Borehole wall

The sidewall sampling tool (Sl/VS) can be used to obtain small plugs (2 cm diameter, 5 cm length, often less) directly from the borehole wall. The tool is run on wireline after the hole has been drilled. Some 20 to 30 individual bullets are fired from each gun (Fig. 5.35) at different depths. The hollow bullet will penetrate the formation and a rock sample will be trapped inside the steel cylinder. By pulling the tool upwards, wires connected to the gun pull the bullet and sample from the borehole wall. 

In a more recent development a new wireline tool has been developed that actually drills a plug out of the borehole wall. With sidewall coring (Fig. 5.36) some the main disadvantages of the SWS tool are mitigated, in particular the crushing of the sample. Up to 20 samples can be individually cut and are stored in a container inside the tool. 

Permeable intervals can be identified from a number of logging tool measurements, the most basic of which is the caliper tool. The caliper tool is used to measure the borehole diameter which, in a gauge hole, is a function of the bit size and the mudcake thickness. Mudcake will only build up across permeable sections of the borehole where mud filtrate has invaded the formation and mud solids (which are too big to enter the formation pore system) plate out on the borehole wall. Therefore the presence of mudcake implies permeability. 

The method of action of the polymers is thought to be encapsulation of drill cuttings and exposed shales on the borehole wall by the nonionic materials, and selective adsorption of anionic polymers on positively charged sites of exposed clays which limits the extent of possible swelling. The latter method appears to be tme particularly for certain anionic polymers because of the low concentrations that can be used to achieve shale protection (8). 

The lateral shocks, the most damaging for the MWD/LWD equipment, are more severe in vertical holes than in horizontal holes. In a vertical hole, the collars or stabilizers hit the borehole wall hard because the gravity does not pull the collar on the low side as in deviated or horizontal boreholes. 

Water tight borehole wall A water tight borehole would mean the same problem as in the casing, but overpressure is more likely to occur in the casing. 

Jet u 200 M drilling fluids required smearing of borehole walls in fine-grained soils and sediments causing sealing Diameter limitations fluid level (water and NAPL) difficult sampling accuracy limited produced fluids... 

Bucket auger u 100 R,I require handling (hazardous if NAPL is encountered) Sampling of borehole wall samples easy can install large-diameter well difficult to control caving... 

CA 73, 5578m (1970) [Explosives sheathed with. resite salt were tested for use in dusty coal mines using simulated mine drifts and boreholes. It was found that for a borehole with hard walls, the best dispersion on detonation of the sheath was obtd when there was an air gap betw the sheath and borehole walls. For boreholes In soft materials, such as clay, the sheath could fit. tightly in the borehole]... 

Another technique to control solids inflow is to maintain a high level of grain-to-grain stress within the borehole wall. This technique was made possible by the practicality of long inflatable formation packers (5, 91). 

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. 

The other extreme of behavior is represented by shales that are highly cemented and unable to swell (awsh constant) and the response to the imbalance of chemical potential is an increase in Psh. The shales will tend to exhibit tensile failure by fracturing and large angular fragments may detach from the borehole wall. These shales, which are usually geologically older, contain mostly illite and kaolinite clays. Intermediate behavior of shale hydration has been observed (157). 

---