Finding Oil and Gas _Hyne

8/2/2019 Finding Oil and Gas _Hyne

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Finding and Producing Oil and Gas

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Contents:

Chapter 1 - Gas and Oil

Chapter 2 - Petroleum Geology

Chapter 3 - Petroleum Exploration - The Prospect

Chapter 4 - Land - Leasing and Site Preparation

Chapter 5 - Drilling the Well

Chapter 6 - Testing the Well

Chapter 7 - Completing the Well

Chapter 8 - Producing Gas and Oil


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Chapter 1

Gas and Oil

Both natural gas and crude oil are called hydrocarbons because both are composed of carbon andhydrogen atoms. These hydrocarbon molecules range in size from one carbon to over sixtycarbon atoms in length. On the surface of the ground, under surface temperature and pressure,natural gas is composed of a mixture of the short hydrocarbon molecules that range from one tofour carbons in length. In contrast, liquid crude oil is composed of a mixture of hydrocarbonmolecules that have five or more carbon atoms.

There are many different types of crude oils. One of the most common ways to describe andcompare oils is by their densities. Some oils are light and others are heavy. The oAPI scale isused to describe oil densities. Average weight oils have 25 to 35 degrees API gravities. They

tend to be black with a possible greenish tint and are fluid like water. Light oils have highdegrees API gravities (above 35o). They are very fluid and can be produced at a high rate. Lightoils can be transparent with a yellowish tint. They are rich in gasoline and refineries often pay apremium price for them. Heavy oils have degrees API gravities below 25. They are black in colorand are very viscous and difficult to produce. Heavy oils are rich in asphalt and refineries do notpay a good price for them.

Some crude oils contain a significant amount of the impurity sulfur. Because the sulfur must beremoved from the crude oil before the oil can be refined, the refiner pays less for oil that containssulfur. Oil that contains little or no sulfur is called sweet crude. Oil that contains more than 1%sulfur is called sour crude and the refinery will pay $1 to $2/barrel less for sour crude.

West Texas Intermediate (WTI) is a benchmark crude oil used by the United States to set pricesand compare other oils. It has 38 to 40o API gravity and 0.3% sulfur.

Natural gas is a mixture of four hydrocarbon gasses. The most abundant is methane (CH4) withone carbon atom. In order of lesser abundance is ethane (C2H6),propane (C3H8) and butane(C4H10). Natural gas from a well is sold to a pipeline - to be taken to market and burned. Allfour hydrocarbon gasses burn. Propane and butane are the hottest burning but are least abundantin natural gas. The natural gas coming out from a well, however, contains other gasses that donot burn and are called inerts. They detract from the heat content of the natural gas and areunwanted. Some common inerts are steam (water vapor), carbon dioxide and nitrogen.

Some natural gasses contain hydrogen sulfide. This is a poisonous gas and will corrode metal.Unfortunately there is a lot of metal equipment in and on a gas well such as pipes and valves.Production of natural gas that contains hydrogen sulfide will cause production problems. Afterproduction, the hydrogen sulfide must be removed from the gas by expensive equipment at thewellsite before the gas can be sold to a pipeline. Natural gas that contains detectable amounts ofhydrogen sulfide is called sour gas and natural gas without it is called sweet gas.


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In some subsurface natural gas reservoirs, where temperatures are very high, some naturallyliquid hydrocarbons occur as a gas mixed in with natural gas. When the gas is produced on thesurface, these hydrocarbons condense out of the natural gas as a liquid called condensate.Thiscondensate has the chemical composition of gasoline and is often called natural gasoline. Like

gasoline, condensate is very fluid, light in weight (high degrees API gravity) and is transparentwith a yellowish tint. The refinery pays a price similar to crude oil for condensate. Because theoctane of condensate is very low, the price will be slightly less than that for crude oil. Naturalgas that contains this condensate is called wetgas. Dry gas is almost pure methane gas.

Oil is measured in barrels containing 42 U.S. gallons (Figure 1-1). The abbreviations b/dorbopd is used to describe production from a well in barrels of oil per day. Natural gas is measuredin volumes of 1,000 cubic feet (Figure 1-2). The abbreviation Mcf/d is used to describeproduction from a well in thousands of cubic feet per day. MMcfand Bcfareused for million andbillion cubic feet. The natural gas is also tested for the amount of heat it produces where burned.The heat is measured in Btus (British Thermal Units). One Btu is about the amount of heat given

off by burning one wooden match. One cubic foot of natural gas commonly has from 900 to1200 Btus of heat. In general, the higher the Btu content of the gas, the higher the price for thegas.


