Scratching devices as casing scratchers are external devices designed to remove immobile mud from the wellbore and mud cake from the wellbore wall (Fig. 1). These devices are categorized by how aggressive they are.
Casing Scratchers Types
The basic springloaded bristle scratcher is more aggressive than the cable scratcher (also known as a wellbore wiper). Scratchers are also designed for casing reciprocation and rotation. Reciprocating devices, consisting of a collar with radial wires or cables, are designed to remove mud (check also: mud conditioning) during pipe reciprocation. The up-and-down movement causes the wires or bristles to disturb the wellbore wall.
Rotating devices are straight strips of steel containing longitudinal wires or cables. They are attached lengthwise to the casing to remove mud while rotating. Scratchers and wellbore wipers are most effective when the casing is centralized and manipulated before and during cementing in drilling.
Casing Scratchers Spacing
To prevent solids or mud cake buildup, scratchers should be spaced to ensure overlapping of areas worked by adjacent scratchers. It is very important that circulation be established before pipe movement. Reciprocating scratchers may be allowed to float between the stop collars. Rotating scratchers are attached with special clamps that are usually different from normal stop collars.
Take The Scratchers Advantage
The use of scratching devices has declined in recent years. The problem is not related to their effectiveness sometimes they work too well. Mud filter cake that is removed from the wellbore wall while running the pipe adds solids to the displaced mud column and increases fluid loss to the formation as the cake is rebuilt. Both of these occurrences increase the hydrostatic pressure exerted by the fluid column.
Casing scratchers are most effective when the drilling engineer takes their effects into account. About every 10 to 15 joints of pipe, one should stop and circulate the annulus to move the displaced solids up into the previous casing string or to surface. During the displacement of the primary cementing process, pipe movement is still important because cement filter cake takes the place of the removed mud filter cake.
Conclusion
Scratchers and hole wipers are another subject of controversy amongst drillers. There are various types that require reciprocation or rotation of the casing string. As originally conceived they were supposed to remove wall cake from the wellbore to allow good bonding of the cement to the wellbore wall. Probably no one still believes that anymore because the wall cake reforms almost immediately, but they do definitely free up large accumulations of mud that has gelled and will not move under normal circulation conditions. For that reason alone, they are quite useful and should be run except in horizontal wells or highly deviated wells where pipe movement is not possible.
Notes:
- Scratchers and wipers are of little benefit, unless they are put to work by casing movement.
- Use scratchers and centralizers on the tailpipe when the hole is not excessively washed out.
- To minimize cement channeling, Jones and Berdine (1940) proposed casing centralization. They also found effective ways to remove the mud filter cake, including fluid jets, scrapers or scratchers, casing reciprocation, and pumping acid ahead of the cement slurry.
- Mechanical devices such as scratchers, scrapers, and cable wipers (Chapter 11) also improve the efficiency of the displacement process when used in combination with casing movement (Jones and Berdine, 1940; Teplitz and Hassebroek, 1946). Especially, In cases in which mud removal difficulties are expected (e.g., hole irregularities, high gel strength, poor fluid-loss control, and poor casing centralization)
References:
- Petroskills Casing Cementing Course
- Well Cementing Second Edition – Erik B. Nelson and Dominique Guillot