Roland Gritto

Pressure and fluid saturation prediction in a multicomponent reservoir using combined seismic and electromagnetic imaging

This paper presents a method for combining seismic and electromagnetic (EM) measurements to predict changes in water saturation, pressure, and CO2 gas/oil ratio in a reservoir undergoing CO2 flood. Crosswell seismic and EM data sets taken before and during CO2 flooding of an oil reservoir are inverted to produce crosswell images of the change in compressional velocity, shear velocity, and electrical conductivity during a CO2 injection pilot study. A rock‐properties model is developed using measured log porosity, fluid saturations, pressure, temperature, bulk density, sonic velocity, and electrical conductivity. The parameters of the rock‐properties model are found by an L1‐norm simplex minimization of predicted and observed differences in compressional velocity and density. A separate minimization, using Archies law, provides parameters for modeling the relations between water saturation, porosity, and electrical conductivity. The rock‐properties model is used to generate relationships between changes in...

Tube‐wave suppression in single‐well seismic acquisition

Single‐well seismic imaging is significantly hampered by the presence of borehole tube waves. A tube‐wave suppressor has been tested using single‐well seismic equipment at the Lost Hills (California) oil field. The suppressor uses a gas‐filled bladder kept slightly above borehole fluid pressure. Field tests show a measurable reduction in tube‐wave energy as compared to body waves propagating in the surrounding reservoir rock. When using a high‐frequency (500–4000 Hz) piezoelectric source, the P‐wave–tube‐wave amplitude ratio was increased by 33 dB. When using a lower frequency (50–350 Hz) orbital vibrator source, the S‐wave–tube‐wave amplitude ratio was increased by 21 dB while the P‐wave–tube‐wave amplitude ratio was increased by 23 dB. These reductions in tube‐wave amplitudes significantly improve single‐well data quality.

Estimating subsurface topography from surface-to-borehole seismic studies at the Rye Patch geothermal reservoir, Nevada, USA

A 3-D surface seismic reflection survey, covering an area of over 7.7 km2, was conducted at the Rye Patch geothermal reservoir (Nevada, USA) to explore the structural features that may control geothermal production in the area. In addition to the surface sources and receivers, a high-temperature three-component seismometer was deployed in a borehole at a depth of 1250 m within the basement below the reservoir, which recorded the waves generated by all surface sources. The objective of this study was to determine the subsurface structure of the reservoir based on this surface-to-borehole dataset. A total of 1959 first-arrival travel times were determined out of 2134 possible traces. Two-dimensional ray tracing was performed to simulate wave propagation from the surface sources to the receiver at depth. The ray tracing was based on a 2-D laterally homogeneous velocity model derived from results of a vertical-seismic-profile (VSP) experiment recorded in the same well. The method is an approximation where it is assumed that differences in travel time between the observed and modeled data are caused by structural deviations from a homogeneously layered model as estimated by the VSP profile, and thus are mapped into topographic changes at depth. The results indicate, to first order, the presence of two dominant geologic features. The first observation is consistent with the regional trend of the geologic units in the Basin and Range province with a north-south strike and dip to the west, as expected for this area west of the Humboldt Thrust Range. The second is a local feature in the form of an east–west ridge. The geometry of the structure is corroborated by results from a seismic-reflection survey, and by results of a gravity survey conducted in the area above the reservoir.

