Formation evaluation of naturally fractured reservoirs

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Development Geology Reference Manual
Series Methods in Exploration
Part Wireline methods
Chapter Formation evaluation of naturally fractured reservoirs
Author Roberto Aguilera
Link Web page
Store AAPG Store

There are many logging tools and many methods that can be used for detection and evaluation of naturally fractured reservoirs (see Aquilera.[1]) However, there are no panaceas, and tools and methods that work well in one reservoir can fail miserably in the next one. (For information on standard logging tools, see Basic open hole tools).

Well logs can be considered as an indirect source of information. Sometimes they are used in efforts to detect where the fractures are located (qualitative analysis) and sometimes they are used to attempt to quantify the degree of fracturing. (For details of other methods used to evaluate naturally fractured reservoirs, see Evaluating fractured reservoirs.)

Qualitative analysis

Sonic amplitude log

Sonic amplitude logs are frequently used in attempts to detect fractures. Through laboratory work and experience, it has been found that the compressional wave amplitude is generally more attenuated by vertical and high angle fractures, while the shear wave amplitude seems to be more attenuated by horizontal and low angle fractures. Care must be exercised when using this tool because other features such as a rough borehole, shales, and changes in porosity, lithology, and tool centralization might produce amplitude drops that have nothing to do with the presence of fractures.

However, solid-to-solid contact along the plane of a fracture might reduce the degree of acoustic discontinuity. In this case, the fracture is present but is not detected by the sonic amplitude log.

Variable intensity log

Variable intensity logs are presented commercially as a recording of depth versus time after the initiation of an acoustic pulse at the transmitter. Amplitude changes are indicated by a succession of varying shades across the film track. The darkest areas correspond to the largest positive amplitudes, while the lightest areas correspond to the largest negative amplitudes.

When the tool is run through an unfractured section of reasonably constant porosity, lithology, borehole rugosity, and tool centralization , the shades of dark and light colors give the impression of banding. If a fracture is present, drastic breaks in the banding occur.

Sonic log

In some cases, the presence of natural fractures might produce cycle skipping in sonic logs. Other potential sources of cycle skipping include operational methods of the logging unit and gas in the borehole. Long spacing sonic logs are also useful for locating fractures. This tool operates with transmitter-receiving spacings of 8, 10, or 12 ft3.658 m
144 in
and emits acoustic energy that propagates radially through the formation via the mud and back to the receiver via the mud.

Caliper log

Sometimes the borehole gets enlarged in the presence of natural fractures. Thus, caliper tools (two-, three-, four-, or six-arm) can provide good indications of fractures.

Resistivity log

All kinds of resistivity logs can be used under the right circumstances for locating natural fractures. For example, induction logs can show drastic increases in resistivity in wells drilled with oil-based muds. In wells drilled with muds of relatively low resistivity, a shallow laterolog will indicate less resistivity than the induction log. In the same way, a microspherical log will show less resistivity than a deep laterolog. In general, all methods involving resistivity logs depend on a marked contrast between shallow and deep resistivities.

Pe log

In wells drilled with muds that contain barite, the Pe measurement might locate natural fractures. If the barite penetrates the fractures, the Pe curve shows considerable increases.

Borehole televiewer

The borehole televiewer presents an acoustic picture obtained with a rotating ultrasonic scanner. As such, it can sometimes “see” natural fractures. Some of the limitations include the necessity of a low solids content in the borehole fluid, a very slow constant logging speed, and a near-perfect tool centralization. Fractures wider than 1/32 in. can probably be seen by the borehole televiewer. In some cases, damage may make the fracture appear wider than it actually is. (For more information on the borehole televiewer, see Borehole imaging devices.)

Dipmeter

The dipmeter log has been used in some instances to indicate zones of vertical fracturing and their orientation. In general, the continuous four-pad dipmeter detects vertical fractures in opposite curves, that is, in curves one and three or curves two and four. This type of response depends on invasion of the fracture by a conductive fluid. A two-directional caliper run with the dipmeter can sometimes show an elliptical hole in front of the fractured interval (see Dipmeters).

Formation MicroScanner

Formation MicroScanner<xref ref-type="fn" rid="FormationEvalfn3">1</xref> is an extension of the dipmeter in which an array of electrodes is used to produce an electrical image of the formation. In the four-pad configuration, each pad contains 16 buttons that cover approximately 44% of an 8-in.-diameter borehole. Obtaining a good electrical image depends on invasion of the fracture by a fluid of relatively low resistivity. (For more on this device, see Borehole Imaging Devices.)

Spontaneous Potential (SP)

In some naturally fractured intervals, a hacked SP log develops in which each pip most likely corresponds to a streaming potential effect resulting from mud filtrate into the fractures.

Correction curve on the compensated density log

The Δρ (delta rho) curve corrects the density log for the effect of rough borehole and mud cake. As such, the correction curve might be affected by roughness due to fractures or mud in the fractures even if the hole is in gauge. Unfortunately, the correction detector diameter is only about 2 in.0.0508 m
0.167 ft
, and it can easily miss the fractures in more than 90% of the borehole.

Borehole gravity meter

Due to its large radius of investigation, the borehole gravity meter has proven useful in detecting large fractured bodies, especially in highly brittle materials.

Uranium index

Since uranium is highly soluble in water, it is commonly contained in groundwater. An increase in uranium content with respect to a shale volume (calculated from sources that are independent of natural formation radioactivity) might be an indication of natural fractures. This method, however, does not distinguish between open and mineralized fractures.

Temperature and noise logs

This combination has provided excellent results especially when dealing with underpressured gas reservoirs that are drilled with air. For these conditions, the temperature decreases drastically and the noise log detects the noise made by gas when it is flowing to the wellbore via the fractures. (For more details on these logs, see the chapter on Production logging.)

Quantitative analysis

Neutron, density, and sonic logs

In this combined method, the assumption is made that neutron and density logs read total porosity while sonic logs read only matrix porosity. The difference is taken as secondary porosity, which might include fractures. These logs are also used in the lithology-density crossplot that allows estimates of lithology variations and secondary porosity.

Dual porosity models

Dual porosity models are based on the observation that the cementation exponent of the fractures (mf) should be very close to 1.0. This is much smaller than the cementation exponent of the matrix (mb). The cementation exponent of the composite system (m) varies between mf and mb. The smaller the degree of fracturing, the closer the value of m is to mb. The larger the degree of fracturing, the closer the value of m is to mf. These models permit quantification of fracture porosity.

See also

References

  1. Aquilera, R., 1980, Naturally fractured reservoirs: Tulsa, OK, PennWell Books, 703 p.

External links

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