A_Guide_to_Fracturing
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Fracturing•What are you calling fracturing?–Transport of materials to the well site?–Surface equipment and pumping operations?–Surface pressure control of the well during drilling or production?–Well construction issues where insufficient barriers fail?–Fracturing the pay zone?•Simple fact–1million frac jobs have been pumped since the late1940’s and there is no documented case of a frac breaking up through thousands of feet of rocks to pollute a fresh water zone.•However,there are cases of:–Poorly constructed or maintained wellbores leaking into an aquifer,mostly during production.–Leaks at surface during pumping due to failure of surface equipment.–Infrequent spills of water and some chemicals at surface from transport accidents or equipment failures.•Questions to ask:What is being spilled?What is the frequency?Can it be prevented?How is it being cleaned up?This Series of Guides Addresses:•Fracturing–what it is,what it isn’t and is it necessary?•Chemicals in Fracs–what,how much what happens to it?•Water Management–use,backflow,content,treating,risks.•Drilling–how the well is drilled,what is used,risks.•Well Construction–effective barriers and monitoring.•Multi-Well Pads–advantages and disadvantages.•Surface Pressure Containment–blow out preventers or BOPs.•How Fluids Flow in the Reservoir.•How Fluids Flow in the Well.Each guide uses basic terms and references risks,monitoring actions and performance indicators.Why Fracture?•Fracturing is required in nearly all currently developing on-shore US gas reservoirs to make the wells commercial.•Without fracturing,an average gas shale well will not produce more energy than it takes to keep it flowing(running pumps to keep the condensed water out).•A fractured shale gas well may produce½billion scf of gas in a5year period or enough to power,cool and heat~500homes for5years or to run two thousand cars for five years(10,000miles/car/yr).•Without fracturing,the US gas production is predicted to fall by an estimated17%per year.•With fracturing and horizontal wells,the recoverable US gas production will rise by88%in the next5years.•The US is extremely rich in natural gas that is locked in low permeability reservoirs.This requires fracturing to produce.The effect of fracturing on production Basically,fracturing in low and ultra-low permeability formations(shale)is the only effective means of generating a flow rate that achieves both well unloading(stable production)and economic payback.What does a fracture look like?•Fractures are breaks in the rock,created by fluid under pressure,overcoming the insitu formation stresses and starting a small crack atthe wellbore,then enlarging that crack by pumping water at high rates.Right:a fracture formingperpendicular to leastprinciple stress s hx.Left:a real fracture,<0.1”wide,at a depth of1740ft.These vertical fracs can behalted at even small barrierssuch as the shale streak andneed access to the formationto start again.What limits a fracture’s growth?•A fracture may grow outward(length),up and down(height)and slightly wider until it meets a barrier or runs out of driving force(fluid pressure).1.frac barrier is another formation with sufficientlyAdifferent properties that it will not fracture with theavailable forces in the frac.2.Driving force is supplied by the injection rate:whenthe leakoff rate into the formation equals the fractureinjection rate,the fracture stops growing.If barriers are present?•Frac barriers may contain a frac so that it does not generate significant height growth.This is common in the Barnett shale formation sequence and in all other formations.•How is it monitored?Microseismic,logging tools,tilt meters,mine-back experiments,pressure behavior,etc.SPE115766Fracs Self-Limited by Frac Fluid Leakoff •Take the previous fracture:with2wings(each side of the wellbore)and2formation faces exposed to leakoff along the frac.•The area is650’x95’x2x2=247,000ft2•At a pump rate(100bbl/minute),the water highinjected would be0.29ounces per minute per ft2 of formation area.•If the leakoff into the matrix or the natural fractures is more than this,the frac will stop growing.The bottom line?•It is very difficult to get a fracture to grow to more than300ft of total height.•Nearly all gas productive shales are at depths of4000to12000ft deep.The depth of the deepest fresh or even usable water zone is about800ft.How is a Frac Designed?•Refined and complex fracture models predict fromrock mechanics how fractures will form.