Daño a La Formación (Paper)

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SPE-179011-MS

Near Wellbore Damage and Types of Skin Depending on Mechanism of Damage

Mahesh Chandra Patel, and Aaditya Singh, Gubkin Russian State University of Oil and Gas, Moscow

Copyright 2016, Society of Petroleum Engineers

This paper was prepared for presentation at the SPE International Conference & Exhibition on Formation Damage Control held in Lafayette, Louisiana, USA, 24–26

February 2016.

This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contents

of the paper have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflect

any position of the Society of Petroleum Engineers, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written

consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may

not be copied. The abstract must contain conspicuous acknowledgment of SPE copyright.

Abstract

Near wellbore damage has been a topic of great attention over last many decades. Typical measures of

formation damage is skin factor (S) which is a hydrodynamic parameter characterizing additional

resistance to flow of fluids in the near borehole zone of the reservoir, leading to reduced production (yield)

compared to the perfect (ideal) wells. This has been a great attention of topic because it directly effects

on formation fluid productivity from the concerning well. Reasons behind formation of this damage can

be combinations of several mechanisms are clay particle swelling, fluid loss or change in formation water

saturation, wettability reversal, Emulsion blockage, Mutual precipitation of soluble salts in wellbore-fluid

filtrate and formation water, Deposition of paraffins or asphaltenes, Fines migration etc. Operations which

are usually leads to those mechanisms to happen are drilling, cementing, well completion, work-over

operations, production of fluids, injection of fluids and Operations to isolate water production etc.

This paper describes mechanisms of the formation damage. And introduces types of skins depending

on the mechanism of damage and source operation, and Types of skin are

1. The mechanical or formation damage skin factors (Sd )

2. Completion pseudo skin factor (S p)

3. Partial penetration skin factor (S pp)-

4. Geometrical pseudo skin factor (Sg)

5. Multiphase pseudo skin factor (Sm)

6. Non-Darcy flow or Rate-dependent high velocity or turbulent flow pseudoskin factor (Sturb)

This paper also describes all type of skin in detail with mathematical models given to calculate the skin

individually as well as effective skin.

Introduction

Formation damage can happen at any given time in the lifecycle of a well: drilling, completion, production

or work over operations too. As exploitation activities are becoming more challenging and in more

complex, tighter and from deeper reservoirs, these days much greater importance is being associated with

understanding the formation damage mechanism, its type and cause since it is detrimental to the well



productivity. Studying the near wellbore damage is of utmost significance on open hole completions.

Formation damage basically includes flow restrictions caused by a reduction in permeability in the

near-wellbore region, changes in relative permeability to the hydrocarbon phase, and unintended flow

restrictions in the completion itself. The global cost of Formation Damage is difficult to measure but it

is estimated that billions of dollars per annum are lost through deferred production, remedial treatments

and irrecoverable damage. Therefore, if not avoidable, diagnosing, assessing, quantifying and remediating

the formation damage are among the most important issues to be resolved for efficient and more profitableexploitation of hydrocarbon reservoirs. The influence of damage of bottom-hole zone of the formation is

estimated using well testing or hydrodynamic investigations of wells. Damage of the bottom-hole zone

acts as a choke, restricting the flow of fluid into the well and gives additional pressure loss. The degree

of damage of bottom-hole zone can be evaluated by conducting a well testing.

The degree of damage of bottom-hole zone is given by hydrodynamic parameter Skin factor(S) and

pressure loss is denoted as Pskin.

Generally, If skin factor is positive ( 0), the damage of bottom-hole formation zone exists, and the

relative magnitude of the skin factor indicates the degree of damage. A negative skin factor ( 0) shows

how increased effective radius of the well after the stimulation.

Formation Damage

The simplest way to define Formation Damage is that it is any process that leads to a reduced natural

productivity of that formation or decrease in water or gas injectivity into the formation by the injection

wells.

Mechanisms of Formation Damage

Formation Damage can be classified into four types based on their damage mechanism:

1. Chemical

2. Thermal

3. Mechanical

4. Biological

1. Chemical Damage MechanismsThey include mechanisms in which there is an interaction between the rock formation and the

injected fluids or between the formation fluids and the injected fluids. Some of the common types

of chemical damage mechanisms are:

● Clay Deflocculation: This is caused when the electrostatic forces between the clay molecules

and also between the walls of the formation and the clay units, which hold them together are

altered or disrupted. Some of the causes are rapid changes in salinity or pH.

● Clay Swelling: Interaction of hydrophilic clays with fresh water or low salinity water leads to its

hydration and expansion of these clay units that causes permeability reduction in the medium.

