The Origins, Migration and Trapping of Petroleum and Exploring For It

The Origins, Migration and Trapping of Petroleum and Exploring For It.

 

THE ORIGIN OF PETROLEUM

During certain geologic ages, when the climate was suitable, petroleum began as organic material derived from plants and animals which grew in abundance. As these organisms went through their cycles of growing and dying, buried organic material slowly decayed and became our present-day fossil fuels: oil, gas, coal and bitumen. Oil, gas and bitumen were dispersed in the sediments (usually clay-rich shales). Over millions of years, these organic-laden shales expelled their oil and gas under tremendous pressures from the overburden. The oil and gas migrated into permeable strata below or above them, then migrated further into traps that we now call reservoirs. It’s interesting to note that the word “petroleum” is derived from the Latin words for “rock” (petra) and “oil” (oleum), indicating that its origins lie within the rocks that make up the earth’s crust.

These ancient petroleum hydrocarbons are complex mixtures and exist in a range of physical forms — gas mixtures, oils ranging from thin to viscous, semi-solids and solids. Gases may be found alone or mixed with the oils. Liquids (oils) range in color from clear to black. The semi-solid hydrocarbons are sticky and black (tars). The solid forms are usually mined as coal, tar sand or natural asphalt such as gilsonite.

As the name “hydrocarbon” implies, petroleum is comprised of carbon atoms and hydrogen atoms bonded together; the carbon has four bonds and the hydrogen has one. The simplest hydrocarbon is methane gas (CH4). The more complex hydrocarbons have intricate structures, consisting of multiple carbon-hydrogen rings with carbon-hydrogen side chains. There are often traces of sulfur, nitrogen and other elements in the structure of the heavier hydrocarbons.

 

THE MIGRATION AND TRAPPING OF PETROLEUM

Sedimentary rocks. Oil is seldom found in commercial amounts in the source rock where it was formed. Rather, it will be found nearby, in reservoir rock. These are normally “sedimentary” rocks — layered rock bodies formed in ancient, shallow seas by silt and sand from rivers. Sandstone is the most common of the sedimentary rock types. Between the sand grains that make up a sandstone rock body there is space originally filled with seawater. When pores are interconnected, the rock is permeable and fluids can flow by gravity or pressure through the rock body. The seawater that once filled the pore space is partially displaced by oil and gas that was squeezed from the source rock into the sandstone. Some water remains in the pore space, coating the sand grains. This is called the reservoir’s connate water. Oil and gas can migrate through the pores as long as enough gravity or pressure forces exist to move it or until the flow path is blocked. A blockage is referred to as a trap.

Carbonate rock, limestones (calcium carbonate) and dolomites (calciummagnesium carbonate) are sedimentary rocks and are some of the most common petroleum reservoirs. Carbonate reservoirs were formed from ancient coral reefs and algae mounds that grew in ancient, shallow seas. Organic-rich source rocks were also in proximity to supply oil and gas to these reservoir rocks. Most limestone strata do not have a matrix that makes them permeable enough for oil and gas to migrate through them. However, many limestone reservoirs contain fracture systems and/or interconnecting vugs (cavities formed when acidic water dissolved some of the carbonate). These fractures and vugs, created after deposition, provide the porosity and permeability essential for oil to migrate and be trapped. Another carbonate rock, dolomite, exhibits matrix permeability that allows fluid migration and entrapment. Dolomites also can have fracture and vugular porosity, making dolomite structures attractive candidates for oil deposits.

Salt domes. A significant portion of oil and gas production is associated with salt domes which are predominately classified as piercement-type salt intrusions and often mushroom shaped. Piercement-type domes were formed by the plastic movement of salt rising upward through more dense sediments by buoyant forces resulting from the difference in density.

 

EXPLORING FOR PETROLEUM

Locating petroleum. Knowing that petroleum traps exist is one thing, but pinpointing traps far below the earth’s surface is quite another. Many methods have been used to locate petroleum traps, but the most important methods are aerial surveying, geological exploration, geophysical (seismic) exploration and exploratory drilling.

Aerial and satellite. Surveys from high altitudes give a broad picture of a geographic area of interest. Major surface structures such as anticlines and faulted regions can be clearly observed by these methods. This information helps locate areas where more detailed study is warranted. In the early years of petroleum exploration, visualization from an aircraft or mapping river and creek drainage patterns were successful surveying techniques. Modern aerial and satellite surveying is more sophisticated allowing a number of features to be evaluated, including thermal anomalies, density variations, mineral composition, oil seepage and many others.

Surface geological exploration.

Observations by trained geologists of rock outcrops (where subsurface layers reach the surface), road cuts and canyon walls can identify lithology and assess the potential for hydrocarbon source rocks, reservoir-quality rocks and trapping mechanisms in an area under study. Much has been learned about ancient deposits from studying modern river deltas, for example. Detailed geologic maps, made from these observations, show the position and shape of the geologic features and provide descriptions of the physical characteristics and fossil content of the strata.

