125
Exploration, Recovery, and Transportation
6.3.4 drIllIng
Drilling an oil well is tedious and is often accompanied by difficulties. Some problems result
from formation penetration and the occurrence of high-pressure gas, fissures, or unexpected high
pressures in permeable rock. Others result from metallurgical or mechanical failures in the bit,
the drill stem, the draw works, or the mud system. Many tools and techniques have been developed to solve most problems; probably the best known is the fish used to recover broken bits.
Another technique involves intentional deviation of the borehole to avoid difficult formations, to
go around an unrecoverable fish, or sometimes to restore the direction of the hole after an accidental deviation.
Once the rig is set up, drilling operations commence. First, from the starter hole, a surface hole is
drilled to a preset depth, which is somewhere above where the oil trap is located. There are five basic
steps to drilling the surface hole: (1) place the drill bit, collar, and drill pipe in the hole, (2) attach
the kelly and turntable and begin drilling, (3) as drilling progresses, circulate mud through the pipe
and out of the bit to float the rock cuttings out of the hole, (4) as the hole increases in depth, add new
sections (joints) of drill pipes, and (5) remove (trip out) the drill pipe, collar and bit when the preset
depth (anywhere from a few hundred to a couple-thousand feet) is reached.
When the preset depth is reached, the casing pipe section is run into the hole and cemented to
prevent the hole from collapsing. The casing pipe has spacers around the outside to keep it centered in
the hole. The cement is pumped down the casing pipe using a bottom plug, a cement slurry, a top plug,
and drilling mud. The pressure from the drilling mud causes the cement slurry to move through the
casing and fill the space between the outside of the casing and the hole. Finally, the cement is allowed
to harden and then tested for such properties as hardness, alignment and a proper seal.
Drilling continues in stages and when the rock cuttings from the mud reveal the oil sand from
the reservoir rock, the final depth may have been reached. At this point, the drilling apparatus is
removed from the hole and tests are preformed to confirm that the final depth has been reached.
These tests include (1) well logging: lowering electrical and gas sensors into the hole to take measurements of the rock formations there, (2) drill-stem testing: lowering a device into the hole to
measure the pressures, which will reveal whether reservoir rock has been reached, and (3) core
sampling: taking samples of rock to look for characteristics of reservoir rock.
6.4 WELL COMPLETION
Once the final depth has been reached, the well is completed to allow oil to flow into the casing
in a controlled manner. First, a perforating gun is lowered into the well to the production depth.
The gun has explosive charges to create holes in the casing through which oil can flow. After
the casing has been perforated, a small-diameter pipe (tubing) is run into the hole as a conduit
for oil and gas to flow up the well and a packer is run down the outside of the tubing. When the
packer is set at the production level, it is expanded to form a seal around the outside of the tubing. Finally, a multivalve structure (the Christmas tree; Figure 6.4) is installed at the top of the
tubing and cemented to the top of the casing. The Christmas tree allows them to control the flow
of oil from the well.
Tight formations are occasionally encountered and it becomes necessary to encourage flow.
Several methods are used, one of which involves setting off small explosions to fracture the rock.
If the formation is mainly limestone, hydrochloric acid is sent down the hole to dissolve channels
in the rock. The acid is inhibited to protect the steel casing. In sandstone, the preferred method is
hydraulic fracturing.
A fluid with a viscosity high enough to hold coarse sand in suspension is pumped at very high
pressure into the formation, fracturing the rock. The grains of sand remain, helping to hold the
cracks open.
Exploration, Recovery, and Transportation
6.3.4 drIllIng
Drilling an oil well is tedious and is often accompanied by difficulties. Some problems result
from formation penetration and the occurrence of high-pressure gas, fissures, or unexpected high
pressures in permeable rock. Others result from metallurgical or mechanical failures in the bit,
the drill stem, the draw works, or the mud system. Many tools and techniques have been developed to solve most problems; probably the best known is the fish used to recover broken bits.
Another technique involves intentional deviation of the borehole to avoid difficult formations, to
go around an unrecoverable fish, or sometimes to restore the direction of the hole after an accidental deviation.
Once the rig is set up, drilling operations commence. First, from the starter hole, a surface hole is
drilled to a preset depth, which is somewhere above where the oil trap is located. There are five basic
steps to drilling the surface hole: (1) place the drill bit, collar, and drill pipe in the hole, (2) attach
the kelly and turntable and begin drilling, (3) as drilling progresses, circulate mud through the pipe
and out of the bit to float the rock cuttings out of the hole, (4) as the hole increases in depth, add new
sections (joints) of drill pipes, and (5) remove (trip out) the drill pipe, collar and bit when the preset
depth (anywhere from a few hundred to a couple-thousand feet) is reached.
When the preset depth is reached, the casing pipe section is run into the hole and cemented to
prevent the hole from collapsing. The casing pipe has spacers around the outside to keep it centered in
the hole. The cement is pumped down the casing pipe using a bottom plug, a cement slurry, a top plug,
and drilling mud. The pressure from the drilling mud causes the cement slurry to move through the
casing and fill the space between the outside of the casing and the hole. Finally, the cement is allowed
to harden and then tested for such properties as hardness, alignment and a proper seal.
Drilling continues in stages and when the rock cuttings from the mud reveal the oil sand from
the reservoir rock, the final depth may have been reached. At this point, the drilling apparatus is
removed from the hole and tests are preformed to confirm that the final depth has been reached.
These tests include (1) well logging: lowering electrical and gas sensors into the hole to take measurements of the rock formations there, (2) drill-stem testing: lowering a device into the hole to
measure the pressures, which will reveal whether reservoir rock has been reached, and (3) core
sampling: taking samples of rock to look for characteristics of reservoir rock.
6.4 WELL COMPLETION
Once the final depth has been reached, the well is completed to allow oil to flow into the casing
in a controlled manner. First, a perforating gun is lowered into the well to the production depth.
The gun has explosive charges to create holes in the casing through which oil can flow. After
the casing has been perforated, a small-diameter pipe (tubing) is run into the hole as a conduit
for oil and gas to flow up the well and a packer is run down the outside of the tubing. When the
packer is set at the production level, it is expanded to form a seal around the outside of the tubing. Finally, a multivalve structure (the Christmas tree; Figure 6.4) is installed at the top of the
tubing and cemented to the top of the casing. The Christmas tree allows them to control the flow
of oil from the well.
Tight formations are occasionally encountered and it becomes necessary to encourage flow.
Several methods are used, one of which involves setting off small explosions to fracture the rock.
If the formation is mainly limestone, hydrochloric acid is sent down the hole to dissolve channels
in the rock. The acid is inhibited to protect the steel casing. In sandstone, the preferred method is
hydraulic fracturing.
A fluid with a viscosity high enough to hold coarse sand in suspension is pumped at very high
pressure into the formation, fracturing the rock. The grains of sand remain, helping to hold the
cracks open.