Well logging operation

Well Logging Operations

Introduction

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. In the oil and gas industry, the term wire line usually refers to a cabling technology used by operators of oil and gas wells to lower equipment or measurement devices into the well for the purposes of well intervention and reservoir evaluation.

Production Logging uses high technology sensors and software to acquire down-hole data from producing and injecting wells as an input for reservoir management, or for diagnosis of problem wells. This allows the recording of pressure, temperature and fluid-flow parameters directly at the point where the reservoir has been penetrated by the well. Through building knowledge of pressure and temperature regimes, plus understanding different fluid profiles, the production regime can be controlled.

Braided(Wire line cable) line can contain an inner core of insulated wires which provide power to equipment located at the end of the cable, normally referred to as electric line, and provides a pathway for electrical telemetry for communication between the surface and equipment at the end of the cable.

Production logging tools (PLT) can be run either in memory mode on slick line or coiled tubing. A PLT string may comprise a range of several different sensors, as circumstances demand, including (a variety of) flow meters, fluid density, and capacitance, pressure, temperature, gamma ray and casing collar locators. Some sensors use different techniques to derive the same measurement. This data is recorded either at surface (real-time mode), or downhole (memory mode) to electronic data format and then either a printed record or electronic presentation called a well log.

Well logging can be done during any phase of a well's history; drilling, completing, producing and abandoning.

Open hole operations, or reservoir evaluation, involve the deployment of tools into a freshly drilled well. As the tool string traverses the wellbore, the individual tools gather information about the surrounding formations. A typical open hole log will have information about the density, porosity, permeability, lithology, presence of hydrocarbons, and oil and water saturation.

Casedhole operations, or production optimization, focus on optimizing the completed oil well through mechanical services and logging technologies. At this point in the well's life, the well is encased in steel pipe, cemented into the well bore and may or may not be producing. A typical cased hole log may show cement quality, production information, and formation data. Mechanical services use jet perforating guns, setting tools, and dump bailers to optimize the flow of hydrocarbons.

Logging Unit

Logging service companies utilize a variety of logging units, depending on the location (onshore or offshore) and requirements of the logging run.

Each unit will contain the following components:

• Logging unit
• Logging cable & Tools
• Winch to raise and lower the cable in the well
• Self-contained 120-volt AC generator
• Set of surface control panels
• Set of downhole tools (sondes and cartridges)

These components along with other important equipments involved will be discussed below.

Logging Units

Most logging units offer the following features:

• All logs are directly recorded on digital magnetic tape or hard disk.
• Digital telemetry between the tool and the recording unit.
• Computer control of the data gathering allows logs to be recorded either logging up or down, with all curves mutually on depth.
• Calibrations are performed under programmed control more quickly and accurately than in conventional units.
• Logs can be played back with enhanced vertical resolution on a variety of scales (both depth and response scales).
• Well site computation of raw data ranges from completion aids (hole volume integration for cement volumes) to dipmeter computations, full waveform acoustic processing, and complete log analysis

Logging Cable

Logging cables are typically classified as either monoconductor or multiconductor. The monoconductor cables, with a diameter of 1/4 inch, are used for completion services, such as shooting perforating guns, or setting wire line packers and plugs, and for production logging surveys, such as flow meters and temperature logs in producing wells. Multiconductor cables, with a diameter of about 1/2 inch, are used by most logging service companies for recording openhole surveys. The multiconductor cables contain 6 or 7 individual strands of insulated conductors in the core.

The outer sheath is composed of two counter-wound layers of steel wire. Such cable typically has a breaking strength of between 14,000 and 18,000 pounds, and weighs between 300 and 400 pounds per 1000 feet. It is quite "elastic" and has a stretch coefficient of around 1 x l0-6 ft/ft-1b.

