PowerPoint Presentation

Lecture 2

Well logging and interpretation2nd year, By: Dr. Pavel Spirov1

Outline:

-Logs-Borehole environment-Well logging tools2

LOGGING BASICSWell logging is the process of recording various physical, chemical, electrical, or other properties of the rock/fluid mixtures penetrated by drilling a borehole into the earth's cruste. A log is a record of a voyage, similar to a ship's log or a travelog. In this case, the ship is a measuring instrument of some kind, and the trip is taken into and out of the wellbore.

In its most usual form, an oil well log is a record displayed on a graph with the measured physical property of the rock on one axis and depth (distance from a near-surface reference) on the other axis. More than one property may be displayed on the same graph.

Well logs are recorded in nearly all oil and gas wells and in many mineral and geothermal exploration and development wells. Although useful in evaluating water wells, few are run for this purpose. 3logs

Mechanical Methods - Caliper LoggingAcoustic Logging Electrical Methods Radioactive MethodsDensity Logging

45Intro to Well Logging Restored

Mechanical ToolsCaliper

Lithology ToolsSpontaneous PotentialGamma Ray

Fluids Identification ToolsResistivityLaterologInductionPetrophysical ToolsPorosityNeutronDensitySonic

Lecture 4

Kellyville Training CenterPage # 15The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples or with low magnification microscopy, such as colour, texture, grain size, or composition.[1][2][3] It may be either a detailed description of these characteristics or be a summary of the gross physical character of a rock.[4] It is the basis of subdividing rock sequences into individual lithostratigraphic units for the purposes of mapping and correlation between areas. In certain applications, such as site investigations, lithology is de6Intro to Well Logging Restored

Lecture 4

Acoustic Logging Electrical Methods Radioactive MethodsDensity Logging

Kellyville Training CenterPage # 16The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples or with low magnification microscopy, such as colour, texture, grain size, or composition.[1][2][3] It may be either a detailed description of these characteristics or be a summary of the gross physical character of a rock.[4] It is the basis of subdividing rock sequences into individual lithostratigraphic units for the purposes of mapping and correlation between areas. In certain applications, such as site investigations, lithology is deCreating of well log

Up-hole instruments

Down-hole instruments

The Downhole logging instrument

Basically, the equipment needed for wireline logging consists of the following:Up-hole instruments (Logging Unit and Rig)Down- hole instruments (Sondes and cables)A logging unit and an instrument for taking measurements. For land rigs, the unit is mounted on a truck. The unit has a winch and an electrical wireline cable. The Downhole logging instrument, called a sonde, is attached to the end of the cable. The unit also includes equipment to power the sonde and to make a permanent record of the log.

7A well log recording

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Borehole environment & drilling

The nature of the fluids in the pore space immediately surrounding the borehole depends upon the amount and type of mud filtrate that invades the formation.

Wireline log measurements respond to rock properties and the properties of the fluids in the pore space. The nature of the fluids in the pore space immediately surrounding the borehole depends upon the amount and type of mud filtrate that invades the formation.During the drilling process, drilling mud is pumped down the drill pipe and up the annulus between the drill pipe and the formation. In formations with sufficient permeability, water will infiltrate the formation, leaving mud cake on the side of the well bore.This occurs because the pressure in the mud column is greater than the formation pressure.11Drilling fluidsDrilling mud - RmMud filtrate - RmfMud cake Rmc

Drilling fluids generally affect logging tool response. The logging tool response is effected by: -Tool designBorehole sizeMudcake thickenessDepth of invasionMud type: oil, water

Electric properties of mud differ from formation, creating a considerable resistivity contrast. Borehole size and shape

In gaugeBorehole diameter is equal to drilling bit size

Borehole reductionMud cake buildup, precursor to permeable formation

Borehole enlargementSwelling and sloughing of shalesCollapse of poorly cemented porous rockDissolution of salts, evaporates

Logging tools are typically calibrated on 8 borehole. Correlations necessary for smaller and larger borehole sizes.