Figure 1—Clay Swelling – unexpanded and expanded clay with water molecules (hydration). (Source – petrowiki.org).

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● Chemical Adsorption: Injection fluids often contain heavy or high molecular weight com-

pounds and polymers. When these compounds come in contact with the formation, some-

times, get attracted and adsorbed on the surface of the formation. This restricts the flow area

thereby decreasing the formation permeability.

● Emulsions: Most commonly produced oilfield emulsions are water-in-oil emulsions. Water-

in-oil emulsions consist of water droplets in a continuous oil phase. They are also troublesome

as they increase the viscosity up to two to four orders more than clean and non emulsified oil.

In case of heavy oils, an emulsion block may be created, reducing the permeability of the

medium.

● Paraffins and Waxes: Crystallization of long alkane chains of hydrocarbons into waxes is

common in oils having a low cloud point temperatures. This forms paraffin or wax plugs

resulting in permeability reduction in the formation.

2. Thermal Damage Mechanisms

High temperatures in the formation are a consequence of thermal recovery processes like steam

injection and in situ combustion and the resulting damage may be of the following types:

● Dissolution: With the rise in temperature, solubility of minerals also increases. When theinjected high temperature fluid with dissolved minerals from the matrix, flows deeper into the

formation, the injected fluid cools down and the dissolved minerals may get precipitated back

and plug the flow channels.

● Mineral transformations: Some clays swell only at high temperatures. So a clay unit which is

not reactive at normal temperatures may get ionized at very high temperatures and get

hydrated and swell up to reduce the formation permeability.

● Reduction in absolute permeability: High temperatures associated with thermal processes in

deep reservoirs with high overburden pressures can result in expansion of grains in the rock

matrix and consequently narrowing down of pore space.

3. Mechanical Damage Mechanisms

Mechanical Damage of the formation is based on the physical interaction of the formation and the

equipments or fluids used or injected during the well operations. Common Mechanical damage

mechanisms include:

● Migration of Fines: The movement of naturally existing fine clay, quartz particles or similar

particulates within the reservoir formation system due to drag forces during production may

result in this kind of formation damage.

● Solids Invasion: This is a common type of drilling induced formation damage as drilling

fluids contain very fine solids which may move into the formation and occupy the pore

space in the matrix especially in the case of overbalanced drilling. Its severity is very high

in open hole completions.

SPE-179011-MS 3



● Geomechanics: During the production life of a well, the void created by the extraction of

formation fluids may disturb and drastically alter the geomechanical stress profile of the area

near the wellbore. It may lead to change in the geometry of the pores and hence interrupt the

permeable flow channels.

4. Biological Damage Mechanisms

In cases where nutrients and bacteria are injected in the formation, there are three basic types of

damage process:

● Corrosion: Some types of microorganisms are responsible for pitting and hydrogen stress

cracking on downhole metallic equipments. This is a result of electrokinetic hydrogen

reduction reaction caused by certain types of bacteria.

● Toxicity: Sulphate Reducing Bacteria initiate sulphate reduction reactions of sulphates present

in the formation or in the injected fluids, giving rise to toxic Hydrogen Sulphide gas.

● Plugging: Bacteria generally produce a thick polysaccharide polymer during their lifecycles

which gets absorbed in the matrix and blocks the area of flow, thus reducing the permeability

of the formation.

Types of skin factor Yildis(2003) noticed that skin factor should be expressed as a proper function of the various skin factors,

whereas it has been usually expressed inaccurately as a simple linear sum of all the contributing skin

factors.

Yildis (2003) stated that the interaction between the total skin factor and its individual components is

mostly non-linear and the decomposition of the total skin factor to its elements requires an accurate

mathematical model.

Skin can be divided into following types

1. The mechanical or formation skin factors (Sd )

2. Completion pseudo skin factor (S p)

3. Partial penetration skin factor (S pp)-4. Geometrical pseudoskin factor (Sg)

5. Multiphase pseudoskin factor (Sm)

6. Non-Darcy flow or Rate-dependent high velocity or turbulent flow pseudoskin factor (Sturb)

The mechanical skin factors (Sd)

Alteration of pore structure occurs as a result of adverse formation damage processes such as formation

fine migration, filtrate invasion, rock compaction and deformation, acid stimulation and other mechanical

processes which lead to formation permeability impairment. The parameter to estimate the intensity and

extent of the damage is given by mechanical skin factor (Sd ).

Figure 2—Solids Entrainment in the pore spaces. (Source – petrowiki.org).

