WHAT IS AN OIL AND GAS WELL?
Natural gas provides electricity as well as fuel for home heating and cooking.
il and gas are essential ingredients for a comfortable modern life. Oil is needed to create many products such as motor fuels, road tar, and lubricants. Natural gas is burned directly in cooking, to heat homes, and as a feedstock for plastics. Petroleum (oil and natural gas) are produced through wells that are connected to reservoirs thousands of feet deep in the ground.
Motor fuels like gasoline and diesel and many other useful products are made from oil.
Producing wells can be classified as gas producing, oil producing, or both oil and gas producing. After the oil and gas has flowed out of the well it is piped into vessels, separated into components, and sold. A vast network of pipelines, trucks, refineries, and terminals deliver the end products to all consumers.
Hydraulic Fracturing Pre-Test ◼ Question 1
◼ Question 7
This discipline is concerned with uncontrolled flow of oil and gas releases after a frac job: A. Flow Control B. Well Control C. Environmental Control
Prior to starting a frac job extra formation strength information can be obtained from a: A. Microfrac B. Net pressure plot C. Fluid sample D. Geologic map
◼ Question 2
◼ Question 8
fluid is potentially flammable. A. Kill B. Completion C. Frac D. Hydraulic
The deadliest position in hydraulic fracturing is loading and arming the perforating guns. A. True B. False
◼ Question 3
◼ Question 9
Induced fractures typically turn more the farther they are from the treating well. A. True B. False
Multiple operations taking place on a single location are called: A. Multi-Jobs B. SIMOPS C. CLO-Jobs D. Multis
◼ Question 4 Natural fractures are caused from fluid pressures, just like from a frac job. A. True B. False
◼ Question 5
◼ Question 10 The frac well must be hooked to the pipeline LACT prior to flowback. A. True B. False
Frac sand is safe to handle without wearing special protection. A. True B. False
◼ Question 6 A chiksan is a high-pressure swivel joint. A. True B. False
Oil and gas wells producing
All onshore oil and gas wells have the same basic process throughout their lifetimes:
wells producing from shale formations.
The United States has approximately
Large earth moving equipment is used to clear a pad to prepare for drilling and completions
Pad construction is clearing and flattening a piece of land of enough size to start drilling operations. Large earth moving equipment are used to create roads, remove surface obstructions, and flatten the surface dirt into a firm and flat pad. Installation of electric lines or pipelines may also be done at this time. Once pad construction is complete the drilling rig is moved in to drill the well.
Drilling creates the well which connects the underground reservoir to the surface. A sharp heavy piece of metal called a drill bit makes the hole that will later become the oil and gas well. As gravity pulls the bit into the ground it is rotated to grind the rock up into tiny pieces called cuttings. The cuttings are pumped to surface and drilling continues until the bit reaches the total planned depth. Steel casing is lowered into the well and cemented into place permanently.
Completion describes the different processes performed after drilling to open flow into the well and divert the flow paths into the properly sized tubing string. Stimulation is a completion process that increase the rate of flow from the reservoir into the well. A minimum flow rate is required to make the new well profitable. Frac is one stimulation method although there are others.
PRODUCTION Production is the most important phase because this is when the well begins to return its investment. Oil and gas are produced through valves in a wellhead at the surface and then sold. Some wells also produce saltwater which must be treated and disposed. When the well first starts production it also tends to produce oil and gas at its highest flow rate. Over time as the well depletes this flow rate will drop and eventually cease. Installing pumps and other lift equipment can temporarily increase the depleted flow rates, but eventually the well will become too depleted to produce economically and must be abandoned. A drilling rig â€œmakes holeâ€?, creating the borehole for the oil and gas well.
Drill bits crush and grind the rock which is then circulated out of the well with drilling mud.
ABANDONMENT Abandonment removes the equipment in the well and permanently plugs the hole. Cement forms a permanent barrier and the surface wellhead is also removed. The ground is filled back in and the land reclaimed for other uses.
PRESSURE PUMPING EQUIPMENT
Nylon webbing is often used as hose restraints.
hydraulic fracturing treatment consists of injecting a mixture of sand and water at high rate and pressure into an oil or gas well. The operation is called a frac job and the mixture of solids (such as sand) and water is referred to as a slurry. Sand is stored on location in boxes, larger compartmentalized containers, or silos. Conveyers carry the sand from these containers into a blender unit where it is mixed in with water at the specified pounds per gallon. Water is stored in 500-barrel tanks called â€˜frac tanksâ€™ or in lined pits. The sand-water slurry is injected at low pressure into a manifold. This manifold is
often called a missile because it looks similar to its namesake military equipment. Frac pumps add pressure to the slurry by pulling fluid from the low-pressure line on the manifold and reinjecting it into the highpressure line. The high-pressure line is connected by iron pipe to the treating well. Hammer unions connect each piece of the iron pipe to each other and high-pressure swivels (genericized: Chicksans) allow the pipe to make the turns it needs to reach the well. Hose restraints, clamps, and hobbles are used to reduce whip damage in the case that the pipe connections suddenly fail.
