Borehole Gravity Measurement (BHGM) tools have been available for several decades, but until now, widespread applications were not possible. The current BHGM tools are too bulky to be run in almost any well without pulling the tubing. In addition, BHGM tools are limited to vertical wells and the existing sensors fall short on stability for permanent installation.

BHGM tools could be very useful for reservoir and production monitoring. First, the depth of investigation is enormous when compared to many other measurements. Advancing flood fronts, not only gas-water but even oil-water, could be detected from a distance of tens, hundreds or even beyond thousand feet from a well, if suitable BHGM sensors were available. Second, only very few parameters are needed to convert the raw gravity measurements, via a bulk density, to the main output: saturation. This avoids the challenges with several special parameters which are needed to arrive at saturation when using resistivity logs. Third, the interpretation models are simple: for gravity interpretation Newton's laws are perfectly adequate, while for resistivity and acoustic measurements, research on response functions is still ongoing.

We have modelled the response of BHGM tools in a number of typical Saudi Arabian conditions, including advancing oil/water fronts, coning and near well bore monitoring, proving the significant potential for BHGM technology. Hence, as there is a need for improved production and reservoir monitoring, further BHGM development becomes very attractive.

From an analysis of current day gravity sensors, several possible further development routes can be painted, each with its own advantages. It is clear that if our industry desires additional monitoring capabilities, it will be relatively simple to achieve intelligent monitoring by selecting the most appropriate route and developing those BHGM sensors, tools and related systems.

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