Skin Factor and Wellbore Storage

Characterizing Damage and Stimulation

Characterizing Damage and StimulationInstructional Objectives1.List causes of damage skin2.List causes of geometric skin3.Calculate skin from pressure drop4.Calculate flow efficiency from skin5.Calculate skin factor and wellbore radius 6.Convert skin to fracture half-length

Damage Caused by Drilling Fluid

Damage Caused by Production

Damage Caused by Injection

Reservoir Model of Skin Effect

Reservoir Pressure Profile

Skin and Pressure Drop

Skin and Pressure Drop

Skin Factor and Properties of the Altered Zone

Skin Factor and Properties of the Altered Zone

Effective Wellbore Radius

Minimum Skin Factor

Minimum Skin Factor Example

Geometric Skin - Converging Flow to Perforations

Geometric Skin - Partial Penetrationhhp

Partial Penetration

Partial PenetrationApparent Skin Factor

Geometric Skin - Deviated Wellbore

Deviated Wellbore Apparent Skin Factor

Geometric Skin - Well With Hydraulic Fracture

Skin Factor and Fractured Wells

Completion Skin

Gravel Pack SkinLgCement

Productivity Index

Flow Efficiency

Flow Efficiency and Rate

Exercise 1

Damage and Stimulation CalculationsCalculate1.Pressure drop2.Flow efficiency3.Apparent wellbore radius4.New skin factor

Wellbore Storage

Wellbore StorageInstructional ObjectivesDefine wellbore unloadingDefine afterflowCalculate wellbore storage (WBS) Coefficient for wellbore filled with A single-phase fluidCalculate WBS coefficient for rising liquid level

Fluid-Filled Wellbore - Unloading

Fluid-Filled Wellbore - Afterflow

Rising Liquid Level

Wellbore StorageFluid-filled wellboreRising liquid levelGeneral

Wellbore Storage DefinitionFluid-filled wellboreRising liquid level

Exercise 2Calculate WBS coefficient for a wellbore filled with a single phase liquid

Exercise 3Calculate WBS coefficient for a rising liquid level

Exercise 4Calculate WBS coefficient for a wellbore filled with a single phase gas

Skin Factor and Wellbore StorageSkin Factor and Wellbore StorageSkin Factor and Wellbore StorageUpon completion of this section, the student should be able to:1.List and describe 5 factors that cause skin damage.2.List and describe 3 factors that cause a geometric skin factor.3.Given formation and fluid properties, be able to calculate skin factor from additional pressure drop due to damage and vice-versa.4.Calculate the flow efficiency given the skin factor, the wellbore pressure, and the average drainage area pressure.5.Express skin factor as an apparent wellbore radius and vice-versa, given the actual wellbore radius.6.Convert skin factor to an equivalent fracture half-length for an infinite-conductivity fracture and vice-versa.

Skin Factor and Wellbore StorageMud filtrate invasion reduces effective permeability near wellbore.Mud filtrate may cause formation clays to swell, causing damage.Skin Factor and Wellbore StorageIn an oil reservoir, pressure near well may be below bubblepoint, allowing free gas which reduces effective permeability to oil near wellbore. In a retrograde gas condensate reservoir, pressure near well may be below dewpoint, allowing an immobile condensate ring to build up, which reduces effective permeability to gas near wellbore.Skin Factor and Wellbore StorageInjected water may not be clean - fines may plug formation.Injected water may not be compatible with formation water - may cause precipitates to form and plug formation.Injected water may not be compatible with clay minerals in formation; fresh water can destabilize some clays, causing movement of fines and plugging of formation.Skin Factor and Wellbore StorageConsider an undisturbed formation of thickness h and permeability k.

Assume that something (drilling the well, producing fluid from the well, injecting fluid into the well) changes the permeability near the wellbore. One simple model of this effect is to assume that this altered zone has uniform permeability ka and radius ra, and that the rest of the reservoir is undisturbed.

For generality, we allow the permeability in the altered zone to be either smaller or larger than the permeability in the undisturbed formation.

