Borehole Caliper

Basic Concept

The caliper log is a simple, straightforward, and informative wireline-borehole method. Using one or more tensioned-mechanical arms, caliper tools can continuously measure and record the size and/or shape of a borehole over its length. The caliper tool has value as a stand-alone log but is frequently used in conjunction with other borehole geophysical methods.

The caliper log can provide both direct- and indirect information of the borehole and borehole- intersected formations. Borehole geophysical data are commonly affected by the borehole environment (e.g., diameter, mudcake thickness, rugosity etc.). As such, the caliper log is often an essential component of many suites of borehole logs. Furthermore, information of borehole diameter variations allows for inferences of lithology and borehole condition.

Theory

Standard mechanical caliper tools employ one or more spring-loaded arms that are fully contracted while the tool is lowered down the borehole prior to logging. As the tool is pulled up the borehole, the arms expand and contract to maintain physical contact with the borehole wall where it enlarges or constricts. Each arm is connected to a device that converts its movement to an electrical signal that is calibrated to borehole diameter.

The types of mechanical caliper tools available vary in the number and type of arms as well as the method of recording borehole size. Some caliper tools incorporate the movement of all arms and produce a log of average diameter with depth. Others record the movement of each arm to produce logs of variations in size and shape with depth. Regardless of the method used, all caliper sondes must be calibrated to caliper rings, gage, or casing with known dimensions.

The resolution and sensitivity of a caliper tool depends highly on the number of arms, the contact area of each arm, and the pressure exerted by each arm. Caliper tools may employ one, two, three, four, or six arms, and each of these provides a successively more comprehensive borehole picture. Calipers containing more arms (i.e., four or six) are often standalone tools, whereas those with fewer arms are often components of other borehole tools (Collier, 1993).

Boreholes are often irregular, and the side with which a caliper arm contacts can vary with tool type and each log run. One- and two-arm tools tend to align with and, thus, measure the larger borehole “diameter”. One- and two-arm calipers are often included on other pad-type tools, and the mechanical arm(s) also maintain(s) sidewall contact and collect data. Three-arm calipers measure one diameter that is generally somewhere in between the maximum and minimum values (Collier, 1993).

Four-arm calipers display two perpendicular measurements that typically represent the values of the minimum and maximum axes. The measurements collected by the six-arm caliper are divided into three pairs, and three measured “diameters”, which are recorded in three directions, are averaged out. Each tool type and log collected provides a slightly different borehole picture. Thus, it is useful to display and compare all collected caliper logs (USBR, 2001).

Additionally, the caliper-arm type can affect the log results. Rod-type arms, which are under high-pressure and have a small contact area, tend to slice through the borehole mudcake. Alternatively, pad-type and bowspring arms tend to ride over the mudcake unless they have an abnormal pressure increase and/or area decrease relative to borehole conditions. Thus, the type of caliper employed is related to whether or not mudcake is identified completely, partially, or not at all (Collier, 1993).

Applications

Borehole diameter is one of the major factors that affect the responses of other geophysical logs, and caliper data is primarily used for correcting logs for borehole diameter. Additionally, caliper logs are often used to locate open fractures in hard-rock wells and support flowmeter data interpretation. Furthermore, changes in borehole diameter may also be related to and used to infer changes in lithology, material properties, borehole construction, and hole integrity.

Data are typically displayed as diameter versus depth but can be shown relative to hole (i.e. drill bit) size, which is denoted as “zero”. In such, increases in diameter are plotted to the right of center as positive values, whereas decreases are plotted to the left as negative values. The single- and composite logs, which are produced by more advanced tools (e.g., 6-arm tool), quite accurately depict the borehole geometry and aid visual analyses and interpretations.

