Podberezhniy, M.Y. (Institute of Geophysics SB RAS) | Nefedkin, Y.A. (Institute of Geophysics SB RAS) | Kulikov, V.A. (Institute of Geophysics SB RAS) | Sibiryakov, E.B. (Institute of Geophysics SB RAS)
In the paper we describe a series of experiments on studying seismic anisotropy and nonlinear effects (due to electric-seismic effects) detected in the laboratory physical models of liquid saturated friable media under atmosphere pressure. Two- and one-dimensional sand and clay models were saturated with liquids of different conductivity. The seismic parameters of longitudinal and shear waves in acoustical frequency range of 250Hz-5 kHz were studied under influence of external direct electric current of different value. Direct current causes significant variations in seismic amplitudes and velocities depending on the direction of electric current passing through the medium saturated by the conductive liquid and depending on geological type and faction of rock. At the same time, there are no considerable changes of seismic parameters in case of medium saturated with non-conductive liquid for all types of rocks. Weak electric fields and currents render essential influence on a degree of medium nonlinearity: the factor of nonlinearity becomes 1.5 times higher. We discuss a possible mechanism of these effects.
Conventional migration assumes that every point in the earth''s subsurface is a potential scatterer, generating an idealized broad-band impulse response. Such a model incorrectly treats energy contained the more realistic bandlimited seismic source wavelet, spreading it out in image space and giving rise to wavelet stretch of the farther offsets. We use an alternative assumption that along a given horizon the earth''s subsurface can be approximated by a discrete reflector whose energy is carried by the band-limited seismic wavelet. This horizon-oriented Kirchhoff migration provides stretch-free far-offset data appropriate for reservoir characterization studies. Such data can then be included in the stacking process to obtain improved images from prestack migrated data having increased vertical and lateral resolution. Because we no longer mute the long-offset data, we now need to deal with non-hyperbolic moveout to fully achieve the greatest potential of this technique.
Beresnev, Igor (Department of Geological and Atmospheric Sciences, Iowa State University) | Vigil, Dennis (Department of Chemical Engineering, Iowa State University) | Li, Wenqing (Department of Chemical Engineering, Iowa State University)
Elastic waves have been observed to increase productivity of oil wells, although the reason for the vibratory mobilization of the residual organic fluids has remained unclear. Residual oil is entrapped as ganglia in narrow pore constrictions because of resisting capillary forces. A finite external pressure gradient, exceeding an “unplugging” threshold, is needed to carry the ganglia through. We show that vibrations help overcome this resistance by adding an oscillatory inertial forcing to the external gradient; when the vibratory forcing acts along the gradient and the threshold is exceeded, instant “unplugging” occurs. This mechanism predicts the mobilization effect to be proportional to the amplitude and inversely proportional to the frequency. We observe this dependence in a laboratory experiment, in which residual saturation is created in a glass micromodel, and mobilization of ganglia is monitored using digital photography. We also directly demonstrate the release of an entrapped ganglion by vibrations in a computational fluid-dynamics simulation.
Alpak, Faruk O. (The University of Texas at Austin) | Mallan, Robert K. (The University of Texas at Austin) | Hou, Junsheng (The University of Texas at Austin) | Torres-Verdín, Carlos (The University of Texas at Austin)
Marine controlled-source electromagnetic (CSEM) surveying has emerged as a new tool for remotely detecting reservoired hydrocarbons offshore. The technology was pioneered by university and government researchers over the past 25 years, and recently has benefited from development by contractors and the oil industry, including ExxonMobil. When integrated carefully with other geoscience information, primarily seismic data, marine CSEM shows promise for adding considerable value in Upstream applications. An extensive survey program was conducted over the past three years to evaluate marine CSEM. Remote reservoir resistivity results from a recent West Africa survey demonstrate the viability of the method.
