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 |  موضوع: Fundamentals of Finding & Producing Oil & Gas in the Illinois Basin  الجمعة يناير 07, 2011 9:57 pm | |
| Fundamentals of Finding & Producing Oil & Gas in the Illinois Basin
Hydrocarbons
- crude oil and natural gas - are found in certain layers of rock that
are usually buried deep beneath the surface of the earth. In order for a
rock layer to qualify as a good source of hydrocarbons, it must meet
several criteria.
Characteristics of Reservoir Rock
For one thing, good reservoir rocks (a reservoir is a
formation that contains hydrocarbons) have porosity. Porosity is a
measure of the openings in a rock, openings in which petroleum can
exist. Even though a reservoir rock looks solid to the naked eye, a
microscopic examination reveals the existence of tiny openings in the
rock. These openings are called pores. Thus a rock with pores is said to
be porous and is said to have porosity (Figure 1).
Figure 1: Porosity
Another
characteristic of reservoir rock is that it must be permeable. That is,
the pores of the rock must be connected together so that hydrocarbons
can move from one pore to another (Figure 2). Unless hydrocarbons can
move and flow from pore to pore, the hydrocarbons remain locked in place
and cannot flow into a well. In addition to porosity and permeability
reservoir rocks must also exist in a very special way. To understand
how, it is necessary to cross the time barrier and take an imaginary
trip back into the very ancient past.
Figure 2: Permeability
Imagine
standing on the shore of an ancient sea, millions of years ago. A small
distance from the shore, perhaps a dinosaur crashes through a jungle of
leafy tree ferns, while in the air, flying reptiles dive and soar after
giant dragonflies. In contrast to the hustle and bustle on land and in
the air, the surface of the sea appears very quiet. Yet, the quiet
surface condition is deceptive. A look below the surface reveals that
life and death occur constantly in the blue depths of the sea. Countless
millions of tiny microscopic organisms eat, are eaten and die. As they
die, their small remains fall as a constant rain of organic matter that
accumulates in enormous quantities on the sea floor. There, the remains
are mixed in with the ooze and sand that form the ocean bottom.
As the
countless millennia march inexorably by, layer upon layer of sediments
build up. Those buried the deepest undergo a transition; they are
transformed into rock. Also, another transition occurs: changed by heat,
by the tremendous weight and pressure of the overlying sediments, and
by forces that even today are not fully understood, the organic material
in the rock becomes petroleum. But the story is not over.
For,
while petroleum was being formed, cataclysmic events were occurring
elsewhere. Great earthquakes opened huge cracks, or faults, in the
earth’s crust. Layers of rock were folded upward and downward. Molten
rock thrust its way upward, displacing surrounding solid beds into a
variety of shapes. Vast blocks of earth were shoved upward, dropped
downward or moved laterally. Some formations were exposed to wind and
water erosion and then once again buried. Gulfs and inlets were
surrounded by land, and the resulting inland seas were left to evaporate
in the relentless sun. Earth’s very shape had been changed.
Meanwhile,
the newly born hydrocarbons lay cradled in their source rocks. But as
the great weight of the overlying rocks and sediments pushed downward,
the petroleum was forced out of its birthplace. It began to migrate.
Seeping through cracks and fissures, oozing through minute connections
between the rock grains, petroleum began a journey upward. Indeed, some
of it eventually reached the surface where it collected in large pools
of tar, there to lie in wait for unsuspecting beasts to stumble into its
sticky trap. However, some petroleum did not reach the surface.
Instead, its upward migration was stopped by an impervious or
impermeable layer of rock. It lay trapped far beneath the surface. It is
this petroleum that today’s oilmen seek.
Types of Petroleum Traps
Geologists
have classified petroleum traps into two basic types: structural traps
and stratigraphic traps. Structural traps are traps that are formed
because of a deformation in the rock layer that contains the
hydrocarbons. Two common examples of structural traps are fault traps
and anticlines.
An anticline is an
upward
fold in the layers of rock, much like an arch in a building. Petroleum
migrates into the highest part of the fold, and its escape is prevented
by an overlying bed of impermeable rock (A).
A fault
trap occurs when the formations on either side of the fault have been
moved into a position that prevents further migration of petroleum. For
example, an impermeable formation on one side of the fault may have
moved opposite the petroleum-bearing formation on the other side of the
fault. Further migration of petroleum is prevented by the impermeable
layer (B).