4 SPE-179011-MS





And pressure drop due to mechanical formation damage assuming radial single phase flow is given as

(Hawkins 1956)

Where

K d is permeability of damaged zone

K is permeability of undamaged formation

r d is radius of damaged zone

r w is radius of well

Completion pseudo skin factor (Sp)

It is very important for oil production to provide effective communication of the reservoir with the well.

The inflow to the wellbore can be difficult due to damage of the bottom-hole zone, which can be removed

by using methods of stimulation. The effectiveness of any type of treatment of bottom-hole area depends

on the perforations. The detonation of the old charges, perforators, giving misfires, inaccurate perforation

of a certain interval, all affect the productivity of wells.

An improper well completion technique which includes penetration of high speed metal jets or

perforation by different fracturing methods leads to reservoir deformation, compaction and crushing of the

rock. The scale of the damage resulted from improper completion can be given by completion pseudo skin

factor (Sc).

A semianalytical perforation skin model was presented by Karakas and Tariq (1991)

According to which the perforation skin (S p) excluding the mechanical damage skin can be calculated

Figure 3—The influence of mechanical skin factor (Sd

).

Figure 4—Perforation spacing and geometry. (Source – Jonathan bellarby, Well completion design 2009 first edition).

SPE-179011-MS 5



Sh is horizontal skin

Swb is wellbore skin

Sv is vertical skin(Sv)

And Sc is crushed zone skin

Parameters C1, C2, x, a1, a2, b1 and b2 are empirical function of the gun-phasing angle. (table 2)

Table 1—Perforation input parameters. (Source-Jonathan

bellarby, Well completion design 2009 first edition).

Table 2—Gun –phasing parameters for karakas and tariqperforation model. (Source-Jonathan bellarby, Well completion

design, 2009, first edition).

6 SPE-179011-MS



Partial penetration skin factor (Spp)

To prevent formation from water coning and gas coning well are often completed partially, this producing

fluid partially penetrated wells experiences additional pressure drop due restricted region accessible to

fluid flow which contributes to increase skin factor.

Skin due to partial penetration can be calculated using the Odeh correction (1980) shown below

Here r wc is effective radius of the well

Typically partially penetration skin ranges from 0 to 30.

Geometrical pseudoskin factor(Sg)

When the well is horizontal or deviated, the contact area between the producing formation and the well

increases many times. The benefits of an additional area can be determined using the values pseudo-skin factor.Since the horizontal and deviated wells are expected to give higher productivity than vertical, the value of skin

factor Sg is negative.This type of skin are mainly depends on anisotropy (k v /k h) of the formation.

The pseudoskin resulting from well inclination can be given as (Burton et al.,1998):

Where h D is the depth, and r w is the wellbore radius.

’w and h D can be given as

Figure 6 —Inclined well.

Figure 5—Partially penetrated well. (Source- www.fekete.com).

SPE-179011-MS 7

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Here, w inclination angle of the well, K h and Kv are horizontal and vertical permeabilities.

For short horizontal wells as compared to reservoir dimensions Joshi(1988) presented an approxi-

mated skin factor

Here

Later Joshi’s equation was corrected by Kuchuk et al.(1990) and confirmed by Besson (1990).

This equation is valid for only shorted well lengths and thicker reservoirs.

Multiphase pseudoskin factor (Sm)

The multiphase flow usually occurs in reservoirs at pressure below the bubble point pressure or in gas

reservoirs at pressure below dew point pressure or in hydrocarbon reservoirs with below water bearing zones.

Figure 8—horizontal well geometry. (Source- - Jonathan bellarby, Well completion design, 2009, first edition).

Figure 7—Fully completed slanted well. (Source-Jonathan bellarby, Well completion design, 2009, first edition).

8 SPE-179011-MS



The multiphase flow develops additional pressure losses resulting from the relative permeability effect

of various fluid phases present in the multiphase fluid system.(Yildiz, 2003).

Skin factor resulting from oil-water phase simultaneous flow can be expressed as (Bratvold and Horne

1990):

Where y f is the value of Boltzmann variable at the displacement front between water and oil,

Here r D and tD denote the dimensionless radial distance and time. Total mobility can be given by

0.5772 is Euler’s constant

M is end point mobility ratio water to oil. And is mobility of the water at residual oil saturation.

is diffusivity and is end point diffusivity ratio.

Non-Darcy flow pseudoskin factor(Sturb)

Flow regimes changes into non-Darcian from the Darcian due to convective acceleration or deceleration phenomenon involving in the fluid motion in the near wellbore region during production of the fluids

especially in cases of gas wells.

The produced turbulence mainly depends on the tortuosity effect or inertial coefficient which is a

function of average path length that a fluid particle must travel and the straight minimum length. This

turbulence results in additional pressure drop near wellbore region where the flow is also known as inertial

turbulent flow.

