Well stimulation - petroleum engineering

SORAN UNIVERSITYSchool of Engineering

Department of Petroleum Engineering

Dr. Muhammad Amin

Well Stimulation

Prepared by:Rebaz Abdulqadr HamadAras Bahri SalimDlvin Taher AbdullahZhidar JargisRawa AsaadOmer Muhemmed

Well Stimulation Some, petroleum exists in a formation

but is unable to flow readily into the well because the formation has very low permeability.

• Natural low permeability formation.• Formation damage around the wellbore caused by invasion of perforation fluid and charge debris.

Well Stimulation Formation damage: the reduction of permeability in a reservoir rock caused by the invasion of drilling fluidand treating fluids to the section adjacent to die wellbore. It is often called skin damage.

Well Stimulationany of several operations used to

increase the production of a well or a treatment

performed to restore or enhance the productivity of a well such as: 1) Acidizing2) Fracturing

Well StimulationAcidizing • The pumping of acid into the wellbore to remove near-well formationdamage and other damaging substances.• This procedure commonly enhances production by increasing theeffective well radius.

Well Stimulation The two basic types of acidizing are

characterized through injection rates and pressures:

Injection rates below fracture pressure are termed Matrix acidizing.

Injection rates above fracture pressure are termed Fracture acidizing.

Well StimulationMatrix acidizing Matrix acidizing is applied primarily to

remove skin damage caused by drilling , completion, workover , well-killing, or injection fluids, and by precipitation of scale deposits from produced or injected water.

During matrix acidizing the acids dissolve the sediments and mud solids within the pores that are inhibiting the permeability of the rock.

mostly used in sandstone formations.Due to the extremely large surface area

contacted by acid in a matrix treatment, spending time is very short. Therefore, it is difficult to affect formation more than a few feet from the wellbore.

Well Stimulation Fracture acidizing Fracture acidizing is an alternative to hydraulic

fracturing and propping in carbonate reservoirs. In fracture acidizing, the reservoir is hydraulically fractured an then the fracture faces are etched with acid to provide linear flow channels to wellbore.

As such, the application of acid fracturing is confined to carbonate reservoirs and should never be used to stimulate sandstone, shale, or coal-seam reservoirs.

Long etched fractures are difficult to obtain, because of high leak off and rapid acid reaction with the formation

Well Stimulation

Well StimulationWELL STIMULATION ACIDS The basic types of acid used are:• Hydrochloric• Hydrochloric-Hydrofluoric• Acetic• Formic• Sulfamic• Fluoboric Also, various combinations of these acids are

employed in specific applications.

Well StimulationACID ADDITIVES: Acidizing can cause a number of well problems. Acid may :(1) release fines(2) create precipitants(3) form emulsions(4) create sludge(5) corrode steel Additives are available to correct these

and a number of other problems

Well Stimulation• Surfactants should be used on all acid jobs

to reduce surface and interfacial tension, to prevent emulsions, to water-wet the formation, and to safeguard against other associated problems.

Suspending Agents: Most carbonate formations contain insolubles which can block formation pores or fractures if fines released by acid are allowed to settle and bridge.

Well Stimulation Suspension should be differentiated

from dispersion. Dispersed particles usually settle in a short time.

A suspending surfactant, such as Halliburton's HC-2, in concentrations of about five gallons per 1,000 gallons of acid may suspend fines for more than 24 hours, and possibly as long as seven days. Suspending agents are usually polymers or surfactants

Well Stimulation Anti-Sludge Agents: Some crudes, particularly heavy asphaltic crudes, form an insoluble sludge when contacted with acid, with greater problems experiences with high strength acid. Dissolved Fe(III) in acid appreciably increases the possibility of sludge.

The primary ingredients of a sludge are usually asphaltenes.

Well Stimulation Sludges may also contain resins and

paraffin waxes, high-molecular weight hydrocarbons, formation fines, clays, and other materials.

The addition of certain surfactants can prevent the formation of sludge by keeping colloidal material dispersed. These sludge-preventing surfactants usually prevent an emulsion

Well Stimulation Corrosion inhibitors for acid are

chemical additives that reduce the rate of corrosion of steel by acid.

