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Figure Captions
Figure 1: A schematic of the  borehole gravity logging tool.
Figure 2: A model of the distance sensitivity of borehole
gravity.(courtesy of L.A. Beyer, adapted from A.K. Shultz).
Figure 3: Gas coning models around producing wells.
Figure 4: Measured borehole gravity logs from the Cannae #1 well,
East Texas (courtesy of D.G. Ziegler and Broughton Operating
Corporation).
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General Statement
Borehole gravity is a density logging method
that has been available for over 30 years – yet today it is a
seldom-used technique, even though there are numerous published case
studies that demonstrate its value. By one estimate, it is thought that
borehole gravity has directly or indirectly contributed to the
cumulative discovery of more than one billion barrels of oil equivalent.
The physical principles behind borehole
gravity logging are quite simple. By measuring minute differences in the
earth’s gravity field at different downhole locations, formation
densities can be directly computed. In order to do this, the borehole
gravity sensor must be capable of sensitivities on the order of one part
per billion. It is quite difficult to build such sensors, yet 16
borehole gravity meters have been built to these specifications since
1970. To put this incredible sensitivity into perspective, it is
equivalent to trying to weigh an entire whale in order to determine the
weight of one of its whiskers.
The physics behind this particular sensor
gives it advantages over other density logging tools. For example:
- The
computation from downhole gravity to density is direct,
straightforward and does not require a lithology-dependent
calibration.
- The
entire system is passive and does not rely upon radioactive sources.
- And
even more valuable is the depth of penetration of the sensor.
Borehole gravity can easily see beyond casing,
cement, hole rugosity and near-borehole formation damage. It works just
as effectively in cased holes as it does in open holes. It is a bulk
density method, so when coupled with a near-borehole imaging nuclear
density method such as a gamma-gamma log, the formation porosity can be
computed with great accuracy. Moreover, the pore-filling fluids can
often be determined with some reliability. The best results occur with
gas vs. liquid, but at higher sensitivities it may be possible to see
oil-water contacts.
Tool and Its Resolution
Figure 1 shows
the gravity sensor that can detect beyond casing and cement using an
inclined spring, lever arm mass. The lever arm mass pivots and stretches
the spring under the influence of the local gravity field. This operates
similar to a common bathroom scale.
The local gravity field measurements are
directly related to nearby formation densities. It is often asked how
far can the borehole gravity tool “see.” This question is akin to asking
how far can the human eye see. Obviously, the sensor has greater
reliability at near-well distances, but if the object is large (such as
a reef or salt) it may be resolved at greater distances. Typically, it
can easily resolve densities within the first 50 feet of the well, but
it is not uncommon to image features 100 to 500 feet away. The distance
resolution is a tradeoff between downhole resolution. In other words, as
the object gets more distant, its signal becomes broader in wavelength.
Thus, the method is tunable for more distant objects – but with the
price of poorer vertical resolution.
Another question often asked is whether the
direction to the object can be determined. Unfortunately, without a
multi-well logging program (akin to radio beacon triangulation), this is
not currently possible. With single wells it is only possible to
determine radial distance away from the borehole, but not direction.
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Examples of Resolution
Figure 2 shows
an example of the distance resolution from borehole gravity. In Case 1,
there is no distant porous mass; therefore the near-borehole detecting
gamma-gamma log and the far-detecting borehole gravity log show no
difference. In Cases 2-4 the distant porous mass is increasingly moved
closer to the well bore.
The wavelength of the density difference
decreases as the amplitude increases. Thus the amplitude and wavelength
of the density difference is diagnostic in determining the distance of
the remote mass away from the borehole. Notice that this model is
sensitive to a distant mass as far away as 200 feet from the borehole.
Current and Future Use
Current uses of the borehole gravity method
include formation evaluation, reservoir monitoring, determining
behind-casing bypassed gas and exploration for distant structures.
Figure 3 shows
a conceptual new use of the method for detecting the size and shape of a
gas cone from a producing well. Borehole gravity is not only capable of
imaging past a gas cone to determine the true top of the gas-oil
contact, but it is also capable of determining the shape of the cone, as
these models show.
Figure 4 shows
how borehole gravity can contribute to a gas discovery. A set of actual
logs, acquired in 1996, is shown. The deviation between the
near-borehole detecting gamma-gamma log and the far-imaging borehole
gravity log (in yellow) helped determine that the Cannae #1 well was 300
feet away from a porous reef in the east Texas Cotton Valley trend.
Based on this information, the well was sidetracked, and a commercial
discovery was made.
Why hasn’t borehole gravity become a more
widespread logging method? This is due primarily to engineering
limitations of the present tool. Currently it must be used in nearly
vertical, large-diameter boreholes, and it takes on average about five
to seven minutes to acquire a data point. The only commercially
available borehole gravity sensors are manufactured by LaCoste & Romberg
of Austin, Texas. LaCoste & Romberg has begun to develop a new borehole
gravity sensor that is expected to overcome the engineering limitations
of the present tool, using advances in digital electronics and materials
science.
We predict that these advances will lead to
renewed interest in the technology from oil and wireline companies.
Innovative new uses, such as time-lapse reservoir surveillance and gas
storage monitoring, are also expected to generate new demands for the
technology. We foresee a day when borehole gravity logs will be as
common as any other wireline log. While this technology is old and
well-developed, it now has new life.
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