INTRODUCTION TO BOREHOLE GRAVITY

written by: Alan T. Herring, EDCON, Inc.
February, 1990

SUMMARY

The borehole gravity meter (BHGM) can be described simply as a deep-investigating, density logging tool. Application range beyond this simple description to include detection of oil and gas-filled porosity, detection and definition remote structures (e.g. salt domes, faults, reefs, etc.) and non-Newtonian gravity experimentation.

One of the great advantages of the BHGM as a density logging tool is that it is practically unaffected by nearhole influences which are the scourge of nuclear tools: casing, poor cement bonding, rugosity, washouts, and fluid invasion. Another advantage is the fundamental simplicity of the relationships between gravity, mass, rock volume and density. Complex geology can be easily modeled so that the response of a range of hypothetical models can be studied and understood before undertaking a survey.

What is actually measured is referred to as BHGM apparent density which is a simple function of the measured vertical gradient of gravity. To obtain an apparent density measurement, gravity is measured at two depths. The accuracy of the computed density depends on the accuracy of both measured differences: gravity and depth. Operationally, BHGM surveys resemble VSP (vertical seismic profiling) surveys. The BHGM is stopped at each planned survey level and a five-to-ten-minute reading is taken. The blocky appearance of the log reflects the station interval, Figure 1.

The log is not continuous. BHGM measurements are taken at discrete depths usually at intervals of 10 to 50 feet, depending on the vertical and density resolution required. While the BHGM has remarkable resolution in the measurement of density over intervals of 10 feet or more (less than 0.01 g/cm³), surveys requiring closer vertical resolution must sacrifice density resolution.

Figure 1. Example BHGM Log
The sharp difference in density between 6330 and 6370 is caused by porosity not detected by the gamma-gamma density log. The Broader difference anomaly observed over the length of the logged interval is explained by the structural influence of the reef complex.

APPLICATIONS

The spectrum of BHGM applications is defined on one extreme by density logging and on the other by remote sensing of structure. The first extreme sometimes focuses strictly on formation and reservoir evaluation questions, the other extends to basic exploration. The example shown in Figure 1 can be used as an example of both applications. In fact, the purpose of the survey was to detect carbonate porosity in a reef environment that was missed by the other logs. For this objective, the useful radius of investigation of the measurement is on the order of 50 feet. The sharp negative density anomaly observed between 6330 and 6370 suggests porosity obscured by near borehole effects or poor volume sampling. On the other hand, the broad departure of the BHGM and gamma-gamma logs over the depth range of the logged section is typical of a structural effect, in this case the edge of the reef complex which is within a few hundred feet.

Density Logging

Borehole gravity density measurements are unhindered by casing, poor hole conditions, and all but the deepest fluid invasion. The BHGM measurement samples a large volume of rock which provides a density-porosity value that is more representative of the formation. This is especially beneficial in carbonate and fractured reservoirs. BHGM surveys have been used to find hydrocarbon-filled porosity missed by other logs in both open and cased holes. Gas-saturated sands are a particularly easy target.

The wide radius of investigation has also been successfully used to determine gas-oil and oil-water contacts in reservoirs where other measurements have been ineffective. BHGM density measurements have been used to calculate hydrocarbon saturations: the larger the fluid density contrast, the larger the measured effect. Gas saturations are therefore the easiest to measure. The difference in densities measured by the gamma-gamma log and the BHGM can be used to calculate the difference in oil saturation between the invaded and undisturbed zones, which can in turn give an estimate of movable hydrocarbons.

Remote Sensing

A useful and practical rule of thumb for BHGM remote sensing applications is that a remote body with sufficient density contrast can be detected by the BHGM no farther from the well bore than one or two times the height of the body. A salt dome with 15,000 feet of vertical relief would have a definitive signature a few miles away. A channel sand 20 feet thick would be detectable no more than 40 feet away. Local geology, and in particular the thickness of local density units, defines the effective radius of investigation of the BHGM.

Computer modeling of BHGM measurments can be used to develop relatively detailed salt-dome-flank or reef-flank model interpretations. Modeling is particularly effective where seismic data can be integrated into the modeling process; a model is sought that is consistent with both data sets. In one case, the presence of an imbricate thrust sheet was confirmed by the BHGM and led to a sidetracked hole and an economic discovery.

THEORY

Measurements of gravity differences repeatable to about 3 microgals have been achieved using a LaCoste and Romberg borehole gravity meter. One gal (after Galileo) is 1 cm/sec². The earth's gravity field varies from the equator to the pole from about one part in 10⁹ of the earth's field. The BHGM is clearly a remarkably sensitive instrument.

The underlying assumption in computing apparent density is that of an earth model made up of a layer cake of horizontal infinite slabs. For such a model, the density of any slab is exactly given by the gravity gradient through that slab; the gradient measured at any point within the slab is constant; and the slabs above and below it have no effect on the gradient within it. The derivation of the density of an infinite slab is shown in Figure 2. This simple assumption serves effectively in a majority of cases. Modeling of more complex geometries is not difficult and is routinely used in computing structural corrections to apparent density.

Figure 2. Density of an Infinite Slab from Borehole Gravity
G is the Universal Gravitational Constant.

The formula for apparent density is given approximately by the following (there are small corrections for latitude and elevation):

r = 3.6824 - .03913 Dg/Dz

where r is in g/cm³, Dz is in feet, and Dg is in microgals. The constant density term compensates for the earth's normal vertical gravity gradient.

OPERATING LIMITATIONS

The gravity meter itself is a delicate spring balance which measures changes in weight of a small proof mass. The meter must be leveled at each station, and it is accurately thermostatted at a temperature of about 126° C. The present LaCoste and Romberg meter will not operate in a well bore deviated from vertical by more than 14° . For higher operating temperatures the meter is operated inside a Dewar flask. Using the flask, operating temperatures up to 260° C are possible. BHGM surveys have been carried out from floating platforms, but usually at depths no shallower than 4000 feet.

Operating Conditions Summary
(LaCoste and Romberg through Number 14)

REFERENCES

Gournay, Luke S. and Maute, Robert, E., 1982, Detection of bypassed gas using borehole gravimeter and pulsed neutron capture logs: The Log Analyst, v. 23, No. 3, p. 27-32.

Case history demonstrates quantitative analysis of BHGM data in combination with other logs.

LaFehr, T. R., 1983, Rock density from borehole gravity surveys: Geophysics, v. 48, No. 3, p. 341-356.

Theoretical development examines the response of the BHGM in the presence of anomalous masses.

Lines, L. R., Schultz, A. K., and Trietel S., 1988, Cooperative inversion of geophysical data: Geophysics, v. 53, No. 1, p. 8-20.

BHGM data figure prominently in this integrated interpretation study that also utilizes seismic and surface gravity data.

Rasmussen, Noel F., 1975, The successful use of the borehole gravity meter in Northern Michigan: The Log Analyst, v. 16, No. 5, p. 3-10.

Case history study also includes review of theory, interpretation and operations.

Robbins, Stephen L., 1980, Bibliography with abridged abstracts of subsurface gravimetry (especially borehole) and corresponding in-situ rock density determinations: USGS Open File Report 80-710.

Smith, Neal J., 1950, The case for gravity data from boreholes: Geophysics, v. 15, No. 4.

Imaginative classic paper provides a review of basic theory as well as a still useful source of ideas for applications and interpretive approaches.