Wireline log measurements respond to rock properties and the properties of the fluids in the pore space. The nature of the fluids in the pore space immediately surrounding the borehole depends upon the amount and type of mud filtrate that invades the formation.During the drilling process, drilling mud is pumped down the drill pipe and up the annulus between the drill pipe and the formation. In formations with sufficient permeability, water will infiltrate the formation, leaving mud cake on the side of the well bore.This occurs because the pressure in the mud column is greater than the formation pressure.13Logging speedLogging speeds are by no means the same for all types of log.Since natural and induced radio-active phenomena are random by nature, it is necessary to accumulate count data over a period of time and compute the mean in order to obtain a representative reading. This accumulation or sampling period corresponds tothe Time constant

Other factors which limit logging speed are galvanometer inertia (they must have sufficient time to deflect to the full value, which precludes very highlogging speeds where high contrasts in readings are to be encountered), and various safety considerations, particularly cable tension and the risk of damage to pad-type equipment

Hostile environment

Well-bore temperature and pressure increase with depth as a function of the geothermal gradient, and mud density, respectively (Figs. 2-10 and 2-1 1). Logging tools must be able to withstand extreme hole conditions which might be encountered. (Freshly circulated drilling mud may initially be considerably cooler than the formations with which it is in contact.

The warming-up period may provide a short safe period for logging very hot wells.)

The presence of corrosive gases, such as hydrogen sulphide, may require special precautions, and resistant equipment (H,S-proof cable, for instance).

Logging companies provide the operating limits for each tool. Outside these limits, there is the risk of breakdown or destruction of the equipment by temperature failure of electronic components, leaking of mud past pressure seals, collapse of the pressure housing, and so on.InvasionThe pressure differential between the mud in the annulus and the formation fluid pressure, forces drilling fluid into the formation.

Invasion

Formation damage near the wellbore

Invasion environment

The saturation profile of the invaded zone must be taken into account when interpreting wireline logs.

The invaded zone is composed of a flushed zone and a transition zone, and the dimensions of the transition zone will vary with time due to diffusion. In a water-bearing formation, the mud filtrate will completely flush the formation water near the well bore, and there will be a transition zone between the flushed zone and undisturbed formationFluid composition of the invaded zone. In a permeable water-bearing formation, the mud filtrate will completely flush the formation water near the well bore, and there will be a transition zone betweenthe flushed zone and the undisturbed formation

19Invasion

Invasion profile water zone

20Invasion

Invasion profile oil zone

21TEMPERATURE MEASUREMENTS (TEMPERATURE LOGS)Generally temperature increases with depth and in undisturbed conditions it has a rate of increase with depth known as the geothermal gradientThis gradient varies according to the geographical location and the thermal conductivity of the formation.

The gradient is generally low in formations of high thermal conductivity (saltor anhydrite for example) and high in the opposite case (e.g. shales)

Thermometry

Application of geothermal gradient

Open hole

Temperature measurements allow the definition of changes in the geothermal balance (geothermal energy, thermal flux, maturing organic matter, etc.). We can then define the average geothermal activity of awell or a zone.

The thermal equilibrium destroyed by drilling reestablishes itself more or less quickly according to thethermal conductivity of the rock. Variations in temperature can then give some indication of lithology

We can locate lost circulation zones or, on the other hand, fluid flow into the well, and in particular, gas, which is detected by the cooling effect brought about by gas expansion.

Application of geothermal gradient

Example:

Application of geothermal gradient

Cased hole

The main applications of temperature logs are in cased holes and in particular in production logging:

(a) detection of the cement height behind casing (Fig. 18-12) and channeling zones (fluid circulation behind pipe);

(b) detection of producing zones (Fig. 18-13);

(c) determination of the depth of the bubble point;

(d) detection of zones of fluid injection entry

Pressure change with depth

Fluid Pressure

Types of logs30

Opened hole logsCased hole logs

Mud log no cable

The geology log

Measurements while drilling (MWD) or logging while drilling (LWD)

composite log Logs run in a hole which has just been drilled, and before it is cased, are called open-hole logs. Logs run after the well is cased are called cased-hole logs. Open hole logs are mainly used to determine the petrophysical properties of the rocks. Some cased hole logs are used for the same purpose. Others are used to assess the integrity of the well completion; others are used to assess fluid flow into the well

