Discover Additional Reserves in a Mature Field

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From DGInsider, Q2 2000

EarthVision’s spatially consistent geologic models enable you to thoroughly investigate the production potential of complicated reservoirs by carefully evaluating the geometric relationships between layers and fault blocks. In the last issue of DGInsider, we discussed how the 3D Viewer can assist you in the process of looking for bypassed oil in a single fault block. With this second application, we will take the same model and see how it might be used to discover additional hydrocarbon reserves in surrounding fault blocks.

As you may recall, this particular model contains four thrust faults and 12 horizons, including an unconformity and a sand-filled channel extending across the model area. We will first evaluate the potential for structural closure in an adjacent lower fault block, then establish the possibility for layer connectivity, and finally, plan a sidetrack path from an existing well into the new trap.

For review, the sand channel is shown first in “zone chair” view (Figure 1) to show the sand in the context of surrounding geology, then in an isolated view (Figure 2) to show only the producing sand (block 1) and adjacent areas of potential production (blocks 3 and 4).

Verify Structural Closure

To identify structural closure in adjacent blocks, we will first generate a contour map of the sand layer. This map can be requested as one of the model outputs in the WorkFlow Manager. Remember that we are generating a map from a 3D model. As such, even though our contour map is in “2D,” we retain the inherent accuracy and characteristics of the 3D model. This mapping process has very important implications, particularly in cases of reverse faulting and repeated sections. In the map of the sand top (Figure 3), you can see that portions of the surface are potentially hidden by overhangs as a result of reverse faulting. In such cases, the hidden contours still appear as dashed lines. As such, we can retain important information in the vicinity of reverse faults, even in this “2D” contour map.

What we see in this case is a subtle structural closure in the lower fault block 3. Let us assume that additional analysis of seismic data has revealed the presence of this structure and that we need to evaluate its production potential. This trap could contain hydrocarbons that might be tapped cost-effectively by drilling a sidetrack path from an existing well in the upper block (Figure 4).

Identify Layer Connectivity

Establishing the presence of layer connectivity for the sand channel between fault blocks 3 and 4 could be important to the economic viability of this new prospect (Figure 5). If the fault is non-sealing and allows the migration of hydrocarbons from block 4 into 3, then the prospect in block 3 will be continuously recharged. Thus, both blocks can be produced by drilling only into block 3.

We are now going to identify the fault that separates blocks 3 and 4, and then display the outline of layers in block 4 on the fault surface. The resulting Allan diagram (Figure 6) is used to show which layers in block 4 are actually in contact with layers in block 3 across the fault. In other words, what you are seeing is the transparent “footprint” of adjacent blocks posted on the currently viewed block. If the sand channel is in contact with itself across the fault, then the potential for hydrocarbon migration across the fault exists.

The required 3D Viewer command sequences for the 3D model analysis described above are provided below. The use of hotkeys is noted in places where they greatly speed up the model analysis and manipulation.

• Zone (Hot key 4) –> Fault block display –> Block3; Turn on 3D Cursor (Hot key k); Activate Snap to Surface (Hot key ^j); Place cursor on fault surface and click middle mouse button; (fault name is listed next to surface, 2 in this case).
  

• Allan Fault (Hot key ^8 ) –> Allan fault plane display –> 2 (fault name); Display Allan zones; Remove all Allan zones except sand channel and layer3.

Note that we have displayed the layer above the sand channel to help identify where the top of the sand channel is located. The solid yellow color on the fault surface represents the amount of overlap of the sand channel with itself across fault 2 (Figure 7).

Using the 3D Viewer function called Mark/Unmark, we will measure the amount of overlap to determine whether or not the quantity is sufficient for meaningful hydrocarbon migration. Based on this technique, we conclude that there is a maximum of ~85 feet of thickness and ~1500 feet of width for the sand channel at the fault boundary.

• Place the cursor at the bottom of the solid yellow band and click the middle mouse button; Mark the location (Hot key Alt + m).
  
• Place the cursor at the top of the solid yellow band and click the middle mouse button; Read the value next to Mark distance.

Plan a Sidetrack Well Path

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We are now going to plan a sidetrack well path into the prospect from an existing well. We can use the 3D geologic model as a spatial guide to digitize critical path points (Figures 8, 9, 10). These points are not the well plan but are points through which the well path must pass. The hot keys, mouse buttons, and typed commands required for digitizing this sidetrack are identical to those described in the last DGInsider (available on-line at www.dgi.com). The procedure, however, can be summarized with the following steps:

1. Begin line digitizing.
2. Zoom close to well (D-1) to begin sidetrack and save cursor location as first point in new well path.
3. Add second point just below and to the north of the first point.
4. Snap 3D cursor to near top of prospect and save data point.
5. Snap 3D cursor to other side of prospect and save data point.
6. End line digitizing.
7. Save data file to a new file name.
8. Interpolate additional well points at desired interval.

In summary, we now have solved some asset management problems and helped reduce risk by using a 3D spatially consistent geologic model to help identify a new prospect in another fault block. In the process, we verified the structural closure, and established the presence of layer connectivity which will enable hydrocarbon migration if the fault is not sealing. To conclude the process, we planned a sidetrack well path into the prospect from an existing well.

 
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