Schema of a skid mounted triplex pump for drilling
A centrifugal pump.
Pump types Pressure pumping is achieved through two basic types of pumps: centrifugal and reciprocating. Reciprocating type pumps use pistons that move back and forth in a cylinder to add energy in the form of pressure to the frac fluid. The reciprocating pump may be either a triplex with 3 cylinders or a quintplex with 5 cylinders. These pumps are positive displacement, meaning that they deliver a fixed volume of fluid with each stroke. This style of pump is preferred for delivering high pressures and
constant flow rates. These reciprocating pumps supply most of the pressure to the frac fluid. Centrifugal pumps are used to pull water from the frac tanks into the blender. These pumps are not positive displacement and are only used on the low-pressure side of the frac job. Since they are not positive displacement they can operate at a constant speed with a varying flow rate.
Sand mover with spilled frac sand.
Sand Many tons of sand are needed for each frac job. The sand must be trucked in either from local mines for in-basin sand or from rail terminals that bring the sand from across the country. These trucks are either pneumatic or boxes. The pneumatic trucks use a motorized blower to push pressurized sand while unloading and the
boxes are unloaded onto the frac site using a forklift. The sand is then stored in vertical silos, horizontal sand movers containing multiple compartments, or individual boxes that load directly onto a conveyor. Each sand handling method has pros and cons including the size of its footprint, equipment needed, and the amount of dust generated.
A hydration unit mixes dry guar into water to create gel. Note the large tub on the unit.
Blender Sand and water are mixed together in the blender unit. Dry sand moves from the storage containers on conveyer belts into a hopper at the rear of the blender. Screws pull the sand from the bottom of the hopper and into the blender pot. The sand-water slurry may also be mixed with small concentrations of other chemicals if special
properties are needed. If a thicker gel slurry is required, a hydration unit may be needed to mix the dry powder into the base fluid to create a gel. The mixture containing sand, water, and chemicals at the proper concentrations is injected at low pressure into the frac manifold.
Frac Pump Pumps are used to increase the amount of hydraulic energy in a flow stream. The amount of energy at surface is governed by the desired flow rate and pressure of the
fluid. Frac jobs are pumped at high pressures, sometimes over 10,000 Pounds Per Square Inch, requiring multiple pumps to achieve.
Frac Manifold Manifolds are used throughout the industry. Their purpose is to bring fluids in from one or more flow streams and divert them out to one or more flow streams. The frac manifold is commonly called a missile due to the appearance of its profile. Low pressure sand/water slurry is injected into
the frac manifold from the blender. The low-pressure flow stream is diverted out to all the pump trucks where it is pressurized and returned to the manifold at treating pressure. The high-pressure slurry is then piped to the well.
Frac Iron High pressure iron pipe controls the slurry flow after it leaves the pumps and until it enters the treating well. Each joint of pipe is connected to the next using a hammer union. In order to make turns and bends, curved swivel joints are added into the string of frac iron. These high-pressure swivel joints have a common and
genericized name â€œChicksanâ€? due to the manufacturer of the same name. In order to minimize whip damage in case of a broken joint, restraints of various design are used. These restraints may include a combination of nylon webbing or metal clamps.
US Patriot missile system
Frac iron delivering high pressure slurry from the pumps to the frac manifold.
Frac Head The frac head is where the high pressure frac lines connect to the well. Sometimes this piece of iron is called a â€˜goat headâ€™ as the pipe curling out of the head resemble the horns of a goat. The frac head is bolted on top of the frac valve.
The frac valve acts as a high-pressure master valve for the well during the high pressure frac treatment. It is generally not a good idea to use the wellâ€™s normal master valve because production equipment is typically not pressure rated for frac treating pressures.
Frac Labeling Exercise Label the frac job equipment below.
Label: Sand Movers Sand Trucks Water Tanks Frac Pumps Blender Missile Wellhead
Rig Up Frac Pad Name: ___________________________________________
Given Show the equipment layout for a single well pad using 2,000 HP frac pumps. Treating pressure is 12,000 PSI at 80 BPM. HHP = BPM x PSI x 0.0245 50,000 Gallon Stages, 75,000# Sand Stages 450’ x 450’
Wireline Completions Completions is the process, after the well has been drilled and cased, of opening flow paths from the reservoir into the well. The well is cased with steel and cement which is impermeable to flow from the reservoir. To allow flow from the reservoir into the
well a completion method called perforation is used. Perforations are placed in the wellâ€™s casing with an explosive charge that has been lowered into the well via wire cable.
Plug and perf In shale wells the permeability of the reservoir is still too low to flow economic quantities of oil and gas. After each section of the well is perforated it must be stimulated to make it economic. Many sections throughout the length of the well are perforated and stimulated. To make the frac stimulation effective, each section must be isolated from each other. If all the sections were stimulated at the same time, then the treatment would mainly be effective for one or two sections
Loading a perf gun.
and not affect the others. Zonal isolation can be achieved with a technology called plug and perf. Plug and perf uses wireline to both perforate the well, opening the zone up to frac stimulation, and to plug each stimulated zone so that only new perforated zones are stimulated. Each zone is stimulated individually until all the target zones are treated. Once all the zones are treated, the plugs are drilled out using a coiled tubing.
