Drilling fluids hole cleaning

Hole Cleaning. A major function of the drilling fluid is to clean the wellbore by carrying the drilled rock cuttings to the surface. The... 

The objectives of a drilling hydraulics programme are to ensure good hole cleaning, to minimize frictional losses in the flow in the drill pipe and annulus and to minimize the erosion of open hole formations by the shear stress exerted on the borehole wall. Several texts have described drilling fluid (or non-Newtonian fluid) hydraulics in some detail (82-85). The bulk flow properties of the drilling fluid are calculated from the models r = f(y) used to fit the experimental rheological data. 

A mathematical model was developed to describe the effect of viscosity on the stability of fiber-containing drilling fluid sweeps. In order for the fiber to perform as efficiently and effectively as possible as a hole-cleaning aid, it is important that the fiber be homogeneously distributed throughout the base fluid, i.e. minimal separation of the fiber occurs under downhole conditions. Thus, a theoretical study was conducted to determine the desirable base fluid properties to formulate sweep fluids that are stable under downhole conditions. 

The chemistry of cement slurries is complex. Additives will be used to ensure the slurry remains pumpable long enough at the prevailing downhole pressures and temperatures but sets (hardens) quickly enough to avoid unnecessary delays in the drilling of the next hole section. The cement also has to attain sufficient compressive strength to withstand the forces exerted by the formation over time. A spacer fluid is often pumped ahead of the slurry to clean the borehole of mudcake and thereby achieve a better cement bond between formation and cement. 

Tube holes cannot be drilled very close together, since this may struc-tually weaken the tube sheet. The shortest distance between two adjacent tube holes is called the clearance. Tubes are laid out in either square or triangular patterns as shown in Figure S-.i. The advantage of square pitch is that the tubes are accessible for external cleaning and cause a lower pressure drop when shell-side fluid flows perpendicularly to the tube axis. The tube pitch is the shortest center-to-center distance between adjacent tubes. The common pitches tor square putienis arc i-in. OD on... 

These distributors are fabricated of pipe lengths tied to a central distribution header (usually) %vith orifice holes drilled in the bottom of the various pipe laterals off the header. This style of distributor can be fed by pressure or gravity for clean fluids. The gravity feed is considered better for critical distillation application when uniformity of the flow of the drip points (or flow points) through out the cross-section of the tower is extremely important, and is excellent for low flow requirements such as below 10 gpm/ft2 [131]. 

Beyond the close-off depth large drills, either electromechanical (32) or thermal (33), are used. Once more, due to the technical operations, the ice cores suffer contamination problems. In fact, to counteract the enormous pressure encountered at great depth and prevent closure of the hole, it is necessary to fill it with special fluids (usually kerosene mixed with freon or n-butyl acetate), so it is unavoidable that some fluid gets into the core. The contaminated outside layer, once roughly cleaned, can be used to measure oxygen or hydrogen isotopes, but for trace element it is necessary to decontaminate the ice core with the procedure described below. 

Rock cores are examined in two different states. One is a preserved state where attempts are made to retain the original conditions of fluid saturation. This poses problems because the environment of the samples alters from high pressure and temperature to lower values when lifting the rock cores out of the drill hole to the earth s surface. The other approach uses cleaned cores which have been extracted with solvents to remove the reservoir fluids and impurities. These cores are subsequently resaturated with selected fluids to gain the desired measurement conditions. 

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