Borehole enlargements often occur in weak or weathered formations with closely spaced fractures or dissolution cavities and progress over time. Borehole diameters equal to the bit size are often associated with consolidated formations with lower permeability. Borehole diameters that are less than the bit size result from swelling, shale sloughing, and/or mudcake buildup. However, the excessive buildup of mud filter, which occurs over intervals of higher porous and permeable formations, is most common (Collier, 1993).

The caliper log also aids the distinction of lithology, which is especially useful in places where there is core loss (i.e., material is not recovered during drilling). Some formations have diagnostic signatures in their borehole diameters due to their lithology and/or degree of cementation (e.g. some washout while others retain integrity). Thus, knowing the local lithology along with the associated material properties allows for rough correlations of site lithology (Collier, 1993).

Caliper logging has proven valuable as both a stand-alone method and one that provides ancillary information to aid the interpretation of data collected using other geophysical methods. Additionally, because caliper data can provide both direct- and indirect information of the borehole and intersected formations, caliper logging has been and continues to be diversely applied. As such, mechanical caliper borehole logging has assisted in the following:

Correcting other logs for borehole environment
Evaluating overall borehole condition
Indicating areas of porosity in non-shales
Calculating integrated borehole volume (IHV)
Estimating volumes of material (e.g., cement, grout, gravel) needed for well completion
Selecting locations of packer seats or water well screens
Correlating/determining lithology
Detecting fractures, cavities, caverns, and solution openings
Identifying locations of washouts or swelling
Aiding the placement of packers or submersible pumps within the borehole
Estimating grout volumes in solution or washout zones
Relating borehole elongation to stress fields (four- or more arm caliper)

Examples/Case studies

Dembicki, E.A. and Machel, H.G., 1996, Recognition and Delineation of Paleokarst Zones by the Use of Wireline Logs in the Bitumen-Saturated Upper Devonian Grosmont Formation of Northeastern Alberta, Canada: AAPG Bulletin, v. 80, no. 5, p. 695-712, doi:10.1306/64ED8886-1724-11D7-8645000102C1865D.

Abstract: The Upper Devonian Grosmont Formation in northeastern Alberta, Canada, is a shallow-marine carbonate platform complex that was subaerially exposed for hundreds of millions of years between the Mississippian(?) and Cretaceous. During this lengthy exposure period, an extensive karst system developed that is characterized by an irregular erosional surface, meter-size (several feet) dissolution cavities, collapse breccias, sinkholes, paleosols, and fractures. The karsted Grosmont Formation, which contains giant reserves of bitumen, subcrops beneath Cretaceous clastic sediments of the giant Athabasca tar sands deposit. The paleokarst in the Grosmont Formation can be recognized on wireline logs in relatively nonargillaceous carbonate intervals (<30 API units on the gamma-ray log) as excursions of the caliper log, off-scale neutron-density porosity readings, and severe cycle skipping of the acoustic log. The paleo–karst is more prevalent in the upper units of the Grosmont Formation, and the effects of karstification decrease toward stratigraphically older and deeper units. The paleokarst usually occurs within 35 m (115 ft) of the erosional surface. The reservoir properties of the Grosmont Formation (e.g., thickness, porosity, permeability, and seal effectiveness) are significantly influenced by karstification. Depending upon the location, karstification has either benefited or degraded the reservoir characteristics. Benefits include porosity values greater than 40% (up to 100% in caverns) and permeability values of 30,000 md in severely fractured intervals. Detrimental reservoir characteristics include erosion, porosity and permeability reduction, and seal ineffectiveness.

Hilchie, D.W., 1968, Caliper Logging – Theory And Practice: The Log Analyst, v. 9, no. 1, 10 p.

Abstract: Caliper logging has changed a great deal since its introduction as a commercial well service in 1938. Guyod in a series of paper ( l 4 ) in 1945, outlined some uses of caliper logs in exploration and exploitation. Since 1945 no effort has been made to keep the literature up to date with the modern aspects of caliper logging. This paper covers today's caliper logs, classifying them in terms of the way the measurements are made, explains the different variables which influence the readings and presents examples of different caliper logs presently available.