Other types of logs require no cable, such as a mud log which may record up to 5 or 10 properties of the drilling fluid, or a drilling log which records the rate of penetration and other functions of the drilling process

The geology log, often called the stratigraphic log, strat log, or sample description log, is a record of the rock samples retrieved from the drilling mud, and is one of the primary sources of rock and fluid descriptions for the well. It consists of a verbal description of the rock type as well as qualitative or interpretive data coincerning evidence of the fluid content of the rock. These are all useful logs and are used in any analysis of a well, if they are available

Most logs can now be recorded while drilling is going on or while tripping the drill pipe. This is called measurements while drilling (MWD) or logging while drilling (LWD). Open-hole logs require that the drill string be removed from the well bore before the logging tools can be lowered into the hole. MWD does not have this need, so measurements are available continuously as drilling proceeds.

A composite log is made up of measurements and interpretations from many sources of data. It is usually made up in the office in a standard format (for the company or agency who owns the well). Since it compresses a great deal of data onto one log, it is often one of the most used items in the well file31

Most open and cased-hole logs are recorded continuously as the tools are pulled out of the hole. A few logs, however, may only be recorded when the tool is stationary in the hole, such as the gravity meter survey. Such logs are called station-by-station logs as opposed to continuous logs

Most open hole logs are run in a conductive mud system. Muds with relatively high resistivity are called fresh muds, and those with low resistivity are called salt muds

Salt muds may be salted on purpose to reduce erosion in shales or solution of salt beds while drilling through them.

Oil-base muds are non-conductive and cause a few problems, but not many are serious. You cannot run SP, microlog, microlaterolog, or laterlog because they need conductive mud. Dipmeter and Formation Micro Scanners need scratcher pads but otherwise operate normally. Sonic, density, neutron, gamma ray, NMR, caliper, induction logs all work normally.

Logs are used for a variety of purposes depending on the nature of the data gathered. Correlation from well to well is the oldest and probably the most common use of logs. It allows the subsurface geologist to map formation depths and thicknesses and then to identify conditions that could trap hydrocarbons. 32

One of the important uses of logs today is the determination of rock porosity. This measurement is significant because it tells how much storage space a rock has for fluids. No log actually measures porosity directly, but many analytical methods are available to help estimate this important property

Another of the routine uses of logs is the determination of the water, oil, or gas saturation in the rock pores

One of the older, but very useful, surveys is the caliper log. In open hole logging, it is used to determine hole volume and aids the engineer in designing a cementing program.It also indicates mud cake build-up and hole wash-out. Both of these indications are of interest to the log analyst when he considers the other logs. In cased hole work the caliper is often used to find casing damage and separated casing.

A more recent development in logging is fracture-finding

When it is time to perforate the casing to allow fluid to flow into the well, there may be some doubt about how well the perforator depths match the log depths. To overcome this uncertainty, a casing collar gamma ray log is often run

Another common use for this type of measurement is the determination of water input profiles in water injection wells.Usage of logs33The temperature log is commonly used to indicate the top of cement behind a newly cemented string of casing.

Another use for the temperature log is the location of points of fluid entry in a well bore or of fluid flow behind casing. As the fluid enters the well it expands and cools creating abnormally low temperature in the well at the point of entry.

Acoustic noise logs also find flow entry and flow behind pipe by the noise caused by the flowing fluid.

The most significant change in the use of logs, in recent years, is production monitoring. The thermal decay time log (often called a pulsed neutron log) allows for the interpretation of porosity and fluid saturation behind casing.

Neutron log:The fluid saturation will change over time as a reservoir is depleted by production, and the changes may be monitored by logging at regular intervals, say once a year. If the production pattern is not as predicted, remedial action may be possible. The log is also used to provide porosity and fluid saturation data in wells which are not, or could not, be logged in the more conventional open-hole manner.

