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| Well logging methods |
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A caliper log is a well logging tool that provides a continuous measurement of the size and shape of a borehole along its depth [1] and is commonly used in hydrocarbon exploration. The measurements that are recorded can be an important indicator of wash-outs, cave ins or shale swelling in the borehole, which can affect the results of other well logs.
The caliper tool measures the variation in borehole diameter as it is withdrawn from the bottom of the hole, using two or more articulated arms that push against the borehole wall. Each arm is typically connected to a potentiometer which causes the resistance to change as the diameter of the borehole changes, creating a varying electrical signal that represents the changing shape of the borehole. This variation in output is translated into changes of diameter after a simple calibration and the caliper log is printed as a continuous series of values of hole diameter with depth.
Caliper logs may also be created from both acoustic and electrical borehole image logging. Electrical borehole imaging logs are typical created with 4, 6, or 8 armed tools, which can also be used to create a caliper log. Acoustic borehole imaging tools can create the most accurate and detailed caliper logs, using the pulse echo transit times with an independent acoustic measurement of the borehole drilling fluid velocity, but the range of this type of measurement is often limited by the high acoustic attenuation of the drilling fluid.
The decentralization arm typically present on a density logging tool may also be used to create a crude caliper log, which is limited along a single, high-side axis.
Known challenges with caliper logging include borehole spiralling. The position of the drill bit may precess as it drills, leading to spiraling shapes in the wellbore wall, as if the hole had been drilled by a screw. If the arms of the caliper log follow the grooves of the spiral, it will report too high an average diameter. Moving in and out of the grooves, the caliper will give erratic or periodically varying readings.
The borehole can also change to an oval shape while drilling, which typically occurs in doglegs where the axis of borehole is bending to change the direction of the well. This oval shape can cause a 2-arm caliper log to overestimate the size of the borehole, if its arms get stuck in the long-axis.
In most cases, the borehole's circumference will not be a perfect circle and therefore a caliper tool with several arms is required to obtain a true understanding of the size and shape of the borehole. The current state of the art is six independent mechanical arms that simultaneous record the radial distance from the center of the tool. Abrupt changes in borehole diameter, such as a washout at the bottom of casing, or thin fractures may not be accurately measured due to the geometry or range of the measurement arms.
In petroleum exploration and development, formation evaluation is used to determine the ability of a borehole to produce petroleum. Essentially, it is the process of "recognizing a commercial well when you drill one".
In the oil and gas industry, the term wireline usually refers to the use of multi-conductor, single conductor or slickline cable, or "wireline", as a conveyance for the acquisition of subsurface petrophysical and geophysical data and the delivery of well construction services such as pipe recovery, perforating, plug setting and well cleaning and fishing. The subsurface geophysical and petrophysical information results in the description and analysis of subsurface geology, reservoir properties and production characteristics.
Directional drilling is the practice of drilling non-vertical bores. It can be broken down into four main groups: oilfield directional drilling, utility installation directional drilling, directional boring, and surface in seam (SIS), which horizontally intersects a vertical bore target to extract coal bed methane. In highway engineering these are used on hard rock formations and in a rush highway by the cliff excavation, these are bored to the rocks and then TNTs are inserted.
Well logging, also known as borehole logging is the practice of making a detailed record of the geologic formations penetrated by a borehole. The log may be based either on visual inspection of samples brought to the surface or on physical measurements made by instruments lowered into the hole. Some types of geophysical well logs can be done during any phase of a well's history: drilling, completing, producing, or abandoning. Well logging is performed in boreholes drilled for the oil and gas, groundwater, mineral and geothermal exploration, as well as part of environmental and geotechnical studies.
A mud engineer works on an oil well or gas well drilling rig, and is responsible for ensuring the properties of the drilling fluid, also known as drilling mud, are within designed specifications.
A core sample is a cylindrical section of (usually) a naturally-occurring substance. Most core samples are obtained by drilling with special drills into the substance, such as sediment or rock, with a hollow steel tube, called a core drill. The hole made for the core sample is called the "core hole". A variety of core samplers exist to sample different media under different conditions. More continue to be invented on a regular basis. In the coring process, the sample is pushed more or less intact into the tube. Removed from the tube in the laboratory, it is inspected and analyzed by different techniques and equipment depending on the type of data desired.
In geotechnical engineering, drilling fluid, also called drilling mud, is used to aid the drilling of boreholes into the earth. Often used while drilling oil and natural gas wells and on exploration drilling rigs, drilling fluids are also used for much simpler boreholes, such as water wells. One of the functions of drilling mud is to carry cuttings out of the hole.