Geophysical exploration. Through the use of sensitive equipment and analytical techniques, geophysicists learn a great deal about the subsurface. Chief among these techniques is seismic exploration in which shock waves, generated at the surface and aimed downwards, are reflected back to the surface as echoes off the strata below. Because rocks of varying density and hardness reflect the shock waves at different rates of speed, the seismologist can determine depth, thickness and type of rock by precisely recording the variances in the time it takes the waves to arrive back at the surface. Continual improvements in seismic measurement and the mathematical methods (algorithms) used to interpret the signals can now give a clearer “picture” of subsurface formations. Other geophysical methods use variations in the earth’s gravity and magnetic properties to detect gross features of subsurface formations.

 

Drilling for Petroleum

 

DRILLING METHODS

When it has been established that a petroleum reservoir probably exists, the only way to verify this is to drill. Drilling for natural resources is not a new idea. As early as 1100 A.D., brine wells as deep as 3,500 ft were drilled in China, using methods similar to cable tool drilling.

Cable tool drilling. This was the method used by pioneer wildcatters in the nineteenth and early twentieth centuries and is still used today for some shallow wells. The method employs a heavy steel drill stem with a bit at the bottom, suspended from a cable. The tool is lifted and dropped repeatedly. The falling steel mass above the bit provides energy to break up the rock, pounding a hole through it. The hole is kept empty, except for some water at the bottom. After drilling a few feet, the drill stem (with its bit) is pulled out and the cuttings are removed with a bailer. The cable tool method is simple, but it is effective only for shallow wells. Progress is slow because of the inefficiency of the bit and the need to pull the tools frequently to bail out cuttings.

Rotary drilling. Rotary rigs are used for a variety of purposes — drilling oil, gas, water, geothermal and petroleumstorage wells; mineral assay coring; and mining and construction projects. The most significant application, however, is oil and gas drilling. In the rotary method (introduced to oil and gas drilling in about 1900), the drill bit is suspended on the end of a tubular drillstring (drill stem) which is supported on a cable/pulley system held up by a derrick (see Figure 3). Drilling takes place when the drillstring and bit are rotated while the weight of the drill collars and bit bears down on the rock.

To keep the bit cool and lubricated, and to remove the rock cuttings from the hole, drilling fluid (mud) is pumped down the inside of the drillstring. When it reaches the bit, it passes through nozzles in the bit, impacts the bottom of the hole and then moves upward in the annulus (the space between the drillstring and the wellbore wall) with the cuttings suspended in it. At the surface, the mud is filtered through screens and other devices that remove the cuttings, and is then pumped back into the hole. Drilling mud circulation brought efficiency to rotary drilling that was missing from cable tool drilling — the ability to remove cuttings from the hole without making a trip to the surface.

Equipment for rotary drilling is illustrated in Figure 3.

 

THE DRILLSTRING

Starting at the bottom, a basic drillstring for rotary drilling consists of the (1) bit, (2) drill collars and Bottom-Hole Assemblies (BHAs), and (3) drill pipe (see Figure 5).

The BHA is located just above the bit and consists of drill collars combined with one or more bladed stabilizers (to keep the BHA and bit concentric), possibly a reamer (to keep the hole from becoming tapered as the bit diameter wears down) and other tools. MWD tools and mud motors are generally located low in the BHA, usually just above the bit. Sometimes, a set of “jars” is located near the top of the BHA. Jars can free stuck pipe by giving a hammering action when they are set-off by pulling hard.

Drill collars are thick-walled, heavy joints of pipe used in the BHA to provide weight to the bit. Usually, one of the collars is made of non-magnetic metal so that a magnetic compass tool (survey tool) can be used to determine the inclination of the lower BHA and bit without interference from magnetic metals.

Each joint of drill pipe is approximately 30 ft long, and has a box (female connection) welded onto one end and a pin (male connection) welded to the other. These threaded couplings (tool joints) must be strong, reliable, rugged and safe to use. They must be easy to make up (connect) and break out (disconnect). Outer diameters for drill pipe range from 23⁄8 to 6 5⁄8 in.

The hollow drill string provides a means for continuous circulation and for pumping drilling mud under high pressure through the bit nozzles as a jet of fluid. The blast of mud knocks rock cuttings from under the bit, gives a new rock surface for the cutters to attack and starts the drill cuttings on their trip to the surface. This transmission of hydraulic horsepower from the mud pumps to the bit is a very important function of the mud.