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Chapter Two

Petroleum Geology

The surface of the land where a well is drilled is composed of layers ofsedimentary rocks,thousands of feet thick (Figure 2-1). These rocks were deposited millions of years ago whenshallow seas repeatedly covered the surface of the land. Sand grains, deposited along ancientbeaches, formed sandstone rock, mud particles deposited on the sea bottom formed the rockshale and limestone rock was formed by shell beds and reefs. The crust of the earth where thewell is drilled contains hundreds of sedimentary rock layers of sandstones, shales and limestones.Below the sedimentary rock layers, is formerlymolten rock. It is unproductive for oil and gas andis called basementrock.

Gas and oil is found in sedimentary rocks. Where the sedimentary rocks are very thick (10 to

50,000 feet) is where the most gas and oil is found. These areas are called basins. The Anadarkobasin of Oklahoma and the Williston basin of Montana are examples. In other areas, such aseastern Canada, there are no sedimentary rocks and the basement rock is on the surface. There isno gas and oil in these areas.

In many sedimentary rocks, tiny spaces between rock particles calledpores are found (Figure 2-1). These pores are located between sand grains in sandstones and between shells in limestones.In the subsurface, these pores are filled with fluids, usually water. In some areas, however, thepores are filled with gas or oil. This is the spot where you want to drill a well.

In order for a well to be a success, two subsurface conditions must be met. First, there must be a

reservoir rock - a sedimentary rock layer that can store the gas or oil. The reservoir rock musthave both porosity and permeability. Porosity is a measure of the holes the pores in a rock. It isexpressed as a percent volume which indicates the storage capacity of the reservoir rock for gasand oil. Porosity of reservoir rocks generally ranges from 30 to 5% with the higher percentagesbeing more favorable. Permeability is a measure in which the fluids can flow from pore to porethrough the rock (and into the well). It is measured in units called millidarcies (md). Thepermeability of reservoir rocks ranges from 1,000 to 10 md with the higher numbers being thebest. Most reservoir rocks are sandstones and limestones. Shales lack permeability and are not


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reservoir rocks. Each sedimentary rock layer has been given a formal name by geologists such asTuscaloosa Sandstone, San Andreas Limestone or Coffeyville Formation.

The second criteria for a successful well is a trap on the reservoir rock. The trap will concentrate

the gas and oil in the pores of the reservoir rock into commercial quantities. A trap is a high areain the reservoir rock (Figure 2-2). All reservoir rocks originally contained water. Gas and oil islighter than water and will flow through the reservoir rock from pore to pore in an upwarddirection toward the surface. The trap is as far up in the reservoir rock as the gas and oil canflow. The trap in figure 22 is an circular uplift in the reservoir rock called a dome. Directlyoverlying the reservoir rock in the trap must be a cap rock. The cap rockis impermeable andforms a seal that does not allow fluids to flow through it. Without a cap rock, the gas and oilwould leak to the surface. Two common sedimentary rock layers that act as cap rocks are shalesand salts.

A petroleum trap can be either structural or stratigraphic (Figure 2-3). A structural trap isformed by deformation of the reservoir rock. Examples are an anticline ( Figure 2-3a), a long,upward arch, or afault(Figure 2-3b), a break in the rocks along which there has beenmovement. A stratigraphic trap is formed during the deposition of the reservoir rock. Thereservoir rock is encased in shale which holds the gas and oil in the reservoir rock. Examples area limestone reef ( Figure 2-3c), a sandstone river channel (Figure2-3d) or a sandstone thatpinches out in an upward direction (Figure 2-3e).


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In the trap, the gas, oil and water separate according to density. Natural gas, the lightest, goes tothe top of the trap where it fills the pores of the reservoir rock to form thefree gas cap (Figure2-2). Oil fills the pores of the oil reservoir. Salt water, the heaviest, goes to the bottom. Theboundary between the free gas cap and the oil reservoir is called the gasoil contactand theboundary between the oil reservoir and the water is the oilwater contact( Figure 2-2). Both arelevel.

Oil and gas occur in the tiny pores of the reservoir rock. The oil and gas, however, share eachpore with some water. There is never 100% oil or gas in the pores. The amount of water thatshares the pores with oil or gas varies from field to field and is called saturation. Saturation isexpressed as a percentage of oil and water or gas and water that occupies the pores and always

adds up to 100%. For example, an oil reservoir could have 50% oil and 50% water saturation.Typical oil field saturation is 50% oil 50% water to 80% oil 20% water saturation. This explainswhy most oil wells also produce very salty water called oilfield brine. This is the salt water thatalso shares the pores with the oil or gas.


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Chapter Three

Petroleum Exploration The Prospect

It is the job of a geologistto select a drill site. Usually the identity of the potential reservoir rocksare known from wells that have already been drilled in this area. The geologist explores for a trapon the reservoir rock using two subsurface methods correlation and mapping.

Correlation is the matching of rock layers from one area to another of the subsurface. This isdone between wells that have already been drilled in the area. Whenever a well is drilled, a veryaccurate record of the rock layers in the well is made on a well log. By correlating rock layersbetween well logs (Figure 3-1), a cross section can be made to locate any traps.