Reel of wireline cable.
Pump down In a vertical well gravity is the only force required to pull the wireline tools down to the target zone. In a horizontal well gravity no longer pulls the tool along the wellbore, a pushing force is required. To push the
plug along the horizontal wellbore a plug is attached to the end of the wireline. At surface a pump is used to push the plug along the horizontal section.
Depth control Each of the sections to be completed in the well are carefully selected by geologists for their potential productivity. Lowering the wireline tools into the exact position in the well is a challenge due to the changing stretch of the wire and wear on the sheaves measuring the depth of the tools. Depth control is maintained by matching downhole measurements with the analysis
made by the geologists. Classically, this is done using gamma ray measurements reviewed by the geologist and made during the original drilling of the well compared with gamma ray measurements made at the time the well is perforated. The measurement of depth is therefore not made from surface but rather made from readings downhole.
Worker reviewing a wireline log.
Wireline Quiz Name: ___________________________________________
◼ Question 1 Cased hole logging is not able to read gamma ray signals from the formation. A. True B. False
◼ Question 2 The is designed to be long enough to contain the wireline tool string prior to removing it from the well. A. Lubricator B. Strip C. Master Valve D. Wireline BOP
◼ Question 3 Wireline depths are always tied back into ground level. A. True B. False
◼ Question 4 Perforations in the wrong location are easy to repair. A. True B. False
◼ Question 5 Placing the perforation at the correct depth is done using. A. Perf Control B. Depth Control C. Counting collars from surface D. Hydrostatic Surge
Coiled Tubing After all the stages have received a frac treatment there is more work before the well can be turned on. The frac job has left behind large volumes of excess sand and water. The sand and plugs are removed using coiled tubing. A large reel of coil is hooked up to the well with a spool long enough to reach end of the well when uncoiled. Completion fluids, water with protective salts, are pumped down the coiled tubing to lift sand up and out of the well. Once the well is cleaned out it is ready for flowback.
Well control When oil and gas flows from the reservoir to the surface they bring pressure with them. Prior to completion there is no flow path into the well from the reservoir. After completion, these paths are open and the surface equipment must be up to the job of controlling the well. An uncontrolled release of oil and gas is called a blowout, which must be prevented. A wireline
Blowout Preventer (BOP) uses an annular element to hold back pressure while the wireline is in the hole. To insert and remove the wireline tools a lubricator is used. The lubricator is a large cylinder longer than the wireline toolset than can quickly be removed and replaced onto the well.
Flowback When the well is first turned on it is not quite ready for its normal production operation because it still contains an excess of water and other frac fluids. The normal production equipment is not capable of handling these fluids so the initial flowback is sent to temporary tanks. It is also important that the initial production is closely monitored so additional well tests
will be performed as well. Once the frac fluids have been flowed back then the well can be hooked up to the surface production facilities. The oil is then sent to a Lease Automated Custody Transfer (LACT) where it is purchased by a pipeline company and sent to refineries all over the world.
*Blowout Prevention Equipment (BOPs) used in the drilling process.
Flowback Quiz Name: ___________________________________________
◼ Question 1 Find the wellhead pressure of a well with 9PPG completion fluid and a formation EMW of 16PPG and a top perf at 10,000’ TVD. __________________________________________________________________________ __________________________________________________________________________ __________________________________________________________________________ __________________________________________________________________________ __________________________________________________________________________ __________________________________________________________________________
◼ Question 2 Find the minimum kill mud weight for a 8,000’ TVD with a 7.7PPG fluid and 1200PSI WHP. __________________________________________________________________________ __________________________________________________________________________ __________________________________________________________________________ __________________________________________________________________________ __________________________________________________________________________ __________________________________________________________________________
◼ Question 3 What is a reasonable frac EMW for the well in question 2? __________________________________________________________________________ __________________________________________________________________________ __________________________________________________________________________ __________________________________________________________________________ __________________________________________________________________________ __________________________________________________________________________
Darcy components It has been known for decades that many shale formations contain large amounts of oil and gas. Prior to the shale boom in the United States, the production from these formations was considered too low to be economical. Two technologies, hydraulic fracturing and horizontal drilling, have since increased production from shale until it has become the majority source of oil and gas produced in the United States. Production in any well is driven by three factors that increase production rate: permeability, pressure, and formation thickness; and two factors that reduce production rate: fluid viscosity and drainage area. Each of these factors are initially limited by the rock formation but can be optimized with different well production methods. In shale, or other tight oil formations, the low production has been
due to the nano-darcy permeability of the rock itself. Modern shale production uses horizontal drilling and hydraulic fracturing to effectively increase the formation thickness and permeability of an oil and gas well. Increasing either one of these components tends to increase the production proportionally. This means that a doubling of either the effective permeability or a doubling of the effective formation thickness will double the productivity of the well. Doubling both the effective permeability and formation length increases the well productivity to four times. The formation height is increased in a straightforward manner using horizontal drilling. Consider a hypothetical formation that is 100 feet thick. A vertical well in such a formation is limited to a 100 foot
completion thickness. A horizontal well is limited to the state of drilling technology and not limited by the thickness of the formation. A horizontal well in our hypothetical 100 foot formation could be drilled to 200 feet lateral length, an effective doubling of the formation length and productivity over the vertical well. If our hypothetical well needs an even larger productivity increase it could be likewise increased to 1,000 feet for a ten-times increase, 5,000 feet for a 50-times increase, or a 10,000 foot lateral for a 100times increase in effective formation height and productivity.