Julian, J.Y., Cismoski, D.A., Younger, R.O., Burton, J.P., and Lawrence, M., 2007, Use of 3D Visualization Software for Multi-finger Caliper Analysis at Prudhoe Bay, Alaska, in Proceedings, SPE/ICoTA Coiled Tubing and Well Intervention Conference and Exhibition: The Woodlands, Texas, Society of Petroleum Engineers, 7 p., doi:10.2118/106625-MS.

Abstract: Multi-finger calipers have provided an excellent overview of internal tubing condition for the past fifty years. However, understanding the fine details of pipe condition over short intervals required familiarity and training in interpreting the raw curves produced by these tools. The development of digital caliper data and 3D visualization software has dramatically altered pipe condition evaluation at Prudhoe Bay, Alaska. 3D visualization software allows the average engineer to quickly and accurately understand the details of localized tubing damage. Additionally, computer processing of digital caliper data supports quick correlation of tubing damage to wellbore geometry, other leak detection logs, previous caliper data of the same well interval, and cross-correlation of data from different wells. This has resulted in dramatic improvements to recent well work decisions, allowing surgical placement of patches, plugs, and whipstocks. Caliper logs have been used to assess scale buildup prior to coiled tubing scale removal operations and have been run after removal to evaluate job effectiveness. Additionally, areal corrosion trends and velocity effects have been identified, resulting in recommendations to deter damage. This paper presents several examples of how 3D visualization software has led to improved well work operations based on a study of approximately 500 calipers run in the Eastern Operating Area of Prudhoe Bay, Alaska.

Reinecker, J., Tingay, M., Müller, B., and Heidback, O., 2010, Present-day stress orientation in Molasse Basin: Tectonophysics, v. 482, no. 1-4, p. 129-138, doi:10.1016/j.tecto.2009.07.021.

Abstract: The present-day state of stress in Western Europe is considered to be controlled by forces acting at the plate boundaries. It is assumed that the Alpine orogen only influence the regional pattern of present-day stress in Western Europe within the Alps themselves. We examine the present-day maximum horizontal stress orientation in the Molasse Basin in the Alpine foreland in order to investigate the possible influence of the Alps on the far-field stress pattern of Western Europe. Four-arm caliper and image logs were analysed in 137 wells, in which a total of 1348 borehole breakouts and 59 drilling-induced fractures were observed in 98 wells in the German Molasse Basin. The borehole breakouts and drilling-induced fractures reveal that stress orientations are highly consistent within the Molasse Basin and that the present-day maximum horizontal stress orientation rotates from N–S in southeast Germany (002°N ± 19°) to approximately NNW–SSE in southwest Germany and the Swiss Molasse Basin (150°N ± 24°). The present-day maximum horizontal stress orientation in the Molasse Basin is broadly perpendicular to the strike of the Alpine front, indicating that the stress pattern is probably controlled by gravitational potential energy of Alpine topography rather than by plate boundary forces. The present-day maximum horizontal stress orientations determined herein have important implications for the production of hydrocarbons and geothermal energy in the German Molasse Basin, in particular that hydraulically-induced fractures are likely to propagate N–S and that wells deviated to the north or south may have reduced wellbore instability problems.

Saada, M., Ismail, M., Lu, Y., Zhang, W., Rourke, M., and Abdelmoula, M., 2018, Measure the Effectiveness of Frac Sleeves Integrity in Multi-Stage Completions Using Multi-Finger Caliper and Acoustic Flow-Analyzer, in Proceedings, SPE Kingdom of Saudi Arabia Annual Technical Symposium and Exhibition: Dammam, Saudi Arabia, Society of Petroleum Engineers, 10 p., doi:10.2118/192372-MS.

