Geoexploration最新文献

Seismic velocities are influenced by many enhanced oil recovery (EOR) techniques. The crosshole seismic survey is well suited to the mapping of injection fluids or fracture patterns following reservoir stimulation. Both traveltime tomography and wave-equation techniques can be used to monitor and map changes in the seismic velocities, given pre-stimulation and post-stimulation crosshole seismic data.

In order to evaluate algorithms for EOR mapping using crosshole seismic surveys, data were obtained from a scale model of a crosshole seismic survey. The epoxy resin model contained simulated geological structures with strong velocity contrasts. Two versions of the same model were constructed, both with and without a simulated “flood” zone of a known geometry. Traveltime tomography and tow wave-equation algorithms, the inverse generalized Radon transform (inverse GRT) and frequency-domain wave-equation imaging, were used to attempt to locate the extent and velocities of the perturbation.

The results of this experiment show that traveltime tomography suffices to locate the flood zone and to determine the magnitude of the velocity perturbations. However, images that resolve the geometry of the flood zone were only obtained when the full waveform was utilized, using either the inverse GRT or frequency-domain wave-equation imaging. In this experiment the best images of the flood zone were obtained using frequency-domain wave-equation imaging. This result is due to the (realistic) complexity of the model, which supports wave types not accounted for by the acoustic ray approximation used in tomography and in the inverse GRT.

Image quality depends on how the input data to the full waveform schemes are generated. For the inverse GRT the change in interface reflectivities due to the flood zone could be detected using preprocessed, “scattered” wave fields as input data. However, better images of the geometry of the flood zone were produced when the input data consisted of “difference” wave fields (the subtraction of preflood data from post-flood data). Although the inverse GRT contains an “obliquity factor” that will normally ensure a high image quality, a further result of the experiment is that the obliquity factor in the inverse GRT needed to be suppressed to image the flood zone directly from the difference data.

The Bureau of Mines is conducting research to develop improved methods for predicting and monitoring the flow of leach solution during in situ mining. Potential benefits include higher metal recovery through better solution distribution and more cost-effective environmental monitoring.

The ability of seismic tomography to detect fractured zones and saturated areas was field tested for applications in predicting flow patterns and in monitoring leach solution above the water table. Seismic refraction tomography located fractured zones in a shallow refracting rock layer. A crosshole field test located water injected between source and receiver boreholes. In field tests at the University of Arizona's San Xavier experimental mine, tomograms of the seismic velocity distribution indicated dipping rock layers of contrasting seismic velocities consistent with borehole logs. Comparing data collected before and during water injection located wetted regions.

Six electromagnetic methods for determining where high-conductivity leach solution has replaced groundwater were tested at the San Xavier mine in cooperative research with the University of Arizona and Sandia National Laboratory. The methods were variations of surface and borehole electromagnetic induction and controlled source audio-frequency magnetotellurics. They were tested by conducting surveys both before and during injection of a brine solution. The salt-water brine was injected into boreholes and allowed to permeate the surrounding rock, creating a conductive plume. The surface methods located the brine at the water table, and the borehole methods located brine-filled fractures. The analysis is not complete, but preliminary results appear promising for applying these methods to monitoring leach solution.

A new method determines the top and bottom of the hydraulic fracturing in a cased treatment well from the microseismicity induced within the fracturing. The method uses a wall-locking sonde to passively record three-component data over a range of depths within and outside the fracturing immediately following either the fracture treatment or subsequent fluid injection into the fractured formation. The processed data (i.e., the background motion after removing the obvious events) show an anomalous inversion as a function of depth that delineates the fracture height. Specifically, the ratio of the horizontal motion component, H, to the vertical component, Z, inverts subdividing the recording traverse into three regions: those above and below the affected zone defined by $\text{H}\text{Z}\text{< 1}$ and the affected zone with $\text{H}\text{Z}\text{>1}$. The hydraulic fracture treatment creates the horizontally elongated, in situ stress-aligned affected zone that is comprised of a dilatant communicating network of new and preexisting fractures, joints, pores, and weaknesses, nucleates from the casing perforations. The zone has anomalously reduced elastic properties (i.e., seismic velocities) and acts as an embedded seismic and hydraulic waveguide with the borehole running through it. Pressure gradients, temperature gradients, and stress recovery within the low-velocity zone induce a pervasive microseismic cloud for several hours after the pressurization. The microsource cloud and the low-velocity zone create the data anomaly and its alignment with the extent of the affected zone. Computer simulations of the in situ setting and recording corroborate the $\text{H}\text{Z}$ inversion and its interpretation.

A series of shallow-depth hydraulic fracturing experiments was carried out in the summer of 1988 at the Elda landfill near Cincinatti, Ohio and mise-à-la-masse (MLM) borehole-to-surface resistivity measurements were obtained in an attempt to detect the fracturing. The well casing of an injection borehole was energized and potentials were measured at various points on the surface near the borehole before and after hydraulic fracturing was performed with a conductive fluid. Forward and inverse modeling algorithms based on the DC alpha center method were developed and tested with synthetic data to create a tool for interpretation of the experimental data. The alpha center forward algorithm incorporates a vertical line source of current to model an energized steel well casing. The advantages of the alpha center method are its speed and simplicity, and its ability to handle 3D geometry and indicate positions of conductive inhomogeneities. The forward solution is incorporated into an iterative least-squares inversion algorithm, with constraints applied to the alpha center parameters to facilitate modeling of fractures.