Logging Tools

Logging tools are cylindrical tubes containing sensors and associated electronics that can be attached to the logging cable at the logging head. Although there are wide variations in sizes and shapes, a typical logging tool is 3 5/8 in. in diameter and from 10 to 30 ft long. They are built to withstand pressures up to 20,000 psi and temperatures of 300 to 400 F. The internal sensors and electronics are ruggedly built to withstand physical abuse. Modern tools are "modularized" to allow combination tool strings. By appropriate mixing and matching, various logging sensors can be connected with each other. Among the obvious limitations to this method are the difficulty in handling very long tools and the limited information-transmitting power of the cable conductors.

Because logging tools have multiple sensors at different points along their axes, their respective measurements have to be memorized and placed on a common depth reference. Thus, the signal from the sensor highest on the tool must be remembered until the signal from the lowest sensor arrives from the logging depth being memorized.

Cable Head

The cable head is the upper most portion of the tool string on any given type of wire line. The cable head is where the conductor wire is made into an electrical connection that can be connected to the rest of the tool string. Cable heads are typically custom built by the wire line operator for every job and depend greatly on depth, pressure and the type of wellbore fluid.

Electric line weak points are also located in the cable head. If the tool is to become stuck in the well, the weak point is where the tool would first separate from the wire line. If the wire line were severed anywhere else along the line, the tool becomes much more difficult to fish.

Tractors

These are electrical tools used to push the tool string into hole, overcoming wire line’s disadvantage of being gravity dependent. These are used for in highly deviated and horizontal wells where gravity is insufficient, even with roller stem. They push against the side of the wellbore either through the use of wheels or through a wormlike motion.

Measuring Head

A measuring head is the first piece of equipment the wire line comes into contact with off the drum. The measuring head is composed of several wheels which support the wire line on its way to the winch and they also measure crucial wire line data.

A measuring head records tension, depth, and speed. Current models use optical encoders to derive the revolutions of a wheel with a known circumference, which in turn is used to figure speed and depth. A wheel with a pressure sensor is used to figure tension.

Pressure control equipments

The pressure control employed during wire line operations is intended to contain pressure originating from the well bore. During openhole electric line operations, the pressure might be the result from a well kicking. During casedhole electric line, this is most likely the result of a well producing at high pressures. Pressure equipment must be rated to well over the expected well pressures. Normal ratings for wireline pressure equipment are 5,000, 10,000, and 15,000 pounds per square inch.

Flange

A flange attaches to the top of the Christmas tree, usually with some sort of adapter for the rest of the pressure control. A metal gasket is placed between the top of the Christmas tree and the flange to keep in well pressures.

Wireline Valve

A wire line valve, also called a wire line blowout preventer (BOP), is an enclosed device with one or more rams capable of closing over the wire line in an emergency. A dual wire line valve has two sets of rams and some have the capability of pumping grease in the space between the rams to counterbalance the well pressure.

Lubricator

Lubricator is the term used for sections of pressure tested pipe that act to seal in wire line tools during pressurization.

Pump-In Sub

Pump-in subs (also known as a flow T) allow for the injection of fluid into the pressure control string. Normally these are used for well site pressure testing, which is typically performed between every run into the well. They can also be used to bleed off pressure from the string after a run in the well, or to pump in kill fluids to control a wild well.

Grease Injector Head

The grease injector head is the main apparatus for controlling well pressure while running into the hole. The grease head uses a series of very small pipes, called flow tubes, to decrease the pressure head of the well. Grease is injected at high pressure into the bottom portion of the grease head to counteract the remaining well pressure.

Pack-Off Sub

Pack-off subs utilize grease pressure on a rubber sealing element to create an impermeable seal around the wire line. Pack-off subs can be hand pumped or compressed through a motorized pumping unit.

Line Wiper

A line wiper operates in much the same way as a pack-off sub, except that the rubber element is much softer. Grease pumps exert force on the rubber element until a light pressure is exerted on the wire line, cleaning grease and well fluid off the line in the process.

Quick Test Sub

Quick Test Sub (QTS) is used when pressure testing the pressure control equipment (PCE) that will be used during explosives operations. The PCE is pressure tested and then broke at the QTS. The explosives are then attached to the tool string and pulled back in to the lubricator. The PCE is then reconnected at the QTS. The QTS has two O-rings where it was disconnected that can be tested with hydraulic pressure to confirm the PCE can still hold the pressure it was tested to.

