DENSITY BOREHOLE GRAVITY METER (BHGM)
DERIVED DENSITY MEASUREMENT
The accuracy of the Deep Density BHGM™
(Borehole Gravity Meter) measurement depends on the
accuracies of the depth and gravity differentials. The
combined effect of these sources of error is illustrated in
figures 1 and 2. For a set gravity and depth error level,
increased density accuracy is achieved by using larger depth
increments. Useful density accuracy for depth increments of
less than 2.5 meters requires the use of the Shuttle
Sonde™. In time-lapse surveys, repeatability is
confirmed by making density measurements opposite tight or
water saturated zones.
An advanced electrostatic feedback positioning circuit,
coupled with capacitance position sensing, controls the
gravity sensor. Innovative design work has significantly
reduced the electronic noise level providing improved gravity
reading accuracy and survey reliability.
On surveys where the depth increment is small, gravity
readings are repeated a minimum of 3 times, reducing depth
and gravity uncertainties. With proper survey design and tool
preparation, gravity reading standard deviations of 3 ?Gal
(about 3 parts in 109 of the earth's gravity
field) are attainable.
For reservoir monitoring applications, depth increments of
2.5 meters or less may be desirable. To minimize depth
errors, a Shuttle Sonde™ has been developed which
enables the gravity sensor to be moved within the tool. The
depth increment error is reduced to +/- 1 mm over the
2.5-meter range of the Shuttle Sonde™.
The absolute depth of the gravity sensor is provided by
wireline odometer measurements. In time-lapse applications,
gravity measurements must be repeated at the same absolute
depth. This is achieved using high resolution Casing Collar
Locator logs recorded at 1 mm sample intervals (Fig. 3).
Gravity records are made with the tool held stationary.
The time between records is from 8 to 14 minutes, depending
on the distance between stations and the background noise.
COMBINATION VSP AND BHGM
Deep Density BHGM and downhole seismic surveys are a
natural combination since both entail multiple stationary
recordings. Two complementary data sets are acquired for the
rig time cost of one. Tests conducted in Texas in 1993 show
that the presence of the BHGM tool does not degrade the
signal from the Schlumberger CSI tool and the seismic signal
does not degrade the BHGM recording.
This approach is recommended for mapping deep salt
Figure 1 - Density accuracy
for various levels of Dg
Figure 2 - Density accuracy for various levels of
Figure 3 - Absolute Depth
location of gravity tool for repeat surveys
Deep Density BHGM and Gamma-Gamma densities differ because
- Investigates a greater volume of rock
- Sees little effect from near-well environment
- Is not related to electron density
Deep Density BHGM provides better average density
- Through casing (e.g., in old wells)
- In wash-outs, rugose or fracture zones
- Where formation is damaged (e.g., by drilling or
- In invaded zones where mud filtrate density is
different from original fluid density
- Where mud additives or solids may alter the apparent
density near the borehole (e.g., barite mud, solids
- Where acidization products in the formation affect
- For accurate determination of overburden pressures
for compaction studies
The Deep Density BHGM is influenced by structures which
may be remote from the wellbore.
- Mapping of salt flanks, flares and overhangs
- Detection of faults, especially overthrusts
- Mapping reef structures
- Evaluation of fracture systems and porosity away from
The Deep Density BHGM measure changes in fluid saturation
- Through casing
- Over a large volume
- Away from formation damage
- In low salinity waters
Time-lapse monitoring of
- Gas caps
- Steam flood fronts
- Gas storage reservoirs
- Oil/water contacts in high porosities
- Hydrocarbon/water discrimination in freshwater sands
ENHANCING VSP, SURFACE GRAVITY AND SEISMIC
The average density profile with depth given by the Deep
Density BHGM complements VSP and surface seismic and gravity
information in modeling the subsurface.
- Combination with surface seismic and gravity for
- Investigation of the lateral extent of structural
anomalies in combination with VSP (e.g., salt flares
and reef boundaries)
Figure 4 - BHGM sensing element - Maximum
tilt and operating deviation (about 14?) is limited by
Figure 5 - BHGM sensor transport case - The
BHGM sensor travels with the operator.
SPECIFICATIONS AND PRINCIPLES
|Small Hi-Temp Sonde
|Large Hi-Temp Sonde
Micro-g LaCoste's BHGM sensors travel with the operator as shown in
figure 5. The sensor is interchangeable among all the above
sondes. The BHGM equipment is readily heliportable as shown
in figure 6. Uphole equipment supplies and spare parts are
shipped in carrying cases weighing up to 120 lbs with
dimensions of 28" x 24" x 18".
Figure 6 - BHGM support equipment
THE DEEP DENSITY MEASUREMENT WITH THE BHGM
The BHGM is a miniature version of the LaCoste &
Romberg land gravity meter. The sensor is a sensitive spring
balance which measures changes in weight of a proof mass due
to changes in gravity. It is precisely leveled for each
recording and maintained at a constant temperature close to
By Poisson's equation, the vertical gradient of gravity in
a uniform medium is proportional to the medium's density. The
BHGM density is calculated from the measured gravity
r = 3.6827 -
0.03913 Dg / Dz
where r is in g/cm?, Dz is in feet and Dg
is in ?Gals. The constant density term compensates for the
earth's normal vertical gravity gradient (the free-air
RULE OF THUMB
A useful and practical rule of thumb for Deep Density BHGM
applications is that a remote body with sufficient density
contrast may be detected at a distance of between one and two
times the height of the body.