Skin Factor and Wellbore StorageSkin Factor and Wellbore StorageWe define the skin factor in terms of the additional pressure drop due to damage.As defined, the skin factor is dimensionless -- it has no units.Nomenclaturek=mdh=ftq=STB/DB=bbl/STBps=psi=cpSkin Factor and Wellbore StorageThe skin factor equation may be rearranged to give the additional pressure drop caused by a given skin factor.Skin Factor and Wellbore StorageThe skin factor may be calculated from the properties of the altered zone.If ka < k (damage), skin is positive.If ka > k (stimulation), skin is negative.If ka = k, skin is 0.Skin Factor and Wellbore StorageThe equation on the previous slide can be rearranged to solve for the permeability in the altered zone. If we know the reservoir permeability and the skin factor and can estimate the depth of the altered zone, we can estimate the permeability of the altered zone using this equation.

Skin Factor and Wellbore StorageIf the permeability in the altered zone ka is much larger than the formation permeability k, then the wellbore will act like a well having an apparent wellbore radius rwa.The apparent wellbore radius may be calculated from the actual wellbore radius and the skin factor.Skin Factor and Wellbore StorageThe minimum skin factor possible (most negative skin factor) would occur when the apparent wellbore radius rwa is equal to the drainage radius re of the well.

Skin Factor and Wellbore StorageFor a circular drainage area of 40 acres (re = 745 feet) and a wellbore radius of 0.5 feet, this gives a minimum skin factor (maximum stimulation) of -7.3.

Skin Factor and Wellbore StorageWhen a cased wellbore is perforated, the fluid must converge to one of the perforations to enter the wellbore. If the shot spacing is too large, this converging flow results in a positive apparent skin factor. This effect increases as the vertical permeability decreases, and decreases as the shot density increases.

Skin Factor and Wellbore StorageWhen a well is completed through only a portion of the net pay interval, the fluid must converge to flow through a smaller completed interval. This converging flow also results in a positive apparent skin factor. This effect increases as the vertical permeability decreases and decreases as the perforated interval as a fraction of the total interval increases.

Skin Factor and Wellbore StorageSkin Factor and Wellbore StorageSkin Factor and Wellbore StorageWhen a well penetrates the formation at an angle other than 90 degrees, there is more surface area in contact with the formation. This results in a negative apparent skin factor. This effect decreases as the vertical permeability decreases, and increases as the angle from the vertical increases.

Skin Factor and Wellbore StorageSkin Factor and Wellbore StorageOften to improve productivity in low-permeability formations, or to penetrate near-wellbore damage or for sand control in higher permeability formations, a well may be hydraulically fractured. This creates a high-conductivity path between the wellbore and the reservoir. If the fracture conductivity is high enough relative to the formation permeability and the length of the fracture, there will be virtually no pressure drop down the fracture. This distributes the pressure drop due to influx into the wellbore over a much larger area, resulting in a negative skin factor.

Skin Factor and Wellbore StorageIf the fracture conductivity is high enough relative to the formation permeability and the length of the fracture, there is virtually no pressure drop down the fracture, and the apparent wellbore radius and the fracture half length are related by the equations shown on this slide.If the fracture conductivity is lower, there is still a negative skin factor, but a longer fracture half-length is required to give the same apparent wellbore radius as a high conductivity fracture. A high-conductivity fracture is one where the dimensionless fracture conductivity Cr is greater than 100.

This is not a conversion of the area of the fracture open to flow to an equivalent circumference. It is a rigorous equation that applies whenever the fracture is high conductivity and the pressure transient has moved some distance beyond the tips of the fracture.Skin Factor and Wellbore StorageFor a well that has been perforated, there is an additional pressure drop across a zone surrounding the perforations. This pressure drop may be calculated using the radial flow form of Darcys law.NOTE: This expression does not include the effects of non-Darcy flow, which may be extremely important, especially in high-rate gas wells!Nomenclature:sp- geometric skin due to converging flow to perforationssd - damage skin due to drilling fluid invasionsdp - perforation damage skinkd - permeability of damaged zone around wellbore, mdkdp - permeability of damaged zone around perforation tunnels, mdkR - reservoir permeability, mdLp - length of perforation tunnel, ftn - number of perforationsh - formation thickness, ftrd - radius of damaged zone around wellbore, ftrdp - radius of damaged zone around perforation tunnel, ftrp - radius of perforation tunnel, ftrw - wellbore radius, ftSkin Factor and Wellbore StorageWhen a well is gravel packed, there is a pressure drop through the gravel pack within the perforation.

