Chapter 1 - Well Stimulation

1. WELL ST STIMULA IMULATION TION

By: Engr. Saleem Qadir Tunio

TUNIO, May' 2011

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Overview of  Stimulation in an Oil Well

Well stimulation economics

Overview of  acidizing

1. Well Stimulation

General safety and work guidelines

Fundamentals of fracturing

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Well Stimulation Well stimulation involves techniques to optimize well performance. performance. This may include pumping of  acids acids,, en ener ergi gizzed fl flui uids ds,, and various other chemicals to improve formation flow characte characteristics ristics..

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OVERVIEW OF STIMULATION IN AN OIL WELL

• Du Durin ring g dr drill illin ing g an and d co comp mple letio tion n op oper era atio tions ns,, formation damage are due to: Soli lid d bl bloc ock k po porre th thrroa oats ts – mu mud d so soli lid d in inv vas asio ion n  –  So leading to blocked pore throats (reduce permeability) Mud d fi filt ltrrat ate e in inv vas asio ion n – cl cla ay sw swel elli ling ng (r (red educ uce e  –  Mu permeability)

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OVERVIEW OF STIMULATION IN AN OIL WELL

pres essu sure re an and d te temp mper erat atur ure e  –  Scale – due to pr chan es. Salts in form chan ormati ation on wa water ter become become over saturated and precipitate. precipitate.

 –  Wax  –  Asphaltenes – precipitate due to pressure drop

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OVERVIEW OF STIMULATION IN AN OIL WELL • Ma Matr trix ix tr trea eatm tmen entt an and d ac acidi idizi zing ng in pa parti rticu cular lar,, aims to rem aims emov ove e th the e exce cess ss fl flow owin ing g pr pres essu surre drop (p) created by the presence of volume rock wh whic ich h ha hass su sufffered for orm mati tion on da dama mag ge near wellbore area.

• Da Dama mage ged d zon one e ha hass lo lowe werr pe perm rmea eabi bili lity ty th than an original.

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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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OVERVIEW OF STIMULATION IN AN OIL WELL • The removal of this formation damage will restore the ‘natural’ well productivity. • The Hawkins formula;

s

k 

r d 

   1  ln r w  k d   

is con onv ven enie ient nt too ooll for an anal alyz yzin ing g th the e in infl flue uenc nce e of  varying levels and depth of formation damage. TUNIO, May' 2011,,, Courtesy AP Aung

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OVERVIEW OF STIMULATION IN AN OIL WELL

• Matrix

stimulation treatments increase well prod pr oduc ucti tivi vity ty by pu pump mpin ing g a sp spec ecia iall lly y for ormu mula latted treatment fluid (normally acid acid))

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

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Well stimulation types • Acid Fracturing/Acidizing/ Matrix Acidizing • H dra draulic ulic Fra Fracturin cturin

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Acidizing • A type of stimulation treatment, acidizing is performed below the reservoir fracture pressure in an effort to restore the natural permeab abiili lity ty of the reserv rvo oir .

• Well acidizing is achieved by

pumpin pump ing g ac acid id in intto th the e wel elll to dissolve formations like limest lime stone one,, dol dolomit omite e and cal calcit cite e ceme ce ment nt be betw twee een n th the e se sedi dime men nt grains of the reservoir rocks TUNIO, May' 2011,,, Courtesy AP Aung

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OVERVIEW OF ACIDIZING

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OVERVIEW OF ACIDIZING

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Organic Acid (Source: Wikipedia)

orga ganic nic aci acid d is an or An or orga ganic nic com compou pound nd wi with th ac acidi idicc prop pr oper ertie ties. s. Th The e mos mostt co comm mmon on or org gani anicc ac acid idss ar are e th the e carboxylic acids, whose acidity is associated with their carboxyl group -COOH. Sulfonic acids, containing the group -SO2OH, are relatively stronger acids. ew co comm mmon on ex exam amp p es es nc u e: • Lactic acid • Acetic acid • Formic acid • Citric acid • Oxalic acid • Uric acid TUNIO, May' 2011,,,

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Organic Acid (Source: Wikipedia)

• Simple organic acids like formic or acetic acids are used for oil and gas well stimulation treatments. treatments . These organic acids are much less reactive with hydr droc ochl hlor oric ic ac acid id (H (HCl Cl)) or mi mixt xtur ures es of HC HCll an and d hyd ydrrofl ofluor uoric ic ac acid id (H (HF) F).. Fo Forr th this is re reas ason on,, organic acids are used at high temperatures or when long contact times between acid and pipe are needed. needed .

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In-organic Acid (Source: Wikipedia)

mineral al acid or (inor inorgan ganic ic acid) is an acid derived • A miner

from one or more inorganic compounds. compounds. A mineral acid is not organic and all mineral acids release hydrogen ions when dissolved in water.

• Commonly used mineral acids are su sulf lfur uric ic ac acid id,, hydrochloric acid and nitric acid (They are also known as bench acids). acids). Mineral acids range from acids of great stre st reng ngth th (e (exa xampl mple: e: su sulfur lfuric ic acid acid)) to ve very ry we weak ak (bo (boric ric acid). Mineral acids tend to be very soluble in water and insoluble in organic solvents. solvents. TUNIO, May' 2011,,,

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In-organic Acid (Source: Wikipedia)

• • • • • • •

Hydrochloric acid Nitric acid Phosphoric acid Sulfuric acid Boric acid Hydrofluoric acid Hydrobromic Hydrobr omic acid TUNIO, May' 2011,,, Courtesy AP Aung

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FUNDAMENTALS OF FRACTURING •In fr frac actu turi ring ng,, co cond nduc ucti tiv ve path is created by injecting fracturing fluid into the .