Figure 3-1 Correlation

The geologist also uses three types of maps to locate traps.A base map of the area will show thelocation of all the wells that have already been drilled. Map symbols (Figure 3-2) will show ifthe well is an oil producer, gas producer or dry hole (not commercial). The cumulativeproduction of each well should be listed next to the well.

Figure 3-2 Well symbols

Two types of subsurface maps are structural and isopach (Figure 3-3). The structuralmap usescontour lines to show the depth in feet below sea level to the top of the potential reservoir rock.This map is used to locate high areas (traps) on the reservoir rock. The isopachmap uses contourlines to show the thickness of the reservoir rock. It can be used to estimate the amount of oil orgas in the reservoir.


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Figure 3-3 Subsurface map

The data for correlating and drawing subsurface maps comes from wells that have already beendrilled. This is called well control In general, the more the wells, the better the well control andaccuracy of the correlations and maps.

Seismic is an exploration technique run by geophysicists. A seismic crew will put an impulse ofsound into the subsurface (Figure 3-4). Some of the sound will be reflected off subsurfacesedimentary rock layers to return to the surface as echos. These echos are recorded by detectorson the surface to image the shape of the rock layers and locate traps.

Figure 3-4 Seismic

Two common types of seismic sources used on land today are explosives and vibroseis.Vibroseis uses a vibratortruckto shake the ground for a period of time. The air gun whichreleases a high pressure bubble of air is commonly used as a seismic source in the ocean. Thedetectors used for seismic on land are called geophones orjugs and those in the ocean are calledhydrophones. The seismic is printed on a seismic record (Figure 35) that is similar to a crosssection.


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Figure 3-5 Seismic record

A recent development is 3Dseismic. This gives a three-dimensional view of the subsurface on acomputer monitor that is similar to a hologram (Figure 3-6 ). Slices at various levels of thesubsurface are called time or horizontal slices. It is very expensive but is more accurate anddetailed than 2D seismic. Seismic significantly decreases the number of dry holes drilled.

Figure 3-6 3D seismic with time slices

An authority for expenditure (AFE) ismade that lists all the estimated costs of drilling andcompleting the well. Costs are listed two ways. First as a dry hole that is drilled and then plugged

and abandoned and secondly as a completed well. An estimate called reserves is also made ofhow much gas and oil the proposed well will produce.

Part of the proposal to drill should be prospect economics. Two common factors are payout andreturn on investment. Payout(PO) is the time that it takes the oil and gas revenue to equal thedrilling and completion costs. It is expressed in months or years.Return on investment (ROI) isthe net oil and gas revenue divided by the costs of drilling and completing. The shorter thepayout and the higher the return on investment, the better the economics of the deal.


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Another important factor is riskor chance of success. This is a measure of how probable it is thatthe well will be a producer and not a dry hole. It is expressed as a percentage with the higherpercentages being more favorable.

Two parties to the deal are the royalty owners and the working interest owners. The royaltyowners, such as the land owner, get their share of the gross production revenue before any costsare taken out and are not liable for any well costs. The working interest owners get their share ofthe net production revenue after costs and are liable for all well costs.


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Chapter Four

Land - Leasing and Site Preparation

A wildcator exploratory well isa well drilled to find a new gas or oil field. A developmentalwell is drilled in the known extent of a field. The chance of success of a developmental well isgreater than a wildcat well but the rewards (oil and gas revenues) are usually smaller.

If the land on which the well is to be drilled is privately owned and located in the United Statesor western Canada, it is called fee land. Fee landhas two, separate, legal ownerships to theland the surface and mineral rights. The surfacerights ownercan ranch, farm or build on thesurface of the land. The mineral rights ownercan explore and drill for gas and oil on the landand owns the gas and oil. The surface and mineral rights owners are often the same but theownership can be separated and different parties can own each. Legal arrangements must be

made with the mineral rights owner before a well can be drilled. In every other country of theworld, the federal government owns the mineral rights. It is a job of a landman to locate theminerals rights owners. They are on file in the county or parish courthouse. The landmanapproaches the mineral rights owner with a legal document called a lease.

By signing the lease, the mineral rights owner, called the lessor, willreceive both a bonus and aroyalty.A bonus is front end money for signing the lease (for example: $50 per acre). A royaltyis a promised fraction of the gross oil and gas revenue from that land (for example: 1/8). Theowner of the lease, called the lessee, has the legal right to explore and drill for gas and oil on thatland for the life of the lease. The lease has a definite time limit for exploration called theprimaryterm. It is commonly three, five or ten years. If commercial production from the land is not

established before the primary term expires, the lease becomes invalid. If commercial productionis established, the life of the lease is extended to cover all the future production from the landduring the secondary term of the lease.