Hydraulic fracturing effectively increases the permeability of the reservoir around the well, although this effect is much more complex than the increase from horizontal drilling. In shale reservoirs the natural permeability is low because the rock matrix is made of the smallest particles. During a frac job, high pressures break open the rock and create large pathways for the oil and gas to freely flow. Relatively large sand grains are left behind in these fractures in order to hold the fractures open with a limited reduction in permeability. Larger fractures with larger and betterquality sand grains will tend to increase the effective permeability of the fracture.
FACT: High temperature steam is used to increase the production of thick heavy oils. These oils have a high viscosity and flow slowly like molasses. Raising the temperature lowers the viscosity of the oil causing it to loosen and flow more quickly. An increase in:
Causes production to:
Rock Stresses After the frac job is pumped and the proppant is left behind the fracture will try to close in response to the bleeding down of the high pressure from the treatment. The fracture, however, can not close because of the proppant left behind. While the proppant is holding open the fracture the stress compresses the sand together. This compression causes the sand to compact, reducing its permeability. After spending all that time and money to pump the frac treatment and now the new permeability is immediately being crushed away by rock stresses! This effect can be reduced, but not eliminated completely, by using stronger sand. Stronger, more crush resistant sand, is more expensive and is optimized for the rock stress and the amount of permeability reduction that can be tolerated.
The rock stress is driven by the weight of all the earth overlaying the reservoir. Geologists call this effect overburden. Overburden, because it is caused by gravity, creates stress in the vertical (up and down) direction. Compaction of any material will force it to deform in the horizontal directions just like how clamping down on a toothpaste tube causes the paste to squeeze out. Reservoir rock, however, will push out against the neighboring rock as it has nowhere to deform to. This creates a gradient of stresses in the horizontal directions. On the way the rock deforms under stress there will be one horizontal direction where the stress is greatest, or maximum. Perpendicular to that direction the rock stress will be at its lowest, or minimum.
As a fracture is pumped and expands it will grow out away from the well. Although this growth is complex, it tends to be perpendicular to the direction of minimum stress. Knowing which direction the fracture will grow in is important in order to plan the drainage of the reservoir.
Treating pressure is the surface pressure during the pumping of a frac job. This pressure is ideally measured as close to the wellhead as possible. The surface treating pressure is related to the downhole pressure adjusted for the hydrostatic pressure of the fluid column in the well as well as the friction pressure on the tubulars in the wellbore.
Tortuosity If we understand the stress regime when we fracture our well we should understand which direction the fractures will tend to grow. Ideally the horizontal section of the wellbore is placed so that the fractures grow perpendicularly away from it. If they do not then this can impact the productivity of the well. At the beginning, where the fracture has just left the wellbore, the fracture will begin turning to find its natural propagation direction. This turn happens close to the well and contributes to an impediment known as Near Wellbore Tortuosity Turns and bends within any flowing conduit contributes to increased friction, as it does in near wellbore tortuosity. The increase in friction both decreases the effectiveness of the resulting frac and hampers production when the well is put online.
Breakdown pressure Before starting the fracture the pressure must be increased above frac pressure for a moment. This is known as the breakdown pressure. Once breakdown pressure is reached the fracture will initiate and grow causing the pressure to drop back to frac pressure.
Closure pressure When the pumps stop and the pressure bleeds down the fractures will close. The pressure at which the fracture closes is closure pressure.
Frac Pressure Frac pressure is defined as the minimum pressure to sustain a hydraulic fracture. We already know about the stress holding the rock together. When this stress is exceeded by the pressure pumped into the rock the fracture will grow. Although minimum formation stress and frac pressure will not be exactly equal, they are closely related.
Screen out During the pumping of a frac job we want to understand how the frac is growing. If the fracture is growing in length then this bodes well the future productivity of our well. If the fracture is only growing in width but not length then it is not creating additional contact with the reservoir, this is termed screening out.