Stratigraphic
traps are traps that result when the reservoir bed is sealed by other
beds or by a change in porosity or permeability within the reservoir bed
itself. There are many different kinds of stratigraphic traps. In one
type, a tilted or inclined layer of petroleum-bearing rock is cutoff or
truncated by an essentially horizontal, impermeable rock layer (C).
Or
sometimes a petroleum-bearing formation pinches out; that is, the
formation is gradually cut off by an overlying layer. Another
stratigraphic trap occurs when a porous and permeable reservoir bed is
surrounded by impermeable rock. Still another type occurs when there is a
change in porosity and permeability in the reservoir itself. The upper
reaches of the reservoir may be impermeable and nonporous, while the
lower part is permeable and porous and contains hydrocarbons.
SECURING LEASES
Once a
likely area has been selected, the right to drill must be secured before
drilling can begin. Securing the right to drill usually involves
leasing the mineral rights of the desired property from the owner. The
owner may be the owner of all interest in the land, or just the mineral
rights. As payment for the right to drill for and extract the oil and
gas, the owner will usually be paid a sum call a "lease bonus" or a
"hole bonus" for every well drilled on the leased land. He will also
retain a royalty on the production, if any, of the leased property. The
royalty is the right to receive a certain portion of the production of
property, without sharing in the costs incurred in producing the oil,
such as drilling, completion, equipping and operating or production
costs. The costs are borne by the holder of the right to drill and
extract the mineral, which right is usually referred to as the working
interest.In many cases the procurement of the lease from the land owner
is accomplished by a lease broker who will, in turn, offer and then
assign the lease to an operator such as Maverick Energy, Inc. Maverick
Energy is very selective in choosing leases for drilling. The lease
broker usually retains an overriding royalty on the working interest as
compensation for his services. In the case of Maverick’s leases, there
generally is a retained land owner’s royalty of 1/8 of all production
and a 1/16 overriding royalty on the working interest, retained or
granted to one or more persons who may have acted as lease brokers.
DRILLING
Once an
area has been selected and the right to drill thereon has been obtained,
actual drilling may begin. The most common method of drilling in use
today is rotary drilling. Rotary drilling operates on the principle of
boring a hole by continuous turning of a bit. The bit is the most
important tool. The rest of the rig ( a derrick and attendant machinery)
is designed to make it effective. While bits vary in design and
purpose, one common type consists of a housing and three interlocking
movable wheels with sharp teeth, looking something like a cluster of
gears. The bit, which is hollow and very heavy, is attached to the drill
stem, composed of hollow lengths of pipe leading to the surface. As the
hole gets deeper, more lengths of pipe can be added at the top. Almost
as important as the bit is the drilling fluid. Although known in the
industry as mud, it is actually a prepared chemical compound. The
drilling mud is circulated continuously down the drill pipe, through the
bit, into the hole and upwards between the hole and the pipe to a
surface pit, where it is purified and recycled. The flow of mud removes
the cuttings from the hole without removal of the bit, lubricates and
cools the bit in the hole, and prevents a blow out which could result if
the bit punctured a high pressure formation. (See the drilling rig to
the right.)
The
cuttings, which are carried up by the drilling mud, are usually
continuously tested by the petroleum geologist in order to determine the
presence of oil.
DRILLING TO TOTAL DEPTH
The
final part of the hole is what the operating company hopes will be the
production hole. But before long, the formation of interest (the pay
zone, the oil sand, or the formation that is supposed to contain
hydrocarbons) is penetrated by the hole. It is now time for a big
decision. The question is, "Does this well contain enough oil or gas to
make it worthwhile to run the final production string of casing and
complete the well?"
EVALUATING FORMATIONS Examining Cuttings
To help
the operator make his decision, several techniques have been developed.
One thing that helps indicate whether hydrocarbons have been trapped is a
thorough examination of the cuttings brought up by the bit. The mud
logger or geologist (Remember him? He’s been there all along, monitoring
downhole conditions at the location.) catches cuttings at the flow
ditch and by using a microscope or ultraviolet light can see whether oil
is in the cuttings. Or he may use a gas-detection instrument.
Well Logging
Another
valuable technique is well logging. A logging company is called to the
well while the crew trips out all the drill string. Using a portable
laboratory, truck-mounted for land rigs, the well loggers lower devices
called logging tools into the well on wireline. The tools are lowered
all the way to bottom and then reeled slowly back up. As the tools come
back up the hole, they are able to measure the properties of the
formations they pass.Electric logs measure and record natural and
induced electricity in formations. Some logs ping formations with sound
and measure and record sound reactions. Radioactivity logs measure and
record the effects of natural and induced radiation in the formations.
