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A Statistical Approach to Depth Uncertainty Analysis for Model Integrity

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depth uncertainty analysis

3D Depth Uncertainty Model: Depth uncertainty imported into well paths and displayed as cones of uncertainty indicating positional uncertainty to 1 Standard Deviation.

Dealing with the fundamental uncertainty of subsurface environments and their hydrocarbon resources is one of the major industry challenges. Seismic and borehole technologies improve our ability to acquire greater volumes of geologic information. Ever-increasing processor power crunches these data more efficiently to create models to help geoscientists and reservoir engineers understand the interaction between geology and hydrocarbon resources. However, regardless of the volume of data acquired, depth uncertainty analysis regarding horizons, faults, strata, and other subsurface formations continues to be a challenge.
Knowing accurate depths is critically important. It affects anything and everything regarding well planning, drilling, and production. If an oil-bearing layer was estimated to be 7,000 feet deep in very soft rock and it turned out to be 1,000 feet deeper with various hard layers to drill the result of the inaccurate estimation could be huge cost overruns due to extra rig time, drill pipe, and expensive drill bits.
Depth uncertainty analysis is critical to minimizing the risk of costly planning and drilling mistakes throughout every phase of reservoir development.

A Better Approach to Depth Uncertainty Analysis

CoViz 4D makes it possible for geoscientists and reservoir engineers to integrate, analyze, and visualize a wide variety of subsurface datasets, regardless of the original source, to better understand how development decisions affect reservoir performance.
One of its many capabilities is the standard velocity modeling module that calibrates existing velocity models to minimize the depth residual (error) at the wells. This results in a more accurate velocity model that can be used to transform seismic data into the depth domain. A byproduct of the process is a depth uncertainty model. Depth predictions from this model can minimize the risks of drilling wells in previously undrilled regions. Customizable workflows enable an efficient modeling process and allow for rapid updates as additional well data are acquired during the course of field development.

More Accurate Depth-Interpreted Horizons

CoViz 4D inputs a series of 2D time horizons to build a 3D structural model in time populated with interval velocities. These velocities are averaged to depth convert the time horizons. The CoViz 4D velocity modeling module overcomes the problem of tying the interpreted horizon to the well pick. The typical method of layer-by-layer produces a corrected horizon but fails to correct the problem of inaccurate average velocity volume (time interpreted horizons) or the PSDM seismic volume (depth interpretations). CoViz 4D’s velocity modeling module delivers a more accurate velocity model, allowing seismic volumes to be converted from time to depth and providing more accurate depth-interpreted horizons.

Corrected Velocity Model—Layer by Layer

CoViz 4D achieves a better level of depth uncertainty analysis by first calculating a scalar value from well pick residuals. It then applies this scalar value to the interval velocity layer by layer to produce a corrected velocity model. The interval and average velocities are continually refined as the process iterates from the top to the bottom of the structural stack. The result is an average velocity volume that can convert time surfaces to depth with reduced residuals at the wells.
depth uncertainty analysis 2

Corrected Velocity Model Incorporating Additional Data from New Wells.

CoViz 4D provides several velocity model output options:
  • The Compute Well Drop-out Residuals option systematically removes a well and generates a velocity model excluding specific well(s) from the analysis.
  • The Show Detailed Computing Log provides detailed information regarding the velocity building process.
  • Depth Uncertainty uses the residual results of the velocity model to produce a 3D model.

Compute and Visualize a 3D Depth Uncertainty Model

CoViz 4D takes the residual results calculated by the velocity model and computes a depth uncertainty model. In the process information regarding residuals and timing/velocity errors are displayed allowing the user to make adjustments as needed. The resulting output is a 3D model of depth uncertainty that can be visualized to explore various depth uncertainty scenarios.
As additional wells are drilled, data from these wells can be incorporated to improve the depth uncertainty analysis. Rock layer depth markers obtained from well logs help correlate and correct the depth uncertainty derived from uncalibrated velocity models. Velocity and depth conversion accuracy continue to improve as more wells are drilled and models are recalculated. Customizable workflows make it easy to automatically run (and re-run) these calculations, changing parameters to evaluate various scenarios, and continually refine the models.

Customizable workflows make it easy to automatically run (and re-run) these calculations, changing parameters to evaluate various scenarios, and continually refine the models.

Better Analysis and Understanding of Depth Uncertainty

Seismic imaging is the foundation for understanding subsurface formations and planning field development, yet it rarely provides the depth accuracy that geoscientists and reservoir engineers need for cost-effective field development. CoViz 4D offers a velocity modeling approach that offers considerable flexibility in how geoscientists define horizons, specify grid resolution, select well picks, and determine scaling factors. Using the resultant velocity model CoViz 4D then generates a 3D depth model that gives reservoir teams the ability to more accurately understand subsurface environments, and continually improve understanding as additional well data are incorporated into the model.

CoViz 4D, a data visualization analytics software from Dynamic Graphics Inc., gives geoscientists and reservoir engineers the ability to easily access and combine all relevant data associated with subsurface environments. Powerful analytic capabilities enable users to explore data relationships, analyze depth uncertainty, and visualize geologic formations in context to facilitate decisions that positively impact profit and reduce operational risk. To learn more about CoViz 4D contact our team.

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