Coiled-tubing drilling. This method employs a continuous string of coiled tubing and a specialized, coiled-tubing drilling rig. Rather than drilling with separate joints of the traditional, largediameter, rigid drill Rather pipe, the drillstring is smaller-diameter, flexible tubing. Unlike drill pipe which is screwed together to form the drillstring, and which must be disconnected into stands that are racked in the derrick during trips, the tubing comes rolled on a reel that unwinds as drilling progresses and is subsequently rewound onto its spool during trips. The coiled-tubing method greatly facilitates lowering and retrieving the drilling assembly.

Traditionally, coiled-tubing rigs have been used for workover and completion operations where mobility and compact size were important. With the development of downhole mud motors which do not require the use of a rotating drillstring to turn the bit, coiledtubing units are now functioning as true drilling rigs.

 

DRILL BIT ROTATION

Regardless of bit type, it must be rotated in order to drill the rock. There are three methods used to turn the bit downhole:

1. The drillstring and bit are turned by a rotary table and kelly.

2. The drillstring and bit are rotated by a “top-drive” motor.

3. Only the bit is rotated by a hydraulic mud motor in the drillstring. (The drillstring can be held still or rotated while using a mud motor, as desired.)

Rotary table and kelly. A rotary table is a gear- and chain-driven turntable mounted into the rig floor that has a large open center for the bit and drillstring. The rotary table kelly bushing is a large, metal “donut” with a 4-, 6- or 8- sided hole at its center. This bushing can accept a special piece of 4-, 6- or 8-sided pipe, called the kelly. The kelly, which is about 40 ft long, is turned by the Kelly bushing in the rotary table, just as a hex nut is turned by a wrench. The kelly is free to slide up and down in the Kelly bushing so it can be raised while a 30-ft joint of drill pipe (the topmost joint in the drillstring) is attached to its bottom. The drill pipe is then lowered into the hole until the bit touches bottom, and the kelly can be rotated. The driller starts the rotary table, and as the bit drills down, the kelly moves down, too. When the top end of the kelly is level with the bushing (at rig floor level), the kelly is broken out from the drill pipe, raised while another joint is added, and the process of drilling down is repeated. In order for the drilling mud to get into the drillstring, a rotary hose and mud swivel are attached to the top of the kelly to supply mud from the mud pumps. The swivel is a hollow device that receives mud from the stand pipe and rotary hose and passes it through a rotating seal to the kelly and into the drillstring. One disadvantage of the kelly/rotary arrangement is that while pulling pipe with the kelly disconnected, no mud can be pumped and pipe rotation is minimal.

Top drive. A top-drive unit has important advantages over a kelly/rotary drive. A top-drive unit rotates the drillstring with a large hydraulic motor mounted high in the derrick on a traveling mechanism. Rather than drilling one 30-ft joint before making a connection, top drives use 3-joint (90-ft) “stands” of drill pipe and greatly reduce the number of connections and the time to make a trip. One key advantage — the driller can simultaneously rotate the pipe while going up or down over a 90 ft distance in the hole and circulate mud. This allows long, tight spots to be quickly and easily reamed without sticking the pipe. Due to these advantages, top drive units are being installed on most deep rigs and offshore rigs.

Mud motor. While the first two rotation methods involve turning the drill pipe in order to turn the bit, this method is different. In this case, there is a hydraulic motor (turbine or positive-displacement mud motor) mounted in the BHA near the bit. During drilling, hydraulic energy from the mud passing through the motor turns the bit. This is achieved through the use of multiple rotor/stator elements inside the motor which rotate a shaft to which the bit is attached. This offers several advantages. Mud motors can achieve much higher bit rotational speeds than can be achieved by rotating the entire drillstring. Less energy is required to turn just the bit. The hole and casing stay in better condition, as does the drillstring, when only the bit (and not the pipe) rotates. Higher bit RPM results in improved Rate of Penetration (ROP), and vibration is less of a problem. Mud motors are used extensively for directional drilling where it is essential to keep an orienting tool positioned in the desired direction.

 

BLOWOUT PREVENTERS

A drilling mud should have sufficient density (mud weight) to prevent (hydrostatically) any gas, oil or saltwater from entering the wellbore uncontrolled. Sometimes however, these formation fluids do enter the wellbore under great pressure. When this happens, a well is said to “take a kick.” It is especially risky if the fluid is a gas or oil.

To guard against the dangers of such events, rigs are usually equipped with a stack of Blowout Preventers (BOPs). Depending on the well depth and other circumstances, there will be several BOP units bolted together and then to the surface casing flange. One or more of these BOPs can be engaged to seal off the wellbore if a kick occurs. Multiple BOPs in the stack provide flexibility and redundancy in case of a failure.

