Improved Hydrocarbon Reservoir Evaluation Through Use of Borehole-Gravimeter Data

Abstract

The borehole gravimeter was developed to improve subsurface remote-sensing capabilities and reservoir-analysis capabilities. Successful applications of borehole gravimetry illustrate the sensitivity and versatility of the device. Its success is attributed to its large volumetric investigation capability and the absence of wellbore and near-wellbore effects. Introduction Within the past 3 years, substantial amounts of oil and gas reserves have been discovered by a unique application of the time-worn but sound technique of gravity measurement. In many instances, the hydrocarbon discoveries were made after conventional logging devices had condemned the reservoir or had failed to provide sufficient data to properly evaluate the reservoir potential. The instrumentation and methods used in applying potential. The instrumentation and methods used in applying subsurface measurement of gravity gradients to reservoir analysis is the subject of this paper. The purpose of the paper is to initiate further development of borehole gravity devices and to expand the application of gravimeter data. Tool Development The development of the borehole gravimeter used in the following examples was the result of the efforts of a committee representing six major oil companies; the instrumentation used was patterned after an original tool designed for the USGS. The first successful application of borehole gravimetry to reservoir analysis is credited to McCulloh, who, along with Hammer, Smith, Gilbert, and Goodell and Fay, proposed various applications for borehole gravimetry. McCulloh et al. presented the first paper devoted-primarily to applications of borehole-gravimeter data to reservoir analyses. The suggestions and comments of these authors have been most helpful in the successes noted in this paper. Objectives of Tool Development The basic purpose for developing the borehole gravimeter was twofold: to improve subsurface remote-sensing capabilities and to improve reservoir-analysis capabilities. Four primary applications were anticipated:(1)deep porosity-investigating capabilities;(2)accurate porosity determinations in heterogeneous reservoirs;(3)porosity determinations in heterogeneous reservoirs;(3)detection of hydrocarbons behind pipe and beyond deep invasion; and(4)detection of density anomalies remote from the wellbore. Applications of borehole gravimetry have been successful in all four categories; and the examples included at the end of this paper demonstrate the extreme sensitivity and versatility of the device. The success of borehole gravimetry is attributed to its large volumetric investigation capability and the absence of wellbore and near-wellbore effects. In heterogeneous, vuggy, fractured, and invaded reservoirs, the capability of the borehole gravimeter for large depth of investigation has proven to be more compatible with those resistivity devices that possess deep, "large-volume" investigating capabilities than with the conventional, shallow, "low-volume" investigating porosity devices. Evidence gathered from numerous porosity devices. Evidence gathered from numerous applications of the borehole gravimeter in reservoirs of various types has indicated that pairing low-volume investigating porosity devices with large-volume investigating resistivity devices can result in serious errors in the evaluation of hydrocarbon potentials. JPT P. 709