Donald C. Adams

4D Seismic Data Processing Issues And Examples

4D seismic data acquisition efforts can be divided into three major categories: “legacy”, “re-shoot” and “4Ddesign ” projects. In legacy 4D seismic projects, multiple vintages of overlapping 3D seismic data sets are analyzed for time-lapse effects, but none of the 3D surveys were originally acquired with a 4D application in mind. In reshoot 4D seismic projects, the baseline 3D seismic survey was not acquired for 4D purposes, but the subsequent reshoot 3D survey was designed, at least in part, with a 4D objective in mind. Finally, in 4D-design projects, at least two of the time-lapse 3D seismic surveys were specifically designed and acquired to optimize the subsequent reservoir monitoring analysis.

Estimation of Reservoir Pressure And Saturations By Crossplot Inversion of 4D Seismic Attributes

We present a method to simultaneously estimate pressure and saturation changes in a producing hydrocarbon reservoir using time-lapse (4D) seismic attributes. 4D seismic attributes are displayed in a crossplot domain, where pressure and saturation axes can be estimated deterministically or interpretively. These axes form the basis for a linear or nonlinear coordinate transformation to the pressure-saturation domain. A final calibration to production data is required to convert the qualitative results to quantitative estimates of pressure and saturation. Our method is applied to 4D seismic data sets acquired over producing North Sea reservoirs in the Schiehallion and Gullfaks fields.

Seismic monitoring of CO2 geo-sequestration: realistic capabilities and limitations

Summary Seismic is useful for monitoring and verification of subsurface geo-sequestration CO2 injection and storage projects. The physical properties of CO2-saturated rocks can vary strongly, thus the resulting seismic wavefield can be rich and complex, posing significant challenges to obtain accurate CO2 images and quantitative inversion results. We discuss the seismic rock and fluid properties of CO2-saturated rocks under various realistic pressure and temperature conditions, show the effects of CO2 injection with realistic 3D models and finite-difference simulated seismic data, and compare simulation data images and inversion results to real seismic data at the Sleipner CO2 sequestration site. Rock and fluid physics In this paper we focus on the geo-sequestration application of injecting CO2 into subsurface porous and permeable brine-saturated rock formations for long-term storage. For many geo-sequestration sites and timescales of interest, the depth, pressure and temperature of the reservoir storage rocks will imply that the bulk of injected CO2 will be in an immiscible supercritical phase above the critical point (Pc = 7.38 MPa, Tc = 31.1 o C) in the phase diagram, thus having physical properties of both a gas and fluid. Figure 1 shows our calculations of the bulk modulus (incompressibility) and density of CO2 at various pressure and temperature conditions. CO2 is a gas below the red line, a fluid above the red line, and supercritical to the right of the red line in the phase diagram. The (P,T) conditions for Sleipner are plotted on the diagrams (red circle) and clearly fall within the supercritical region, as most sequestration projects are likely to do (dashed yellow box). Note that the compressibility and density of CO2 can vary as much as one order of magnitude across the geo-sequestration (P,T) range, especially at higher pressures (depths). Figure 2 shows seismic P- and S-wave velocity (Vp, Vs) and density curves for saturated Sleipner-type sandstone rocks as a function of the CO2/brine fluid mixture, when the CO2 behaves as a dense supercritical “fluid” (at 37 o C, 10MPa) and as a light supercritical “gas” (at 44 o C, 7.5 MPa). In the supercritical “fluid” case, the bulk density of the saturated rock is a weak linear function of the CO2 saturation, and Vp is a nonlinear function showing a strong decrease for CO2 saturations up to about 30% but little or no change for larger CO2 saturations. In the supercritical “gas” case, the bulk density of the rock is a strong measurable and linear function of the CO2 saturation, and Vp is a nearly binary function showing a strong decrease for small amounts of CO2 < 5-10% but little or no change for larger CO2 saturations. In both supercritical cases, Vs is fairly insensitive to CO2 saturation. 3D models and seismic simulations We built a 3D earth model loosely based on Sleipner logs, rock and fluid properties, and geologic/seismic structure, as shown in Figure 3. We introduced porosity heterogeneity using spatial statistics from another reservoir to examine its effects on seismic imaging and inversion. We created three fluid and pressure distributions in the model: (i) brine only, i.e. before CO2 injection (T0), (ii) after limited injection has created one layer of CO2 just below the impermeable cap rock (T1), and (iii) after further injection has created two vertically stratified layers of CO2, one beneath the cap rock as in T1 but with stronger saturation, and a second weaker saturated layer trapped below a deeper shale (T2). We generated synthetic seismic shot-gather data sets for each of the three CO2 injection scenarios acquired along a 2D line through the 3D model, using a visco-elastic finite-difference (FD) algorithm running on our parallel cluster. Figure 3 shows the z-component data (mainly P-waves) and the x-component data (mainly S-waves) of the elastic wavefield for a single shot gather. Note that a single layer of CO2 (at T1) generates a complex coda of many strong events in the shot-gather difference data (T1-T0) of Figure 3! The wavefield differences are even more complex at T2 for two stratified layers of CO2, as we show in the presentation. Seismic imaging We performed industry-standard prestack depth migration velocity analysis and image processing of the data sets for all three scenarios. Aside from the changes in CO2 saturation and pressure in the geo-sequestration layers, these data sets are perfectly repeatable from a time-lapse perspective. Figure 4 shows the P-wave depth image before and after CO2 injection (T0 and T1), and the time-lapse image (T1-T0). Figure 5 shows a zoom of the time-lapse difference image (T1-T0) for P-waves (left), converted PS-waves (right) and a comparison to a real time-migrated Pwave difference image from Sleipner (center). Note that even with our best velocity analysis and depth imaging in

