The borehole gravity logging tool, used as a deep investigation density method that can image through casing, is enjoying a renaissance. Originally designed in the early 1970s, the tool suffered from hardware limitations, such as size, deviation, and speed, which now keeps it out of the majority of wells drilled. These artificial limitations can now be overcome with modern engineering and new electronic components - technology that was unavailable a few years ago. New advances in gimbals and leveling move borehole gravimetry forward into the 21st century. The fundamental principles of borehole gravity remain a sound and valuable technology for all phases in the life of an oil field. Numerous examples and case studies can be cited which attest to borehole gravity's value in exploration, formation evaluation, reservoir monitoring, and missed pay opportunities.
Introduction
The borehole gravity logging tool was originally designed in the early 1970s. Sixteen tools were manufactured and they have logged more than 2,200 wells. The earliest model which we term BHGM Model I was large diameter, comprising of serial numbers 1–3. The second generation, BHGM Model II comprises of serial numbers 4–16 and are "slim-hole" tools. BHGM II, the most recent model up until last year, is based upon Amoco's electronic systems developed beginning in 1975 (Table 1). At any one time, only about 6 tools were ever used in the oil industry, yet by one U.S. Geological Survey estimate, more than 1 billion bbls of oil equivalent have been discovered with this method.1
The borehole gravity logging method, which we term Deep-Penetration Density™ logging or Deep Porosity-Density™ logging (depending on the application) or DPD™ for short, is essentially a density logging method. It is a start-and-stop tool which measures the earth's gravity at two or more downhole locations. The in situ bulk density between measurement intervals can be directly determined to high accuracies - typically higher than can be achieved with any other logging method (Fig. 1). The depth of penetration is tunable to some degree depending upon the downhole measurement interval. The larger the measurement interval, the farther out it samples. A good rule of thumb is that 90% of the effect is measured at a distance of five times the measurement interval, assuming the interval has a density contrast and assuming the well penetrates the density boundaries. For example, if the measurement interval is 100 feet, the method can sense about 500 feet from the well (Fig. 2).
Like most downhole tools, it is an omni-directional measurement. Other information must be brought to bear on the interpretation to obtain direction. As an example, Amoco routinely ran a dipmeter with their borehole gravity tool.2 Or, if three wells are logged, then it may be possible to triangulate the results to obtain direction.3
This large radius of investigation is one of the powerful advantages of the DPD method. Because it samples such a large volume of rock, the method is generally not affected by such near wellbore effects as casing, cement, hole rugosity, formation damage, mud infiltration, and fluid invasion. Thus, it works equally well in either open-holes or cased-holes. It is a passive sensor that measures deviations in the earth's gravity field. It directly measures density regardless of the lithology, so it does not have to be tuned or recalibrated depending on the lithology. There are no downhole radioactive sources, nor are there any exotic materials in the tool.
In order to measure densities at sensitivities of 0.01 g/cm3 or better, the sensor must be able to measure the gravity field at sensitivites on the order of 2–4 µGal. This represents a measurement of 1 part per billion of the earth's gravity field, an incredible level of sensitivity no other mechanical device man has ever built can achieve. It must do this reliably and in harsh downhole conditions. Yet, these tools can routinely achieve these sensitivities.