•Conductivity is maintained by propping agent(sand or glass beads)to hold the fracture faces apart. TUNIO, May' 2011,,, Courtesy AP Aung

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Frac. Fluid and Proppant Fracturing fluid The fluid used during a hydraulic fracture treatment of oil, gas or water wells. The fracturing fluid has two major functions 1) Open and an d ex exte tend nd th the e fr frac actu ture re;; 2) Tra rans nspo port rt th the e pr prop oppa pant nt al alon ong g th the e fracture length. Proppant Suspended particles in the fracturing fluid that are used to hold frac fr actu ture res s op open en af afte terr a hy hydr drau aulilic c fr frac actu turi ring ng tr trea eatm tmen ent, t, th thus us producing a conductive pathway that fluids can easily flow along. Naturally occurring sand grains or artificial ceramic material are common proppants used.

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FUNDAMENTALS OF FRACTURING Fracture Initiation

• Hydraulic fracture treatment is accomplished by pumpin pump ing g a su suit itab able le fl flui uid d in into to th the e for orma mati tion on at a rate faster than the fluid can leak off into the rock.

• Injecting fluid pressure is built up sufficient to overcome the earth compressive stress holding the rock material together. The rock then parts or fractures along a pl plan ane e pe perp rpen endi dicu cula larr to th the e mi mini nimu mum m co comp mprres essi siv ve stress in the formation matrix. TUNIO, May' 2011,,, Courtesy AP Aung

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FUNDAMENTALS OF FRACTURING  A Fracture Extension:

•As injection of frac fluid continues, the fracture tends to grow in width as fluid pressure in the fracture works againstt the elasticity of the rock material. agains material.

• After sufficient frac fluid has been injected to open the fracture wide enough to accept proppant, sand is added to the frac fluid and is carried into the fracture to hold it open after the job. TUNIO, May' 2011,,, Courtesy AP Aung

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FUNDAMENTALS OF FRACTURING When sufficient sand has been injected:

• pumps are shut down •

ressure ress ure in in the fra fractur cture e dro dro s

the e ea eart rth h com ompr pres essi siv ve str tres esse sess cl clos oses es th the e • th fracture on the proppant.

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FUNDAMENTALS OF FRACTURING Sand-Out Treatment:

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

• As sand enters the fracture and is deposited, more fluid flui d pr press essur ure e is re requ quir ired ed to cr crea eate te gr grea eate terr st stre ress ss against the fracture face to increase the frac width.

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FUNDAMENTALS OF FRACTURING • If the required fluid pressure cannot be applied due to equipmentt or casing limita equipmen limitations: tions:

• fluid injection rate slows down • sand drops drops out of the fluid fluid at a more rap rapid id rate rate • sand-out in the fracture occurs

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FUNDAMENTALS OF FRACTURING Fracturi Fra cturing ng is beneficial beneficial in following following case cases: s:

1. If the reservoir is composed of a lowpermea per meabi bilit lity y, hom homoge ogeneo neous us ro rock, ck, fr fract acturi uring ng is . . ., for orme merl rly y fl flow owed ed th thro roug ugh h th the e lo loww-pe perm rmea eabi bili lity ty rock becomes able to move into the high cap apac aciity fr frac actu turre at so som me di disstan ance ce fr from om th the e well.)

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FUNDAMENTALS OF FRACTURING 2. Fr Fracturi acturing ng will will elimina eliminate te forma formation tion damage damage due to - in inva vasio sion n of dri drill lling ing mud mud,, - depo depositi sition on of mine mineral ral mat matter ter - sw swel elli ling ng of cla clays ys If damage is only in the immediate vicinity of the well bore, only a mild fracture treatment is required.

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FUNDAMENTALS OF FRACTURING 3. Fracture radiati tin ng fr fro om th the e well bore act as a gathering lines connecting permeable and porous systems that are otherwise isolated from the well .

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MATRIX ACIDIZING VS. HYDRAULIC HYDRAU LIC FRACTU FRACTURING RING

• Matrix acidizing – removal near wellbore damage • Hydraulic fracturing – improving well inflow performance Skin

Permeability

Treatment

High

Low

Fracturing (matrix possible)

Low

Low

Hydraulic Fracturing

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MATRIX ACIDIZING VS. FRACTURING TREATMENTS

• Matrix acidizing aims to;  –  Increase the PI  –  By reducing the skin  –  through

dissolving formation damage components and rock near wellbore region

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MATRIX ACIDIZING VS. FRACTURING TREATMENTS

• Hydraulic fracturing aims to:  –  Increase the PI  –  By increasing effective wellbore radius  –  through creation of high conductivity fractures from the wellbore

 –  Fracture bypass the damage zone and extend to a greater depth into the reservoir than can be reached by acidizing

 –  Suitable for low permeability with or without skin TUNIO, May' 2011,,, Courtesy AP Aung

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General safety and work guidelines & Well Stimulation Ec Economics onomics

ASSIGNMENT NO. 01 Deadline : Wednesday ednesday,, 15/06/2011 TUNIO, May' 2011,,, Courtesy AP Aung

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