Logging while drilling (LWD) is a technique of conveying well logging tools into the well borehole downhole as part of the bottom hole assembly (BHA).
A drilling rig is used to create a borehole or well in the earth's sub-surface, for example in order to extract natural resources such as gas or oil. During such drilling, data is acquired from the drilling rig sensors for a range of purposes such as: decision-support to monitor and manage the smooth operation of drilling; to make detailed records of the geologic formations penetrated by a borehole; to generate operations statistics and performance benchmarks such that improvements can be identified, and to provide well planners with accurate historical operations-performance data with which to perform statistical risk analysis for future well operations. The terms measurement while drilling (MWD), and logging while drilling (LWD) are not used consistently throughout the industry. Although these terms are related, within the context of this section, the term MWD refers to directional-drilling measurements, e.g., for decision support for the wellbore path, while LWD refers to measurements concerning the geological formations penetrated while drilling.
Geosteering is the optimal placement of a wellbore based on the results of realtime downhole geological and geophysical logging measurements rather than three-dimensional targets in space. The objective is usually to keep a directional wellbore within a hydrocarbon pay zone defined in terms of its resistivity, density or even biostratigraphy. In mature areas, geosteering may be used to keep a wellbore in a particular section of a reservoir to minimize gas or water breakthrough and maximize economic production from the well. In the process of drilling a borehole, geosteering is the act of adjusting the borehole position on the fly to reach one or more geological targets. These changes are based on geological information gathered while drilling. Originally only a projected target would be aimed for with crude directional tools. Now the advent of rotary steerable tools and an ever-increasing arsenal of geophysical tools enables well placement with ever-increasing accuracy. Typically a basic tool configuration will have directional and inclination sensors, along with a gamma ray tool. Other options are neutron density, look ahead seismic, downhole pressure readings et al. Due to the vast volume of data generated, especially by imaging tools, the data transmitted to surface is a carefully selected fraction of what is available. Data is collected in memory for a data dump when back on surface with the tool.
Petrophysics is the study of physical and chemical rock properties and their interactions with fluids.
Spontaneous potential log, commonly called the self potential log or SP log, is a passive measurement taken by oil industry well loggers to characterise rock formation properties. The log works by measuring small electric potentials between depths with in the borehole and a grounded electrode at the surface. Conductive bore hole fluids are necessary to create a SP response, so the SP log cannot be used in nonconductive drilling muds or air filled holes.
Resistivity logging is a method of well logging that works by characterizing the rock or sediment in a borehole by measuring its electrical resistivity. Resistivity is a fundamental material property which represents how strongly a material opposes the flow of electric current. In these logs, resistivity is measured using four electrical probes to eliminate the resistance of the contact leads. The log must run in holes containing electrically conductive mud or water, i.e., with enough ions present in the drilling fluid.
Sonic logging is a well logging tool that provides a formation’s interval transit time, designated as , which is a measure of a how fast elastic seismic compressional and shear waves travel through the formations. Geologically, this capacity varies with many things including lithology and rock textures, most notably decreasing with an increasing effective porosity and increasing with an increasing effective confining stress. This means that a sonic log can be used to calculate the porosity, confining stress, or pore pressure of a formation if the seismic velocity of the rock matrix, , and pore fluid, , are known, which is very useful for hydrocarbon exploration.
Well completion is the process of making a well ready for production after drilling operations. This principally involves preparing the bottom of the hole to the required specifications, running in the production tubing and its associated down hole tools as well as perforating and stimulating as required. Sometimes, the process of running in and cementing the casing is also included. After a well has been drilled, should the drilling fluids be removed, the well would eventually close in upon itself. Casing ensures that this will not happen while also protecting the wellstream from outside incumbents, like water or sand.
Oilfield terminology refers to the jargon used by those working in fields within and related to the upstream segment of the petroleum industry. It includes words and phrases describing professions, equipment, and procedures specific to the industry. It may also include slang terms used by oilfield workers to describe the same.
In the oil industry, a deviation survey, or simply a survey, is the measurement of a borehole's departure from the vertical, expressed in degrees (°).
The Conductor Pipe is a large diameter pipe that is set into the ground to provide the initial stable structural foundation for a borehole or oil well. The Conductor pipe is the first string of casing and is the largest diameter casing to be installed in a well. It can also be referred to as a drive pipe because it is often driven into the ground with a pile driver.
Ice drilling allows scientists studying glaciers and ice sheets to gain access to what is beneath the ice, to take measurements along the interior of the ice, and to retrieve samples. Instruments can be placed in the drilled holes to record temperature, pressure, speed, direction of movement, and for other scientific research, such as neutrino detection.
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