Leases are real property; they can be bought, sold and traded. If someone has a lease you want,you might be able to persuade them to give (farmout) the lease to you for consideration such asmonies or a royalty.

The geologist selects a drill site and a drilling target. A drilling targetis a potential reservoirrock. It is identified such as the Bartlesville Sandstone and the depth is estimated. A contractmust now be signed with a drilling contractor to drill the well. Three types of drilling contracts

are footage, day rate and turnkey. Afootage contractcharges on a per foot basis down to thedrilling target, such as $50/foot. It is commonly used in welldrilled areas. A day rate contractcharges for each day the rig is drilling to the drilling target such as $5,000 per day. It is used inareas where subsurface conditions might unexpectedly extend the anticipated drilling time andon all offshore drilling rigs. A turnkey contractincludes the entire cost of drilling and completingthe well such as $200,000. It is often used by drilling funds to control expenses.


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Before the well can be drilled, it must be staked. Lawrequires the well site to be accuratelysurveyed and a map, called a plat, to be registered with the state regulatory agency. The state willalso award well spacing such as 40 acres. This is the area upon which only one well can bedrilled. The state uses well spacing to control production and prevent wastage that was common

in the days before state control.

Before the rig can be brought onto the well site, a bulldozer will probably have to make anaccess road and level off the well site. The reserve pit might have to be dug to store excessdrilling mud.

If the well is going to be relatively deep, there is a space provided for mounting the blowoutpreventers on the well below the drill floor.

A rectangular pit, called a cellar,isdug in the ground. It is lined with boards or cement.

Loose soil could collapse into the well as it is being drilled. To prevent this, a truck with anauger drills a large diameter hole into the ground. Large diameter (about 20 inches), steel pipecalled conductor casing, is lowered and cemented into the hole. The blowout preventers can nowbe bolted to the top of the conductor casing and the large rig can be brought onto the drill site.When the big rig starts to drill the well, it is called spudding the well.


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Chapter Five

Drilling a Well

Today, the well is drilled with a modern rotary rig (Figure 5-1). It is called a rotarydrilling rigbecause there is a long length of steel pipe with a drill bit on the end suspended in the hole. Thepipe is rotated from the surface to rotate the bit and cut the hole on the bottom. The rotatingdrillpipe and the bit is called the drillstring.

The most common bit is the tricone bit(Figure 5-2) with three rotating cones mounted onbearings on the bottom of the bit. On the cones are teeth that are designed to flake the rocks onthe bottom of the well into well cuttings, small rock chips.


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The drillpipe comes in thirty foot sections which are threaded on both ends and are calledjoints.As the well is drilled deeper, another joint of drillpipe must be added each thirty feet in an

operation called making a connection. After an average of 24 to 48 hours of rotating, the drill bitbecomes worn out. To change the bit, all the drillpipe must be pulled out of the hole (trippingout), the bit changed and all the drillpipe put back in the hole (tripping in). This procedure takestime, and the deeper the well, the longer it takes.

The power to the drilling rig is supplied by diesel engines located on the ground next to thedrilling rig. They are connected to the drilling rig by a mechanical transmission. On some of themore modern, large land rigs and on all offshore drilling rigs there are still diesel engines butthey drive a generator that makes electrical power. The electrical power is fed to the rig byelectrical cable to drive electrical motors. It is called a dieselelectric system.

The hoisting system on a drilling rig (Figure 5-3) is used to raise and lower equipment such asthe drillstring in the well. It includes a steel tower located directly above the well, calledthederrick. On the drill floor is the drawworks which contains a reel of hoisting line made ofbraided, steel cable. The hoisting line goes through a series of wheels in a steel frame at the topof the derrick called the crownblockand through another series of wheels in a steel frame beingsuspended in the derrick called the traveling block. Below the traveling block is a hook to attachequipment to be raised or lowered in the well such as the drillstring. When the hoisting line iswound back onto the reel in the drawworks, the traveling block goes up and the drillstring israised from the well. When the hoisting line is let off the reel in the drawworks, the travelingblock goes down and the drillstring is lowered into the well.


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Drilling mudismade at the drillsite by mixing fresh water with dry clay called bentonite thatcomes in sacks. It is pumped down the rotating drillstring (Figure 54). The drilling mud jets outof nozzles on the bottom of the bit and picks the well cuttings off the bottom of the well. Thedrilling mud and well cuttings then flow up the well in the space called the annulusbetween therotating drillstring and the sides of the well. On the surface, they flow through the mud returnline and onto the shale shakerwhich is located on the mud tanks. Screens in the shale shakerseparate the well cutting from the drilling mud which flows through the screens and into the mudtanks where it is stored. Pumps next to the mud tanks, called mud hogs,then pump the drillingmud back down the drillstring. The drilling mud is continuously being cleaned and repumpeddown the well by the mud circulating system.

