Tomographic Fracture Imaging (TFI)

Typical cloud of frac induced micro-earthquakes (MEQs) seen with hypo-central techniques Well bores This is a typical micro-earthquake (MEQ) cloud produced during a frac that hypo-central techniques provide. Somewhere in these clouds of MEQs are the frac induced or activated fractures. As you can see there is virtually no hope of finding the actual fracture locations. Note the distinction between “induced” and “activate” fractures. “Induced” are new fractures created by the frac. “Activated” are pre-existing fractures that the frac has stimulated. TFI can distinguish between these two. Hypo-central studies cannot. Further the “activated” fractures often are at the kilometer scale and are expressed by failure events too small to be resolved, Only TFI can image this very weak signal.

Frac induced fracture as imaged with TFI TFI provides an objective image of the fracture surface in comparison with the highly subjective one that can be extracted from the MEQ cloud. This surface is a tessellated TIN made from the body centers of the voxels comprising the medial surfaces of the semblance clouds. The medial surface is one voxel thick. The precision depends on the size of the voxel. Minimum voxel size is 8 meters, so the error is +/- 4 meters. Accuracy is known from FMI and trace gas data. TFI clients have all tested the technology independently via blind tests. TFI has passed all. 50 meters Laterallw

TFI: The Physical Basis The Earth’s Brittle Crust is in a near critical state. Stress drops of <0.02 bar associated with background MEQs (Ziv and Rubin; 2000) All faults/fractures are surrounded by damage zones of pre-existing fractures. Fracture density increases geometrically as discontinuity is approached => increase seismic energy. Weak but geographically stable signals can be acquired by high sampling rate and “stacking” over time. This is an outline of the Physical Basis for TFI. Details are in Geiser et al 2012.

TFI: Dimensions = X, Y, Z, t, E This image shows the same fracture but contoured for energy on the left and for time on the right. The semblance amplitude is used as a proxy for energy while the time contour shows the fracture growth with time. In the image shown the fracture grew very rapidly on the left hand side of the image and more slowly on the right hand side. TFI is the only technology capable of showing this important property.

Semblance Stratigraphy Reservoir Seal This image is a projection of a 3D volume and demonstrates the accuracy of TFI by showing that the difference in rock properties is reflected by the TFI . In this case the more brittle rocks of the reservoir produce greater seismic energy than do the more ductile rocks of the reservoir seal. The location of the reservoir seal is known and terminates the gas migration columns. The data is from W. Virginia equivalents of the Marcellus. TFI – Gas Migration Fracture Columns

Semblance Stratigraphy Marcellus Formation - Pennsylvania Similar to the previous slide, Rock strength reflected by semblance data agrees with stratigraphy.

TFI Permeability Field Imaging This shows 3 different images of the same well but at different times in the production cycle. It demonstrates that 1] TFI can directly image the permeability field; 2] most of the production volume is associated with pre-existing reservoir scale fracture zones. BEFORE image is a horizontal or “map” slice that includes the lateral (red line) and the Reservoir Scale (RS) natural fracture zones with the highest permeability zones (black lines) embedded in the semblance clouds associated with the “damage” zones. This image is called the “ambient” activity field. “Ambient” activity is produced by the continuous passage of stress waves through the Earth’s crust. DURING image is a vertical slice through the semblance field generated by the entire frac. It shows the seismic activity generated by the frac operations. This is often referred to as the “Stimulated Reservoir Volume” (SRV). It bears only an indirect relationship to the reservoir permeability field as it includes failure events induced by seismic waves emitted by other induced failures which are not necessarily part of the permeability field and therefore will not produce. AFTER is a projection of a 3D image that shows the volume of the reservoir that is stimulated during production. Note that it is quite different from the SRV and the largest volumes are those associated with the RS natural fractures. This also demonstrates that failure on pre-existing fractures can be induced by either positive or negative changes in fluid pressure as well as seismic waves.