NOTE: This expression does not include the effects of non-Darcy flow, which may be extremely important, especially in high-rate gas wells!

Nomenclaturesgp - skin factor due to Darcy flow through gravel packh - net pay thicknesskgp - permeability of gravel pack gravel, mdkR - reservoir permeability, mdLg - length of flow path through gravel pack, ftn - number of perforations openrp - radius of perforation tunnel, ftSkin Factor and Wellbore StorageThe productivity index is often used to predict how changes in average pressure or flowing bottomhole pressure pwf will affect the flow rate q.The productivity index is affected byReservoir quality (permeability)Skin factor

Skin Factor and Wellbore StorageWe can express the degree of damage on stimulation with the flow efficiency.For a well with neither damage nor stimulation, Ef = 1.For a damaged well, Ef < 1For a stimulated well, Ef > 1

Skin Factor and Wellbore StorageWe can use the flow efficiency to calculate the effects of changes in skin factor on the production rate corresponding to a given pressure drawdown.

qnew =Flow rate after change in skin factorqold =Flow rate before change in skin factorEfnew =Flow efficiency after change in skin factorEfold =Flow efficiency before change in skin factor

Skin Factor and Wellbore StorageExercise 1Damage and Skin Factor Calculations

1.Calculate the additional pressure drop due to skin for a well producing at 2,000 STB/D. Oil formation volume factor is 1.07 RB/STB, viscosity is 19 cp, permeability is 5400 md, net pay thickness is 175 ft, skin factor is 11, and porosity is 1.2%.

2.Calculate the flow efficiency for the well in Problem 1, if the average reservoir pressure is 1,800 psi and the flowing bottomhole pressure is 1,600 psi.

3.Calculate the apparent wellbore radius for the well in Problem 1, if the bit diameter is 8 in.

4.Calculate the new skin factor if we create a 100-ft fracture in the reservoir in Problem 1.

Skin Factor and Wellbore StorageSkin Factor and Wellbore StorageSkin Factor and Wellbore StorageSkin Factor and Wellbore StorageSkin Factor and Wellbore StorageSkin Factor and Wellbore StorageSkin Factor and Wellbore StorageNomenclature:C -Wellbore storage coefficient, bbl/psiq -Flow rate out of wellbore at surface, STB/Dqsf -Flow rate into wellbore at sand face, STB/DB -Formation volume factor, bbl/STBpw -Wellbore bottomhole pressure, psit -Time, hrsAwb-Area of wellbore, ft2gc-Gravitational constant, 32.2 lbf ft/s2/lbmg-Acceleration of gravity, 32.2 ft/s2wb-Density of fluid in wellbore, lbm/ft3cwb-Compressibility of fluid in wellbore, psi-1Vwb-Wellbore volume, bbl

Skin Factor and Wellbore StorageExercise 2Calculate WBS Coefficient For Single-Phase Liquid

Calculate the wellbore volume and WBS coefficient for a wellbore filled with a single phase liquid. The well is 2600 ft deep and has 6 5/8, 24 lb/ft casing (5.921 ID). The bottomhole pressure is 1,690 psi. If the well is filled with water (cw = 4 x 10-6 psi-1) what is the wellbore storage coefficient?Skin Factor and Wellbore StorageExercise 3Calculate WBS Coefficient For Rising Liquid Level

Calculate the cross-sectional area and wellbore storage coefficient for a wellbore with a rising liquid level. The well is 2600 ft deep and has 6 5/8, 24 lb/ft casing (5.921 ID). the bottomhole pressure is 750 psi. If the well has a column of water of density 1.04 g/cm3, in it, what is the wellbore storage coefficient?Skin Factor and Wellbore StorageExercise 4Calculate WBS Coefficient for Single-Phase Gas

A wellbore is filled with a single-phase gas. the well has 7200 ft of 2 7/8 tubing (2.441 ID) and 375 ft of 6 5/8, 24 lb/ft casing (5.921 ID). the average temperature in the wellbore is 155F, and the average pressure is 2,775 psia. If the wellbore is filled with gas having 0.77 gas gravity and 0.2% CO2, what is the WBS coefficient?