chapter 1 - well stimulation

1. WELL STIMULATION

By:

Engr. Saleem Qadir Tunio

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1. WellStimulation

Overview ofStimulation in

an Oil Well

Overview ofacidizing

Wellstimulationeconomics

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Stimulation

Fundamentalsof fracturing

General safetyand workguidelines

Well Stimulation

Well stimulation involves techniques tooptimize well performance. This may includepumping of acids, energized fluids, andvarious other chemicals to improve formationvarious other chemicals to improve formationflow characteristics.

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OVERVIEW OF STIMULATION IN AN OIL WELL

• During drilling and completion operations,formation damage are due to:

– Solid block pore throats – mud solid invasion– Solid block pore throats – mud solid invasionleading to blocked pore throats (reducepermeability)

– Mud filtrate invasion – clay swelling (reducepermeability)

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– Scale – due to pressure and temperaturechanges. Salts in formation water become over

OVERVIEW OF STIMULATION IN AN OIL WELL

changes. Salts in formation water become oversaturated and precipitate.

– Wax

– Asphaltenes – precipitate due to pressure drop

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• Matrix treatment and acidizing in particular,aims to remove the excess flowing pressuredrop (p) created by the presence of volumerock which has suffered formation damagenear wellbore area.

OVERVIEW OF STIMULATION IN AN OIL WELL

near wellbore area.

• Damaged zone has lower permeability thanoriginal.

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OVERVIEW OF STIMULATION IN AN OIL WELL

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OVERVIEW OF STIMULATION IN AN OIL WELL

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rk

• The removal of this formation damage will restorethe ‘natural’ well productivity.

• The Hawkins formula;

OVERVIEW OF STIMULATION IN AN OIL WELL

w

d

d r

r

k

ks ln1

is convenient tool for analyzing the influence ofvarying levels and depth of formation damage.

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• Matrix stimulation treatments increase wellproductivity by pumping a specially formulatedtreatment fluid (normally acid)

OVERVIEW OF STIMULATION IN AN OIL WELL

treatment fluid (normally acid)

• The fluid is designed to remove (normally dissolve)the formation damage.

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Well stimulation types

• Acid Fracturing/Acidizing/ Matrix Acidizing

• Hydraulic Fracturing• Hydraulic Fracturing

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Acidizing

• A type of stimulation treatment,acidizing is performed below thereservoir fracture pressure in aneffort to restore the naturalpermeability of the reservoirrock.rock.

• Well acidizing is achieved bypumping acid into the well todissolve formations likelimestone, dolomite and calcitecement between the sedimentgrains of the reservoir rocks

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OVERVIEW OF ACIDIZING

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OVERVIEW OF ACIDIZING

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Organic Acid(Source: Wikipedia)

An organic acid is an organic compound with acidicproperties. The most common organic acids are thecarboxylic acids, whose acidity is associated with theircarboxyl group -COOH. Sulfonic acids, containing thegroup -SO2OH, are relatively stronger acids.

A few common examples include:A few common examples include:• Lactic acid• Acetic acid• Formic acid• Citric acid• Oxalic acid• Uric acid

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• Simple organic acids like formic or acetic acids areused for oil and gas well stimulation treatments.These organic acids are much less reactive withmetals than are strong mineral acids like

Organic Acid(Source: Wikipedia)

metals than are strong mineral acids likehydrochloric acid (HCl) or mixtures of HCl andhydrofluoric acid (HF). For this reason, organicacids are used at high temperatures or when longcontact times between acid and pipe are needed.

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• A mineral acid or (inorganic acid) is an acid derivedfrom one or more inorganic compounds. A mineral acidis not organic and all mineral acids release hydrogenions when dissolved in water.

In-organic Acid(Source: Wikipedia)

• Commonly used mineral acids are sulfuric acid,hydrochloric acid and nitric acid (They are also knownas bench acids). Mineral acids range from acids of greatstrength (example: sulfuric acid) to very weak (boricacid). Mineral acids tend to be very soluble in waterand insoluble in organic solvents.

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• Hydrochloric acid

• Nitric acid

• Phosphoric acid

• Sulfuric acid

In-organic Acid(Source: Wikipedia)

• Sulfuric acid

• Boric acid

• Hydrofluoric acid

• Hydrobromic acid

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•In fracturing, conductivepath is created by injectingfracturing fluid into theformation.

