Borehole direction

The drill string is defined here as a drill pipe with tool joints and drill collars. The drill stem consists of the drill string and other components of the drilling assembly that includes the kelly, subs, stabilizers, reamers as well as shock absorbers, and junk baskets or drilling jars used in certain drilling conditions. The drill stem (1) transmits power by rotary motion from the surface to a rock bit, (2) conveys drilling fluid to the rock bit, (3) produces the weight on bit for efficient rock destruction by the bit, and (4) provides control of borehole direction. 

Borehole direction Refers to the azimuth in which the borehole is heading. 

Borehole directional survey Refers to the measurements of the inclinations, azimuths and specified depths of the stations through a section of borehole. 

J. Dusek R. Mecir, Rudy(Prague) 14(3), 72—4(1966)(Czech) CA 65, 5292(1966)[Brief info is given on tests in Czechoslovakia of the mixt called DAP-1 in overcast large diam (>75 mm) borehole blasts. DAP-1 is prepd by simultaneously adding 95% AN and 5% fuel oil to the borehole directly before blasting, without any special mixing. By tests with 4... 

Do not spot exploration boreholes directly over the proposed HDD path— offset each 25 feet to 30 feet laterally... 

If a shallow kick off in soft formation is required (e.g. to steer the borehole away underneath platforms) a technique using jet bit deflection or badgering is employed (Fig. 3.16). A rock bit is fitted with two small and one large jet. With the bit on bottom and oriented in the desired direction the string is kept stationary and mud is pumped through the nozzles. This causes asymmetric erosion of the borehole beneath the larger jet. Once sufficient hole has been jetted, the drill bit will be rotated again and the new course followed. This process will be repeated until the planned deviation is reached. 

The basic data gathering methods are direct methods which allow visual inspection or at least direct measurement of properties, and indirect methods whereby we infer reservoir parameters from a number of measurements taken in a borehole. The main techniques available within these categories are summarised in the following table ... 

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 preparation for a field wide quick look correlation, all well logs need to be corrected for borehole inclination. This is done routinely with software which uses the measured depth below the derrick floor ( alonghole depth below derrick floor AHBDFor measured depth , MD) and the acquired directional surveys to calculate the true vertical depth subsea (TVSS). This is the vertical distance of a point below a common reference level, for instance chart datum (CD) or mean sea level (MSL). Figure 5.41 shows the relationship between the different depth measurements. 

Besides readings of Earth s surface temperatures taken with standard glass thermometers, direct readings of atmospheric temperatures have been taken with satellites and weather balloons. In addition to direct measurements of Earth s recent temperatures, proxy measurements of temperatures from farther in the past can be derived from borehole temperature measurements, from historical and physical evidence regarding the e xtent and mass of land and sea ice, and from the bleaching of coral reefs. 

Despite the forces of wind, waves and ocean currents, at a water depth of. S,000 ft (1,526 in), a dynamic positioning system can reliably keep a drill ship within 50 ft (15.2 m) of the spot directly over the borehole. 

Air (or Gas) Downhole Motors. Some positive displacement mud motors can be operated on unstable foam. In general, these mud motors must be low-torque, high-rotalional-speed motors. Such motors have found limited use in air and gas drilling operations where directional boreholes are required. Recently a downhole turbine motor has been developed specifically for air and gas drilling operations. This downhole pneumatic turbine motor is a high-torque, low-rotational-speed motor. 

If the circulating air has been saturated, then any formation water entering the annulus will be carried to the surface as droplets and will not reduce the temperature of the air and thereby reduce kinetic energy of the air as it expands in the annulus. The amount of formation water that can be carried from the borehole annulus by the real amount of air circulating is directly related to the additional air that is being circulated above that minimum value necessary to clean the hole of rock cuttings. 

Every accelerometer has a response curve of the type shown schematically in Figure 4-222. Instead of having an ideal linear response, a nonlinear response is generally obtained with a skewed acceleration for zero current, a scale factor error and a nonlinearity error. In addition, the skew and the errors vary with temperature. If the skew and all the errors are small or compensated in the accelerometer s electronic circuits, the signal read is an ideal response and can be used directly to calculate the borehole inclination. If not, modeling must be resorted to, i.e., making a correction with a computer, generally placed at the surface, to find the ideal response. This correction takes account of the skew,... 

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 sensors function in a pulse-echo mode that allows the direct measurement of stand-off, from which short and long axes of the borehole diameter are computed. The vertical resolution is 1 in. (25 mm) and accuracy of the diameter measurement is 0.1 in. (2.5 mm). 

Corrective jetting runs Action taken with a directional jet bit to change the direction or inclination of the borehole. 

Dogleg severity A measure of the amount of change in the inclination and/ or direction of a borehole usually expressed in degrees per 100 ft of course length. 

Hole curvature Refers to changes in inclination and direction of the borehole. 

Original hole direction and inclination is N20°E and 10°, respectively. It is desired to deviate the borehole so that the new hole inclination is 13° and direction N30°E. 

There has been an increasing level of nitrate contamination of borehole supplies in the east of England, because of the use of agricultural fertilisers since the Second World War . Nitrates are known to exacerbate certain corrosion processes e.g. at soldered joints however the maximum value allowed for this ion by the EC drinking water directive (50 mg NO3 r ) should limit its significance. 

The utility of c) as a thermometer gains further support from direct measurements showing correlation of temperature and 3 of precipitation through time over seasonal cycles (Shuman et al, 1995). At longer temporal scales, temperature measurements in boreholes and gas composition measurements both provide temperature information which can be compared to 3. Results from such comparisons have so far... 

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