The mud circulating system does three things. It removes the well cuttings from the bottom ofthe hole to preventing them from clogging up the bottom of the hole and stopping the drilling.Secondly, it cools and lubricates the bit. The drilling mud also prevents any water, gas or oil


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from flowing out of the rocks and into the well. This is because the weight of the drilling mudexerts a greater pressure on the drilling mud at the bottom of the well than the pressure of anywater, gas or oil in the rocks. Therefore, instead of the fluids flowing into the well, some drillingmud is forced into the rocks surrounding the wellbore. As the mud is forced into the surrounding

rock, the clay in the mud is plastered onto the sides of the wellbore. This forms a mudorfiltercake which is as hard as cement and stabilizes the sides of the well. If gas or oil flowed out of therocks and up the well onto the drill floor, it could catch fire. If water flowed out of the rocks intothe well, the sides of the well could cave or sluff in. As long as the well is being drilled, the mudmust be kept circulating.

Because a drilling rig is very expensive, it must be kept operating 24 hours a day. Threeeighthour crews of workers called tours are used each day. On rigs in remote areas or offshorerigs, only two twelvehour tours are used each day. The drilling contractoristhe company whoowns and operates the drilling rig. The operatoristhe oil company who contracts for the drillingof the well. The one person who is ultimately in charge at the drillsite is called the toolpusher

He is an employee of drilling contractor and lives at the rig site in a trailer. Each morning thetool pusher puts together the dailydrilling or morning report. It is a summary of the last 24 hourson the drilling rig. The company man is an employee of the operator and works with the toolpusher to make sure the well is being drilled right. On the floor of the drilling rig, the drillergives the orders to the crew and operates the machinery. There are usually two to fourroughnecks on the floor of the drilling rig who do the general work under the command of thedriller.

A mud man periodically checks the viscosity and density of the drilling mud. The mud manconditions the mudby adding water and/or chemicals called additives.


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Chapter Six

Drilling Problems and Techniques

Usually, a well is drilled down tocontract depthwith no problems. As the well gets deeper,both the temperature and pressure increase. Both are usually predictable. The temperatureincreases at a rate of about 1.4 F per 100 feet and the pressure on the fluids in the pores of therocks increases at a rate of 45 pounds per square inch. For example, in Oklahoma where thesurface temperature averages 55 F, the temperature in a well at 10,000 feet would be 195 F andthe pressure would be 4,500 pounds per square inch.

Every so often, however, there is a problem during drilling. The most common problem is thatsomething will break and fall to the bottom of the well. For example, the drillpipe twists off andfalls down the well or a cone falls off the tricone bit. This is metal and you cannot drill through it

and it is calledjunkorfishat the bottom of the well. Drilling must be suspended and a specialtool called afishingtoolhas tobe leased from a service company. The fishing tool is thenlowered down the well to grapple for the fish in a process calledfishing. Fishing can take days.While you are fishing, you are not drilling the well but you are still being charged for the drillingrig. This charge can be avoided iffishinginsurance is bought before the well is drilled.

Another problem, called lostcirculation, is caused by drilling into a very permeableformation. The drilling mud is flowing into the lost circulation zonewithout building up a filtercake along the wellbore. Very little, if any drilling mud is circulating back to the surface. This isbad for two reasons. First, the drilling mud cost money. Secondly, there is only a limited amountof drilling mud at the wellsite. When you run out of drilling mud, you have to stop

drilling. Service companies sell finegrained, fibrous particles called lost circulation materialthat can be pumped down the well to clog up the lost circulation zone.

The most serious problem is caused by drilling into anabnormal high pressure zonewhere thepressure on the fluid (water, gas or oil) in the pores of the rock is much higher than expected.The drilling mud pressure cannot control it and fluids will flow into the well in what is called akick. Usually the driller has ample warning that there is a kick on the bottom of the well becausethe drilling mud will be flowing out of the top of the well at a very high rate and automaticdevices warn him. The drillerkills the wellby closing the blowout preventer. Very heavydrilling mud calledkill mudisthen pumped down the well to bring the well back under control.The well is then drilled deeper using the kill mud. Sometimes, however, the well is not killed in

time and natural gas flows onto the floor of the drilling rig and catches fire during ablowout. The rig and well are usually lost during a blowout.

When a well is drilled nearly straight down, it is called a straight hole. The well, however, nevergoes straight down because the drillstring is being turned clockwise (to the right) on a rotarydrilling rig. Instead, the well corkscrews down. If it is kept in a 5 cone, it is considered astraight hole. The well can be surveyed using a compass or gyroscope to make a directional log,a map of the well location at various depths.


