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Log Types

• A general explanation of log types, their uses and potential problem areas.

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Cased Hole – Fluid Composition

3/14/2009 2George E. King Engineering

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Cased Hole – Casing/Cement Inspection


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Cased Hole – Correlation/Evaluation


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Locating a Tubing Leak

• High resolution temp tool shows leak by friction heating (high rate leak)

• PLT w/RST waterflow logging tool. Intro trace chemical into the annulus and read with tool.

• Pony tail (frayed nylon rope) on a WL cutter tool (moderate rate leak)

• Run and set a plug going down the well and pressure test above plug until leak is located

• Run downhole camera –inject liquid into annulus & watch for entry into tbg.

• Plug tubing at bottom and fill tbg w/ water. Displace with gas and locate top of liquid. If gas N/A, then use a pump down dart.


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Detection of Crossflow or Underground Blow out

Temperature Survey

Difference in slope of temperature gradient – will detect flow rates down to 25 BPD if liquid and temperatures of fluids are different. Figures on temperature vs. flow distance help estimate water flow in the annulus.

Noise Log Best performance of noise logs is with gas flow. Gas flow to about 10 actual ft3/D (Note – not standard ft3/day). At very low gas flow rates (q<400 actual ft3/D), gas flow can be estimated from millivolts of noise between the 200-Hz and 600-Hz frequencies: q = 0.35 (N200 – N600). Where q is the actual gas flow in ft3 and N = noise log cut at that frequency.

Oxygen Activation Survey

Open hole or channels behind single string. Accuracy is sharply reduced for investigating channels behind two strings (use temp or noise tools). 3/14/2009 6

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Location of Cement Top


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Evaluation of Cement Placement and Bond


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Determination of External Casing Corrosion


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Flow Behind Pipe


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Location of Source of Annular Leak


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Quick View - Openhole Logging• Principally a formation evaluation service

– Performed before setting casing or liner

– Several tools can be run simultaneously

• Two distinct types of application for openhole CTdeployed logging:– In highly deviated and horizontal wellbores

• toolstring can no longer be lowered into well by gravity

– Special applications in vertical wellbores


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Openhole Logging Tools –Quick View

– Gamma ray• lithology identification and correlation

– Dual induction• measures formation resistivity

– Litho density• measures porosity and identifies lithology

– Compensated neutron• measures porosity and identifies lithology• locates gas and fluid contacts

– Borehole seismic• recovers seismic data


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Openhole Logging Tools - Quick View

– Sonic measurement• measures acoustic velocity for porosity• identifies lithology

– Stratigraphic• identifies bed orientation, fracture location, hole direction and

geometry

– Rock sampling• provides side wall cores

– Fluid sampling• retrieves fluid samples under reservoir conditions• estimates permeability


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SP

• One of the oldest and simplest logs• Measures voltage between formations and the fluid

in the wellbore. Potential differences arise due to the difference s between salinity of the formation and the wellbore fluids.

• SP is used for qualitative permeability, reservoir quality evaluation, Rw calculations and zone shaliness estimation.

• Check Rmf value and correct for formation temperature to determine if SP deflections really are a valid indicator of permeability.


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Gamma Ray Tool (Basically a Geiger Counter)

• Uses a scintillation crystal and a photomultiplier tube to measure naturally occurring and artificially induced gamma-ray radiation.

• The gamma-ray radiation is a signature of the formations in a well – very useful in depth control. Used in open hole or pipe

• Also used to spot changes in radiation (NORM scale) and radioactive tracers.

• GR is sensitive to logging speed and factors/statistics that govern radioactive decay.

• Used as one measurement of shaliness, correlation tool for stacked beds and in shales, seal/barrier location, id of organic rich zones, gross and net reservoir thicknesses.


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Capacitance Tool

• Measures the fluids capacitance – uses the wellbore fluid as the fluid between plates of a capacitor. – Determine water entry points

– Identify static fluid interfaces

– Assists production logs


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Resistivity

• Measures resistance difference between formation and wellbore fluids to various depths in the formation.

• The shallowest measurements are indicative of severely invaded zone and the deepest measurements are most reflective of actual formation fluids.


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Resistivity (continued)

• Generally the rock matrix is not conductive, so the resistance of the fluids in the pore space are reflective of invasion.

• Used to establish qualitative permeability, correlate markers, and determine water and hydrocarbon bearing zones.


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ML - Microlog

• A special resistivity tool that measures the resistivity of the mud cake on one curve and the resistivity of the fluids in the formation, but near the wellbore, on a different curve.

• Separation between the curves is an indication of permeability since mud cake builds on permeable zones.

• Tight zones usually show high resistivity readings.


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Density Tool

• Measures electron density of the formation. A Co60

or Cs137 source bombards the formation with gamma-rays. A higher degree of Compton scattering on return is associated with higher density formations.

• Used for porosity estimation, density measurement, lithology determination, indication of gas bearing zones (effect of free gas on density shows a higher porosity / lower density reading than when the formation is wet).


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Density Tool (continued)

• Readout is displayed as bulk density (g/cc) and density porosity (%).

• The porosity reported is a derivation based on departure from an assumed matrix density.

• The pad readouts are very sensitive to hole roughness. Check hole for roughness as part of log Q/C. Tool reads high porosity / low density when pad does not contact the formation. Density variances also cause log readout problems.


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Sonic

• A sonic log measures the interval travel time (delta t) of compression sound waves moving through one foot of a formation (milliseconds/ft). If matrix velocity is known, porosity can be calculated since sound waves travel slower in porous media (travel slower in less dense materials such as fluids).

• Sonic logs are used for porosity determination, pressure determination in shale, as a correlation log, and a gas detector.


