Borehole Systems

Drillship-wireline in situ testing is conducted using a carrier tool that latches into a special drill collar and using one of two methods of inserting the test rod and sensor into the soil below the drill bit. One method is to continuously jack the test rod and sensor out of the carrier tool to a depth of 1.5 or 3 m below the drill bit. The other is to latch the carrier tool and an extended test rod and sensor into the drill collar, when the drill bit has been lifted a small distance above the bottom of the hole, and then to lower together the drill string and the attached tool, which forces the extended test rod and sensor into the soil. 

A typical stationary seabed system utilizing a stabilized drill string is the Wison wire-line cone penetrometer. 

Entire drill string jacked by seabed jack 

Test rods jacked or motor driven Test rods attached on vessel Test rods integral with seabed unit which is fully remotely operated Manned 

Jack operated by free divers Jack and divers in a diving beU Vessel jack Submersible unit 

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Figure 57. Outline of borehole system for heat extraction...

The groundwater level is important in the design of borehole systems since the depth of the borehole below the groundwater level is the so-called active borehole depth, i.e., the part of the hole that provides the heat. If... 

Neighboring systems If a neighboring borehole system is located closer than 15-20 m the system will influence each other, which shows in a lower ground temperature than in a single borehole case. The lower ground temperature increases the risk of freezing. 

The studied freezing problem in borehole systems is rare, but it causes big problems for those individual who are affected. It is also a problem for the... 

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 . 

If a situation arises whereby formation fluid or gas enters the bore bole the driller will notice an increase in the total volume of mud. Other indications such as a sudden increase in penetration rate and a decrease in pump pressure may also indicate an influx. Much depends on a quick response of the driller to close in the well before substantial volumes of formation fluid have entered the borehole. Onoe the BOP is closed, the new mud gradient required to restore balance to the system can be calculated. The heavier mud is then circulated in through the kill line and the lighter mud and influx is circulated out through the choke line. Once overbalance is restored, the BOP can be opened again and drilling operations continue. 

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. 

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. 

Higher annular velocity-low viscosity system or low annular velocity-higher viscosity systems can be selected. Annular velocity can be substituted for viscosity in lifting particle. Annular velocity at 150 ft/min should be sufficient for borehole cleaning with 1 cp viscosity clear salt water. 

If the acceleration is variable, as in sinusoidal movement, piezoelectric systems are ideal. In case of a constant acceleration, and hence a force that is also constant, strain gages may be employed. For petroleum applications in boreholes, however, it is better to use servo-controlled accelerometers. Reverse pendular accelerometers and single-axis accelerometers are available. 

An MWD system is lowered at the end of a 4.5-in. drillstring in an 8-in. borehole. Neglect the drill collar section of the string. The following data are available ... 

In the top part of Figure 4-283 the stand-off is constant during the rotation. In the oval borehole represented in the lower part of the figure, the stand-off is excessive when the density system is oriented up and the normal Ap correction is not enough. 

When the MWD systems are battery powered and have a downhole recording capability or use an electromagnetic telemetry, logging measurements can be repeated each time the bit is pulled out or run into the borehole. This new capability provides a way to map the progression of the filtrate front in the permeable formations. 

The high cost of platinised materials for use in borehole groundbeds as opposed to conventional silicon-iron anodes may also be offset by the reduction in required borehole diameter, hence lower installation cost, with the relative economics between the different systems dependent upon a combination of both material and installation costs. 

Pb, Zn) sulfides occur in deeper part as in Broadlands (New Zealand) and in Okuaizu (Japan). Pb and Zn sulfides occur dominantly at 400 m from the surface in Broadlands. In the boreholes of Broadlands, concentrations of precious and base metal elements in the boreholes change with depth as studied by Ewers and Keayse (1977), Au, As, Sb and T1 decrease with depth, but Ag increases with depth. In active geothermal systems in Japan, Au, Te, Se, T1 and Hg are enriched on the surface in the Osorezan hydrothermal system. 

The two most promising options are storage in aquifers (ATES) and storage through borehole heat exchangers (BTES). These concepts have already been introduced as commercial systems on the energy market in several countries. Another option is to use underground cavities (CTES), but this concept is so far rarely applied commercially. 

In systems where the borehole temperature is below freezing (0 °C) the borehole groundwater will freeze to ice. This freezing occurs from the top and down into the borehole. This is normally not a problem and is even an advantage because ice conducts heat more easily than water. 

Lowflow in the brinefilled pipe system A low flow rate though the borehole pipes indicates a lower fluid temperature and a greater risk of freezing. 

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