Net pressure To classify this growth during the pumping of the frac job we look at “how hard” the formation is being fractured via the Net Pressure. The net pressure is the net of the current frac pressure minus the closure pressure. In the next chapter we will show how monitoring net pressure throughout the life of the frac job gives us information about the fracture’s behavior. Leakoff
While a frac is being pumped not all the frac fluid will stay in the fracture itself. A growing fracture will also be flowing fluid into the rock matrix of the formation. The increasing fracture length and reservoir contact will also increase the leakoff.
Skin effect Producing wells often have damage to the completion that makes it harder to produce. This damage is caused when particles are left behind from the drilling process or when production causes tiny clay particles to invade the near wellbore area. The corresponding production reducing effect is termed skin effect. Frac jobs and other forms of stimulation remove or bypass the skin in an affected well.
*Migrating clays within the near wellbore contribute to a skin effect that reduces production.
Frac Principles Quiz Name: ___________________________________________
◼ Question 1 Hydraulic fractures grow at a 90 degree angle to the direction of minimum rock stress. A. True B. False
◼ Question 2 The following increase well inflow except: A. Permeability B. Tortuosity C. Pressure Drawdown D. Completion Length
◼ Question 3 Immediately after shutting the frac pumps off and closing the well, surface pressure will drop to ISIP. A. True B. False
◼ Question 4 Net pressure is the surface treating pressure minus friction plus hydrostatic head. A. True B. False
◼ Question 5 Screen out describes when the frac fluid begins to leakoff. A. True B. False
◼ Question 6 Frac treating pressure should remain constant throughout the job. A. True B. False
Net Pressure Plotting Earlier the concept of net pressure was discussed. Net pressure during the frac job is graphed on a Nolte Plot to describe the behavior of the frac. Stable and gradual increases in the plot indicate a healthy growing fracture. A rapid increase in the plot gives the pumpers an early indication
of a screen out. When a screen out has been detected early then countermeasures can be employed to prevent the plugging of the well with frac sand. A decrease in the plot indicates a loss of zonal containment, excessive leakoff, or a loss of pump horsepower.
Micro frac Frac jobs are, on daily spend basis, perhaps the most expensive operation in the onshore oil and gas industry. To spend that money wisely and reduce mistakes it is an advantage to have more data ahead of time about how the frac job will perform. Indirect measurements and computer simulations can only give a limited
inference about the future frac job. To obtain direct frac measurements prior to pumping the frac we can pump a microfrac. The micro-frac gives us a variety of directly measured information such as breakdown pressure, closure pressure, friction losses, and formation permeability.
Step rate test A step rate test finds the friction lost to the near wellbore and the fracture gradient if we do not already know it. It is performed by pumping at constant flow rate for a short
period of time then increasing the rate to new several new levels for the same period of time. For each period the stabilized pressure is plotted and analyzed.
G-Function When pumping has finished on the micro frac the well is immediately shut in. The Initial Shut In Pressure ISIP is then recorded at surface. At the point pumping stops the fractures are still open. The ISIP, therefore, does not represent closure pressure of the fracture. It better
represents a static frac pressure without the interference of friction. While the well is shut-in it is leakoff to the formation that bleeds the pressure in the frac down until closure is reached. This pressure over time is plotted through G-Function analysis.
Diversion In the completions section we discuss plug and perf as a routine method used for diversion. There are times when it is impossible or impractical to use plug and perf or other mechanical diversion means. In these cases we have the option of pumping the diversion in during the frac
job. One method includes pumping â€œfrac ballsâ€? designed to temporarily seal the perforations that have already been treated. If the well does not use a perforated completion another option is to pump gypsum flakes to temporarily seal the area already stimulated.
Frac balls: Rubber balls may be pumped in during the frac job to plug thief perfs and allow the frac treatment to treat less productive perfs.
Proppants Proppants are required to maintain an open flow path after the frac treatment has been completed. Sand is the most commonly used proppant. The sand is produced and processed at a mine then loaded into a truck for delivery to the wellsite. Synthetic proppant made from ceramics are also available. These
proppants are much more expensive than sand, but they also have some favorable properties including uniform sorting sizes, ideal grain shape, and higher crush resistance. Natural, in-basin, ceramic
Weights and volumes The amount of sand in the frac slurry is measured in pounds of sand per gallon of frac fluid. A large frac job may pump more than 10 million pounds of sand into a single well. Engineers design the frac job to have enough proppant to maintain good permeability in the frac while being limited
by the carrying capacity of the frac fluid used. Frac sand is trucked onto location in loads of 40,000 to 50,000 pounds each. To complete a frac job using millions of pounds of sand the trucks must keep coming around the clock.
Grain size and sorting A load of sand is graded by the size of the grains that make up the load. Since no two grains of sand are the same size then the loads of sand are graded by the smallest grain allowed. Grains are sized using the mesh system. In the mesh system a sieve of equally spaced wires is constructed and sand passes through the mesh. Any sand that passes through the mesh is not included within the mesh size. The mesh
Proppants are sized based on the smallest grain that will not pass through a sieve.
itself is sized based on the number of openings within a linear inch. A 100-mesh sieve, therefore, has 100 holes within an inch. Grains that are 1/100th of an inch or larger cannot pass through the mesh and are included. The exact size of the opening in a 100-mesh sieve are less than 1/100th of an inch due to the width of the wires making up the mesh.
