Time-domain on top with a velocity cube, seismic cube, and interpreted horizons. Depth domain below with the depth-converted seismic cube, wells, well tops, and reservoir simulation grid derived from the depth-converted horizons. Data used by permission of the owner.
Depth uncertainty is a major issue for geologists and geophysicists in reservoir development planning. The well to seismic tie is important for achieving accuracy in the depth conversion data as well as for characterization of the subsurface and reduction of geological uncertainty. However, the variation in rock properties can affect the conversion process and the ultimate analysis derived from the well tie.
Uncertainty can be reduced with the ability to visualize and make a comparative analysis of 3D seismic attributes and well data in a single platform. Accurate depth conversion of 3D seismic data can be achieved with a comprehensive 3D depth and velocity model. Data visualization and integration software can facilitate this correlation analysis, allowing asset teams to make an economic decision in well planning and development.
Depth Conversion of 3D Seismic Data
In the development of reservoirs and well planning, the understanding of the geologic structure is the most important aspect. The 3D seismic volume provides a detailed reflection of subsurface geology in the oil and gas field. However, for engineers to make decisions regarding the drilling of wells, the time domain of the seismic data requires precise conversion to the depth domain.
Accurate depth conversion is crucial to ensure there is no miscalculation when estimating the drilling depth to reach fluids. But the conversion process can be complex. Even a subtle variation in subsurface rock properties can cause miss-ties between seismic depth converted data and well log data. For instance, a gas-charged reservoir significantly alters the velocity of the rock such that reserve calculations are off by millions of dollars. Also, drilling a new well into that reservoir could encounter the top at an unexpected depth leading to long downtime at significant expense. Inaccuracy such as this can be economically burdensome for the project. To eliminate such errors and variation in seismic depth data, a correlation between 3D visualization of depth conversion and well data can prove to be advantageous.
Time and depth data visualization and integration software platforms allow geologists, geophysicists, and reservoir engineers to perform time-to-depth conversion by creating an effective velocity model incorporating the various geological and petrophysical data and interpretations. The most effective software has layer-by-layer, as well as fault block-by-fault block, modelling capabilities which allow efficient calculation of the residual difference between seismic depth conversion and well tops.
Time-to-Depth Conversion and Velocity Modeling
Coviz4D’s time-to-depth capabilities assist asset teams by providing fast and efficient time-to-depth conversion using an existing average or instantaneous velocity model. Additional velocity calibration can be done as new well tops are brought in so your average velocity model always stays up-to-date. The velocity calibration workflow allows engineers to:
- Analyze the 3D depth uncertainty model to cross-correlate data and analyze different depth conversion error scenarios.
- At each well location, calculate the residual difference between existing depth conversion and well tops to estimate discrepancies.
- Apply the changes in the velocity model with any adjustment or addition in data from the drilling of new wells.
- Recalculate the depth conversion to keep the data updated and accurate for future well planning.
CoViz 4D: Enhancing the Depth Conversion of 3D Seismic Data
CoViz 4D provides the interpreter with various time-to-depth calculation options along with the robust visualization capability required to quickly cross-correlate differing depth converted seismic data with well data to reach the best final depth conversion. The software provides geoscientists with options to:
- Define horizons, import well tops/ picks data.
- Back-interpolate well tops onto a seismic interpreted horizon to calculate the velocity estimate.
- Integrate 2D, 3D grids, seismic interpretations of faults and horizons, or cellular grids with an average velocity grid for depth conversion.
- Color code the well tops to efficiently visualize the residual.
- Use symbol size to scale the residual.
These features allow asset teams to get a comprehensive view of the subsurface and enable them to spot the variations and inaccuracies in the depth conversion of 3D seismic data—which is crucial in minimizing the development and production uncertainties.