Borehole effect

The caliper is used to correct the density and neutron porosity measurements for borehole effects and also can be used as a borehole stability indicator. Figure 4-289 shows an example of comparison between the MWD ultrasonic caliper and the four-arm wireline caliper run five days later. 

Baildown test results do not always conform to the theoretical response anticipated. This is evident from Figure 6.12, which shows much fluctuation as a result of borehole effects. In these instances, maximum theoretical values can be determined by subtracting the static depth-to-product from the corrected depth-to-water. Thicknesses provided in this manner are conservative in that true thicknesses must be less than or equal to these values and, thus, this method overestimates the actual thickness by an amount equal to the thickness of the capillary zone. 

Variable accumulation rates of LNAPL caused by borehole effects ... 

In all the cases the borehole environment is not modeled. The reason for excluding borehole effect is to compute logs that are borehole-corrected like the AIT field logs. 

The pores between the rock components, e.g. the sand grains in a sandstone reservoir, will initially be filled with the pore water. The migrating hydrocarbons will displace the water and thus gradually fill the reservoir. For a reservoir to be effective, the pores need to be in communication to allow migration, and also need to allow flow towards the borehole once a well is drilled into the structure. The pore space is referred to as porosity in oil field terms. Permeability measures the ability of a rock to allow fluid flow through its pore system. A reservoir rock which has some porosity but too low a permeability to allow fluid flow is termed tight . 

The annular space between the outer pipe and the surrounding rock is filled with cement over the whole depth up to the ground in new wells. The purpose of this is to seal the deposits at the top and to keep the fresh water and salt water zones separate. In addition, it serves as a protection against pressure from the rock and as corrosion protection which, however, is only effective so long as there is no current exit caused by extended corrosion of cells or due to foreign anodic influences. The cement Ailing of the borehole casing is usually not uniformly spread over the pipe surface. It has to be remembered that there can be sections which are either not covered or are only thinly covered with cement. 

Electronics Production of circuit boards (producing contacts in boreholes), modified electrolytic condensers, modified field effect transistors, molecular electronics (unidirectional conductors), photostructural lacquers based on ICPs (electron beam lithography), novel photoluminescent diodes (LED), data storage (e.g. spatially resolved eleclrochromics)... 

As indicated in Table 31.4, the potential of ICPs is in somewhat esoteric applications. In some instances the potential has reached commercial realisation. For example, coating the walls of boreholes in circuit boards before electroplating with copper involves fewer stages than with older established processes and is claimed to be cost effective, faster and simpler. ICPs are also now being marketed in Japan for use in solid electrolyte conductors. 

The MWD sensors are located in a nonmagnetic part of the drill collars. The magnetic collars located several meters away still have an effect by creating a perturbation in the direction of the borehole axis. This introduces an error that is... 

The energy of the detected neutrons has an epithermal component because a high percentage of the incoming thermal neutron flux is absorbed as it passes through a 1 in. of drill collar steel. Furthermore, a wrap of cadmium under the detector banks shields them from the thermal neutron arriving from the inner mud channel. This mainly epithermal detection practically eliminates adverse effects caused by thermal neutron absorbers in the borehole or in the formation, such as boron. 

Torsional vibrations are due to the stick-slip effect of the stabilizers in deviated boreholes. They can be seen at surface as large torque oscillations with a period of 3 to 10 s. Figure 4-308 shows a near-bit stabilizer in a deviated borehole. The stick-slip effect increases with WOB and RPM. 

Figure 4-308. Stick-slip effect due to near-bit fraction in a deviated borehole. (Courtesy Anadrill [113].,)...

The major reason for these effects is of a chemical nature, namely the hydration of clays. Borehole instabilities were observed even with the most inhibitive fluids, that is oil-based mud. This demonstrates that the mechanical aspect is also important. In fact, the coupling of both chemical and mechanical mechanisms has to be considered. For this reason, it is still difficult to predict the behavior of rock at medium-to-great depth under certain loading conditions. 

The efficiency of an explosive in any given rock can be determined by cratering experiments. In these, boreholes are drilled vertically downwards into the rock and loaded with increasing charges of explosive. The effects which are produced are shown in Fig. 14.2. The optimum charge produces... 

Accdg to Cook (Ref 17, p 265), in borehole, well drilled hole, and tunnel blasting, the effective initial state of the detonation products in the work integral (as was first derived by A. Schmidt) ... 

OBMs were developed to address the shortcomings of WBMs. OBMs use nonaqueous base oils/fluids such as diesel or mineral oils as a continuous phase with water (or brine). OBMs have several advantages over WBMs they reduce or eliminate certain borehole problems, reduce the risk of anticipated or unpredicted difficulties, particularly in highly structurally complex and water-sensitive formations, and are time and cost-effective in hole stability problem situations [14]. OBM cuttings, being... 

---