Close up of sieve mesh. The size is the number of holes within a linear inch.
Angular grains pack together tighter and reduce the proppant permeability.
Crush resistance When the permeability of a proppant is measured in the laboratory it is first done so without the stresses that the sand would endure when placed in the reservoir. The stresses deep underground push the sand grains closer together and cause them to crush and deform. This effect reduces the permeability of the proppant and the productivity of the well in which it was
placed. Proppants with insufficient crush resistance may only retain a few percentage points of the original, unstressed, permeability. Higher crush resistance will come at a cost therefore the proppant selected will account for permeability desired versus the cost to achieve it.
Angularity As no sand grains are the same size nor are they the same shape. Ideally each sand grain would be a uniform circular shape in order to maximize the permeability of the proppant. Natural processes cannot produce perfect sand,
some will have angular shapes which tend to fit together tightly. Angular sands will have a lower permeability because the compacted grains allow less space for fluids to flow.
Purity In addition to the grain sizes used in a proppant, it is also useful to know how much of the sand is actually sand and to what extent impurities are present. If the sand is dirty it will release dust which has negative consequences. The dirtiness of the sand is described as turbidity and measured in Nephelometric Turbidity Units (NTU). The turbidity is obtained by placing a sample of the sand and pure water into a turbidimeter and measuring how dark the water becomes with the sand in it. Dirty sand will cause the water to become darker and have a higher NTU.
Proppant can also be chemically tested to determine purity by using acid. Sand is made up of the mineral quartz, or silica dioxide, which is highly resistant to hydrochloric acid. The solubility in acid of a sand is what percentage of the proppant will dissolve when placed into hydrochloric acid. Lower acid solubility is desired, indicating less impurities. Note that production operations often use hydrochloric acid during routine stimulation procedures in order to remove production damaging build up.
Chemical tabs and tracers On occasion it may be desired to add special properties to the proppant to either chemically treat the well or to map where the proppant is being placed. Chemical treatment can be achieved by adding solid chemical pellets to the proppant. These pellets will dissolve over time and insert chemicals into the well designed to reduce High turbidity proppant is dirtier and liable to create plugging fines in the completion.
scale, corrosion, or other negative effects of production. To map the proppant, radioactive tracer sands can be added. These sand grains emit small amounts of gamma rays with a unique signal which can be mapped later with wireline logging tools.
Hydrochloric acid, HCl, is a strong acid that dissolves the impurities from sand but not the sand itself.
Proppant Quiz Name: ___________________________________________
◼ Question 1 Ceramic proppants have higher crush resistance. A. True B. False
◼ Question 2 Poor grain size sorting reduces: A. Permeability B. Pump Pressure C. Pressure Drawdown D. Tortuosity
◼ Question 3 Higher turbidity risks creating: A. Extra Permeability B. Frac breakdown C. Fines D. Tortuosity
◼ Question 4 Sub-rounded sand has lower permeability than rounded sand. A. True B. False
◼ Question 5 Tracer sand is radioactive. A. True B. False
Fluids Base fluids A frac fluid can be based with either a liquid or a gas. The purpose of the base fluid is to increase the pressure in the formation high enough to grow a frac as well as to carry the proppant into the frac. Most fracs are either slickwater or gel based. Slickwater fracs are mostly water with added salts. When salts are added to the water it makes them feel slippery or slick. Gel fracs use guar gum or some other polymer. A gel frac is thixotropic, meaning the slower it is pumped the thicker it becomes, a useful property for proppant carrying capacity.
Other possible base fluids include: â€˘ â€˘ â€˘
Nitrogen foam Carbon Dioxide Propane
These gas-base fluids have useful properties such as being easier to unload from the well and causing less clay swelling. Despite these benefits, the use of gas-base fluids in frac is not widespread.
Low pressure hoses send water to the frac manifold.
Gels Gels are commonly added to frac fluids in order to increase their carrying capacity for proppants. The addition of the gel makes the frac fluid thicker, technically more viscous, which holds more proppant when the fluid would otherwise be dropping sand out into the bottom of the well lateral.
Increasing viscosity comes with the downside of increased pump pressure and limits the volume of frac fluid. To get around this limitation operators have used a hybrid slickwater frac with a small amount of gel added.
X-Linkers When the maximum amount of sand carrying capacity is required then a crosslinked gel is used. At a molecular level the cross linking agent binds the individual polymer strands together to increase the viscosity of the frac fluid
further. A delayed cross link action is helpful in frac because it reduces friction while the slurry is pumped at surface and only thickens up downhole when the extra viscosity is needed.
Breakers Leaving viscous gels within the well would plug it up and prevent good production after the frac. Breakers are therefore used
The guar seeds are used as a gelling agent.
in the post frac flush to break down and thin out the cross-linked gel.
Viscosity of the gel is tightly controlled by on-site testing.