There are two primary reasons for using corrosion inhibitors:(1) to protect the acid pumping and handling equipment(2) to protect well equipment.

Well Stimulation Factors that govern the degree

of acid attack on steel are:1) type of steel including hardness2) temperature3) type of acid4) acid concentration5) acid contact time

Well Stimulation CARBONATE ACIDIZING The objective of acidizing limestone and

dolomite wells is to removedamage near the wellbore or to create linear flow channels by fracturing andetching. Acid may also be used in sandstone wells to

dissolve carbonates in the formof sand grain cementing materials, discrete particles, and carbonate scale.

Well Stimulation The time required for a specified volume

and concentration of HCl acid tospend to about 3.2% in a selected formation under given conditions isdefined as Acid Reaction Time.

A major problem in fracture acidizing of carbonate formations is that acids tend to

react too fast with carbonates and spend near the wellbore.

Well Stimulation Factors controlling the reaction rate of

acid are: area of contact per unit volume of acid formation temperature pressure acid concentration acid type physical and chemical properties of

formation rock flow velocity of acid

Well Stimulation Retardation of Acid:To achieve deeper penetration in fracture acidizing, it is often desirable to retardacid reaction rate. This can be done by

gelling, Emulsifying chemically retarding the acid

Well StimulationGelled Acid: The use of gelled acid for fracture

acidizing has increased to the point that it is now the most used technique.

The introduction of more temperature-stable gelling agents with ready application up to temperatures of about 400°F has been a major factor in selecting gelled acid for acid fracturing. Two types of gelling systems, polymers and surfactants, are in common use.

Well StimulationEmulsified Acid: For many years the primary retarded acid

for fracture acidizing was an acid-in-oil emulsion. This type retarded acid is very functional but is no longer the primary fracture acid method used.

It has limited temperature range and stability, with high viscosity and high friction loss.

It does, however, have the ability to restrict contact between the acid and formation, to reduce fluid loss, and to retain large quantities of the treating fluid in the fracture.

Well StimulationChemically-Retarded: Acid-Retardation of HCl is obtained by the

addition of unique surfactants to the acid which form protective films on the surface of limestone or dolomite.

These films retard reaction rate in much the same way that an acid corrosion inhibitor protects metal.

In addition to retarding acid reaction rate, chemical retarders tend to promote nonuniform etching of fracture faces, thus increasing fracture conductivity.

Hydraulic Fracturing Hydraulic Fracturing The objective of hydraulic fracturing

for well stimulation is to increase well productivity by creating a highly conductive path (compared to reservoir permeability) some distance away from the wellbore into the formation.

Usually the conductivity is maintained by propping with sand to hold the fracture faces apart.

Hydraulic Fracturing Acid fracturing involves most of

the same considerations as hydraulic fracturing except that conductivity is generated by removing portions of the fracture face with acid, leaving etched channels after the fracture closes

Hydraulic Fracturing During a fracture job or hydraulic fracturing,

a service company injects large volumes of fracture fluids under high pressure into the well to fracture the reservoir rock . Fracture jobs are done either in an open-hole or a cased well with perforations.

Common fracture fluid is a gel formed by water and polymers, long organic molecules that form a thick liquid when mixed with water.

Oil-based fracture fluid and foam-based fracture fluids using bubbles of nitrogen, or carbon dioxide can also be used to minimize formation damage.

The fracture fluid is transported out to the fracture job in large trailers

Hydraulic FracturingA fracture job is done in three steps: First, a pad of fracture fluid is injected into

the well by several, large, pumping units on trucks to initiate fracturing the reservoir.

Hydraulic Fracturing Next, a slurry of fracture fluid and propping

agents are pumped down the well to extend the fractures and fill them with propping agents. Propping agents or proppants are small spheres that hold open the fractures after pumping has stopped.

Hydraulic Fracturing The propping agents are usually well sorted

quartz sand grains, ceramic spheres, or aluminum oxide pellets. The well is then back flushed in the third stage to remove the fracture fluid.