Ball-Check Valve

If the wire line were to become severed from the tool, a ball check valve can seal the well off from the surface. During wire line operations, a steel ball sits to the side of a confined area within the grease head while the cable runs in and out of the hole. If the wire line exits that confined area under pressure, the pressure will force the steel ball up towards the hole where the wire line had been. The ball's diameter is larger than that of the hole, so the ball effectively seals off pressure to the surface.

Head Catcher

A head catcher is a device placed at the top of the lubricator section. Should the wire line tools be forced into the top of the lubricator section, the head catcher, which looks like a small 'claw,' will clamp down on the fishing neck of the tool. This action prevents the tools from falling downhole should the line pull out of the rope socket. Pressure is bled off of the head catcher to release the tools.

Tool Trap

It has the same purpose as a head catcher in that it prevents the tools from inadvertently dropping down the hole.

Types of Logs

As mentioned above well logging can be done during any phase of a well's history; drilling, completing, producing and abandoning there are variety of logs can be performed starting with a basic log to the highly sophisticated logs. In this section a brief description of all kinds of logging operation are discussed both in cased and uncased holes.

Open Hole Electric Line Logs

Logs which are taken before the drilled hole is cased.

Natural Gamma Ray Tools

Natural gamma ray tools employ a radioactive sensor, which is usually a scintillation crystal that emits a light pulse proportional to the strength of the gamma ray pulse incident on it. This light pulse is then converted to a current pulse by means of a photo multiplier tube PMT. From the photo multiplier tube, the current pulse goes to the tool's electronics for further processing and ultimately to the surface system for recording. The strength of the received gamma rays is dependent on the source emitting gamma rays, the density of the formation, and the distance between the source and the tool detector. The log recorded by this tool is used to identify lithology, estimate shale content, and depth correlation of future logs.

The 3 1/2" tool, combinable with Open Hole or Cased Hole tools, is available in an extended temperature range version. The slim versions are very compact and rugged tools combining high sensitivity with high resolution for Production Logging applications.

Operating Principle

Natural gamma rays are detected by a high temperature sodium iodide crystal and photomultiplier with bi-alkali cathode. Electronic stabilization of detected energy level is incorporated.

Applications

• Lithology identification.
• Correlation.
• Depth determination.

Fluid Density Logs

The Fluid Density Tool, a 1 11/16" O.D. combinable pulse output precision 'gradiomanometer', can be run separately or as part of the Production Combination Tool.

Operating Principle

The pressure difference along the borehole, caused by the well fluid density, is measured by the displacement force on a float. Acceleration forces are cancelled out by a second float immersed in a high density fluid, giving an up thrust equal to the total weight of the float assembly. The assembly is centered and restrained by a linear motion spring, obviating any friction forces. Float movement is sensed by a linear variable differential transformer.

Modern density tools utilize a Cs-137 radioactive source to generate gamma rays. Gamma rays emitted from the source pass into the formation. Depending on the density of the surrounding formation, some of the gamma rays will be absorbed into the rock while others are reflected back to the tool. The ratio of returning gamma rays to absorbed gamma rays is useful in determining formation density.

Applications

• Fluid density measurement.
• Fluid interface depth measurement.
• Location of water or gas entries.

Resistivity Logs

This tool is important in reservoir evaluation for determining the location of the oil-water contact. Water is far more conductive than hydrocarbons and so will give the reservoir rock it saturates a lower resistivity than rock saturated with hydrocarbons. When analyzing a resistivity log, the point where the resistivity undergoes a large change is likely to be the location of the oil-water contact. It is also used as an indicator for permeability. Since most resistivity tools have different depths of investigation, a permeable formation will read different resistivities at different depths.

Operating Principle

The resistivity tool has wide symmetrical entry and exit ports to allow a free flow of well fluid through the sensor element. This ensures that the fluid being measured is representative of the fluid outside the tool. The sensor measures the dielectric constant of the well fluid and the water holdup. The water holdup when used with Flow meter and Fluid Density tools enables the calculation of water/oil/gas flow in three phase systems.