These are only a few of many types of logs available. Since all the
logging tools make a record, which resembles a graph or an
electrocardiogram (EKG), the records, or logs can be studied and
interpreted by an experienced geologist or engineer to indicate not only
the existence of oil or gas, but also how much may be there. Computers
have made the interpretation of logs much easier.
Coring
In
addition to these tests, formation core samples are sometimes taken. Two
methods of obtaining cores are frequently used. In one, an assembly
called a "core barrel" is made up on the drill string and run to the
bottom of the hole. As the core barrel is rotated, it cuts a cylindrical
core a few inches in diameter that is received in a tube above the
core-cutting bit. A complete round trip is required for each core taken.
The second is a sidewall sampler in which a small explosive charge is
fired to ram a small cylinder into the wall of the hole. When the tool
is pulled out of the hole, the small core samples come out with the
tool. Up to thirty of the small samples can be taken at any desired
depth. Either type of core can be examined in a laboratory and may
reveal much about the nature of the reservoir.
COMPLETING THE WELL
After
the operating company carefully considers all the data obtained from the
various tests it has ordered to be run on the formation or formations
of interest, a decision is made on whether to set production casing and
complete the well or plug and abandon it. If the decision is to abandon
it, the hole is considered to be dry, that is, not capable of producing
oil or gas in commercial quantities. In other words, some oil or gas may
be present but not in amounts great enough to justify the expense of
completing the well. Therefore, several cement plugs will be set in the
well to seal it off more or less permanently. However, sometimes wells
that were plugged and abandoned as dry at one time in the past may be
reopened and produced if the price of oil or gas has become more
favorable. The cost of plugging and abandoning a well may only be a few
thousand dollars. Contrast that cost with the price of setting a
production string of casing - $50,000 or more. Therefore, the operator’s
decision is not always easy.
Setting Production Casing
If the
operating company decides to set casing, casing will be brought to the
well and for one final time, the casing and cementing crew run and
cement a string of casing. Usually, the production casing is set and
cemented through the pay zone; that is, the hole is drilled to a depth
beyond the producing formation, and the casing is set to a point near
the bottom of the hole. As a result, the casing and cement actually seal
off the producing zone-but only temporarily. After the production
string is cemented, the drilling contractor has almost finished his job
except for a few final touches.
CEMENTING
After the casing string is run, the
next
task is cementing the casing in place. An oil-well cementing service
company is usually called in for this job although, as when casing is
run, the rig crew is available to lend assistance. Cementing service
companies stock various types of cement and have special transport
equipment to handle this material in bulk. Bulk-cement storage and
handling equipment is moved out to the rig, making it possible to mix
large quantities of cement at the site. The cementing crew mixes the dry
cement with water, using a device called a jet-mixing hopper. The dry
cement is gradually added to the hopper, and a jet of water thoroughly
mixes with the cement to make a slurry (very thin water cement).
Special
pumps pick up the cement slurry and send it up to a valve called a
cementing head (also called a plug container) mounted on the topmost
joint of casing that is hanging in the mast or derrick a little above
the rig floor. Just before the cement slurry arrives, a rubber plug
(called the bottom plug) is released from the cementing head and
precedes the slurry down the inside of the casing. The bottom plug stops
or "seats" in the float collar, but continued pressure from the cement
pumps open a passageway through the bottom plug. Thus, the cement slurry
passes through the bottom plug and continues on down the casing. The
slurry then flows out through the opening in the guide shoe and starts
up the annular space between the outside of the casing and wall of the
hole. Pumping continues and the cement slurry fills the annular space.
A top
plug, which is similar to the bottom plug except that it is solid, is
released as the last of the cement slurry enters the casing. The top
plug follows the remaining slurry down the casing as a displacement
fluid (usually salt water or drilling mud) is pumped in behind the top
plug. Meanwhile, most of the cement slurry flows out of the casing and
into the annular space. By the time the top plug seats on or "bumps" the
bottom plug in the float collar, which signals the cementing pump
operator to shut down the pumps, the cement is only in the casing below
the float collar and in the annular space. Most of the casing is full of
displacement fluid.
After
the cement is run, a waiting time is allotted to allow the slurry to
harden. This period of time is referred to as waiting on cement or
simply WOC.