Acoustic visibility of immiscible liquids in poorly consolidated sand

Summary To investigate the acoustic visibility of non-aqueous phase liquids in poorly consolidated sands, laboratory experiments were conducted in a 0.6 m diameter confining cell with watersaturated sand. Crosshole data was collected before and after dodecane, a lighter than water non-aqueous phase liquid (LNAPL), was injected from the bottom of the cell. These experiments show a strong acoustic sensitivity of dod ecane for transmitted P-wave amplitudes (decreases of up to 65%) and a smaller, but measurable, acoustic visibility for velocity (decreases of up to 2%). Velocity difference tomograms were successful, but limited in resolution; they depict a low velocity region in the tank that corresponds to entrapped dodecane, as revealed by subsequent excavation of the sand cell. Effects of immiscible liquids on acoustic waves in unconsolidated sand Detection and characterization of non-aqueous phase liquids in poorly consolidated, water-saturated sands is of profound importance in petroleum and environmental engineering. Prohibitively expensive drilling procedures as well as the potential for accidental liquid mobilization during drilling procedures make non-invasive testing desirable for subsurface characterization. In poorly consolidated sand with weak intergranular cementation and low effective stresses, acoustic waves are sensitive to pore fluid properties. This is because the fluid phase within the pore space carries a large part of the wave energy as the frame stiffness decreases. Therefore, with sufficient contrast between pore liquids, their distributions can be visualized using tomographic inversion techniques (e.g., velocity contrast for travel time tomography, attenuation contrast for amplitude tomography, or impedance contrast for diffraction tomography). The success of these methods in practice is likely to depend on the fluid properties and the volumetrics and geometry of the target fluid. Ultrasonic measurements by Geller and Myer (1995) and Seifert et al. (1998) have demonstrated that the presence of both lighter-than-water and denser-than water non-aqueous phase liquids (LNAPL’s and DNAPL’s, respectively) in initially water-saturated sand packings produces changes in the P-wave velocities and amplitudes that can be explained and predicted. While the P-wave velocity decreases proportionately to the volume of LNAPL through which it passes, the impedance contrast between different phases does not operate on the velocity as it does on P-wave amplitude. In this manner, velocities are sensitive to liquid volume, whereas amplitudes are sensitive to liquid distribution, and could possibly detect the presence of relatively small volume NAPL fingers that form due to flow instabilities.

Crosswell seismic and electromagnetic monitoring of CO2 sequestration

The quantitative estimation of changes in water saturation (S{sub W}) and effective pressure (P), in terms of changes in compressional and shear impedance, is becoming routine in the interpretations of time-lapse surface seismic data. However, when the number of reservoir constituents increases to include in situ gas and injected CO{sub 2}, there are too many parameters to be determined from seismic velocities or impedances alone. In such situations, the incorporation of electromagnetic (EM) images showing the change in electrical conductivity ({sigma}) provides essential independent information. The purpose of this study was to demonstrate a methodology for jointly interpreting crosswell seismic and EM data, in conjunction with detailed constitutive relations between geophysical and reservoir parameters, to quantitatively predict changes in P, S{sub W}, CO{sub 2} gas saturation (S{sub CO2}), CO{sub 2} gas/oil ratio (R{sub CO{sub 2}}), hydrocarbon gas saturation (S{sub g}), and hydrocarbon gas/oil ration (R{sub g}) in a reservoir undergoing CO{sub 2} flood.

Three-dimensional seismic imaging of the Rye Patch geothermal reservoir

A 3-D surface seismic survey was conducted to explore the structure of the Rye Patch geothermal reservoir (Nevada), to determine if modern seismic techniques could be successfully applied in geothermal environments. Furthermore, it was intended to map the structural features which may control geothermal production in the reservoir. The seismic survey covered an area of 3.03 square miles and was designed with 12 north-south receiver lines and 25 east-west source lines. The receiver group interval was 100 feet and the receiver line spacing was 800 feet. The source interval was 100 feet while the source line spacing was 400 feet. The sources were comprised of 4 vibrator trucks arranged in a box array. Seismic processing involved, among other steps, the picking of over 700,000 of the possible one million traces to determine first arrival travel times, normal moveout correction, 3-D stack, deconvolution, time migration, and depth conversion. The final data set represents a 3-D cube of the subsurface structure in the reservoir. Additionally, the travel times were used to perform tomographic inversions for velocity estimates to support the findings of the surface seismic imaging. The results suggest the presence of at least one dominant fault responsible for the migration of fluids in the reservoir. Furthermore, it is suggested that this feature might be part of a fault system that includes a graben structure.