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Rotary drilling today is such an art that the drilling contractor can almost always guarantee astraightholethat is drilled almost straight down, but also can make the well go off at an angleand end up exactly where you want it by usingdirectional drilling. Directional drilling is donewith a turbine or mud motor and diamond bit on the bottom of the drillstring. A turbine or mud

motor, located just above the bit, is driven by the drilling mud that is pumped down the center ofthe drillstring. A diamond bit is a solid metal bit with no moving parts (Figure 6-1). There arehundreds of small, industrial diamonds that are attached to the bottom and sides of the bit ingeometic patterns.

There are many uses for directional drilling and it is very commonly done today. One of the must

common uses issidetracking, to drill around a fish in the well. An extended reach wellhas avery large horizontal displacement (horizontalreach)from the surface to the bottom of the well(Figure 6-2). The world's record for a horizontal reach is over 6 miles.


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Horizontal drain holes(Figure 6-3), drilled parallel to the oil or gas pay zone, are commontoday. They often produce 3 to 5 times more oil or 5 to 20 times more gas than a straight hole. Adouble wing horizontal drain hole(Figure 6-4) has two horizontal branches going out inopposite directions in the pay zone.

Many wells are not drilled as straight holes and there are two ways to measure the depth of awell (Figure 6-5). Total depth (TD ) is measured along the wellbore. True vertical depth(TVD) is measured straight down. Horizontal reach is the horizontal displacement of adirectional well from the surface to bottom hole location of the well.


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Drilling offshore is very similar to drilling on land. The drilling rig, technique and crew are thesame except that only two 12hour crews of workers are used each day. In shallow, protectedwaters such as a lagoon or canal, the drilling rig is located on a barge. For exploratory drilling inshallow water (up to three hundred feet deep), the drilling rig is mounted on ajackup rig(Figure 6-6), a platform that has legs and sits on the sea floor. The jackup rig is towed to thedrillsite. The lower hull is flooded with water and sits on the ocean bottom. The upper hull isthen jacked up the legs. In deeper water, the drilling is done from a floating drilling platform(semisubmersible)that is anchored to the bottom in water depths up to one to two thousand feet.In very deep waters, drilling is done from a drillship that floats above the drillsite. It is kept onstation by a computer that constantly recalculates the ship's exact location. Propellers on the sideof the drillship called thrusters are used to constantly readjust the ship's location above the wellbeing drilled. This is called dynamic positioning. Drillships are very expensive to operate butcan drill in any water depth. To connect the drillship or semisubmersible to the well on thebottom of the ocean, a marine riser, a flexible, metal tube is used. The drillstring runs down thecenter of the marine riser. The marine riser gives a closed system to circulate drilling mud justas on land.


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Offshore wells are very expensive. It costs about 5 times more to drill the same well to the samedepth offshore than it does on land.


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Chapter 7

Testing the Well

Testing the well after it has been drilled is the most important part of the process. It costs more tocomplete the well than to drill it. Will this well produce enough oil or gas to make it worthwhileto complete the well? This is called thecasing point. Isit worth casing (completing) the well? Inthe early days of drilling, if they struck oil, the oil would often gush out the well and sometimesabove the top of the derrick during agusherThat's because the well was drilled with an earlytype of drilling rig called a cable tool drilling rig. There was no drilling mud in the well and theydidn't have effective blowout preventers. There was nothing on an old cable tool drilling rig thatwould set the oil or gas afire during a gusher. Today, the well is drilled with a rotary drillingrig. The well is kept filled with heavy drilling mud and there are effective blowout preventersinstalled on top of the well. If you get a gusher today, it will catch fire and the well will blowout.

The modern well is evaluated using three types of logs. Asample log(Figure 7-1) is a long stripof paper which shows the composition of the rock layers such as sandstone, limestone and shalein the well. As the well is being drilled, a well siteor well sitting geologistisat the rig. One ofthe geologist's responsibilities is to collect well cuttings at regular intervals. The geologist uses abinocular microscope to examine the well cuttings and make the sample log. A dark coating onthe well cuttings could be oil staining. To test for oil, the cuttings are placed in a tube containinga solvent and then examined under a fluorescent light. Oil will produce a brilliant flash called astreaming cut. The most accurate source of information is to core the well. Acore is a cylinderof rock, 3 to 6 inches in diameter, that is drilled from the well. Porosity, permeability and oilsaturation can be measured from cores. Coring, however, takes longer that just drilling through

the rock. This extra time on the drilling rig is expensive and, thus, cores are expensive. Becauseof the expense, usually only the reservoir rock is cored.


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Figure 7-1 Sample Log

Amud logger is a service company that continuously samples and analyzes the drilling mud andwell cuttings for oil and gas as the well is being drilled. Amud log is made in the mud loggingtrailer at the wellsite. Traces of oil and gas in the drilling mud are calledoil cut andgas cutmud. Traces of oil and gas in the well cuttings are called ashow of oiland ashow of gas. Themud log does not determine the amount of oil and gas (it could be just a trace) but does identifypotential reservoir rocks that deserve further tests.