After
the cement hardens, tests may be run to ensure a good cement job, for
cement is very important. Cement supports the casing, so the cement
should completely surround the casing; this is where centralizers on the
casing help. If the casing is centered in the hole, a cement sheath
should completely envelop the casing. Also, cement seals off formations
to prevent fluids from one formation migrating up or down the hole and
polluting the fluids in another formation. For example, cement can
protect a freshwater formation (that perhaps a nearby town is using as
its drinking water supply) from saltwater contamination. Further, cement
protects the casing from the corrosive effects that formation fluids
(as salt water) may have on it.
Perforating
Since
the pay zone is sealed off by the production string and cement,
perforations must be made in order for the oil or gas to flow into the
wellbore. Perforations are simply holes that are made through the casing
and cement and extend some distance into the formation. The most common
method of perforating incorporates shaped-charge explosives (similar to
those used in armor-piercing shells). Shaped
charges accomplish penetration by creating a jet of high-pressure,
high-velocity gas. The charges are arranged in a tool called a gun that
is lowered into the well opposite the producing zone. Usually the gun is
lowered in on wirelin (1). When the gun is in position, the charges are
fired by electronic means from the surface (2). After the perforations
are made, the tool is retrieved (3). Perforating is usually performed by
a service company that specializes in this technique.
Acidizing
Sometimes, however, petroleum exists in a formation but is unable to
flow readily into the well because the formation has very low
permeability. If the formation is composed of rocks that dissolve upon
being contacted by acid, such as limestone or dolomite, then a technique
known as acidizing may be required. Acidizing is usually performed by
an acidizing service company and may be done before the rig is moved off
the well; or it can also be done after the rig is moved away. In any
case, the acidizing operation basically consists of pumping anywhere
from fifty to thousands of gallons of acid down the well. The acid
travels down the tubing, enters the perforations, and contacts the
formation. Continued pumping forces the acid into the formation where it
etches channels - channels that provide a way for the formation’s oil
or gas to enter the well through the perforations.
Fracturing
When sandstone rocks contain oil or gas in commercial quantities but the
permeability is too low to permit good recovery, a process called
fracturing may be used to increase permeability to a practical level.
Basically, to fracture a formation, a fracturing service company pumps a
specially blended fluid down the well and into the formation under
great pressure. Pumping continues until the formation literally cracks
open.Meanwhile, sand, walnut hulls, or aluminum pellets are mixed into
the fracturing fluid. These materials are called proppants. The proppant
enters the fractures in the formation, and, when pumping is stopped and
the pressure allowed to dissipate, the proppant remains in the
fractures. Since the fractures try to close back together after the
pressure on the well is released, the proppant is needed to hold or prop
the fractures open. These propped-open fractures provide passages for
oil or gas to flow into the well. See figure to the right.
ARTIFICIAL LIFT
After
the well has been perforated, acidized or fractured, the well may not
produce by natural flow. In such cases, artificial-lift equipment is
usually installed to supplement the formation pressure.
Sucker-Rod Pumps
The
artificial-lift method that involves surface pumps is known as rod
pumping or beam pumping. Surface equipment used in this method imparts
an up-and-down motion to a sucker-rod string that is attached to a
piston or plunger pump submerged in the fluid of a well. Most
rod-pumping units have the same general operating principles.
INJECTION WELLS
In the ordinary producing operation only a portion of the oil in place
is recoverable by primary production methods. Such methods include
free-flowing wells and production maintained by pumps. As oil is
extracted from a reservoir or sands the pressure which brings the oil to
the well is reduced. Secondary recovery methods are intended to
increase the recoverable percentage of the oil in place by injecting a
substance such as gas or water into the producing formation. The
injected substance is intended to increase the pressure on the oil in
the formation and drive it toward the well-bore. A well, called an
injection well or water injection well, is usually drilled in order to
inject the substance. Sometimes a previously drilled, abandoned well can
be reworked as an injection well. When water is used as the injectant
it is often produced on the property itself. Excess water produced by
operating wells may be diverted to the injection well and used as the
injectant. This method of water disposal usually alleviates the need for
a separate water disposal well. If the water from the producing wells
does not provide enough injectant to provide proper pressure for
secondary recovery, a water supply well may be required to provide an
adequate supply of water.
