Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Earth science
Reexamination Certificate
2001-05-11
2004-10-19
Barlow, John (Department: 2863)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Earth science
C367S905000
Reexamination Certificate
active
06807487
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the field of estimating in-situ permeability of the reservoir rocks. More specifically, the invention is related to a method of determining the dynamic elastic nonlinear interaction between the Fast Compressional Seismic Wave that travels through the rock matrix and a liquid/solid coupled slower Compressional Seismic Wave that travels through the interconnected fluid-filled pores. The presence of this slower Compressional Wave in a hydrocarbon reservoir formation is a strong indicator of the formation's bulk permeability. In this invention, the slower Compressional Wave that is generated, due to the solid/liquid coupling as the Fast Compressional Wave propagates through a permeable rock formation, is identified as “Drag-Wave.” This Drag-Wave travels at the pore fluid compressional velocity but over a longer distance along the tortuous path of the interconnected pores. The elastic nonlinear interaction between the Fast Compressional Wave and the Drag-Wave, as they propagate through a reservoir formation, generates summed and differenced frequencies of the two waves. From this information the Drag-Wave velocity can be calculated. From the Drag-Wave velocity we can calculate the bulk tortuosity of the formation. Permeability that is dependent on the pore size and the tortuosity of the pores can be determined, once the tortuosity is known.
2. Description of the Related Art
Permeability is often the most important factor in influencing the commercial viability of a hydrocarbon reservoir. So far, permeability cannot be measured directly in-situ in reservoir formations. Downhole tools that measure permeability in a borehole quite often provide ambiguous results, and these results are confined to the immediate vicinity of the wellbore. Measurement or estimation of permeability in carbonate reservoir rocks is even more difficult, since carbonates are more heterogeneous compared to sandstones.
A new seismic method that can estimate the bulk permeability of the reservoir formations between the wells will be extremely useful for implementing an efficient production program for a hydrocarbon-producing field that will optimize the economics of the hydrocarbon recovery.
Biot (1956) proposed a comprehensive theory that explained many important features of the seismic wave propagation in fluid-saturated porous media. One of the important contributions of his theory is the prediction of a Slow Compressional Wave with a speed lower than that of the rock matrix or the pore fluid. The Slow-Wave involves a coupled motion between the fluid and the solid frame. The Slow-Wave's velocity and attenuation depend on the morphology of the pore space and the pore interconnections, which also determine the fluid transport properties such as permeability. The detection of the presence of the Slow-Wave in a reservoir formation between two wells is a strong indicator that the formation is permeable.
The Slow-Wave has been successfully measured under laboratory conditions using samples of glass beads and sand stone samples from typical reservoir formations (Berea and Massillon). Considerable effort has been made to detect the Slow-Wave in in-situ sedimentary rocks. So far this effort has not been very successful.
Since information related to in-situ rock permeability of the reservoir formations is extremely important for developing an accurate reservoir simulation model of its flow units, a new method of estimating the permeability of in-situ reservoir formations has been developed. In this invention, we determine the existence and the properties of the Slow-Wave for estimating the bulk tortuosity and permeability of the in-situ reservoir formations.
SUMMARY OF THE INVENTION
This invention introduces a new method of mapping reservoir flow units by identifying the in-situ permeability of the reservoir formations between the existing wells. To economically produce hydrocarbons from a reservoir, the reservoir rocks have to be porous so that the fluids can be stored in the pores. The pores have to be connected so that the reservoir fluids can flow between the pores. The capacity of transmitting a fluid in a rock depends on the size and shape of the pores, size and shape of the interconnections and their extent, and is known as permeability.
When a pressure wave travels through a rock, the rock matrix and pore fluids are simultaneously compressed. The velocity of the Compressional Wave in the rock matrix is related to the mineral frame and the cementation between the grains, while the velocity of the slower component of the Compressional Wave that travels through the interconnected fluid path is determined by the physical properties of the pore fluids and the tortuosity of the connected pores in the rock.
In the published literature, the Compressional Wave that travels through the fluids in the interconnected pores is identified as Slow-Wave. Slow-Wave has been measured under laboratory conditions in samples of glass beads and different porous and permeable sandstones. The Slow-Wave travels at the fluid compressional velocity but over a longer distance along the tortuous interconnected pores between the two ends of the reservoir formation, which is being measured.
The Slow-Wave is diffusive and highly attenuated. For this reason, it has been difficult to measure the Slow-Wave in-situ in the reservoir rocks. The measurements related to the Slow-Wave provide a unique opportunity to determine the reservoir rock properties such as permeability and tortuosity, which affect the flow mechanism of the reservoir fluids. Since Slow-Wave cannot be measured due to its high attenuation in-situ in the sedimentary rocks of the reservoir, a new method of measuring Slow-Wave has been developed and described in this Patent.
Permeable rocks are elastically nonlinear due to: a) their morphology; b) the microstructures of their pores; c) the pore interconnections; and d) pore fluids. In a permeable rock that is elastically nonlinear, the interactions between two elastic waves can be used in a unique way to map its physical properties. An elastic wave generated within a rock can be made to interfere with an externally generated seismic signal, and their elastic nonlinear interaction can be measured to determine the bulk tortuosity and permeability of a reservoir formation.
When the Primary external signal is a sinusoidal wave of a predetermined frequency and time duration, it creates a moving wave of compressional and rarefaction fronts that are repetitive and travel one wavelength apart. Each compressional front is separated from the next compressional front by a wavelength. Due to the physical coupling between the rock matrix and the fluid-filled pores, a Drag-Wave is generated as the Primary Sinusoidal Wave propagates through the rock matrix. The Drag-Wave propagates through the fluid-filled interconnected pores at the same velocity as the Slow-Wave. This velocity depends on the pore fluid properties and the tortuous path of the pore interconnections.
The Primary Sinusoidal Wave and the Drag-Wave propagate through the rock simultaneously and they elastically interact with each other. Due to the elastic nonlinearity of the permeable rock, the interaction between these two waves can be detected and measured as the elastic nonlinear interaction of the high-frequency Primary-Wave and the low-frequency Drag-Wave.
When two elastically linear seismic waves travel together in a subsurface formation, the principle of superposition holds and there is no interaction between the two waves. However, when they travel through a formation that is elastically nonlinear, then a nonlinear interaction between the two elastic waves occurs, and summed and differenced frequencies are generated.
In a permeable subsurface formation that is nonlinear, the interaction between the high-frequency Primary-Wave and the low-frequency Drag-Wave generates the summed and differenced frequencies of the two seismic signals. These summed and differenced frequencies appear as the side
Barlow John
Khan Tawassul A.
Le Toan M.
McGuire Sofia K.
Nonlinear Seismic Imaging Inc.
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