In most cases, before a Deep Density BHGM survey is
attempted, a model should be prepared of the expected density
anomalies around the borehole to evaluate their effect upon
the apparent density measured by the instrument in the
REMOTE SENSING APPLICATIONS
In this North Sea example (fig. 7), a fault 100 feet from
the well was revealed by the density differences between Deep
Density BHGM and Gamma-Gamma density measurements.
Figure 7 - North Sea fault
The broad departure between the Deep Density BHGM and
Gamma-Gamma density logs in this Michigan reef example (fig.
8) reveals through modeling that the edge of the reef complex
is within a few hundred feet of the well. The overlying low
density zone near the top of the figure is salt.
Figure 8 - Michigan reef. The sharp difference in
density between 6330 and 6370 is caused by a remote higher
porosity zone not detected by the density log. The broader
difference anomaly observed over the length of the logged
interval is Gamma-Gamma explained by the structural influence
of the reef complex.
The model (fig. 9) shows the large expected differences in
density between Deep Density BHGM and Gamma-Gamma
measurements in the vicinity of salt flanks. Comparison of
actual and model data may be used to map the salt boundaries.
Figure 9 - Structural effect of salt overhang
Resolution of seismic methods in salt
environments decreases with increasing depth due to
decreasing velocity contrasts and loss of high frequencies,
whereas the resolution of the BHGM method increases because
of increasing density contrast between salt and surrounding
FORMATION EVALUATION APPLICATIONS
LOGGING THROUGH CASING
The Deep Density BHGM is the only means of making reliable
through-casing density measurements. In old wells without
density logs, zones showing high resistivity and low Neutron
porosity could indicate either hydrocarbon or tight sands.
The dilemma can be resolved by Deep Density BHGM, which shows
low density in hydrocarbons (fig. 10) or high density in
tight formation (fig. 11).
Figure 10 - High resistivity, High N counts, Low
density = Gas
Figure 11 - High resistivity,
High N counts, High density = Tight Zone
AVOIDING INVASION EFFECTS
The Deep Density BHGM at the top of this gas- and
water-bearing sand (fig. 12) is lower than Gamma-Gamma
density which investigates only the invaded zone. At the
bottom, the Deep Density BHGM responds to the (denser) water
in the virgin zone, whereas the Gamma-Gamma density is
affected by the (less dense) filtrate from the oil-base
Figure 12 - BHGM and Gamma-Gamma
density in gas sand showing invasion effects
LOGGING IN FRACTURED ZONES, WASHED OUT
The Gamma-Gamma Density/Neutron logs (fig.
13) are seriously affected by poor hole conditions in this
well intersected by fractures. The Deep Density BHGM is not
affected by hole conditions and generally overlays the
undisturbed Density/Neutron readings to give a good average
Figure 13: 33K
jpeg file - BHGM and
Gamma-Gamma density in fractured zone
DETECTION OF POROSITY NOT PENETRATED BY WELL
The average Deep Density BHGM reading in this fractured
karstic carbonate (fig. 14) is less than that of the
Gamma-Gamma Density/Neutron log, indicating that there are
zones of higher porosity near the well.
The Deep Density BHGM in the Michigan reef example (fig.
8) also shows evidence of by-passed porosity in the zone
between 6330 and 6370, although the Gamma-Gamma density log
indicates little porosity at the wellbore.
Figure 14: 34K
jpeg file - BHGM and
Gamma-Gamma densities in karstic zone
RESERVOIR MONITORING APPLICATIONS
In this Able Sand example (figs. 15-17), a baseline Deep
Density BHGM log was run soon after the start of production,
and a plot of Density vs. Porosity indicated that zones a, c
and d were gas bearing. Subsequent production was made with
increasing water- cut, and a second survey was run 11 months
later. This showed substantial flushing of zones a and d, and
some flushing of c. Zone b, which was shaly, remained
Figure 15 - Production history of
nearby well from interval A, Able Sand (Beginning 1/1/79)
Figure 16 - Reference well, Able
Sand, preproduction and postproduction BHGM logs
Figure 17 - Crossplot of four
intervals of the Able Sand, reference well (after Gournay
and Maute, 1982)
Smith, N. J., The case for gravity data from boreholes,
Geophysics, vol. 15, no. 4, 1950.
Gournay, L.S. & Maute R.E., Detection of bypassed gas
using Borehole Gravimeter and pulsed neutron capture logs,
Log Analyst, May/June 1982.
Gournay, L.S. & Lyle W.D., Determination of
hydrocarbon saturation and porosity using a combination BHGM
and deep investigating electric log, SPWLA 25th Annual
Lines, L.R., Shultz A.K. & Treitel S., Cooperative
inversion of geophysical data, Geophysics, Vol. 33, no. 1,
Deep Density Borehole Gravity Logging, EDCON, Denver.
Schultz, A,K., Monitoring fluid movement with the borehole
gravity meter, Geophysics, vol. 54, no. 10, 1989.
van Popta, J. et al., Use of borehole gravimetry for
reservoir characterization and fluid saturation monitoring,
SPE 20896, 1990.