance regions where it is only weakly attenuated. After the current is redirected by many inhomogeneities, only the vertical current and electric field remain at depths greater than the skin depth. Electric field rotation can be modeled using the known anisotropic electrical properties of the subsurface, which is an inhomogeneous resistor network.
At depth, the vertical electric field interacts with a hydrocarbon reservoir, where part of its energy is converted through the electroseismic effect into a seismic wave. Additionally, this seismic wave is accompanied by an EM field due to the seismoelectric effect. An electric field detector located at the Earth’s surface detects both the source vertical EM field and the response vertical EM field generated from the response seismic wave.
Certain rock properties enable coupling between EM and acoustic energies and, hence, mediate conversion of EM energy into seismic energy and the reverse. Electroseismic conversion in reservoirs occurs because permanent electric fields in the pore fluids interact with the transient source vertical electric field. Sedimentary rock contains two phases, a hard matrix material of varying chemical composition and pore spaces that are filled with fluids such as brine, gas and liquid hydrocarbons. Water lining the rock-matrix/pore-space interface creates an electric dipole layer (Figure 1a). The electric dipole layer is composed of immobile ions of one charge at the surface of the rock matrix and mobile charges of the opposite sign in the water.
An external electric field interacts with the internal electric field of the electric dipole layer. This interaction generates movement of the mobile ions and leads to mechanical stress on the rock matrix. The mechanical stress, in turn, induces a seismic compressive (P) wave that travels to the Earth’s surface. It is important to note that the seismic wave generated by the conversion travels to the surface at seismic speed as opposed to the external vertical electric field traveling to the reservoir at EM speed.
When returning seismic waves reach the surface, the gradient and inhomogeneities in near-surface soil create seismoelectric conversions. Inhomogeneities scatter P waves from the target into an equipartition of all bulk and surface waves. The vertical electric field associated with scattered surface waves, predominantly Rayleigh waves, is detected with the same sensor that detects the source fields.
The seismic waves resulting from the electroseismic effect predominantly arise from the contrasts in resistance between the oil-saturated and brine-saturated pore space.
Example
This technology obviously can be applied to hydrocarbon exploration. Figure 1 displays a transect of ES Xplore survey points that span the Elm Coulee Bakken Field in Montana. There is a well-defined Bakken pinchout line to the south of the field, and the field’s porosity decreases to the north of the field center.
The company collected data at sites along the defined transect. Figure 2 shows a cross section of results displayed in stratigraphic log form, where depth is along the vertical axis and the magnitude of the log deflection is related to the amplitude of the electroseismic response at that depth.
This process delivers improved reliability without requiring an active man-made source or encountering any environmentally intrusive methods. Additionally, it offers the opportunity to upgrade the formation evaluation flow to allow more focused development expenditures.
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