This paper presents a practical technique for estimating the five stiffness coefficients A, C, F, L and M from seismic traveltime measurements for multiple, horizontally-layered, transversely isotropic media. This technique is based on the construction of ray velocity surfaces of elastic wave propagation in terms of five measurement parameters — three from a skewed hyperbolic moveout formula for P waves and two from SH waves. The skewed hyperbolic formula is used for analyzing moveout of signals on multi-channel P-wave surface seismic or VSP data. A model-iterative optimization scheme is then used to invert the five measurement parameters for the five stiffness coefficients in a layer-stripping mode.

Both synthetic model and field experiments are performed to demonstrate the feasibility of the method. Synthetic P-wave model experiments demonstrate that the skewed hyperbolic moveout formula yields an excellent fit to time-distance curves over a wide range of ray angles. The measurement parameters are shown to reflect adequately the characteristics of velocity dependency on ray angle, i.e., velocity anisotropy. Although inversion errors generally increase with increasing number of layers, the proposed method does provide a quantitative measure of velocity anisotropy as valuable additional information that can not be readily obtained from conventional methods. A field VSP data example is also provided to show the correlation between the anisotropy parameters with lithology. Chalk and shale exhibited high degrees of anisotropy, and sands showed low degrees of anisotropy.

Electrical conductivity is an important petrophysical property used to predict lithology and fluid content in petroleum reservoirs. Conductivity distribution between wells can, in principle, be mapped with electrical or electromagnetic (EM) techniques when sources or receivers (or both) are located in the wells. Unfortunately, the resolution of these methods is relatively poor. Resolution is improved, however, in monitoring applications where the response of a dynamic reservoir process is recorded at different times and compared. Examples of such dynamic processes are fluid replacement during primary production, secondary recovery, and enhanced oil recovery (EOR) techniques as fireflooding, steamflooding, and CO₂ flooding. Such measurements can be made with technology currently available within the geophysical industry and at relatively low expense.

A numerical model study of the Holt Sand in situ combustion EOR experiment was conducted to test the feasibility of electromagnetically monitoring the progress of the advancing fire flood. The resistivity and geometry of the burn zone was obtained from pre-burn and post-burn well log data. The study shows that the surface-to-borehole electromagnetic method detects a clear signature from changes in resistivity of the burned reservoir horizon from distances as great as 100 m. Similar conclusions hold for steam flood processes. An important phenomenon which complicates interpretation of EM monitoring of thermal EOR processes is a zone of decreased resistivity adjacent to the reservoir horizon caused by conduction of heat into the bounding shales.

A computer algorithm was developed to model time-domain electromagnetic (TEM) fields for conductive structures radially symmetric about a horizontal loop transmitter. This algorithm improves on previous finite-difference algorithms by calculating the product of the radius times the electric field, which is more accurate than solving for the electric field directly, by using the Crank-Nicholson method of stepping through time which allows for coarser time steps and by using simplified boundary conditions which require less computational effort. These improvements allow models to be calculated on an IBM PC instead of main frame computers.

The finite-difference algorithm was used to calculate results for simplified hot-water flood and streamflood simulations at four different stages of the flood front advance, and for thin layers. All the simulations used small horizontal coil sources and receivers and assumed the oil reservoir to be a resistive, horizontal layer (50 Ω·m) in a conductive (5 Ω·m) background. The TEM responses were dominated by the resistive reservoir layer. In model results, beds as thin as $\text{1}\text{24}$ the distance between the transmitting and receiving wells are detectable.

For both the steam flood and the hot-water flood the greatest change in TEM responses between the flooded and the unflooded reservoirs occurs at early times, from 10 to 100 μs. The early-time responses can be qualitatively described as being influenced by the resistivity changes along a relatively narrow signal path which is a straight line in homogeneous regions and refracts along the high velocity reservoir boundary. As a conductive water flood front moves outward from the source borehole, the TEM response is delayed and attenuated for signal paths which pass through the flood zone but little change is seen for signal paths that do not cross the flood zone.

Cross-borehole tomography and borehole gravity meter (BHGM) surveys allow estimations of seismic velocity and rock density to be made in the vicinity of a borehole. Two such surveys recently conducted in Oklahoma and onshore Gulf Coast sediments show some of the first applications of these combined technologies. The results show an encouraging correlation between density and P-wave velocity for these borehole surveys in shallow clastic sediments. Such a correlation is predicted by Gardner's equation.

The cross-borehole resistivity method measures the electrical potential in one well due to direct-current flowing from electrodes located in another well. This arrangement permits the sensing of regions remote from either well. The paper examines the use of the cross-borehole resistivity method in sensing electrical resistivity perturbations caused by steam-floods, water-floods, and fire-floods.

Our examination consists of three parts. We first estimate the magnitude of resistivity perturbations caused by enhanced oil recovery (EOR) processes. We then calculate the theoretical voltage responses, for several theoretical sweep geometries, for a 2.5 acre well-spacing and a hypothetical shallow, heavy-oil field. For ease of computation, we assume that the swept zone is two-dimensional. Finally, we contaminate the calculated voltages with Gaussian noise with a 5% standard deviation and invert them in a least-squares sense to sweep geometry estimates. The starting models for these inversions are dissimilar to the theoretical sweep geometries. After ten or so iterations the estimated sweep geometries agree well with the theoretical geometries when the models are sufficiently well discretized. This shows that interpretation of cross-borehole data can give information about sweep geometries. We conclude that the cross-borehole resistivity technique has promise in monitoring enhanced oil recovery (EOR) processes, particularly when combined with effective inversion schemes.