Friction reducer When pumping the frac slurry into the well at higher rates the pressure will increase disproportionately due to the increase in pipe friction. This increase in pipe friction wastes horsepower and limits the amount
that can be pumped. To mitigate the loss of power to friction a friction reducer can be added. Friction reducers generally work by coating the walls of the pipe and making it easier for the fluids inside to slip by.
Solvents When thick, viscous oils are located in the reservoir it becomes more difficult to pump fluids into the matrix of the rock. Small amounts of solvents such as toluene can
be added to dissolve the thick oil and help the frac fluid flow within the rock and within the completion.
Demulsifiers Oil and water do not mix well. When oil and water are forced together with a shearing action during pressure pumping they can form an emulsion. Emulsions are bubbles of both oil and water, they make it harder
Each stage may use as many as a half-dozen specialty chemicals.
for both the oil and water to flow back to the surface. Demulsifiers added to the fluid break down these bubbles and make them easier to flow back.
FR is the most common additive in modern fracs, it allows frac fluid to flow more easily through the iron and create less friction.
Emulsions can form when oil and water try to mix, forming a plug in the permeable completion.
Fluids Quiz Name: ___________________________________________
◼ Question 1 Adding gels to a frac fluid increases viscosity as well as the amount of fluid can carry. A. Pressure B. Stress C. Color D. Sand
◼ Question 2 Slickwater is comprised of water and: A. Gel B. Salts C. Friction Reducer D. Oil
◼ Question 3 The following is a type of diverter: A. Frac Balls B. Acid C. Slick Water D. Proppant (Sand)
◼ Question 4 Nitrogen fracs create a foam that increases fluid volume. A. True B. False
◼ Question 5 Breakers reduce: A. Rock Strength B. Pipe Tension C. Viscosity D. Sand Weight
◼ Wellbore Diagram Exercise Draw the following well: Surface Casing: 4000’ 7” 32# P110 LTC Liner Depth: 8680’ 5.5” 23# N80 LTC PBTD: 8653’” Completion Interval: 8425’-8581’ 4SPF 90Deg 0.7”EHD Tubing Tail: 8610’ 2 7/8” 6.5# J55 EUE Anchor: 8400’
◼ Horizontal Wellbore Diagram Exercise Draw the following well: Surface Casing: 2000’ Production Casing: 10200’ 7” 32# P110 LTC KOP: 4580’ Heel: 6830’ Tubing Tail: 4530’ 3 1/2” 9.3# J55 EUE What is the build rate?
Hydraulic Fracturing Pre-Test-KEY ◼ Question 1 This discipline is concerned with uncontrolled flow of oil and gas releases after a frac job: A. Flow Control B. Well Control C. Environmental Control Well control is the discipline, by definition, concerned with controlling the releases of fluids from a well.
◼ Question 2 fluid is potentially flammable.
A. Kill B. Completion C. Frac D. Hydraulic Some hydraulic fluids are flammable and may catch fire if a hose bursts. The other three fluids are commonly water based in completion work.
◼ Question 3 Induced fractures typically turn more the farther they are from the treating well. A. True B. False Direction of induced fractures from the frac job is controlled by the direction of the horizontal rock stresses. At the near wellbore, however, the frac will start in the same direction of the perforation and will quickly reorient itself towards the direction of maximum horizontal stress.
◼ Question 4 Natural fractures are caused from fluid pressures, just like from a frac job. A. True B. False Natural fractures are caused by rock stresses, not pressure build ups.
◼ Question 5 Frac sand is safe to handle without wearing special protection. A. True B. False Frac sand can release airborne crystalline silica during loading and unloading, risking permanent lung damage if inhaled.
◼ Question 6 A chiksan is a high-pressure swivel joint. A. True B. False This is the often genericized term for a high pressure swivel joint.
◼ Question 7 Prior to starting a frac job extra formation strength information can be obtained from a: A. Microfrac B. Net pressure plot C. Fluid sample D. Geologic map A microfrac, or minifrac, may be performed prior to the frac job to obtain breakdown pressure and closure pressure. Net pressure plots are performed during the frac job.
◼ Question 8 The deadliest position in hydraulic fracturing is loading and arming the perforating guns. A. True B. False In the onshore US oil and gas industry, including frac and completion, driving accounts for more fatalities than any other cause.
◼ Question 9 Multiple operations taking place on a single location are called: A. Multi-Jobs B. SIMOPS C. CLO-Jobs D. Multis Simultaneous operations are referred to as SIMOPS
◼ Question 10 The frac well must be hooked to the pipeline LACT prior to flowback. A. True B. False Flowback is pumped into temporary tanks, typically specially purposed frac tanks.
Wireline Quiz-KEY ◼ Question 1 Cased hole logging is not able to read gamma ray signals from the formation. A. True B. False Cased hole logging uses gamma ray tools to tie in the depth to the vertical section of the well.