Abstract: With the increased demand of Hydrocarbons, the industry trend to continually develop the unconventional reserves and/or maximize the HC production; different wellbores are accordingly drilled and completed with multi-stage completions. Frac Sleeve in Multi Stage completion tend to show several integrity issues. Multi-finger caliper tool has a proven capability of assessing the wellbore obstructions and internal corrosive damage to tubulars. Gathering appropriate logging data is important for monitoring well-head and annuli pressures. Evaluating the Frac sleeves integrity whether are closed or open as well as to look for deformation within the sleeves are of client's objectives to confirm its conditions. We will combine the caliper data with passive noise detection tools to confirm and understands the fluid flow behavior in the well-bore and behind casing. Same combination is used to assess the injection sweep efficiency and anisotropy. Caliper 3D visualization and advanced noise detection processing software were used to quickly and precisely understand the condition of the Frac sleeves in several wells for the sake of injection assessment and for future remedial actions to the recommended sleeves. Also, through comprehensive analysis, caliper logs have been used to assess scale buildup, paraffin or other mineral deposits in the wellbore which could coat the perforations, casing, tubulars, valves, pumps and downhole equipment. The proposed paper will summarize field results and data gathered from several passes across the Frac sleeves. Using the logging data, we will measure the effectiveness and integrity of the Frac Sleeve for the purpose of injection/production assessment.

Schwerzmann, A., Funk, M., and Blatter, H., 2006, Borehole logging with an eight-arm caliper–inclinometer probe: Journal of Glaciology, v. 52, no. 178, p. 381-388, doi:10.3189/172756506781828520.

Abstract: eight points of cross-sections of the borehole at 1 cm intervals along its axis. Regression circles and ellipses are calculated and used as obvious hypotheses to be tested in order to obtain information on strain rates in the horizontal planes and isotropic or anisotropic borehole closure. The scratching of identifiable marks into the wall of the borehole enables material displacement to be tracked, in particular in the vertical direction, thus providing information for determining vertical strain rates along the borehole. Measurements in two boreholes drilled on high-altitude glaciers in the Swiss Alps are used to demonstrate both the potential and the limitations of the probe and of the mathematical methods presented.

Syms, M.C., 1982, Down hole flowmeter analysis using an associated caliper log: Groundwater, v. 20, no. 5, p. 606-610, doi:10.1111/j.1745-6584.1982.tb01377.x.

Abstract: From flowmeter interpretation studies using wells with no available caliper data has emerged information which is also pertinent to flowmeter analysis in conjunction with a caliper log. A simple method for interpreting flowmeter and caliper log pairs incorporating this insight has been developed. Location of constant‐flow regions, inflow and outflow zones is done by manually comparing the shapes of the caliper and flowmeter logs. A flowrate log is then produced using quantitative volumetric flowrates calculated for several depths in the well. The log contains all the significant flow information without spurious noise and would be suitable for converting to an apparent hydraulic conductivity log.

White, J.E. and Lessenger, M.A., 1988, Caliper effect on borehole coupling: Exploration Geophysics, v. 19, no. 2, p. 201-205, doi:10.1071/EG988201.

Abstract: The mechanism of low-frequency Borehole Coupling from the literature is here applied to a borehole of non-uniform radius. It is noted that in addition to the pressure signal in a fluid-filled borehole coincident with the passage of an external plane compressional wave, there are waves created at any change in radius which arrive later as coherent noise. This is illustrated first with a single step-change in radius. An example using the caliper log from an oil well shows that these delayed waves create a noise train of substantial amplitude relative to the direct compressional wave. It is pointed out that a down-hole pressure source will radiate a noise train due to caliper changes which will follow the direct compressional wave with exactly the same waveform as that encountered in Borehole Coupling.

References

Collier, H.A., 1993, Caliper Tools, in Borehole Geophysical Techniques for Determining Water Quality and Reservoir Parameters of Fresh and Saline Water Aquifers in Texas: Austin, Texas, Texas Water Development Board, v. 1, p. 257-265.