17. Imaging of a Stratigraphically Complex Carbonate Reservoir with Crosswell Seismic Data

Abstract

A. A. VassiliouWe conducted a crosswell survey to evaluate the stratigraphy and structure in and near a stratigraphically complex limestone reservoir in the Permian Basin of West Texas. Although the reservoir was detected with surface seismic data and was successfully drilled, the surface data had insufficient resolution to image its internal structure and to determine its relationship with nearby sedimentary rocks. This is a common problem in surveys of carbonate strata, where large seismic velocities result in long characteristic wavelengths. The thickness of the reservoir in our example is -75 m, whereas the shortest wavelength in the surface seismic data is -60 m. The high-resolution crosswell seismic data, which contain frequencies more than 10 times greater than those of the surface data, clearly delineate the lateral and vertical extent of the reservoir unit between the wells. The data also reveal the inner stratification of the reservoir and show the stratigraphy of units adjacent to, above, and below the reservoir. Anisotropy in the compressional velocities obtained from the crosswell data is an indicator of the facies present. Our analysis of the P-wave direct arrivals suggests that the reservoir itself has little velocity anisotropy but that a thinly interbedded facies of mixed siliciclastics and carbonates has a horizontal velocity of -5% lower than its vertical velocity. The shale units have an intermediate degree of anisotropy of -3%.

Application of seismic boundary-preserving constrained inversion for delineating reservoir body

“…The inversion zone is discretized in 31 × 61 grids. In the second example, we built the complex model based on velocity model from the previous study in King mountain field in west Texas (Langan et al, 1997). Figure 2a shows the true model.…”

“…For example, the reservoir boundaries in King mountain field in the Permian Basin of west Texas is ambiguous, in particular uncertainties about the bottom of reservoir, which critically influence the location of water well drilling. On the other hand, from previous studies, the lateral extent of reservoir is lost to track when massive carbonate reservoir had detected in another 640 ft well away (Langan et al, 1997). For the further qualitative and quantitative assessment and development of reservoir, sufficient information about the lateral and vertical extent of reservoir is needed.…”

Methods and Applications in Reservoir Geophysics

“…Seismic technologies can be used to monitor the injected CO 2 and its displacement process (Langan et al, 1997) only if the injected CO 2 causes sufficient changes in seismic proper ties of the formation rock. The displacement process can be either mis cible or immiscible depending on pressure and tempera ture.…”

Applications of boundary-preserving seismic tomography for delineating reservoir boundaries and zones of CO2 saturation

“…Crosswell seismic profiling (XSP) as a high-resolution seismic method is used for reservoir management to characterize and delineate reservoirs (e.g., Langan et al, 1997). An XSP includes velocity tomography and reflection imaging (e.g., migration).…”