OIL PRODUCTION
Once
an accumulation of oil has been found in a porous and permeable
reservoir, a series of wells are drilled in a predetermined pattern to
effectively drain this "oil pool". Wells may be drilled as close as one
to each 10 aces (660 ft. between wells) or as far apart as one to each
640 acres (1 mile between wells) depending on the type of reservoir and
the depth to the "pay" horizon. For economic reasons, spacing is usually
determined by the distance the reservoir energy will move commercial
quantities of oil to individual wells.The rate of production is highest
at the start when all of the energy from the dissolved gas or water
drive is still available. As this energy is used up, production rates
drop until it becomes uneconomical to operate although significant
amounts of oil still remain in the reservoir. Experience has shown that
only about 12 to 15 percent of the oil in a reservoir can be produced by
the expansion of the dissolved gas or existing water.
SECONDARY RECOVERY
Waterflooding
is one of the most common and efficient secondary recovery processes.
Water is injected into the oil reservoir in certain wells in order to
renew a part of the original reservoir energy. As this water is forced
into the oil reservoir, it spreads out from the injection wells and
pushes some of the remaining oil toward the producing wells. Eventually
the water front will reach these producers and increasingly larger
quantities of water will be produced with a corresponding decrease in
the amount of oil. When it is no longer economical to produce these high
water-ratio wells, the flood may be discontinued.As mentioned
previously, average primary recoveries may be only 15% of the oil in the
reservoir. Properly operated waterfloods should recover an additional
15% to 20% of the original oil in place. This leaves a substantial
amount of oil in the reservoir, but there are no other engineering
techniques in use now that can recover it economically.In most cases,
oil reservoirs suitable for secondary recovery projects have been
produced for several years. It takes time to inject sufficient water to
fill enough of the void spaces to begin to move very much oil. It takes
several months from the start of a waterflood before significant
production increases take place and the flood will probably have maximum
recoveries during the second, third, fourth, and fifth years after
injection of water has commenced. The average flood usually lasts 6 to
10 years.
WATERFLOODING IN THE ILLINOIS BASIN
Waterfloods
have been highly successful in the Illinois Basin and probably account
for 75% of the total production from the area. Flood recoveries will
generally be an additional 80% to 100% of the primary production.There
are no special problems with floods in the Illinois Basin. Ample
supplies of salt water are generally available and injection pressures
are not too high - 1500 PSI or less. Corrosion is minimal and no
expensive, high-pressure equipment is involved. Sufficient potential
flood properties are available on reasonable terms - especially smaller
areas owned by independent operators who do not have the finances to
support the installation of properly engineered secondary recovery
operations.Waterfloods in the Illinois Basin should return 2 to 3 times
their cost and are considered to be low-risk prospects.
OPERATION
When all
equipment is in place, the oil may begin to flow into the holding tanks
to await pick up. It can be expected that a well will not be in
production for certain times due to adverse weather conditions,
mechanical malfunctions and other unforeseen circumstances. After the
production period commences, it is necessary to incur certain costs in
order to bring the oil to the surface. These costs include normal
maintenance on the pump and other equipment, replacement of any pipe or
tanks as needed, compensation to the operator of the pump, and payment
of any incidental damages to the owner of the surface rights of the
leased property. In some cases, the oil in a pay zone will be mixed with
salt water. In such cases, the oil must be separated from the salt
water and the salt water disposed of in a manner which is not harmful to
the environment. The water may be hauled away by tank truck but often
this phenomenon requires the drilling, nearby the oil producing well, of
another well into which the salt water will be pumped. The cost of this
water disposal well is normally considered to be a cost of operation.
Finally, there may be additional costs incurred in opening up a new pay
zone when any presently producing pay zone becomes economically
unfeasible. Because opening a new pay zone involves the installation of
very little, if any, new equipment, the costs involved therein usually
are not very substantial.
SALE OF OIL
Once the
oil is out of the ground and into the holding tanks, it must be sold.
In most cases each holder of a working interest has the right to take
his portion of production in kind, therefore, make his own arrangements
for its sale. It is not uncommon, however, for all the holders of a
working interest of a well to enter into the same arrangement with the
same buyer of the oil production. These sale contracts are normally
entered into for periods of not longer than a few months but in no case
longer than one year. The buyer of the oil will generally be advised by
the operator of the working interest as to the identity and extent of
ownership of each of the holders of the working interest, as well as the
identity of the royalty holders and the amount of their interests. The
information will be compiled on division orders which are the basis upon
which the buyer of the oil can divide the proceeds of sale among the
various holders. The buyer of the oil will pick up the oil from the
holding tanks at periodic intervals, gauge it and remit the remaining
proceeds in the proper amounts to
the holders of the working interest and the royalties.
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