A Newer Wireline Tool Actually Drills a Plug Out of the Borehole Wall

7/29/2019 A Newer Wireline Tool Actually Drills a Plug Out of the Borehole Wall

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A newer wireline tool actually drills a plug out of the borehole wall, thus avoiding crushing of the sample. Up to 20 samples can be individually cut and are stored inside the tool.

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An engineer or geologist can interpret the log readings to reach certain conclusions about the formation. For example, a decrease in radioactivity from the gamma ray log could indicate the presence of a sandstone formation. An

increase in resistivity may indicate the presence of hydrocarbons. And, an increase in a porosity log might indicate that the formation has porosity and is permeable.

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Allow direct measurement of reservoir propertiesUsed to correlate indirect measurements, such as wireline/LWD logsUsed to test compatibility of injection fluidsUsed to predict borehole stability

Used to estimate probability of formation failure and sand production

Cores from the reservoir allow direct measurements of important reservoir properties. It is important to gather cores from a representative part of the reservoir, as reservoir properties vary horizontally and vertically. If a reservoir is

known to be highly heterogeneous, many core samples will be required to describe the reservoir accurately.

Information from Cores

Standard Analysis

Porosity

Horizontal permeability to air

Grain density

Special Core Analysis

Vertical permeability to air

Relative permeability

Capillary pressure

Cementation exponent (m) and saturation exponent (n)

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Mudlogging is one of the first direct evaluation methods available during the drilling of a well. As such, the mudlog remains an important (but often under-used) source of original information.The rate at which the drill bit penetrates the formation gives qualitative information about the lithology being drilled. For example, in a hard shale the rate of penetration will be slower than in a porous sandstone.

The formation cuttings that are chipped off by the bit travel upward with the mud and are caught and analyzed at the surface. This provides information about the lithology and qualitative indications of the porosity.If hydrocarbons are present in the formation that is being drilled, they will show in the cuttings as o il stains, and in the mud as traces of oil o r gas. The gas in the mud is continuously monitored by means of a gas d etector. This is often

a relatively simple device detecting the total combustible gas content. The detector can be supplemented by a gas chromatograph, which analyzes the composition of the gas.

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All LWD tools have a power supply and data transmission system. Data transmission may be within the downhole assembly from the sensors to a memory device or from the sensors to the surface. The latter is usually achieved by

mud-pulse telemetry, a method by which the data is transmitted from the tool in real time, i.e, as data is being acquired. A pressure pulse in the mud is created by opening and shutting a valve in the drillstring. This is usually

accomplished by using a rotating device at a given RPM. In this manner, a binary data string is transmitted to the surface and is received by a transducer. The binary data string is then interpreted by a computer for log readings

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Well Log Analysis

Openhole logsCased-hole logs


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Wireline logs/LWD represent a major source of data for geoscientist and engineers. Logging tools are used to look for reservoir quality rocks, hydrocarbons and source rocks in exploration wells, support volumetric estimates and

geological modeling during field appraisal and development, and provide a means of monitoring t he distribution of remaining hydrocarbons during the production lifetime.

A large investment is made by oil and gas companies in acquiring log data. Logging expenses can easily amount to 5 to 15 % of total well cost. Accordingly, it is important that the cost be justified by the value of the information

obtained from drilling.

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This figure depicts the basic setup of the logging process. A wireline truck with a spool of logging cable is setup so that the sonde (measuring equipment) can be lowered into the wellbore. The logging tools measure different

properties, such as spontaneous potential and formation resistivity, as the sonde is brought to the surface. The information is processed by a computer in the logging vehicle, and is interpreted by an engineer or geologist

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As mentioned previously, well logs are used to estimate important reservoir properties, such porosity, water saturation, and permeability. The logs can be conveniently grouped into Passive and Active categories.

Passive logs simply measure an existing force or electrical current. For example, the SP log measures a natural electrical current produced when a well full of conductive fluids is drilled through a formation.

Active logs, on the other hand, must initiate a response from the formation. For example, the neutron tool bombards the formation with neutrons, which produces a response that can be measured. The sonic log produces an audio

signal; the travel time of this signal is measured, and reservoir properties can be inferred

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The sidewall sampling tool can be used to obtain small plugs from the formation. The tool is run on a wireline after the hole has been drilled. Some 20 to 30 bullets are fired from each gun at different depths. The hollow bullet will

penetrate the formation and a rock sample will be trapped inside the steel cylinder. When the tool is pulled upwards, wires connected to the gun pull the bullet and sample from the borehole wall.

Sidewall cores are useful for identifying hydrocarbons zones, when viewed under UV light. Qualitative inspection of porosity is possible; however, t he cores may have been crushed during the collection process, so quantitative sampling

of porosity is questionable

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Borehole conditions affect log readings and must be taken into account for accurate log interpretation.As a well is drilled through permeable formations, the drilling fluid (or mud) seeps into the formation and alters the rock properties. A flushed zone occurs where water from the drilling fluid completely displaces the native reservoirfluid. There is also a tr ansition zone where the native

fluids are partially displaced. Usually, an array of resistivity tools is run, each with a different radius of investigation, to account for this phenomena.Drilling mud is specially designed to form a mudcake on the borehole wall to prevent excessive amounts of t he drilling fluid from entering the formation. The resistivity of the mudcake must be accounted for.

Wireline logs are calibrated for a specified hole size. Corrections to the readings must be made for smaller or larger hole sizes.

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Unlike a normal drill bit, which crushes the rock into small pieces, a core bit can be visualized as a hollow cylinder with cutters on the outside. The cylinder of ro ck that is cut by the bit is retained within the core barrel by anarrangement of steel fingers or slips. Core diameters are typically from three to seven inches and are usually about 90 feet long.

Once the core is retrieved from the well, it is common to do a lithologic description at the wellsite. In addition, to keep the core from drying out, it is sometimes sealed in a coating of hot wax and foil.

At the laboratory, the core is photographed under normal and ultraviolet light. (UV light will reveal the presence of hydrocarbons.) Routine and special core analysis may then be conducted

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Well log interpretation can be quite simple, as in the case o f thick, clean sandstone, or it can be very complicated, as in t he case of a f ormation containing several different lithologies and various minerals, or when the bed boundaries

are very close together.

Usually, well logs are analyzed by a computer program, which has been designed to correct the raw data for borehole, bed boundary, and fluid effects. Complex well logs require the attention of an expert to be interpreted properly.

Often, logs are used in combination to arrive at an interpretation. For example, a density-neutron crossplot will give far more information than can be obtained by either log alone

A Newer Wireline Tool Actually Drills a Plug Out of the Borehole Wall

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