Presentation on Caliper log

Md. Zubayer SalehinM.S. (Geophysics)

Department of Geology

University of Dhaka

Bangladesh

What is well Logging

Well log is a continuous record of measurement made in bore hole respond to variation in some physical properties of rocks through which the bore hole is drilled

•Caliper tools measure hole size and shape.

•The simple mechanical caliper measures a vertical

profile of hole diameter.

•The more sophisticated geometry tools records two

simultaneous calipers and give an accurate borehole

shape and orientation.

Caliper Log

Caliper log

showing hole

Diameter and

some Typical

Responses

Schematic Caliper tool showing the conversion of a mechanical movement to an electrical signal using a variable resistance

Function of the Tool:•The mechanical caliper measures variations in bore hole diameter with depth.• The measurements are made by two articulated arms pushed against the borehole wall. •The arms are linked to the cursor of a variable resistance. Lateral movements of the arm is translated into the movements of the cursor along the resistance, and hence variation in electrical out put. •The variations in output are translated into diameter variations after a simple calibration.

•The Caliper Log is a tool for measuring the diameter and shape of a borehole. It uses a tool which has 2, 4, or more extendable arms.

•In the 4 arm (dual caliper) tool, the two opposite pairs work together to give the borehole diameter in two perpendicular directions.

•An example of a 4 arm tool is the Borehole Geometry Tool (BGT).

•This has 4 arms that can be opened to 30 inches (40 inches as a special modification), and give two independent perpendicular caliper readings.

•In the multi-arm tools, up to 30 arms are arranged around the tool allowing the detailed shape of the borehole to be measured.

Various types of caliper

Mechanical Caliper -arm averaging�Ultrasonic Caliper -An Ultrasonic transducer scans �

around the borehole walls, and the reflected travel time is converted to the distance between the sonde and the wall.

Acoustic caliper – calculated from acoustic transit time and velocity

Mechanical 3-arm averaging caliper

Mechanical 2 arm caliper

3D image of the ultrasonic caliper. (FromSchlumberger)

Presentation of the Caliper Logs: 1. in ordinary format; 2. in differential format; BS = Bit size

•The caliper log simply is a measure of continuous value of hole diameter with depth. •The curve is traditionally a dashed line. •The horizontal scale may be in inches of diameter or in differential caliper, expressed as increase or decrease in hole diameter about a zero defined by the bit size. •The ordinary caliper log is accompanied by a reference line indicating bit size.

Caliper Log Presentation

•The caliper logs are plotted in track 1 with the drilling bit size for comparison, or as a differential caliper reading, where the reading represents the caliper value minus the drill bit diameter.

•The 4 arm (or dual caliper) tools are presented in a range of formats, an example of which is given in Figure.

•Data from the caliper pairs are shown together, and that they are different indicating an oval hole.

• This information is useful to estimate the amount of drilling mud in the borehole, and to estimate the amount of cement required to case the hole.

• There are engineering approximation formulas to calculate both of these from caliper data.

Table: Factors influencing caliper responsesHole

DiameterCause Possible Lithologies

On Gauge Well consolidated formations Non-permeable formations.

Massive sandstonesCalcareous shalesIgneous rocksMetamorphic rocks

Larger than BitSize

1. Formation soluble in drilling mud.2. Formations weak and cave in.

1. Salt formations drilled with fresh water.2. Unconsolidated sands, gravels, brittle shales.

Smaller than BitSize

1. Formations swell and flow into borehole.2. Development of mudcake for porous and permeableformations.

1. Swelling shales.2. Porous, permeable sandstones.

Simple 2 arm caliper interpretation

Figure shows a schematic hole with caliper information.

Quality control using Caliper

When caving is serious, the quality of log reading is impaired.

It should be pointed out that simple caliper attached to the open hole tools (microlog, formation density) will be pessimistic in terms of hole condition, because in oval holes a simple caliper will naturally open to the maximum diameter of the borehole.

So, the log measurements recorded will be made across the larger diameter, the hole condition is not as bad as may first appear.

• For this reason, many boreholes can attain an oval shape after drilling.

• This is due to the effect of the pressures in the crust being different in different directions as a result of tectonic forces.

• In oval holes, the two arm caliper will lock into the long axis of the oval cross section, giving larger values of borehole diameter than expected. In this case tools with more arms are required.

Four-arm caliper interpretation

More information can be found by dual(4-arm) caliper than simple caliper tool.

A hole can be seen to be “on gauge” and round, or “washed out” and oval, or enlarged by “key seat”. When oval, the direction of enlargement can be given.

However, much more can be interpreted from borehole shape.

Keyseats: asymmetrical oval hole, formed by wear against the drill string at points where the borehole inclination changes(doglegs).

Washouts: develop from general drilling wear, especially in shaly zones and dipping beds. On the geometry logs, it has a considerable vertical extent and both calipers are larger than the drill bit size with one caliper being much larger than other.

Both have general drilling phenomena, but breakouts have a specific cause.

Breakouts: consider to form as a result of the interaction of stresses induced by drilling and the existing stress regime of the country rock.

• breakouts indicates the present day stress-field orientation and are independent of lithology, dip and existing fracture or joint.

•On global scale, breakout derived stress field orientation are similar to those derived from earthquake studies and tend to indicate intraplate tectonic stresses

•On local scale, it have an importance for field development.

•In a logging context, the greatest application of caliper data is in environmental corrections to other logs that are sensitive to hole size such as Natural gamma ray , Density, Neutron logs.

•Contributory information for lithological assessment.

• Indicator of good permeability and porosity zones (reservoir rock) due to development of mudcake in association with gamma ray log.

•Calculation of mudcake thickness, hmc = (dbit– dh)/2, where h stands for the hole, in inches.

The common uses of the caliper log are as follows:

•Selection of consolidated formations for wireline pressure tests, recovery of fluid samples, for packer seating for well testing purposes, and for determining casing setting depths.

•calipers can be used to center or eccenter logging-tool strings.

•Openhole caliper data are used to estimate the volumes of gravel and cement needed for well-completion planning.

The common uses of the caliper log are as follows: