1. Field of the Invention
This invention relates to measuring gravitational acceleration. In particular, the measuring is performed within a borehole.
2. Description of the Related Art
In exploration for hydrocarbons, it is important to make accurate measurements of geologic formations. The geologic formations below the surface of the earth may contain reservoirs of oil and gas. The geologic formations may include formation bedding planes and various structures. In a quest for oil and gas, it is important to know about the location and composition of the formation bedding planes and the various structures. In particular, it is important to know about the geologic formations with a high degree of accuracy so that reservoir production is optimized. Measuring properties of the geologic formations provides information that can be useful for locating the reservoirs of oil and gas. Typically, the oil and gas are retrieved by drilling boreholes into the subsurface of the earth. The boreholes also provide access for taking measurements of the geologic formations.
Well logging is a technique used to tale measurements of the geologic formations from the borehole. In one embodiment, a “logging instrument” is lowered on the end of a wireline into the borehole. The logging instrument sends data via the wireline to the surface for recording. Output from the logging instrument comes in various forms and may be referred to as a “log.” Many types of measurements are made to obtain information about the geologic formations. Some examples of the measurements include gamma ray logs, nuclear magnetic resonance logs, resistivity logs, and sonic or acoustic logs.
Knowledge of many parameters is useful in well logging. For example, having knowledge of depth of the borehole at various points as well as amounts of injection media contained within the reservoirs provides useful information. Depth of the logging instrument in the borehole can be used to determine the depth of the formation bedding planes.
Typically, a common factor among the logs is that a borehole depth is associated with the measurements. For example, each time a resistivity measurement is made, the measurement is associated with the borehole depth at which the measurement was made. In general, many logs of various measurements are analyzed to male an accurate assessment of the geologic formations. The various measurements may be viewed side-by-side to form a composite picture of the geologic formations. Therefore, it is important to have accurate knowledge of the borehole depth and orientation of the logging tool when each measurement is taken.
The orientation of the logging tool is typically with respect to a vertical axis and magnetic north. Even small errors in the borehole depth determination can corrupt logging data. An assumption that the logging instrument is moving smoothly through the borehole is not always valid due to rugose and sticky borehole conditions. Additionally, tool centralizers and decentralizers may keep the logging tool from moving smoothly. Horizontal deviations of the borehole may also lead to errors in measuring the borehole depth. Generally, the horizontal deviations are not relevant to a “true vertical depth.” It is, therefore, important to have reliable techniques for determining the true vertical depth of the logging instrument. In addition to realizing benefits from knowledge of the true vertical depth, accurately assessing status of injection media provides a number of benefits.
An important parameter when recovering oil and gas is an amount remaining in the formation. When the oil and gas recovered using traditional methods start to decrease, injection media such as water or carbon dioxide may be used to enhance recovery. That is, at least one of the water and carbon dioxide are typically injected into the reservoirs. The injection media contribute to forcing the remaining hydrocarbons out of the reservoirs. It is important to measure amounts of the injection media in the reservoirs so that the amounts can be used to determine the amount of remaining hydrocarbons.
Information regarding these and other parameters may be derived from local measurements of gravity. Therefore, accurate techniques for measuring gravity are desired.
What are needed are reliable and accurate techniques for measuring gravity.