Skin
Subtopics:
Skin Due to Partial Penetration
Choked Fracture Skin (sc)
When a hydraulic fracture is created, some portion of the fracture around the wellbore is damaged due to crushing and embedding in the formation. This is modeled by assuming a choked zone in the fracture as shown below.
This choked zone causes an additional pressure drop that tends to negate some of the benefits of the fracture. Cinco-Ley and Samaniego (1977 and 1981) have defined this pressure drop as a choked skin defined as:
References
1. "Effect of Wellbore Storage on the Transient Pressure Behavior of Vertically Fractured Wells", H. Cinco-Ley and F. Samaniego-V., Paper SPE 6752 presented at 1977 AFTCE, Denver, CO, October 9 - 12.
2. "Transient Pressure Analysis: Finite-Conductivity Fracture Case Versus Damaged Fracture Case", H. Cinco-Ley and F. Samaniego-V., Paper SPE 10179 presented at 1981 AFTCE, San Antonio, TX, October 5 - 7.
3. "Optimization of the Productivity Index and the Fracture Geometry of a Stimulated Well with Fracture Face and Choke Skins", D.J. Romero, P.P. Valko and M.J. Economides, SPEPF (February 2003) 57 - 64.
Effective Wellbore Radius (reff)
Effective wellbore radius is used when dealing with negative skins to account for the increase in sandface area exposed to reservoir flow due to acidizing, or when a well is hydraulically fractured. It cannot be measured directly, but can be calculated.
Pressure Drop Due to Skin (Dpskin)
Pressure drop due to skin represents the total pressure drop caused by apparent or total skin (s'). It is an important value to know in order to determine if corrective action (stimulation) may be warranted. Occasionally, a high skin value may translate into a low pressure drop, such as in a reservoir with a high- flow capacity (kh) in which case stimulation would not be necessary. The additional pressure drop due to skin (Dpskin) is usually a function of the following factors:
- Total Skin (s')
- Rate (q)
- Permeability (k)
- Net Pay (h)
For liquids, this can be expressed as:
For gas, the above formula is used combined with pseudo-pressure as shown below:
Note that this equation is a little more complex since it involves converting the pseudo-pressure term, shown in brackets, to pressure. This can be accomplished by using a pressure versus pseudo-pressure table.
Skin Across the Fault (sfault)
The skin across the fault represents the additional pressure drop that occurs due to fluid flow restriction between the fault and the surrounding reservoir zones. It is defined as:
Skin Due to Damage (sd)
Skin due to damage is a measure of the amount of damage, or improvement to the formation near the wellbore. Damage can be caused by drilling fluids, migration of fines, invasion, etc. and results in a reduced permeability near the wellbore and a positive skin. The magnitude of the positive skin effect is generally 0 to 50, but can be as high as 200. Improvement can be accomplished by acidizing or fracturing, and results in an increased effective permeability near the wellbore and a negative skin. The magnitude of the negative skin effect is generally 0 to -5. In some cases, it can be as low as -6 or -7, which generally implies the presence of reservoir heterogeneities such as natural fractures, or formation permeability contrasts, rather than stimulation effects due to wellbore completion operations.
The skin effect is a dimensionless quantity and is defined as the difference between the actual and the ideal dimensionless pressure drop in a reservoir, or pressure drop due to skin (Dpskin).
When skin is viewed as an actual pressure drop rather than a dimensionless value, it becomes easier to determine whether skin is actually a problem, and if steps need to be taken to correct it.
An alternative concept to the skin effect is the effective wellbore radius (reff).
This states that a well with improvement (negative skin) is equivalent to a well with a larger wellbore radius (rw), while a well with damage (positive skin) has a smaller effective wellbore radius (reff).