FUNDAMENTALS OF FRACTURING

formation.

•Conductivity is maintainedby propping agent(sand orglass beads)to hold thefracture faces apart.

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Frac. Fluid and Proppant

Fracturing fluidThe fluid used during a hydraulic fracture treatment of oil, gas orwater wells. The fracturing fluid has two major functions 1) Openand extend the fracture; 2) Transport the proppant along thefracture length.

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ProppantSuspended particles in the fracturing fluid that are used to holdfractures open after a hydraulic fracturing treatment, thusproducing a conductive pathway that fluids can easily flow along.Naturally occurring sand grains or artificial ceramic material arecommon proppants used.

Fracture Initiation

• Hydraulic fracture treatment is accomplished bypumping a suitable fluid into the formation at a ratefaster than the fluid can leak off into the rock.

FUNDAMENTALS OF FRACTURING

faster than the fluid can leak off into the rock.

• Injecting fluid pressure is built up sufficient toovercome the earth compressive stress holding the rockmaterial together. The rock then parts or fractures alonga plane perpendicular to the minimum compressivestress in the formation matrix.

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A Fracture Extension:

•As injection of frac fluid continues, the fracture tendsto grow in width as fluid pressure in the fracture worksagainst the elasticity of the rock material.

FUNDAMENTALS OF FRACTURING

• After sufficient frac fluid has been injected to openthe fracture wide enough to accept proppant, sand isadded to the frac fluid and is carried into the fractureto hold it open after the job.

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When sufficient sand has been injected:

• pumps are shut down

• pressure in the fracture drops

FUNDAMENTALS OF FRACTURING

• pressure in the fracture drops

• the earth compressive stresses closes thefracture on the proppant.

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Sand-Out Treatment:

• The width of the fracture is related to thedifferential pressure.

FUNDAMENTALS OF FRACTURING

differential pressure.

• As sand enters the fracture and is deposited, morefluid pressure is required to create greater stressagainst the fracture face to increase the frac width.

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• If the required fluid pressure cannot be applied due toequipment or casing limitations:

• fluid injection rate slows down

FUNDAMENTALS OF FRACTURING

• sand drops out of the fluid at a more rapid rate

• sand-out in the fracture occurs

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Fracturing is beneficial in following cases:

1. If the reservoir is composed of a low-permeability, homogeneous rock, fracturing issimilar to increase the size of the hole.(i.e., fluid

FUNDAMENTALS OF FRACTURING

similar to increase the size of the hole.(i.e., fluidformerly flowed through the low-permeabilityrock becomes able to move into the highcapacity fracture at some distance from thewell.)

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2. Fracturing will eliminate formation damage due to

- invasion of drilling mud,

- deposition of mineral matter

FUNDAMENTALS OF FRACTURING

- swelling of clays

If damage is only in the immediate vicinity of the wellbore, only a mild fracture treatment is required.

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3. Fracture radiating from the well bore act as agathering lines connecting permeable and poroussystems that are otherwise isolated from the wellby impermeable barriers.

FUNDAMENTALS OF FRACTURING

by impermeable barriers.

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• Matrix acidizing – removal near wellbore damage

• Hydraulic fracturing – improving well inflow performance

MATRIX ACIDIZINGVS.

HYDRAULIC FRACTURING

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Skin Permeability Treatment

High Low Fracturing (matrix possible)

Low Low Hydraulic Fracturing

• Matrix acidizing aims to;

– Increase the PI

– By reducing the skin

MATRIX ACIDIZINGVS.

FRACTURING TREATMENTS

– By reducing the skin

– through dissolving formation damagecomponents and rock near wellbore region

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• Hydraulic fracturing aims to:

– Increase the PI

– By increasing effective wellbore radius

– through creation of high conductivity fractures from

MATRIX ACIDIZINGVS.

FRACTURING TREATMENTS

– through creation of high conductivity fractures fromthe wellbore

– Fracture bypass the damage zone and extend to agreater depth into the reservoir than can be reachedby acidizing

– Suitable for low permeability with or without skin

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General safety and work guidelines&

Well Stimulation Economics

ASSIGNMENT NO. 01ASSIGNMENT NO. 01

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Deadline : Wednesday, 15/06/2011