The most important tests are made by wireline well logs that are run by a service company. A

logging truck is driven out to the well and is backed up to the well. A long (20 to 70 feet) metalcylinder containing instruments called asondeis unloaded from the truck and is lowered downthe well on a wireline that is wound around a reel in the back of the logging truck. As the sondeis brought back up the well, the instruments in it measure the electric, acoustic and/or radioactiveproperties of the rocks along the wellbore. These measurements are recorded in the loggingtruck on a long sheet of paper called a wireline well log (Figure7-2).


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Figure 7-2 Wireline Well Log

Some types of wireline well logs include electrical, induction, gamma ray, neutron porosity,formation density, sonic and caliper. By interpreting the curves on the wireline well log, thegeologist or engineer can determine the exact elevation and composition of each rock layer,

whether the rock layer has porosity and how much and what is the fluid (water, gas or oil) in thepores of the rock.


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Wireline well logs are made after the well has been drilled. A recent development isMWD(measurement while drilling)andLWD (logging while drilling). These are real time logs thatare recorded as the well is being drilled. The geologist on the floor of the drilling rig can use thisto identify the rocks through which they are currently drilling. The system also makes a

directional log that shows the driller where the drill bit is going. MWD or LWD is used on alldirectional wells.

Adrillstem testis similar to a temporary completion of the well (Figure 7-3). A pipe (drillstem)is run down the well which is still filled with drilling mud. Two, doughnutshaped devices madeof a rubberlike substance calledpackersare attached to it. The drillstem is lowered until onepacker is just below and the other packer is just above the zone to be tested. The packers arethen expanded to seal off that zone from the rest of the well.

Figure 7-3 Drillstem Test

Even though the well is still filled to the top with drilling mud, there is no pressure on the zonebeing tested because of the packers. Any fluids can flow out of the zone and into the well. Avalve is opened in the drillstem and the fluids from that zone flow into the drillstem. Gas willusually flow onto the surface where it is passed through an orifice (choke) of a certain size andthe amount and pressure is measured. Oil usually will not flow all the way up the drillstem to thesurface. The valve in the drillstem is closed and the height of the oil in the drillstem is measured.Fluid pressures are also continuously recorded during the drillstem test. A drillstem test willidentify the fluids (water, gas and/or oil) in the zone tested and determine the rate at which theycan be produced.


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Chapter 8

Completing a Well

After the tests have been run, the well is either plugged and abandoned or completed.Toplug and abandona well (P&A), cement is poured down the well to prevent anysalt water from flowing up the well and polluting fresh waters which occur in the nearsurface rocks. A metal plate is then welded to the top of the well.

Setting pipemeans to complete a well. Large diameter steel pipe calledcasing isscrewed together to form acasing stringthat is run into the well. Wet cement calledslurryis then pumped between the casing and sides of the well (Figure 81) by aservice company during acement job. The casing stabilizes the well, preventing the

sides from caving in, and prevents water from other formations from flowing into thewell.

Figure 8-1 Casing in a well

There usually is acasing programas the well is being drilled. The well is drilled,cased, drilled deeper and then cased again. The well is first drilled down to a certaindepth with a large diameter bit and then the drillstring is run out of the well. Large


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diameter casing, calledsurface casing, is cemented into the well. The well is thendrilled down to the drilling target and tested. A string of smaller diameter casing,calledproductionoroil stringcasing, is then run through the surface casing and into

the well and cemented (Figure 82a). Sometimes, an intermediate diameter casingstring calledprotectionor intermediatecasingisrun between the surface andproduction casing (Figure 82b). Each casing string runs back up the well to thesurface and each deeper casing string is smaller in diameter.

Figure 8-2 Casing programs

The bottom of the well is then completed either open hole or perforated. In anopen

hole completion, casing is run and cemented down to the top of the producing zoneand the bottom of the well is left open. Horizontal drain holes are often completedopen hole. In aperforated completion, casing is run and cemented through theproducing zone. Holes, calledperforations, are then shot into the casing at the level ofthe producing zone by aperforating gunwith shaped explosive charges (Figure 83).


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Figure 8-3 A perforated completion

Relatively small diameter, steel pipe calledtubingis then run down the center of thewell (Figure 83). The produced fluids (salt water, gas and oil) are brought up the wellthrough the tubing string. This is to prevent the produced fluids from coming incontact with the casing and corroding the casing. Because the casing has been

cemented to the sides of the well, it is very difficult, if not impossible, to repair thecasing. The tubing, which is being suspended in the well, is relatively easy to removeand repair during a workover.

The steel casting on the surface above the well is called the wellhead. If this is a gaswell, all gas wells flow to the surface by themselves. They don't have to be pumped.There are some oil wells in which the oil has enough pressure to flow all the way upthe tubing to the surface. These flowing oil wells usually occur only early during thedevelopment of an oil field.

