Downhole rocks

For the downhole cement to be effective, the bond between the cement and the casing steel as well as downhole rocks should be good. Wagh et al. [9] evaluated the Ceramicrete formulations given in Table 15.2 for bond strength. They used pipe sections made of mild steel API 5L for bond strength tests with the casing steel. The sections had an internal... 

The invasion of particles can be eliminated either by using solids-free systems or by formation of a competent filter cake on the rock surface. If the components forming the filter cake are correctly chosen and blended, they will form a very effective downhole filter element. This ensures that colloidal sized clays or polymeric materials are retained within the filter cake and do not enter the formation. Further protection is provided by ensuring that a thin filter cake is formed due to low dynamic and static filtrate losses. Thus, the cake may be easily removed when the well is brought into production. Additionally, the filter cake can be soluble in acid or oil. 

The downhole turbine motors that are hydraulically operated have some fundamental limitations. One of these is high rotary speed of the motor and drill bit. The high rotary speeds limit the use of downhole turbine motors when drilling with roller rock bits. The high speed of these direct drive motors shortens the life of the roller rock bit. 

Bit Nozzle Sizes. The pressure loss through the bit must be 500 psi with a circulation flowrate of 348 gal/min with 11.6-lb/gal mud weight. The pressure loss through a roller rock bit with three nozzles is (see the section titled Drilling Bits and Downholes Tools )... 

Axial vibrations usually occur in vertical boreholes and hard rock drilling with tricone bits. They can cause top drive shaking, Kelly bouncing, and induce downhole shocks. Axial vibrations can be minimized by changing the WOB and rotary RPM after rotation comes to a complete stop. Change the bit type may also help. 

Bit Wear and Rock Type. Using downhole weight-on-bit and torque the roller cone bit wear can be estimated while drilling. The well site computer can be used to compute in real-time a dimensionless torque (T ) and a dimensionless rate of penetration (R ) using the following equation ... 

Drilling surface data such as weight-on-bit and torque were difficult to interpret because they were loosely related to downhole values. MWD for the first time in the history of drilling gives values of parameters measured at the bit or close by. Rock strength, bit wear, drag and friction can be calculated in real time. Shocks, temperature and pressure can also be measured. 

There are items other than tubulars that must be fished from boreholes. These are drill bit parts, pieces of downhole tools left in the borehole and hand tools accidentally dropped in the borehole. Nearly all of these items cannot be drilled by a normal rock bit. Such items are called junk. 

NMR has proven to be a valuable tool for formation evaluation by well logging, downhole fluid analysis and laboratory rock characterization. It gives a direct measure of porosity as the response is only from the fluids in the pore space of the rock. The relaxation time distribution correlates with the pore size distribution. This correlation makes it possible to estimate permeability and irreducible water saturation. When more than one fluid is present in the rock, the fluids can be identified based on the difference in the fluid diffusivity in addition to relaxation times. Interpretation of NMR responses has been greatly advanced with the ability to display two distributions simultaneously. 

By far the most widespread use of NMR in an on-line production environment is the utilization of downhole exploration tools by petroleum service companies such as Schlumberger, Halliburton, and Baker Hughes. Articles on these unilateral NMR systems are found in the patent databases, " academic literature, and on-line resources provided by the exploration companies. The references provided here are just a few examples in a very prolific field. The technique is applied in high-temperature and pressure situations and currently is used down to a depth of about 10 km (6 miles) to produce a picture of water/oil content in the adjacent rock formations as well as to derive permeability, diffusivity, and hydrocarbon chemistry information. Mobile unilateral NMR systems such as the NMR-MOUSE are also being developed in order to take benchtop NMR systems into the field to perform analysis of geological core samples at the drill site. NMR analyzers are also being developed to determine the bitumen content and properties in tar sand production and processing. " " ... 

Trenches excavated across the veins are typically flooded by surface water however, three partially flooded trenches have exposed bedrock. From small (< 5m ) outcrops exposed in three of the trenches, it is apparent that the average grain size of the clastic sedimentary rocks and the abundance of coarse-grained detritus, increases from NE to SW away from the buried Caradocian shale, with arkosic sandstone most abundant in the SW near the Christopher vein. An extensive array of 98 drill holes provides excellent downhole, 3-D control on our understanding of the geology. 

The most common groundwater and gas sampling techniques in crystalline rock feature some form of downhole pumping device that lifts the fluid sample to the borehole collar, where standard flow cells and sophisticated sampling techniques are used to process the samples. A wide variety of devices and techniques are available and good reviews can be found in Bottomley et al. (1984), Ahnen et al. (1986),... 

The stability relationships between calcite, dolomite and magnesite depend on the temperature and activity ratio of Mg " /Ca " (Fig. 5d). Lower Mg/Ca activity ratios are required to induce the dolomitization of calcite and to stabilize magnesite at the expense of dolomite (Fig. 5d) (Usdowski, 1994). Formation waters from the Norwegian North Sea reservoirs have an average log(an g -/ cz- ) - TO to 0.0 and thus fall within the stability field of dolomite. Nevertheless, both calcite and dolomite are common cements in these rocks, indicating that dolomitization is a kinetically controlled reaction. Further evidence of this is revealed from Recent sediments, such as the Fraser River delta in Canada (Simpson Hutcheon, 1995) (log (aMg2+/aca=+) -2.2 to h-1.0), where the pore waters are saturated with respect to dolomite, but it is calcite rather than dolomite that precipitates. Calcite rather than dolomite forms below the deep>-sea floor, yet the pore waters plot at shallow, near sea bottom temperatures in the stability field of dolomite and shift with an increase in depth towards the stability field of calcite (Fig. 5d). This shift is due to a diffusion-controlled, downhole decrease in Mg/Ca activity ratio caused by the incorporation of Mg in Mg-silicate that results from the alteration of volcanic material, a process which is coupled with the release of calcium (McDuff Gieskes, 1976). 

Rock fractures present in this section of Lens IV reservoir provided avenues for movement of fluids through the bed. However, upon heating, reservoir rocks expanded, which in turn tended to close these fractures. Air intake capacity of the bed and therefore of the injection well, also suffered as result. This condition was aggravated by lack of automatic temperature controls on the gas heater installation downhole. Thus, towards the end of each period of operation of the heater, the temperature would rise significantly. 

Liu W T, Song C W Ma Y C 2000. Bends downhole probe testing technology for rock area of damage [J]. Coal Science and Technology, 28(6) 14-18. 

Crosshole, downhole, and uphole shear wave velocity measurements are used to determine the primary and shear wave velocities so as to either determine the elastic soil properties of soil and rock or calibrate seismic survey measurements. With the crosshole technique, the travel time is measured between a source in one borehole and a receiver in a second borehole. This technique can be used to directly measure the velocities of various strata. For downhole and uphole logs, the travel time is measured between the ground surface and a downhole source or receiver. Tests are conducted with the downhole source or receiver at different depths. These measurements should preferably be conducted in cased boreholes. 

Nevertheless, the effect of seismic noise attenuation with depth results in improved signal-to-noise ratios espteciaUy when the final layers of earth are highly lossy (Young et al. 1996). Ideally, a downhole seismometer should be placed in hard rock below a highly lossy overburden. 

---