•Are they accurate?The frac predictions are asprobably as accurate as the input data on formationstresses,barriers,rock types,laminations,etc.Frac Model Shape Prediction OutputRight:width profile of affected area.Far Right:length and heightpredictions.SPE115766Formation Stresses Shape the FractureThe stress profilecreated by theformation is measuredusing sonic logs andthe data is used in thefrac model.Microseismic Monitoring of Fracs in aHorizontal WellMicroseismic listens for the sound of stress release during fracturing(breaking rock)and triangulates to an approximate location of the sound.For this twelve stage frac(SPE119896), the fracture height self-limited at a few hundred feet,even though no fracture barriers were yet encountered.Fracture Growth and Breakout Indicators •Temperature Logs in Vertical Well–see the temperature produced from the water in the frac.•Microseismic–an expensive monitoring method that may be done from the surface in some cases or from another near-by wellbore.•Tilt meters–sensitive enough to detect“earth tides”produced from the moon passing overhead,can show fracture direction and growth.•Tracers–chemicals measured in the parts per billion can be used to measure leakoff and returns or breakthroughs to other zones or wells.•Surface Pressure Behavior–pumping pressure,annulus pressure,adjacent wells,etc.,are the best sources of real-time,direct measurements.In this area,nothing beats experience.Rock Fracturing Risks •Breakout to a nearby well in the pay zone(grows outward,notupward)–relatively common if the wells are close(<500ft)and in line along the frac direction.•Screenout–rare,perhaps~2%of jobs,PRD’s(pressure relief devices)vent the pressure and fluids to a catch tank if needed.•Packer or seal unseats downhole and backside pressures up:rare, probably<0.5%.Generally no losses to the upper formation.•Rupture of the well casing or breakdown of cement–exceedingly rare,perhaps<1in10,000(very few known cases).Job stopsimmediately and fluid lost out of pay is usually<100bbls.•Fracture breaks through rock to surface or fresh water?–No documented case known.Other Risks•Transport–depends on drivers,roads and conditions–area specific,but definitely controllable.•Pumping spills–maybe1%of jobs will have a reportable spill–usually water.•Loss of surface pressure(spill)during plug drill-out or cleanout(BOP on the well)–rare–maybe1in500jobs.A Quick Look at Shale Frac Materials(there is a separate presentation for chemicals)•Frac Fluid–Most fracs fluids are waterbased.•Proppant–sand and a few man madeceramics,washed&graded into sizeranges•Friction Reducer–polymers(food Water(all forms)grade)•Biocides–swimming pool shock is typical•Scale inhibitors,pH adjustments, gelling agents(polymer),cross-linkers, etc.–usually very low concentrations–see the section on chemicals.SandChemicalsWater=95.3%Sand=4.6%Chemicals=0.12%Total weight of chemicals on location is usually20,000to30,000 lb,much of it water base weightWhere do the chemicals end up?Polymers often break down into small chains and are attacked by bacteria in the flowback water or treating plant.Chlorine based biocides spend on biogenic materials and come back as chloride ions.Many chemical additives adsorb on the charged,high surface area clays and do not return in the flowback.Surface active materials commonly adsorb on the mineral surfaces and do not return in measurable amounts.15%to 50%of the water returns over 2to 6weeks,the rest is adsorbed and absorbed into mineral structures in the formation.Proppant–sand or man made(ceramic). Keeps the fracture open to improve flow.There may be several million pounds of sand in a frac.It can be tagged with short life isotopes(zero-wash).Frac Fluid Flowback•Salt water makes up99.9%+of the fluid flowed back after a frac job.•About15to50%of the water comes back and must be separated and treated(another section on this topic).•After dilution and treating much,if not all,of this water can be reused for fracturing.Why is there methane gas in my waterwell?!!!!•Biogenic methane is extremely common in shallow formations, including those that contain fresh water recharged from theatmosphere and anywhere else where biologic activity is occurring.•Both biogenic and thermogenic gas seeps are common throughout North America(1100known on shore and600known offshore Gulf of Mexico).•World wide natural gas seeps are known to contribute50to70 million tons of methane per year to the atmosphere and shallow formations–that’s2600to3600Bcf per yr or7to10bcf/day.•Many of the natural gas seeps are centered in areas of shallow to deep hydrocarbon production including Pennsylvania,New York and Colorado.•BTW-Some of the largest natural oil seeps in North America are in California,many in the Santa Barbara channel.Biogenic vs.Thermogenic Gas •Biogenic methane is isotopically lighter,or more depleted in the carbon13isotopes than thermogenic methane.•Thermogenic methane has more C13but no C14(radioactive isotope)US Natural Gas Use(2008-EIA)•Electric Power Generation 6.7tcf •Industrial 6.7tcf •Residential 4.9tcf •Commercial 3.1tcf •Lease and plant fuel 1.3tcf •Pipeline and distribution0.6tcf •Vehicle Fuel0.03tcf23tcf/yr or63bcf/day for200864.8bcf/d EIA estimate for2010。