Marine CSEM research began more than 80 years ago, but only started to make significant progress in the 1970s through efforts by university and government groups. We began investigating marine CSEM in 1981 using theory and computer modeling. Results looked promising for deep water applications, and field tests were proposed over known reservoirs (Srnka, 1986). However, the time for marine CSEM had not yet arrived: the lack of suitable acquisition equipment, limited deep water opportunities, and the emphasis on the (then) new 3D marine seismic technology resulted in deferring our CSEM research. We re-examined the technology in late 1996. Advances in acquisition and computing capabilities, together with growing opportunities in very deep water, led to the identification of a breakthrough project in CSEM methods constrained by seismic data to deliver the sensitivity and resolution required for Upstream applications. The Remote Reservoir Resistivity Mapping ("R3M") project was born, and by mid-1998 research was underway. From its inception, R3M focussed on full 3D methods for survey design, acquisition, data processing, imaging, and interpretation.
Leveraging and Commercialization
Technology leveraging has been essential for the project''s success. Leveraging included modeling and imaging (inversion) algorithms from universities and national laboratories, seafloor receivers from UCSD Scripps, deeptow sources from the Southampton Oceanography Center, and specialized navigation and positioning systems from commercial contractors. For each of these technology elements, significant upgrades were made in-house to improve their performance for hydrocarbon applications. As the project progressed, the technology elements evolved at varying rates, creating both technical opportunities and project management challenges. The Marine CSEM Method
The most effective data acquisition technique for detecting electrically resistive layers (e.g. hydrocarbon-saturated rocks) buried beneath a conductive seafloor uses a horizontal electric dipole source and electric or magnetic receivers, at or near the seafloor (Chave and Cox, 1983; Srnka, 1986; Chave et al., 1991). This is due to the desirable excitation and subsequent detection of both vertical and horizontal currents in the earth by such acquisition, as well as to several operational considerations. Frequency domain processing of the data is favored over time-domain methods, especially in deep water where the direct source signal is strongly suppressed. The method was first tested successfully for hydrocarbon applications in late 2000 offshore Angola (Eidesmo et al., 2002; Ellingsrud et al., 2002). New data components that are ineffective in land surveys, such as the seafloor vertical electric field, can add valuable information for interpretation of the geology (Srnka, 2003).
Introduction Carboniferous exploration in the Southern North Sea using conventional imaging techniques is hampered by complexities in the Mesozoic overburden and the Zechstein evaporites overlying potential targets (Papouin, et al, 2004). Here we present a case history from the GdF acreage over Quad 43 and 44, where an eight survey merge covering some 1500 sq.km was reprocessed to yield a coherent single input volume for both preSTM and preSDM imaging for exploration evaluation. After an initial model building and migration project was completed using purely gridded tomography, we revisited a portion of the data over a new development well, using a hybrid gridded technique. The layer-based approach has hitherto been commonly used for North Sea type environments, where sedimentary interfaces delimit changes in the velocity field and the geology'lends itself' to a layer-based model representation. In other words, we encourage preconceived bias, as we consider it to be a meaningful ...
Lai, W.L. (Building Technology Laboratory, Department of Building and Real Estate, The Hong Kong Polytechnic University) | Tsang, W.F. (Building Technology Laboratory, Department of Building and Real Estate, The Hong Kong Polytechnic University)
To provide the experimental basis of an in-situ soil characterization technique in terms of its texture, this paper describes an investigation which utilizes Ground Penetrating Radar (GPR) on testing the flow characteristics of soil in different texture (i.e. very fine sand, coarse sand and gravel). This was achieved through the measurement of dielectric constant and controlled cyclic variations of the water contents in these soils. Different groups of soils were tested under cyclic variation of the moisture contents via percolation and dewatering processes. These processes led the soils to undergo a series of partially saturated states to a fully saturated state. Their changes of dielectric constants were simultaneously and continuously monitored by GPR systems. Pairs of dielectric constants and the degree of water saturation from dry to moist stages were obtained, in which the porosities according to Complex Refraction Index Model (CRIM) were computed to assist in the soil texture characterization.