Measuring Fractures – Quality and Quantity

As has been emphasized in the three preceding articles of this series, when a shear (S) wave propagates through a rock unit that has aligned vertical fractures, it splits into two S waves – a fast-S (S1) mode and a slow-S (S2) mode.

The S1 mode is polarized in the same direction as the fracture orientation; the S2 mode is polarized in a direction orthogonal to the fracture planes.

This month we translate the principles established by laboratory experiments discussed in the preceding articles of this series into exploration practice.

Figure 1 displays examples of S1 and S2 images along a profile that crosses an Austin Chalk play in central Texas.

The Austin Chalk reflection in the S2 image occurs later in time than it does in the S1 image because of the velocity differences between the S1 and S2 modes that propagate through the overburden above the chalk. Subsurface control indicated fractures were present where the S2 chalk reflection dimmed but the S1 reflection did not.

This difference in reflectivity strength of the S1 and S2 modes occurs because, as shown last month (June EXPLORER), when fracture density increases, the velocity of the slow-S mode becomes even slower. In this case, the S2 velocity in the high-fracture-density chalk zone reduces to almost equal the S-wave velocity of the chalk seal, which creates a small reflection coefficient at the chalk/seal boundary.

When fracture density is small, S2 velocity in the chalk is significantly faster than the S-wave velocity in the sealing unit, and there are large reflection coefficients on both the S1 and S2 data profiles.

Using this S-wave reflectivity behavior as a fracture-predicting tool, a horizontal well was sited to follow the track of a second S2 profile that exhibited similar dimming behavior for the Austin Chalk.

The S2 seismic data and the drilling results are summarized on figure 2.

Data acquired in this exploration well confirmed fractures occurred across the two zones A and B where the S2 reflection dimmed and were essentially absent elsewhere.

The seismic story summarized here is important whenever a rigorous fracture analysis has to be done across a prospect.

If fractures are a critical component to the development of a reservoir, more and more evidence like that presented here is appearing that emphasizes the need to do prospect evaluation with elastic-wavefield seismic data that allow geology to be imaged with both P waves and S waves.

The value of S-wave data is that the polarization direction of the S1 mode defines the azimuth of the dominant set of vertical fractures in a fracture population, and the reflection strength of the S2 mode, which is a qualitative indicator of S2 velocity, infers fracture density.

The Earth fracture model assumed here is a rather simple one in which there is only one set of constant-azimuth vertical fractures.

What do you do if there are two sets of fractures with the fracture sets oriented at different azimuths?

That situation will be discussed in next month’s article.

Comments (0)


Image Gallery

See Also: Bulletin Article

A new hierarchical architectural classification for clastic marginal-marine depositional systems is presented and illustrated with examples. In ancient rocks, the architectural scheme effectively integrates the scales of sedimentology (core, outcrop) and sequence stratigraphy (wireline-log correlation, reflection seismic). The classification also applies to modern sediments, which allows for direct comparison of architectural units between modern and ancient settings. In marginal-marine systems, the parasequence typically defines reservoir flow units. This classification addresses subparasequence scales of stratigraphy that commonly control fluid flow in these reservoirs. The scheme consists of seven types of architectural units that are placed on five architectural hierarchy levels: hierarchy level I: element (E) and element set (ES); hierarchy level II: element complex (EC) and element complex set (ECS); hierarchy level III: element complex assemblage (ECA); hierarchy level IV: element complex assemblage set (ECAS); and hierarchy level V: transgressive-regressive sequence (T-R sequence). Architectural units in levels I to III are further classified relative to dominant depositional processes (wave, tide, and fluvial) acting at the time of deposition. All architectural units are three-dimensional and can also be expressed in terms of plan-view and cross-sectional geometries. Architectural units can be linked using tree data structures by a set of familial relationships (parent-child, siblings, and cousins), which provides a novel mechanism for managing uncertainty in marginal-marine systems. Using a hierarchical scheme permits classification of different data types at the most appropriate architectural scale. The use of the classification is illustrated in ancient settings by an outcrop and subsurface example from the Campanian Bearpaw–Horseshoe Canyon Formations transition, Alberta, Canada, and in modern settings, by the Mitchell River Delta, northern Australia. The case studies illustrate how the new classification can be used across both modern and ancient systems, in complicated, mixed-process depositional environments.
Desktop /Portals/0/PackFlashItemImages/WebReady/a-hierarchical-approach-to-architectural-classification.jpg?width=50&h=50&mode=crop&anchor=middlecenter&quality=90amp;encoder=freeimage&progressive=true 3769 Bulletin Article

See Also: CD DVD

Desktop /Portals/0/images/_site/AAPG-newlogo-vertical-morepadding.jpg?width=50&h=50&mode=crop&anchor=middlecenter&quality=90amp;encoder=freeimage&progressive=true 4057 CD-DVD
Desktop /Portals/0/images/_site/AAPG-newlogo-vertical-morepadding.jpg?width=50&h=50&mode=crop&anchor=middlecenter&quality=90amp;encoder=freeimage&progressive=true 4437 CD-DVD

See Also: DL Abstract

It is quite common for reservoir engineers to adjust the geological modelling without recoursing to the geologists by multiplying the porosity, the permeability, the anisotropy (kv/kh), the relative permeabilities, the well factors and many other parameters within their numerical world.

Desktop /Portals/0/images/_site/AAPG-newlogo-vertical-morepadding.jpg?width=50&h=50&mode=crop&anchor=middlecenter&quality=90amp;encoder=freeimage&progressive=true 3100 DL Abstract

See Also: Learn! Blog

Desktop /Portals/0/PackFlashItemImages/WebReady/reality-based-reservoir-development.jpg?width=50&h=50&mode=crop&anchor=middlecenter&quality=90amp;encoder=freeimage&progressive=true 20924 Learn! Blog