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Skin, Perforations/Logs, h

Nodal Analysis Workshop

Nodal AnalysisTM is a technique that helps us calculate the rate / pressure relationship of production into the wellbore from the reservoir, and then up the tubing string to the surface, and then through a surface line, if needed.

This technique is generically known as well systems analysis (the Nodal Analysis name has been trademarked by Schlumberger).

Last session, we:1. Reviewed rock and fluid properties2. Reviewed the basic, analytical,

straight line IPR, or the radial form of Darcy’s law

3. Reviewed outflow4. Took a brief tour of SNAP5. Worked a few problems

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Nodal Analysis Workshop ═══════════════════════════════════════════════════════════════════════════════════

©PetroSkills, LLC. All Rights Reserved. _________________________________________________________________________________________________________

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Skin, Perforating / Logs, H

Skin, Perforating

Objectives• Learn the definition of skin• Explain how skin

affects the radial pressure distribution

• Learn the various components of skin, including the effects of perforating

• Understand the complexity of calculating skin

Logs, H

Objectives• Recognize the correct H

to use• Review logs and the

data they provide

1. No casing, cement, drilling fluid, perforations - the reservoir / well bore interface is a clean sand face

2. The entire pay section is open to flow, the reservoir is horizontal and the well is vertical

3. Pressure profile (pressure vs radial distance) is illustrated

4. Note that most of the pressure drop occurs near the wellbore due to the logarithmic term in the equation

5. Single phase incompressible fluid

P = pressure

Pwf = pressure at sandface

141.2 = constant required when oilfield units are used

qo = rate in barrels of oil per day

uo = viscosity of oil in cp

Bo = formation volume factor

r = radial distance in feet

rw = wellbore radius in feet

k = permeability in md

h = pay thickness in feetP = Pwf + 141.2*qo*uo*Bo*ln(r / rw)

k*h

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Ideal Inflow

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1. The drilling process has caused mud filtrate to invade the formation, or the well may have been acidized

2. The well has been cased and cemented

3. The casing and formation have been perforated

4. The perforations may have been packed with gravel pack sand, and a screen installed

5. The entire zone may not have been perforated

6. The wellbore may not be vertical and the reservoir may not be horizontal

7. Flow may be highly turbulent

8. All of these changes MAY create a change to the expected pressure distribution

Actual Inflow

Pwf - ideal

Pwf - actual

r

p

The effects of these deviations from the ideal inflow situation is usually represented as an “additional” pressure drop required to achieve the same production rate. The additional pressure drop is spent overcoming the additional resistance to flow caused by reduced perm from drilling muds, convergence to open perforations, etc.

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Pr – P’wf = 141.2*qo*uo*Bo*ln(r / rw)k*h

Pr – Pwf = 141.2*qo*uo*Bo*[ln(r / rw)+s]k*h

This additional pressure drop, the difference between ideal wellbore flowing pressure and observed well bore flowing pressure, is defined as “skin”.

P’wf

Pwf

Pr

∆Ps = P’wf – PwfS = ∆Ps k*h

141.2*qo*uo*Bo

Skin

Skin = the difference in pressure from an “ideal” IPR and an “actual” IPR.

Skin = kh ∆Ps141.2qouoBo

Skin can be positive, or negative. When Skin is positive, the pressure drop is greater than ideal, and the well is damaged. When skin is negative, the pressure drop is less than ideal, and the well is stimulated.

Skin can be MEASURED via a pressure transient test, and we’ll get to do this in a later section.

+skin

-skinideal

Skin is dimensionless. Do you like dimensionless parameters?

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Skin

Skin is comprised of the following:

Depth and degree of damage caused by drilling (or other) fluid invasion

Depth and degree of damage caused by crushed zone around the perforation

Flow convergence to a perforation

Flow convergence on partially perforated / penetrated intervals

Restriction, if any, caused by the gravel pack sand and screens

Additional pressure drop caused by high velocity flow

Since skin is equivalent to a pressure drop, skin can be “calculated” by figuring out the pressure drop associated with each of these restrictions.

Partial Penetration

A partial completion is very common (to stay away from water or gas, and the result forces the flow to “converge” on the open perforations. This convergence requires an additional pressure drop, and is seen as skin.COPYRIG

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Skin

Apply Darcy’s law from the outer reservoir boundary (Re) to the outer radius of the damaged zone. Then, apply Darcy’s law with the reduced permeability from the outer radius of the damaged zone to the wellbore radius.

Many of these calculations are fairly complex, the process for calculating the skin damage due to fluid invasion is straightforward, and is a good one to investigate and learn the overall process.

Exercise

Using Darcy’s law, and the following information, construct:

A graph of pressure (y axis) vs. radial distance away from the wellbore (x axis)

• What is the skin value?

q = 1000 bopdrd = 2 feet rw = 0.5 feetBo = 1.33u = 0.8K = 750 mdkdmg = 50h = 80 feetPwf = 4000 psi

P = Pwf + 141.2*qo*uo*Bo*ln(r / rw)k*h

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Skin

The difficulty with calculating skin by calculating individual pressure drops is thoroughly knowing the value of the parameters involved. Typically, the flow model is investigating a small area (a perforation, or a radial zone around the wellbore) which, by definition, has deviated from normal conditions. Almost never does the practicing engineer know the value of the involved parameters to a sufficient detail to realistically calculate the pressure drop, and therefore the skin.