A large suite of logging instruments is available to evaluate fluid type, fluid flow, and mechanical conditions in producing or injecting wells.The same logs that are used to evaluate porosity and water saturation in oil and gas wells are also used to evaluate other resources such as ground water, coal, potash, salt, uranium, oil shale, gypsum, sulfur, geothermal energy, tar sands, and hard rock minerals.

Usage of logs34

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LOG SCALES and LAYOUTLogs can be run on a number of vertical (depth) scales and quite a variety of horizontal (curve value) scalesEnglishMetricOften CalledTerminologyTerminologyDetail scale or large scale5" = 100 ft1:240 1:200 is also very commonCorrelation scale or2" = 100 ft1:600 1:500 is also very commonsmall scale1" = 100 ft1:1200 1:1000 is also very commonSuper detail scale10" = 100 ft 1:120 1:100 is also very common*25" = 100 ft1:48 1:50 is also very commonDipmeter scale60" = 100 ft1:20Common Logging Scales36Correlation

The spacing between depth grid lines is 10 feet or 5 meters for correlation scales and 2 feet or 1 meter for other scales. Heavier grid lines appear every 10 feet (5 meters) and every 50 feet (25 meters).

Correlation

Logs presentationLogs are presented in the field in a three track presentation. The pair of tracks 2 and 3 is often called track 4, which is used to record curves with a large range in values

Four (or more) track presentations (with all tracks to the right hand side of the depth numbers) is created in the course of computer processed log analysis. These can be generated in the computer on the logging truck or in the office. Some logs recorded prior to 1946 have only two tracks, and logs run for special purposes (eg. potash) have three tracks all to the right of the depth numbers

Well logging toolsThe invaded zone is composed of a flushed zone and a transition zone, and the dimensions of the transition zone will vary with time due to diffusion. In a water-bearing formation, the mud filtrate will completely flush the formation water near the well bore, and there will be a transition zone between the flushed zone and undisturbed formationFluid composition of the invaded zone. In a permeable water-bearing formation, the mud filtrate will completely flush the formation water near the well bore, and there will be a transition zone betweenthe flushed zone and the undisturbed formation

40CaliperMechanical Methods

Caliper Logging

The caliper tool and log is used to determine the shape and size(diameter) of a drilled hole. It measures variations in the borehole diameter.

- There are two main types of caliper log, the independent caliper and the attached caliper. The first provides detailed information about the conditions of the drilled hole. It usually has small tips or contact and has a large contact pressure for actual hole diameter determination.

- The caliper tool can have from 2 arms to up to several arms. The choice of which caliper tool to use depends on the nature of the borehole, as well ashow critical the value must be. 42Mechanical Methods

Caliper Logging

There are two main types of caliper log, the independent caliper and the attached caliper. The first provides detailed information about the conditions of the drilled hole. It usually has small tips or contact and has a large contact pressure for actual hole diameter determination.43Mechanical Methods

Caliper Logging

Measures the variations in the size of well boreThese variations can affect the accuracy of the other measurementsBased on these measurements, the pc recalculates the readings from the other tools

44Mechanical Methods

Caliper Logging

-in addition to providing information on the diameter of the borehole can also give information on the lithology of the well (if used correctly)

- It can also give details of fractures, given that if a pair of the caliper arms locks into a fracture, the tool rotation ceases.

The caliper log, in addition to providing information on the diameter ofthe borehole can also give information on the lithology of the well (if used correctly). If there is a weak formation, the washouts can provide this data. It can also give details of fractures, given that if a pair of the caliper arms locks into a fracture, the tool rotation ceases.45GEOLOGICAL FACTORS INFLUENCING HE HOLE DIAMETER

The diameter of the hole depends mainly on:

(a) The lithology, since certain rocks may:

(1) be soluble in the drilling mud for example salt;

(2) disintegrate and cave in (for example sands, gravel, shales), in which case hole caves will appear;

(3) flow, as in the case of swelling shales or low compaction shales, when the hole willclose in;

(4) be consolidated, in which case the hole will be in gauge.

(b) The texture and structure of the rock

Caliper log data

Interpretation