Evidence for late Middle Proterozoic extension in the Precambrian basement beneath the Permian basin

Through an integrated interpretation of gravity, magnetic, seismic reflection, and well data we are able to define the geometry, lateral extent, and tectonic setting of late Middle Proterozoic mafic rocks (the Pecos mafic intrusive suite) in the basement of the Permian basin of west Texas and eastern New Mexico. In this region, seismic reflection data are characterized by subhorizontal layered reflectivity within basement that can be tied to compositional variation in a layered mafic intrusion through well cuttings and a synthetic seismogram. In places the intrusion forms a sill, and the base of the sill is imaged in the reflection data. Integration of the seismic interpretation with gravity models shows that the Pecos mafic intrusive suite is 3 – 10 km thick and 50 – 100 km wide. The overall structure of the suite is that of a sill with a locally dikelike keel. North - south trending gravity and magnetic anomalies associated with the Pecos mafic intrusive suite suggest that it is composed of four large intrusive centers which cover a minimum area of 9200 km2, making it one-seventh the size of the Bushveld Complex and one of the largest layered mafic intrusions in the world. We interpret this massive intrusion to be associated with collisional rifting in the foreland of the Grenville orogeny.

Applications of first break turning‐ray tomography to shallow seismic reflection data processing and interpretation

Shallow seismic reflection surveys commonly suffer from poor data quality in the upper 100 to 150 ms of the stacked seismic record due to shot-associated noise, surface waves, and direct arrivals which obscure the reflected energy. Nevertheless, insight into lateral changes in shallow structure and stratigraphy can still be obtained from these data by using first arrivals picks in a refraction analysis to derive a near-surface velocity model. We have used turning-ray tomography to model near-surface velocities from a high-resolution seismic profile recorded in the Trans-Pecos region of west Texas, near Fort Hancock. The result of this analysis is an interval velocity model for the upper 200 ms of the seismic profile that locates several geologic features which were not interpretable on the original stacked record. This model helps locate an unconformity near 70 ms two way travel time and strong lateral velocity variations above water table that may be due to a perched water table or soil horizon development. In addition, the interval velocity model leads to improved time-to-depth conversion, and when changed to stacking velocities may aid in producing a better stacked record at early travel times.