How deep is the invasion?

What is the permeability in the damaged zone?

What is the permof the gravel packsand?

What is the length of the perforation?

What is the extent of restriction caused by the perforation damaged zone?

Exercise

Flow Efficiency is defined as the ratio of actual production to ideal production:

This is often simplified to: 7 / (7 + s) So, a well with a skin of 7 has a FE of 50%.

Using the equations previously discussed, and a short analysis of the ln (re / rw) term, show how the simplification 7 / (7 + s) was derived.

= qactualqidealCOPYRIG

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Resistivity LogsSP Log

Example: Gamma Ray / Resistivity / Porosity Log Interpretation

Log Interpretation

Based upon all data obtained from a selected suite of open hole logs run across an interval, log analysts concur to: core, flow test, perforate and complete, assess hydrocarbons in place, etc.

Typical SP / Gamma Ray / Resistivity / Porosity Log Suite

Gamma Ray log (in API units)Log measures natural background radiation / gamma ray from K, U, Th.

• Lithology indicator in open hole or through casing

SP Log Spontaneous Potential Log (in milliVolts)• Lithology indicator, usually now replaced with the Gamma Ray

Resistivity log (in ohm-meters) Laterolog / induction log (conductivity) Log measures resistance to induced current

• A 1 meter thick shale section measures 6 ohm-meters

• A low porosity zone or fresh H2O or hydrocarbon shows more electrical resistance

Acoustic or Sonic log (in microseconds)Log measures travel time of imposed signal

• Identifies lithologies from differences in travel times

• Differentiates salt and anhydrite• Measures formation

Density log (in grams / cc), Compensated Formation Density LogLog measures back scatter of imposed radiation – gamma rays

• Identifies lithology and porosity

• Combined with neutron logs, is an indicator of gas

List indicates the broad diversity of open hole

log measurements

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Concepts in Net Pay1. Gross Sand Height2. Gross pay thickness3. Net pay thickness4. Perforated interval5. Distance from top of sand

to top perforation

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“Net to Gross” is the ratio of the net pay to the gross pay thickness, and is usually expressed as a percent (example: 80%).

Actual Inflow – Pay ZoneUnlike the ideal case, where the entire sand was open to flow, and contributed to flow, a real reservoir may have non-productive areas, and may be perforated only in part.COPYRIG

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1. The reservoir has a dip (can be calculated from the structure maps, dipmeter logs).

2. Wells may have deviation and intersect the reservoir at an angle.

3. The measured distance is what is seen on the open hole measured depth logs.

4. Correcting for well angle gives TVD.

5. Correcting for well angle AND reservoir dip gives the true reservoir thickness (“h”) used in most flow calculations.

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Concepts in Reservoir Geometry

Skin

Analytically calculating skin is relatively easy to do with most software packages, but the assumptions that must be made render the results a rough approximation at best.

It is always superior to conduct a transient test and measure skin, especially if serious decisions are to be made on the outcome.

We will investigate this further once we start using the program, and see the effects a “reasonable” range of parameters has on skin.

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Review Skin

The ideal flow equation assumes nice, even flow from the reservoir to an undamaged sandface. In reality, we have drilling fluid invasion, cement sheaths, perforations, partially completed intervals, etc. All these act as restrictions to flow, usually very close to the wellbore. The additional pressure drop required to overcome these restrictions is skin. COPYRIG

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So, we have seen two general areas of flow,

1. Inflow2. Outflow

Further, we have seen that the inflow region can be divided into two parts.

A. Flow in the reservoir where Darcy’s law (perhaps adjusted for compressibility / two phase flow) applies.

B. Flow in the near wellbore region where deviations from the ideal model yield additional pressure drops defined as skin.

B

A

2

1

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Perforating Details

Nodal Analysis Workshop

Objectives of Perforating

Perforating establishes communication between the formation and well bore

• Deep and conductive perforations– Maximise well productivity ‒ can yield a negative skin: Sperf can be in

the range + 5 to –2

– Prefer the perforations to bypass the damage and preferably the invaded zone – depends on depth!

• Control fluid migration into and from the well ‒ reservoir management

– Selectivity on flow and clean up

– Selective isolation

In some cases, only a fraction of the total number of perforations actually flow but:

• It depends on reservoir heterogeneity• Effectiveness of perforation clean-up

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Perforating Job Variables

Geometry of perforating• Gun phasing• Shot density• Perforation diameter• Length of perforation

– Crushed zone and plugged tip

• Charge weight• Charge type (RDX, HMX, etc)

Hydraulic conditions• Underbalance ‒ Static or

Dynamic• Overbalance

Gun type

Conveyance system

Explosive Train

Perforating guns comprise different components to form an “explosive train”

Charges are mounted on a carrier strip or encased in a tubular steel carrier

Gun systems use three components:• Detonator – primary high explosive

• Primacord – secondary high explosive

• Shaped charges – main explosive charges

Charge Carrier

Explosive Charge

Detonating Cord

Detonator

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Shaped Charges

Shaped charge/jet perforator actually creates the perforation

Basic elements of a shaped charge:

• Case (Steel, Aluminum Glass or ceramic)

• Major quantity of secondary high explosive and primer

• Conical metallic liner – Solid, compacted or

sintered particles

Case

ExplosCharge

Liner

Primacord

PrimerCharge

Explosive Charge

Main Charge Options – DP vs BH

Deep Penetration

Maximum penetration

Smaller hole size

Reduced angle in cone apex – narrow jet!