The influence of turbulence on the pressure drop and flow rate can be estimated using Equation

Forchheimer (Forchheimer):

Where

P loss of pressure, atm

L the length of the portion on which pressure loss occurs, CM

viscosity, cP

v the rate of flow, cm/sec

v2 hydraulic resistance due to turbulence, atm

the coefficient of turbulence

Figure 9—Example of two phase flow.

SPE-179011-MS 9



density of fluid, gram/cm3

Measurement of skin factor

By the method of pressure build-up

Since the effect of damage on the characteristics of the well can be significant, there have been developed

various methods for determining the skin factor with the help of well testing. Understanding the effectsof skin factor to work well is very important when choosing a method of stimulation and removal of

contamination bottom zone.

Characteristics inflows to the well may be analysed by a steady flow rate and pressure build-up during

shut period. Pressure build up data can be shown on the graph as a function of time. In the early 1950s,

Dr. Horner developed a method of calculating the skin factor of data recovery pressure of the oil well.

From the diagram of pressure as a function of log [(t t) / t], the slope m is a function of the steady

flow rate q, the viscosity of reservoir fluid , volume factor B and capacity reservoir kh. By determining

the constant angle, the total skin factor Stotal can be calculated using the equation of Van Everdingen and

Hurst

Where:

P1 hr extrapolated value of the pressure for t 1 hour from the chart Horner

Pwf bottomhole pressure during the test m slope of the curve in the diagram Horner

k the effective permeability, calculated from the slope

porosity

viscosity of the fluid

c compressibility of the liquid

r w well radius

where:

q flow rate steady

viscosity of the liquid

B volume factor

kh capacity of the reservoir

k effective permeability

h height of the productive interval

Figure10—Horner Plot.

10 SPE-179011-MS



The total skin factor

The value of the skin factor, calculated from bottom-hole pressure build up curve which provides reliable

data on the productivity of the well. As mentioned, a positive skin factor indicates a problem with the

efficiency of production. When a positive skin factor is removed (becomes zero or negative) is achieved

by increasing the productivity of the well.

However, the different components of the skin factors are interlinked. So it is not possible to add the

skin factor components.There are some of the models given by combining some of the skin factors, Pucknell and Clifford

(1991) proposed a simple method to combine skin factors. The model shows that the total skin factor is

sum of mechanical and completions skin factors.

The total skin factor consists of several components and the effective positive skin factor depends on

the skin removal processes (if applied), opening and orientation of the well and flow of reservoir fluids.

The equation shows all components of the total skin factor:

Where:

st skin factor due to damage of bottomhole formation zone

S p skin factor due to ineffective perforations or completion

S pp skin factor due to the partial opening

Sg skin factor due to the inclination or the geometry of the well

Sm skin factor due to multi-phase flow

Sturb skin factor due to turbulence

Conclusions

It is important to do prediction and simulation of all the consequesnses encountered during the life of well

that can arise due to the formation damage from various types of damage processes which can lead

industry to optimise all the efforts and strategies for the reduction or prevention of damage.

By affecting reservoir properties and fluid production, formation damage and skin factor are not limited

upto geologists or reservoir engineers, they consist processes related to many disciplinaries and require

experties from different disciplinaries to overcome the consiquneses, As Faruk civan (2006) also stated

formation damage requires interdisciplinary knowledge and expertise.

This paper describes all type of skin factors depending on completion techniques, well geometry and

common mechanisms of formation damage but still there are many operations and processes resulting to

skin and can affect the total skin factor like chocked fracture skin, skin on fracture face etc.

References1. Skin, http://www.fekete.com/san/webhelp/welltest/webhelp/Content/HTML_Files/Reference_Materials/Skin.htm

2. Faruk CIvan 2007, Reservoir formation Damage, second edition.

3. Drilling problem and solution, http://petrowiki.org/PEH%3ADrilling_Problems_and_Solutions#Damage_

Mechanisms4. Brant Bennion, 1999, Formation damage, JCPT auther series, februry 1999

5. Jonathan bellarby, Well completion design, 2009, first edition

6. Alfred R. Jennings, Jr. P.E., Enhanced Well Stimulation, Inc

SPE-179011-MS 11

http://www.fekete.com/san/webhelp/welltest/webhelp/Content/HTML_Files/Reference_Materials/Skin.htm

http://petrowiki.org/PEH%3ADrilling_Problems_and_Solutions#Damage_Mechanisms




http://www.fekete.com/san/webhelp/welltest/webhelp/Content/HTML_Files/Reference_Materials/Skin.htm

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Daño a La Formación (Paper)