Determining the remaining oil, gas, and water saturations simultaneously, independent of water salinity, is often necessary for EOR. The triple-fluid method determines these saturations quantitatively using the apparent porosities of the electromagnetic propagation tool (EPT), density, and neutron logs. Propagation tool (EPT), density, and neutron logs.

LWD Tool

Log indicates CBL-VDL along with CCL

In depleted reservoirs that are potential EOR candidates, calculations of the remaining oil and secondary gas saturations are essential. In several reservoirs, the natural water flood was followed by the expansion of a secondary gas cap; hence, oil, gas, and water could be found in the same pores.

The conventional method for determining the oil saturation in the invaded zone is to subtract the gas saturation indicated by the density/neutron log separation from the total hydrocarbon saturation derived from a micro resistivity device.

Logging While Drilling (LWD)

Ever since the first electric log was run in 1927, the oil industry relied on wire line-conveyed logging for the acquisition of formation data for petro physical analysis. It was not until 1978 that the first measurement while drilling (MWD) tools was introduced in the field. The industry did not pay too much attention to them then because they did not offer sufficient information for petro physical analysis. Furthermore, the reliability of such tools was not impressive and their cost was unattractive.

However, over the last 6 years, scientists and engineers have developed Logging-While-Drilling (LWD), a type of well logging that incorporates the logging tools into the drill string, administering, interpreting and transmitting real-time formation measurements to the surface. The LWD tools that are built into special drill collars provide measurements of resistivity, neutron, density, and gamma ray. Significant improvements in the reliability of the tools and competitive pricing, as compared to conventional wireline logging, has offered advantages in running LWD in fields that have drilling and completion constraints (e.g., borehole stability) or difficult well trajectories.

Overcoming well logging challenges presented by directional drilling, LWD has revolutionized the well logging concept. By locating well logging tools near the drill bit on the end of the drilling apparatus, LWD enables drillers to log wells that exceed 60 degrees, which makes pushing the tool through the well impossible. Additionally, by providing real-time information, LWD helps drillers and engineers to make immediate decisions about the future of a well and the direction of drilling.

Providing information on porosity, resistivity, acoustic waveform, hole direction, and weight on bit, LWD transmits logging measurements at regular intervals while drilling is taking place. Data is transmitted to the surface through pulses through the mud column (also known as mud pulse of mud telemetry) in real time.

Cased Hole Electric Line Tools

Logs which are taken after the hole is cased with casing of different sizes.

Casing Collar Locators

Casing collar locator tools, or CCL's, are among the simplest and most essential in cased hole electric line. CCL's are typically used for depth correlation and can be an indicator of line over speed when logging in heavy fluids.

Casing collar locators (CCLs) respond to changes in metal volume such as tubing collars and perforations. As the Enhanced Casing Collar Locator (EMCCL) passes a collar or a change in metal volume, lines of magnetic flux are disturbed between two opposing permanent magnets.

A CCL operates on Faraday's Law of Induction. Two magnets are separated by a coil of copper wire. As the CCL passes by a casing joint, or collar, the difference in metal thickness across the two magnets induces a current spike in the coil. This current spike is sent up hole and logged as what's called a collar kick on the cased hole log.

Features

• Simultaneous operation with other production logging tools
• Standard GO or other connections
• Provides options for memory telemetry, SRO telemetry or no telemetry

Benefits

• CCLs have increased sensitivity and produce log compensated for line speed variations (i.e., with the same appearance regardless of tool velocity)
• Range of CCLs is available for a variety of casing diameters

Cement Bond Tools

A cement bond tool, or CBT, is an acoustic tool used to measure the quality of the cement behind the casing. Using a CBT, the bond between the casing and cement as well as the bond between cement and formation can be determined. Using CBT data, a company can troubleshoot problems with the cement sheath if necessary. This tool must be centralized in the well to function properly.

