Wednesday, August 23, 2017

Logging While Drilling Perspective chapter 1

Basic LWD measurements - resistivity, neutron and density porosities and photoelectric factor - have not changed since their introduction, but tool technology has undergone several refinements. These include a range of engineering improvements - from more robust sensor design, to more secure mounting of connectors and integrated circuits. These improvements have led to increased tool durability.

Because the tools are contained in drill collars, hardware takes up space inside the collar, reducing the cross-sectional area available for mud flow. This reduction in area, and erosion of tool components by sand and lost circulation material, mean mud flow rates are lower than in plain collars. However, demand for LWD measurements has increased in wells that require higher flow rates for effective hole cleaning. In response, the maximum mud flow rate for the 6 1/2 -in. tools was recently increased from 450 to 600 gallons per minute (from 28 liters/sec to 38 liters/sec). This upgrade makes the tools practical in wells where mud rate requirements might have precluded them in the past. The upgrade was accomplised by redesigning internal tool parts to allocate more cross-sectional area to mud flow, and by increasing the tool diameter. In 1991, the outside diameter of the CDR (compensated dual resistivity) tool was increased from 6 1/2 and 8 in. to 6 3/4 and 8 1/4 in. , tool is rated to 1200 gal/min (76 liters/sec).

The most fundamental change in the nuclear tool is detector design. The first generation used a combination of helium (He) detectors, also used in wireline tools, and Geiger-Mueller detectors. Compared to Geiger-Mueller detectors, He detectors, have a broader dynamic range, do not need correction for spurious activation, are less affected by borehole salinity and have better statistics, permiting a higher rate of penetration (ROP). But they were not thought to be as rugged as Geiger-Mueller detectors. Field experience proved otherwise, and since 1990 the CDN tool uses helium detectors only. Older tools are being retrofitted.

The CDN tool has a 7.5-curie americium-beryllium neutron source and a 1.7 curie cesium density source, bot connected to a source retrieval assembly. In the first version of the tool, the sources and retrieval head were connected with a flexible steel cable. This has been replaced with a flexible titanium rod, giving more reliable retrieval and more accurate placement of the sources. Also improved is the density detector shielding, which eliminates sensitivity to spurious signals from the mud.

The CDN tool uses a full-gauge stabilizer with windows cut in the blades in front of the density source and gamma ray detectors. When the hole is in gauge, the blades wipe away mud from in front of the sensors, thereby minimizing borehole effects. A locking mechanism is being retrofitted on the stabilizer to increase its resistance to slippage under high torque and jarring.

The caliper is used to correct the density and neutron porosity measurements for borehole effects and can be used as borehole stability indicator. It can be used for downhole detecton of free gas-gas bubbles, not dissloved gas-through a combination of formation and "faceplate" echo signals. The faceplate echo is measured at the surface of the tool, at the mud/window interface. It is affected by gas content in mud, with echo amplitude increasing with gas content. The smallest amount of detectable gas is less than 3% volume of free gas. Real-time transmission of this information can shorten the time needed to detect gas influxes while drilling. This can simplifiy kill operations.

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