At the top of the BOP stack is a bag type preventer commonly referred to as a Hydril. This unit contains a steelribbed, elastomeric insert which can be expanded hydraulically to seal the annulus. Below the bag preventers are the ram-type preventers with hydraulically driven rams that close against the pipe or against themselves, thrusting in from opposite sides of the pipe. These preventers can be pipe, blind or shear rams. Pipe rams have heads with a concave shape to grip the pipe and form a seal around it; they accomplish the same function as the bag preventer but are rated at higher pressure. Blind rams come together over the hole to form a fluid-tight seal against one another in the event the pipe is not in the well or if it has parted and fallen down into the wellbore. Shear rams sever the pipe before sealing together.

Below the blowout preventers is the drilling spool. It has an opening in its side to allow drilling mud and the kick fluids to be pumped out. A high-pressure choke line connects to the spool with a special back-pressure valve (the choke) in the line. During well-control procedures, the choke is used to hold back-pressure on the annulus while heavier mud is pumped down the drillstring to kill the kick. If the invading fluid contains gas, the gas must be removed from the mud exiting the well. Gas-cut mud from the choke is sent to a mud-gas separator vessel. The gas is flared and the mud is returned to the pits for reconditioning.

 

CASING AND LINER

When a well is being drilled, exposed formations must be periodically covered and protected by steel pipe. This is done for several reasons — to keep the hole from caving in, to protect the formations being drilled and/or to isolate different geological zones from each other. These protective pipes are called casings and liners. Casing refers to pipe that starts at the surface or mud line and extends down into the borehole. The term liner applies to pipe whose upper end does not reach the surface or mud line but is inside and overlaps the bottom of the last casing or liner. Casing and liners are either totally or partially cemented in place.

Casing. Two, three or more casing strings may be run in a well, with the smaller pipe being run inside the larger sizes, and the smaller ones going deeper than the larger. The “surface casing” is run and cemented at a depth to protect freshwater aquifers and to avoid mud seepage into shallow sand and gravel beds; it might be set at about 2,000 ft. The next string is the “intermediate” casing. It is run and cemented when there’s a need to change the mud to a density that can’t be tolerated by the exposed formations or by the surface casing. Below the intermediate casing may be another string of casing or a liner.

Liners. It may not be necessary, economical or practical to line the entire, already-cased hole all the way to the surface just to protect the lower open hole. This is especially true as the hole nears total depth and becomes smaller. So a liner is run from the bottom of the hole, up into the casing, overlapping it by several hundred feet. Liners are held in place inside the casing by special tools called liner hangers. The practice of running a liner protects the last open hole interval, which often includes the reservoir section.

 

CEMENTING

After a string of casing or a liner has been properly landed in the hole, a cement slurry is mixed and quickly pumped down the inside of the casing (or liner). Pressure drives it out the bottom and up into the annular space between the pipe and the hole wall. Cement is followed downhole by just enough fluid to push all but the last part of it out of the casing or liner. Once all the cement hardens, that small quantity still inside the casing or liner is drilled out and the hole proceeds into a few feet of new rock beyond the end of the casing. Then the casing or liner is pressure-tested to see how much mud weight it will be able to hold, for future reference. If it fails the test, a remedial cement job (squeeze) may be required. Once the cement job passes the pressure test, drilling can resume.

 

Producing Petroleum

 

WELL COMPLETION

The next step, after setting casings and liners, is the completion phase of a well. Completion simply means making the well ready to produce oil and gas under controlled pressures and flow rates. Figure 7 shows the four common completion techniques. In all four, the casing prevents the formations above the producing zone from collapsing into the wellbore. If the producing formation is strong enough, as in the case of limestone, a length of casing can be cemented immediately above it, leaving the producing formation unsupported. This is called an open hole completion. If the reservoir rock needs support, other methods can be used:

Perforated casing or liner. In this method, casing or liner is run all the way through the producing zone and cemented in place. Then, holes are shot (by explosive charge) through the casing and cement, into the formation. These perforations are created with a perforating gun that is lowered into the hole on a wireline. The gun is then fired electrically, and powerful, shaped charges perforate the pipe and the zone at predetermined intervals. Once the perforations have been made, oil and/or gas can flow into the casing.

Perforated or slotted liner. In the second method, a pre-perforated or slotted liner (with holes or slots that are level with the producing zone) is hung from the bottom of the last string of casing. If the producing formation is weak or poorly consolidated, sand and other solids will be carried into the well as the oil or gas is produced. To prevent this “sand production,” the slotted or perforated liner may contain a wire-wrapped or a prepacked- gravel protective layer to keep the sand from entering the wellbore.

Gravel packing. Another approach that is helpful if the producing formation is weak (such as loose sand), and must be supported or held back, is the conventional gravel pack. A gravel packing operation consists of circulating and placing carefully sized gravel into the annular space between the liner and the wellbore wall. The pack forms a permeable layer to exclude any formation particles from the wellbore that become loose during production.

 

 

Literature.

 

1. “Drilling fluids Engineering Manual”. - Magcobar Division Oilfield Products Group Dresser Industries, INC. Houston, Texas, 2001.