A caliper log is a

well logging Well logging, also known as borehole logging is the practice of making a detailed record (a ''well log'') of the geologic formations penetrated by a borehole. The log may be based either on visual inspection of samples brought to the surface ( ...

tool that provides a continuous measurement of the size and shape of a

borehole A borehole is a narrow shaft bored in the ground, either vertically or horizontally. A borehole may be constructed for many different purposes, including the extraction of water ( drilled water well and tube well), other liquids (such as petrol ...

along its depth Schlumberger Oilfield Glossary http://www.glossary.oilfield.slb.com/Display.cfm?Term=caliper%20log and is commonly used in

hydrocarbon exploration Hydrocarbon exploration (or oil and gas exploration) is the search by petroleum geologists and geophysicists for deposits of hydrocarbons, particularly petroleum and natural gas, in the Earth using petroleum geology. Exploration methods Vis ...

. The measurements that are recorded can be an important indicator of wash-outs, cave ins or shale swelling in the borehole, which can affect the results of other well logs.

Process

The caliper tool measures the variation in borehole diameter as it is withdrawn from the bottom of the hole, using two or more articulated arms that push against the borehole wall. Each arm is typically connected to a

potentiometer A potentiometer is a three-terminal resistor with a sliding or rotating contact that forms an adjustable voltage divider. If only two terminals are used, one end and the wiper, it acts as a variable resistor or rheostat. The measuring instrum ...

which causes the resistance to change as the

diameter In geometry, a diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints lie on the circle. It can also be defined as the longest chord of the circle. Both definitions are also valid fo ...

of the borehole changes, creating a varying electrical signal that represents the changing shape of the borehole. This variation in output is translated into changes of diameter after a simple calibration and the caliper log is printed as a continuous series of values of hole diameter with depth. Caliper logs may also be created from both acoustic and electrical borehole image logging. Electrical borehole imaging logs are typical created with 4, 6, or 8 armed tools, which can also be used to create a caliper log. Acoustic borehole imaging tools can create the most accurate and detailed caliper logs, using the pulse echo transit times with an independent acoustic measurement of the borehole

drilling fluid In geotechnical engineering, drilling fluid, also called drilling mud, is used to aid the drilling of boreholes into the earth. Often used while drilling oil and natural gas wells and on exploration drilling rigs, drilling fluids are als ...

velocity, but the range of this type of measurement is often limited by the high acoustic attenuation of the drilling fluid. The decentralization arm typically present on a density logging tool may also be used to create a crude caliper log, which is limited along a single, high-side axis.

Accuracy

Known challenges with caliper logging include borehole spiralling. The position of the

drill bit Drill bits are cutting tools used in a drill to remove material to create holes, almost always of circular cross-section. Drill bits come in many sizes and shapes and can create different kinds of holes in many different materials. In order ...

may

precess Precession is a change in the orientation of the rotational axis of a rotating body. In an appropriate reference frame it can be defined as a change in the first Euler angle, whereas the third Euler angle defines the rotation itself. In othe ...

as it drills, leading to spiraling shapes in the wellbore wall, as if the hole had been drilled by a

screw A screw and a bolt (see '' Differentiation between bolt and screw'' below) are similar types of fastener typically made of metal and characterized by a helical ridge, called a ''male thread'' (external thread). Screws and bolts are used to f ...

. If the arms of the caliper log follow the grooves of the spiral, it will report too high an average diameter. Moving in and out of the grooves, the caliper will give erratic or periodically varying readings. The borehole can also change to an oval shape while drilling, which typically occurs in doglegs where the axis of borehole is bending to change the direction of the well. This oval shape can cause a 2-arm caliper log to overestimate the size of the borehole, if its arms get stuck in the long-axis. In most cases, the borehole's circumference will not be a perfect circle and therefore a caliper tool with several arms is required to obtain a true understanding of the size and shape of the borehole. The current state of the art is six independent mechanical arms that simultaneous record the radial distance from the center of the tool. Abrupt changes in borehole diameter, such as a washout at the bottom of casing, or thin fractures may not be accurately measured due to the geometry or range of the measurement arms.

References

Petroleum production Well logging {{petroleum-stub