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Neutron Tools

• Source emits neutrons into the formation. Neutrons interact with hydrogen nuclei resulting in an energy loss that is converted to neutron porosity. All hydrogen bearing materials (water, oil, gas) contain hydrogen, but the formation usually does not.

• Amount of hydrogen affects reading, so gas filled porosity creates a lower porosity than oil or water filled porosity.


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Neutron Tools (continued)

• The porosity is calculated based on an assumed matrix density. Variances in density and fluid type alter reading accuracy.

• Used for lithology when compared with other logs (density, PE and GR). Also a measure of shaliness. Useful to spot gas (dry vs. wet). Correlation log, porosity evaluation, qualitative measurement of of reservoir depletion (gas pressure affects readings).


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Compensated Neutron

• Radioactive neutron source bombards the formation with high energy neutrons. The high energy neutrons are slowed and captured by atoms of the formation. The low energy neutrons are reflected back to the tool and counted. The amount of neutrons returning is inversely proportional to the porosity of the formation.


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Compensated Neutron

– Locate gas-liquid and evaluate

– Locate hydrocarbon bearing zone

– Determine lithology

– Structural analysis


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Pulsed Neutron Decay

• A neutron source – measures interactions with the elements in the formation. – Basis of carbon-oxygen logs

– Monitors fluid contacts

– Can be run through casing


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PE – Photoelectric Log

• A direct measurement of values present in formations. Under ideal conditions the PE can be read off the log and compared to tables to determine the mineralogy.

• Bad hole conditions, heavy muds, thick mud cake, barium from drilling fluid additives and multiple formation types (stacked pays) can confuse the PE.


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Caliper (Open Hole)

• Measures the roughness or the rugosity of the borehole wall.

• One to four arm calipers are available. Multi arm calipers give an impression of the shape of the wellbore.

• Used for roughness measurement, gauging washouts, hole “gauge”, diameters for hole volume calculations, establishing mud cake thickness (qualitative permeability), identification of sloughing formations (washouts).


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Multi-sensor caliper (cased hole)

• Multi-arm feeler – gauges the shape and smoothness of the casing id.– Corrosion and pit detection

– Pipe collapse and deformation

– Pipe wear


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Collar Locator

• A simple magnetic field that measures a disturbance of thicker (or thinner) metal mass moving through the field.– Typical use is collar location

– Can see profiles

– Locates packers and hangers

– Locates tubing crossovers

– Can see some corrosion problems

– Can see some perforations


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Cement Bond Tool

• Transmitter-receiver tool measures the signal received back from a segment. Helps assess cement presence and bond to the formation and the pipe.


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Temperature Tool

• A recording of the wellbore temperature.– Identify fluid entry or exit– Gas channeling– Tubular leaks– Hydraulic frac height– Cement top– Fluid levels– Differential temperature curve


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"cellar effect - temperature normally cools from surface temperature to about 100 ft below surface, then temperature begins to increase with increasing depth. Watch for

This response is about 2.1oF per 100 ft


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Temperature log – Saaman Well – Trinidad. Leak at packer using annular injection after base line at same rate.


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Gradiomanometer

• Measures the differential pressure of a 24” column of fluid – calculates the fluid density.– Determine gas or liquid entry points

– Identify static fluid interfaces

– Determine exact depth of fluid density change

– Assist production logs


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Flow meter

• Measures fluids moving past the tool. Moving fluids turn the spinner– To determine fluid entry or exit points

– Evaluate perforations

– Evaluate cross-flow occurrence

– Helps evaluate completion design


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Tension Measurement Device

• A component of the tool string that records the tension during logging in various parts of the wellbore. Helps spot increased drag and break free points (tool jumps) that may signal poor logging tool records.

• Used for quality control on logs, as an indicator of depleted zones (fluid loss sticking), dog-legs, location of sticking (above or below tool when compared with a surface record).


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Logging Behind Casing

• Key Elements:– Understand the condition of the casing

– Understand the connection of casing to the formation – cement bond.

– Inclination, diameter, geometry


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Channel Detection Behind Pipe

• Channel logging, with borax solution injection, is accomplished by pumping the solution down the tubing into the liner, allowing the mix to infiltrate the perforations and any channels in the casing formation annuli.

• The initial pass, before borate injection, is made with a thru-tubing Pulsed Neutron logging (or memory neutron) base pass (no Borax), followed by subsequent passes at 20bbls, 40bbls, and 60bbls as the borax solution is pumped away.

• The borax affects neutron capture, leaving a characteristic signature. Overlays of the Sigma/CNL counts reveal the zones of borax penetration, and identify the channels taking fluid.


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Alaska CTD well with good cement isolation over a shale zone


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Comparison of a memory CNL and PNL method. A channel is apparent in the pre and post squeeze logging tracks.

In some cases, the borax logging technique is sufficiently robust to complement or replace temperature logging when looking for behind the pipe channels. (SPE 25383 for Borax logging)

The technique has not been used widely outside Alaska, due to the problems in making the “Borax Brew” and keeping it hot to avoid precipitation of the Borax at colder temperatures (<100degf).

The Borax needs to be mixed at a near critical saturation in order to give the best result. Typically this is 7lb/bbl Borax Pent hydrate and 7lb/bbl NaCl.

Gamma RayGAPI0 100

TVD DepthFEET8950 8850

9500

9600

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DEPTHFEET

Pos

t-Squ

eeze

Per

fora

tions

MCNL Seawater Near Counts0 10000

MCNL Borax Near Cts0 9200

Pre

-Squ

eeze

Per

fora

tions

MCNL Seawater Near Counts0 10000

MCNL Borax Near Cts0 8500

RST Seawater Form. SigmaCU35 0

RST Borax Form. SigmaCU35 0

Source- Alaska PE manual3/14/2009 51

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