Structure Contour Maps, Seismic, and Geosteering

Structure Contour Maps, Seismic, and Geosteering

Integrating Seismic and Structure Contour Maps to Project Target Interval Configuration and Inform Geosteering During Drilling

Utilization of seismic in geosteering is limited due to resolution of the seismic data. Faults and structures less than 20-50ft in total offset may not be interpretable on a seismic section. However, larger structures, offsets, and high dips can be interpreted and can be quite useful in helping geosteering. Avoiding the larger fault zones, termed “geohazards” is essential. It is not possible to stay in zone through such structures and the faults may preferentially receive the energy of the hydraulic fracturing stages around them so that the fracturing does not propagate out into the target zone as intended. Large fault zones may also be zones of mud loss and even loss circulation so there are ample reasons to avoid them. Other issues with seismic along with resolution include surface displacement where the structural configuration is correct but is laterally displaced, sometimes a considerable distance. Prediction of dips on seismic can also be difficult. Relative dips – rises and drops in dip – are easier to determine than the exact changes in angle.   

For purposes of geosteering, the seismic shotpoints are tied to certain Vertical Section points along the azimuth of the well path. In this way the structures can be anticipated. One can do the same with structure maps. With good well control the structure contour map is the most important data. One can simply make a profile of the structure from the map along the Vertical Section points along the azimuth of the well path, as with seismic. With less well control, seismic mapping is often incorporated into the structure mapping so the actual structural contour map is a composite of both. If this is the case, it is a good idea to note where actual well control points are so that one can keep in mind which fault blocks have actual well control. With parallel offset horizontal wells there is more information. This is extremely useful if one is drilling perpendicular to structure as is often the case in several plays. In such cases one may utilize both the structure map and the parallel well configuration and compare along the way. If there is uncertainty about whether the predicted structure is parallel or if there are indications that the contours may be turning (away from parallel) then this may be determined early by updating the structure map during drilling. In this case one may be able to use one or the other models as primary if they differ and as the structure is being drilled. Both seismic and structure contour maps are predictive models and are only as useful as their resolution and/or well control (and grid spacing) allow them to be. The thickness of the target interval is another factor.   

Below is a very simple example of utilizing a profile of a structure contour map as an aid to geosteering projections:

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