Creates higher PI

Big Hole charges

Reduced penetration

Bigger hole diameter

Bigger cone angle – fatter jet!

Preferred where:• HHP constraints on well

treatment process• Remedial treatments

requiring material transport into tunnel

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Big Hole & Deep Penetrating Charge ‒ 34 gms

Perforating Charge Evaluation

Historically standard approach was API RP 43

Officially replaced in Sept. 2006 by API RP 19B • To improve repeatability of tests and results

Tests cannot replicate actual (and highly variable) downhole conditions but attempt to provide:

• Comparative data on performance of charges at surface conditions

• Replicated stressed sedimentary rock penetration data which can be extrapolated to give an “indication of subsurface penetration” corrected for rock compressive strength

Widely used but not universally preferred

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API RP 19 B Tests

The standard tests aim to gauge:• Section 1 – Charge penetration when shot in casing set within

cylindrical concrete target. Test provides “little useful insight into real rock penetration” but useful for comparing charge types.

• Section 2 – Charge penetration into cylindrical target with simulated radial stress being applied.

• Section 3 – Repeat of Section 2 test at elevated BHT.

• Section 4 – Section 2 test conducted with flow after perforating: measures CFE (comparison of backflow through the perforation to that which would occur down a drilled cylindrical tunnel).

• Section 5 – Evaluation of quantity/nature of charge debris.

• Section 6 – Measurement of gun carrier swelling/distortion.

Typical Charge Performance Sheet

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Penetration as a Function of Compressive Strength

ln Pf = ln Pt + 0.086 X (Ct - Cf)/1000• Ct is the compressive strength of test material – usually

Berea SS = 6500 psi

• Cf is the compressive strength of material penetrated – the formation

• Pt is the penetration acquired in the API RP 43 test, inches

• Pf is the predicted penetration into the selected formation, inches.

• However, this will likely not be 100% accurate, there is a difference between concrete and rock

What is the Perforation Crushed Zone?

The perforation pierces through the casing wall by plastic deformation

Perforating crushes and compacts the region surrounding the perforation –the cement sheath and formation

The crushed zone (permeability is normally estimated to be 20% of the undamaged formation)

In addition charge debris remains in the tunnel tip

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Perforation Clean-up: Drawdown

Sources of drawdown:• Static drawdown – fluid in well, controlled to yield PBH<<PRES

• Perforation surge tool – tubular with low hydrostatic pressure –usually atmospheric

Dynamic Underbalance?• For hollow carrier gun, volume of carrier provides small volume

chamber of air at atmospheric pressure across the perforations –1000–2000psi instantaneous drawdown for a short duration –assists in removal of crushed zone

Underbalanced Perforating ‒ Oil Wells

Underbalanced perforating creates immediate backflow – helps remove crushed rock/charge debris/explosive gas

Oil with a higher viscosity relies on drag

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Perforating Performance ‒ Impact of Length

Graph shows a range of studies utilising different measurement/prediction methods, spanning over 30 years of study

If perforation length is > 6 inches then PR = 1 (i.e., Sperf = 0)

Lp has major impact on PI ‒ look at slope of curves!

Effect of Shot Phasing

Greater geometrical distribution yields higher productivity – to be expected

However this graph assumes all shots have the same penetration –very unlikely!

Reality is a compromise (e.g., small diameter guns)

• Maybe better to use 120°and shoot low side with through tubing guns and get max Lp for most charges

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Effect of Perforation/Tunnel Diameter

Perforation diameter has limited impact on flow capacity ‒ velocities are already low

Better to have greater penetration length

Effect of Shot Density

Observe the major improvement in going to 4 and then 8 shots/ft but thereafter the benefit diminishes incrementally

Strongly linked to orientation and perforation lengthCOPYRIG

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Effect of Permeability Anisotropy

Poor Kv/Kh will reduce the PI

Can be offset by higher shot densities

Perforate Beyond the Damage Zone!

Perforating geometry selection is influenced by formation damage around the wellbore ‒ the damaged zone

Perforating past the damage zone can greatly increase well productivity

Resistivity data estimates the depth of filtrate invasion which is the maximum depth of potential formation damage

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Wellbore Fluids

Clean completion fluids should be used during perforating ‒ dirty fluids can plug perforations and the near wellbore tunnel area

• Difficult to remove debris afterwards

• Often perforations remain permanently plugged

• Re-perforating may be more successful than chemical treatments

Completion fluid may need to suppress clay swelling or avoid the formation of precipitates

We often refer to wellbore damage as being drilling related, BUT this can be completion brine related as well

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