Two of the largest problems found in cement by CBT's are channeling and micro-annulus. A micro annulus is the formation of microscopic cracks in the cement sheath. Channeling is where large, contiguous voids in the cement sheath form, typically caused by poor centralization of the casing. Both of these situations can, if necessary, be fixed by remedial electric line work.

Principle

A CBT gains its measurements by rapidly pulsing out compression waves across the well bore and into the pipe, cement, and formation. The compression pulse originates in a transmitter at the top of the tool, which, when powered up on surface sounds like a rapid clicking sound. The tool typically has two receivers, one three feet away from the receiver, and another at five feet from the transmitter. These receivers record the arrival time of the compression waves. The information from these receivers is logged as travel times for the three and five foot receivers and as a micro-seismogram.

Recent advances in logging technologies have allowed the receivers to measure 360 degrees of cement integrity and can be represented on a log as a radial cement map and as 6-8 individual sector arrival times.

Gamma Perforating Tools

A cased hole gamma perforator is used to perform mechanical services, such as shooting perforations, setting downhole tubing/casing elements, dumping remedial cement, tracer surveys, etc. Typically, a gamma perforator will have some sort of explosively initiated device attached to it, such as a perforating gun, a setting tool, or a dump bailor. In certain instances, the gamma perforator is used to merely spot objects in the well, as in tubing conveyed perforating operations and tracer surveys.

Gamma perforators operate in much the same way as an open hole natural gamma ray tool. Gamma rays given off from naturally occurring radioactive elements bombard the tool. The tool processes the gamma ray counts and sends the data up hole where it is put onto a log. The information is then used to ensure that the depth shown on the log is correct. After that, power can be applied through the tool to set off explosive charges for things like perforating, setting plugs or packers, dumping cement, etc.

Setting Tools

Setting tools are used to set downhole completion elements. Setting tools are typically large steel tools onto which a downhole completion can be screwed. Setting tools are explosively driven devices. A shooting CCL or a gamma perforator is used to apply power to detonate a low explosive in the setting tool. The gas pressure created by the deflagrating low explosive exerts a large force on a piston holding back oil. The pneumatic pressure of the piston pushes the oil, which hydraulically separates the setting tool from the plug or packer. The downhole completion is now set in place.

Not only for completions, can setting tools also run bridge plugs. Which are most commonly used to abandon a well? A certain amount of oil well cement must then be placed on top of the plug. A bond log is also common protocol, the cement must be bonded with the casing to abandon a well, if not, there must be squeeze guns shot. So they can pump cement down the casing and through the squeeze perforations and to the outside of the casing.

Ultra Sonic Imaging Tool

Weatherford’s ultrasonic micro imager (UMI) provides high-resolution acoustic images of the borehole in water- and oil-based muds. It enables the analysis of fractures, stress and borehole stability studies, and structural interpretation.

The UMI uses two ultrasonic transducers—a main measurement transducer located in the high-speed rotating scanning head and a secondary fluid property transducer located in the tool that continuously monitors the sound velocity and acoustic impedance of the wellbore fluid.

The UMI emits high-frequency acoustic pulses from a rotary transducer that measures amplitude and transit-time characteristics of signals reflected from the borehole wall.

The UMI can be configured with four different field-changeable scanning heads, each optimized to function within a specific open hole bit size to ensure precise imaging. Images are orientated with navigation data either from the borehole navigation (HNC) tool or the high-resolution micro imager (HMI) tool.

Applications

• Fracture identification including drilling-induced fractures
• Differentiation of open and closed fractures
• Borehole profiling and calculation of cement volume
• Stress analysis, borehole stability and breakout analysis
• Structural dip analysis

Features, Advantages and Benefits

• Azimuthally coverage 360° provides a complete borehole image, delivering critical information for the well completion and reservoir characterization.
• Interchangeable heads cover a wide range of borehole diameters, optimizing the borehole image in different well types.
• High-speed motor increases image resolution or logging speed, which reduces rig time.
• Advanced telemetry system enables operation on nonconductor cables, increasing conveyance flexibility.