- Branża: Oil & gas
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A record of the holdups of gas, oil and water in a producing well using a combination of nuclear measurements recorded by a pulsed-neutron spectroscopy device. The technique is used mainly in deviated and horizontal wells, where the complex flow regimes cause conventional holdup measurements to be inaccurate. <br><br>The pulsed-neutron spectroscopy measurement is processed to obtain the volume of oil in the borehole rather than the oil in the formation as in a conventional carbon-oxygen measurement. The gas holdup is determined from the ratio of counts received by the near and far detectors in either the inelastic or capture mode, a technique similar to the compensated-neutron log. The water holdup is the remaining fraction. Alternatively, the capture cross section of the borehole can be determined from the pulsed-neutron capture measurement. If the water is saline, and its salinity is known, the water holdup can be determined directly.
Industry:Oil & gas
A record of the fraction of gas present at different depths in the borehole. Although several techniques may be used for this purpose, the term usually refers to logs based on one of two principles. In the first, four or more optical probes are used to detect the passage of gas bubbles at different points across the borehole. As with other local probes, holdup is determined by the fraction of time the probe detects gas. In the second technique, a <sup>57</sup>Co (cobalt) source emits low-energy gamma rays that undergo backscattering and photoelectric absorption in the borehole fluid before being counted in a detector. The number of counts is related to the fluid density, and can be calibrated in terms of gas holdup. <br><br>The first technique produces an image of gas holdup along and around the borehole, while the second technique produces a log of the average holdup along the well.
Industry:Oil & gas
A record of the difference in temperature between two vertical points in a well. Most differential-temperature logs are obtained by differentiating a normal temperature log with respect to depth. Some are obtained by recording the difference in temperature between two vertically displaced sensors. Note that the differential-temperature log and the radial differential-temperature log are not the same.
Industry:Oil & gas
A record of the density, or changes in density, of fluids in a production or injection well. Since gas, oil and water all have different densities, the log can determine the percentage, or holdup, of the different fluids, directly in the case of biphasic flow, and in combination with other measurements for triphasic flow. Fluid density is measured by a gradiomanometer or a nuclear fluid densimeter, and can also be derived from the depth derivative of a pressure sensor.
Industry:Oil & gas
A record of the difference in temperature between the opposite sides of the internal wall of a casing. The log is mainly used to detect a channel in the cement, since the fluid moving in the channel is likely to be cooler or warmer than its surroundings. The two temperature probes are held on arms that are extended to touch the casing wall at depths where a channel is suspected. The assembly is then rotated through 360<sup>o</sup> to give the radial differential-temperature log. A sinusoid indicates a channel. Temperature differences are small, typically 0. 005 to 0. 05<sup>o</sup>F (0. 003 to 0. 03<sup>o</sup>C), but can be enhanced by injecting cooler fluids from surface.
Industry:Oil & gas
A record of the density in and around a completed well using a radioactive source of gamma rays and a detector. The log is recorded with a nuclear fluid densimeter. Originally, photon logs were run to determine the size of salt caverns. More recently, they have been run to evaluate the quality of gravel packs and sand cavities, and are then synonymous with gravel-pack logs.
Industry:Oil & gas
A record of the change in temperature along a well, normally recorded by a fiber-optic cable. The distributed temperature is measured by sending a pulse of laser light down the optical fiber. Molecular vibration, which is directly related to temperature, creates weak reflected signals. These signals are detected at the surface and converted to a log of temperature along the well, sampled approximately every 1 m (3. 28 ft) with a resolution of 0. 1<sup>o</sup>C. The fiber-optic cable is normally installed at the time of well completion, so that the distributed-temperature log can be recorded at any later time without well intervention. <br><br>Introduced in the mid-1990s, the technique can also be used to measure flow rates by creating a temperature transient and observing its movement along the well.
Industry:Oil & gas
A record of one or more in-situ measurements that describe the nature and behavior of fluids in or around the borehole during production or injection. Production logs are run for the purpose of analyzing dynamic well performance and the productivity or injectivity of different zones, diagnosing problem wells, or monitoring the results of a stimulation or completion. The term is sometimes extended to include logs run to measure the physical condition of the well, for example cement bond and corrosion logs. <br><br>The earliest production logs consisted of temperature logs (1930s) and flowmeters (1940s), to which were soon added fluid-density and capacitance logs (1950s). Flow-rate measurements were gradually improved by the development of tracer logs and improvement to the basic spinner flowmeter. <br><br>These techniques were adequate for near-vertical wells with single or biphasic flow, but could be misleading in highly deviated, and especially horizontal, wells. New techniques were developed starting in the 1980s. These techniques focused on local probes to measure holdup at different points in the borehole, nuclear techniques to analyze the total holdup of all three phases, and phase-velocity logs for the analysis of individual fluids. At the same time, complex flow structures and flow regimes have been studied more extensively using flow loops.
Industry:Oil & gas
A record of elemental concentrations derived from the characteristic energy levels of gamma rays emitted by a nucleus that has been activated by neutron bombardment. In the context of production logging, the term normally refers to the activation of silicon and aluminum to determine the quality of a gravel pack. Silicon and aluminum are activated by a neutron source to produce isotopes that decay with a half-life of 2. 3 minutes emitting a 1. 78 MeV gamma ray. These gamma rays are counted in a detector placed below the source, with a high count indicating a high quantity of silicon in a sand pack, or aluminum in a bauxite pack. The log is run slowly so that oxygen and other activated elements have decayed before the detector crosses the activated interval. <br><br>The carbon-oxygen log, elemental-capture spectroscopy log, pulsed-neutron spectroscopy log, aluminum-activation log and the oxygen-activation log are also examples of neutron-activation logs.
Industry:Oil & gas
A reaction by-product. In sandstone acidizing, the reaction between hydrofluoric acids (HF) or spent HF acids with formation minerals can precipitate nondamaging products, such as silica, borosilicates or fluoborates. However, other insoluble or difficult to remove by-products can create formation damage. <br><br>Ferric iron (Fe<sup>+3</sup>) and ferrous iron (Fe<sup>+2</sup>) are potential sources for precipitates. Ferric iron present in some formation minerals, including chlorite and glauconite clays, and in tubing rust (iron oxide) can precipitate as ferric hydroxide (Fe(OH)<sub>3</sub>), which is a gelatinous, highly insoluble mass that can plug pore channels and reduce permeability. The precipitation of ferric hydroxide or ferrous hydroxide (Fe(OH)<sub>2</sub>) depends on the pH of the spent acid. The former needs a pH higher than 2. 2, while the latter requires a pH higher than 7. 7. Since the maximum pH for a spent acid is approximately 5. 3, the precipitation of ferric hydroxide is more common. Iron-sequestering or iron-reducing agents can be used in acid to maintain the ferric iron in solution. <br><br>Calcium fluoride (CaF<sub>2</sub>) precipitates when HF contacts calcite or any other calcium source, and alkali-fluosilicates or iron sulfide form crystal-like by-products that can bridge pore throats. Additionally, some sequestering agents, corrosion inhibitors or friction reducers can also form residues that may plug formation pores. <br><br>The formation of precipitates can be avoided or reduced by using a preflush, which dissolves calcareous material, iron rust or iron scales, and displaces formation brines (K, Na, Ca ions) away from the wellbore, thereby reducing the formation of CaF<sub>2</sub>, ferric hydroxide and alkali-fluosilicates.
Industry:Oil & gas