◼ Question 2 The is designed to be long enough to contain the wireline tool string prior to removing it from the well. A. Lubricator B. Strip C. Master Valve D. Wireline BOP The lubricator must be long enough to contain the wireline tool string in order to allow the well to open and close when inserting and removing these tools.
◼ Question 3 Wireline depths are always tied back into ground level. A. True B. False Tying back wireline depths to the service is a poor practice due to drift on the wireline odometer.
◼ Question 4 Perforations in the wrong location are easy to repair. A. True B. False Perforations are permanent holes in the casing, are challenging to repair, often causing lasting damage to the well.
◼ Question 5 Placing the perforation at the correct depth is done using. A. Perf Control B. Depth Control C. Counting collars from surface D. Hydrostatic Surge Depth control uses wireline data to correlate depths to known points subsurface. 62
Flowback Quiz-KEY Name: ___________________________________________
◼ Question 1 Find the wellhead pressure of a well with 9PPG completion fluid and a formation EMW of 16PPG and a top perf at 10,000’ TVD. 10,000 feet x 16 PPG x 0.052 = 8,320 PSI Formation pressure 10,000 feet x 9 PPG x 0.052 = 4,680 PSI Hydrostatic pressure 8,320 PSI – 4680 PSI = 3,640 PSI Surface wellhead pressure
◼ Question 2 Find the minimum kill mud weight for a 8,000’ TVD with a 7.7PPG fluid and 1200PSI WHP. 1,200 PSI / 8,000 feet = 0.15 PSI / Foot 0.15 PSI / Foot / 0.052 = 2.9 PPG 2.9 PPG + 7.7 PPG = 10.6 PPG
◼ Question 3 What is a reasonable frac EMW for the well in question 2? 13.5 PPG
Frac Principles Quiz-KEY ◼ Question 1 Hydraulic fractures grow at a 90 degree angle to the direction of minimum rock stress. A. True B. False Fracture growth is complex but generally propagation is perpendicular to the direction of minimum horizontal stress.
◼ Question 2 The following increase well inflow except: A. Permeability B. Tortuosity C. Pressure Drawdown D. Completion Length Well inflow is a product of the productivity and pressure drawdown of the well. Increasing the tortuosity of the frac will decrease the productivity and inflow of the well.
◼ Question 3 Immediately after shutting the frac pumps off and closing the well, surface pressure will drop to ISIP. A. True B. False ISIP: Initial Shut-In Pressure
◼ Question 4 Net pressure is the surface treating pressure minus friction plus hydrostatic head. A. True B. False Frac pressure is the surface treating pressure minus friction plus hydrostatic head. Net pressure is frac pressure minus closure pressure.
◼ Question 5 Screen out describes when the frac fluid begins to leakoff. A. True B. False Screen out describes a frac growing in width but not length.
◼ Question 6 Frac treating pressure should remain constant throughout the job. A. True B. False Frac pressure will increase throughout a stage as the growing frac requires increasing energy to stay open.
Proppant Quiz-KEY ◼ Question 1 Ceramic proppants have higher crush resistance. A. True B. False Ceramic proppants are stronger, and more expensive, than natural sand.
◼ Question 2 Poor grain size sorting reduces: A. Permeability B. Pump Pressure C. Pressure Drawdown D. Tortuosity Poorly sorted grains will pack together more tightly and are less permeable than uniform sized grains.
◼ Question 3 Higher turbidity risks creating: A. Extra Permeability B. Frac breakdown C. Fines D. Tortuosity Turbidity is a measure of the dirtiness of the frac sand which can leave damaging fines in the completion.
◼ Question 4 Sub-rounded sand has lower permeability than rounded sand. A. True B. False More rounded sand will be pack together less tightly and have higher permeability.
◼ Question 5 Tracer sand is radioactive. A. True B. False Radioactive properties allow tracer sand to be tracked via gamma ray sensors.
Frac Labeling Exercise-KEY Label the frac job equipment below.
1 4 6
Label: 3 Sand Movers
Fluids Quiz-KEY ◼ Question 1 Adding gels to a frac fluid increases viscosity as well as the amount of the fluid can carry. A. Pressure B. Stress C. Color D. Sand Increasing viscosity makes a fluid ‘thicker’ and able to carry more sand or other solids.
◼ Question 2 Slickwater is comprised of water and: A. Gel B. Salts C. Friction Reducer D. Oil Slickwater feels slick because of the salt dissolved into it.
◼ Question 3 The following is a type of diverter: A. Frac Balls B. Acid C. Slick Water D. Proppant (Sand) Frac balls can be used to divert fluid from perforations taking lots of fluid to those taking less.
◼ Question 4 Nitrogen fracs create a foam that increases fluid volume. A. True B. False The foaming action of nitrogen increases the fluid volume beyond that of the original liquid.
◼ Question 5 Breakers reduce: A. Rock Strength B. Pipe Tension C. Viscosity D. Sand Weight A breaker included in the flush portion of a gel frac breaks down the cross-linked gel and allows the frac fluid a lower viscosity to flow back into the well. Add equations appendix (contents) Index