To complete a gas or flowing oil well, a series of pipes, valves and gauges, called aChristmas tree, isbolted to the wellhead (Figure 84). On most oil wells, however, theoil does not have enough pressure to flow all the way up the tubing.


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Figure 8-4 A Christmas tree on a gas well

Inartificial lift, the oil is pumped up the tubing to the surface. A common artificial lifttechnique is abeam pumperorsucker rod pump(Figure 85).

Figure 8-5 A beam pumper on an oil well


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An electric motor or gas engine on the ground causes a steel walking beamto pivot upand down. Attached to the opposite end of the walking beam is a long, small diametersteel rod called asucker rodstring. Sucker rods come in twenty five foot lengths that

are screwed together to form the sucker rod string. The sucker rod string runs all theway down the well through the tubing to the downhole pump on the bottom of thetubing. The walking beam causes the sucker rod string to rise and fall. This activatesthe downhole pump which lifts the oil up the tubing to the surface.

Most oil wells also produce salt water and natural gas. The salt water, calledoilfieldbrine, shares the pores of the reservoir with the oil. The natural gas which is dissolvedin the subsurface oil, bubbles out of the oil on the surface. The produced fluids flowthough a plastic or steelflowlinefrom the wellhead to a long, metal tank, called aseparator(Figure 8-5). The crude oil is separated by gravity from the oilfield brine

and gas in the separator. The crude oil then flows though a flowline to large, metaltanks,stock tanks, where it is stored.

When the stock tanks are filled with oil, either a service company picks up the oil witha tank truck and takes it to the refinery or the oil is transferred to a pipeline and takento the refinery. In both cases, the service or pipeline company gives the operator of thewell arun tickettelling how much oil was taken, the degrees API gravity andtemperature of the oil. The operator is then paid from the run ticket. A sample of theoil is centrifuged to measure thebasic sediment and water (BS&W) content of the oil.This is a measure of the impurities of the oil. The refinery or pipeline usually willaccept oil with a maximum of about 1% BS&W.


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The gas from a gas well is always sold to a pipeline that takes the gas to a market tobe burned for heat. Most natural gas coming from wells contains impurities that mustbe removed in the field in a process calledgasconditioningbefore the pipeline will

accept it. The gas conditioning equipment is installed in the field next to the gas wells.Water is taken out in a process calleddehydrationand corrosive gasses such ascarbon dioxide and hydrogen sulfide bysweetening.

Some wells produce wet gas containing a valuable liquid hydrocarbon calledcondensate. Wet gas is almost never put in a pipeline. The valuable liquidhydrocarbons are removed in the field by agas processing plant. The wet gas iscooled to remove the condensate, butane, propane and ethane which are callednaturalgas liquids (NGLs). The NGLs are then sold to a refinery and the dry gas (methane) isput in the pipeline.

In some wells there are two or more producing zones. The production from the zonescan be mixed (commingled) and brought up the well through a single tubing string.Production from two producing zones, however, is commonly kept separate by a dualcompletion in the well (Figure 87). Two packers are used to isolate the productionwhich is brought up two tubing strings.


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Figure 8-7 A dual completion

On an offshore field, the wellhead, separation and treatment equipment must go on aproductionplatform(Figure 8-8 ). Most offshore platforms are fixed and have legscalled the steel jacket that are pinned to the ocean bottom. On top of the steel jacket isone or more decks. The field is developed from the production platform usingdeviation drilling so that the field is serviced from one platform. Up to 62 deviatedwells can be drilled from one platform although the number is usually much smaller.The produced fluids from the wells flows onto the platform where it is separated andtreated. The oil and/or oil then goes ashore by submarine pipeline. At least one derrickis left on the offshore platform for well workovers. Equipment for offshore workovers

and well stimulation is skid mounted and brought out to the offshore platform on acrane barge. It is lifted onto the platform by the crane.


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Figure 8-8 An offshore production platform

The deepest water production by a fixed production platform is 1,350 feet of water. Invery deep waters, floating production platforms (tension leg platforms and spars) oranchored ships (FPSO vessels) are used. Some wellheads are located in over 5,000feet of water and the production is brought to a production platform in shallowerwater by flowlines.


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About the Author

Norman J. Hyne is Professor of Petroleum Geology at the University of Tulsa in Tulsa,

Oklahoma. He is also president of NJH Energy, a company that owns and manages the operationof oil wells. Norman is well known for his "Basic Petroleum Geology for Non-Geologists" shortcourse that he has taught through out the world. He is the author of "Non-Technical Guide toPetroleum Geology, Exploration, Drilling and Production" and "Dictionary of PetroleumExploration, Drilling and Production" published by PennWell Books. Norman has also editedfour books on the petroleum geology of the mid-continent.

Documents

Finding Oil and Gas _Hyne