Skin Due to Inclination (sinc)
In many cases, wells do not penetrate a formation perpendicular to the bedding plane. When the angle of inclination through the formation is significant (> 10°), a reduction in pressure drop can occur due to the angle of inclination. This pressure drop is defined as skin due to inclination. Examples of this situation can be:
- A vertical well that penetrates a dipping formation
- A directionally drilled well that penetrates a horizontal formation (see figure below)
For example, in a well that does not penetrate a formation vertical to the bedding plane, the communication (or contact) area with the formation is increased. This reduces the pressure drop required to obtain a flow rate equal to that of a well that penetrates a formation vertical to the bedding plane. Therefore, the flow efficiency is improved, which causes a reduction in the apparent or total skin factor (s'). This reduction in skin factor is referred to as the skin (or pseudo-skin) due to inclination (sinc). Cinco et al. (1975) have provided the following correlation for calculating sinc:
Note that the above equation is valid for 0° >= qw <= 75° and when pseudo-radial flow has been established. Also, the formation must be fully penetrated or completed.
For a vertical well, qw = 0°, and sinc = 0. Thus, the skin due to inclination is always equal to or less than 0.
By rearranging the total skin equation, the skin due to damage (sd) can be determined when skin due to inclination (sinc) and the other skin components are known.
References
"Unsteady-State Pressure Distribution Created by a Directionally Drilled Well", H. Cinco, F.G. Miller and H.J. Ramey, Jr., JPT (November 1975).
Skin Due to Partial Penetration (spp)
When dealing with partially penetrated wells, flow restriction can occur due to the restricted perforation region accessible to fluid flow. As the flow streamlines converge to fit into the effectively perforated area, as shown in the following diagram, the flowing fluid experiences an additional pressure drop. This pressure drop can be represented as a skin factor due to partial penetration (spp) (also called pseudo-skin). By accounting for skin due to partial penetration, the system effectively becomes equivalent to a pure radial system.
Skin due to partial penetration can be calculated using the Odeh correlation (1980) shown below:
Skin due to partial penetration is always greater than 0, and typically ranges from 0 to 30. Note that partial penetration greatly magnifies the effect of any skin due to formation damage (sd).
This is more clearly seen in the definition of total skin (s'). By rearranging the total skin (s') equation, the skin due to damage (sd) can be determined when skin due to partial penetration (spp) and the other skin components are known.
References
1. "An Equation for Calculating Skin Factor Due to Restricted Flow Entry", A.S. Odeh, JPT (June 1980).
2. "Numerical Simulations of the Combined Effects of Wellbore Damage and Partial Penetration", R.M. Saidikowski, Paper SPE 8204 presented at 1979 AFTCE, Las Vegas, NV, September 23 - 26.
Skin Due to Turbulence (sturb)
Skin due to turbulence is additional pressure drop caused by high-gas velocity near the wellbore, and only applies to gas wells. For gas flow, Darcy’s law is valid for the majority of a reservoir, except near the wellbore when gas velocity is high. This non-Darcy effect near the wellbore is known as inertial-turbulent flow. Depending on the rate, this effect can be significant, and must be accounted for.
By definition, this additional pressure drop or skin is a function of gas flow rate (qg) and the turbulence factor (D) of the system expressed as:
Note that skin due to turbulence is always positive and is one component of the total skin (s'). Thus, a production test on a stimulated well can still yield a positive total skin (s') value, even if no skin damage (sd) is present, due to the turbulence component (sturb).
By rearranging the total skin (s') equation, the skin due to damage (sd) can be determined when skin due to turbulence (sturb) and the other skin components are known.
References
"Non-Darcy Flow and Wellbore Storage Effects in Pressure Build-Up and Drawdown of Gas Wells", H.J. Ramey, Jr., JPT (February 1965) 223 - 233.
Skin Due to Xf (sXf)
Skin due to Xf (fracture half-length) is defined as the skin equivalent to the fracture half-length, or is the equivalent skin due to the fracture itself. It is derived as follows:
Starting from the definition of apparent wellbore radius:
Alternately, the apparent wellbore radius can also be defined as:
By substitution, the effective fracture radius can be defined as an effective wellbore radius as follows:
Rearranging to solve for skin (s) gives:
Finally, this skin represents the skin due to Xf defined as:
As shown, it is always a negative skin since fracturing is a stimulation technique used to reduce total skin (s') to improve fluid flow.
Skin on Fracture Face (sf)
When a hydraulic fracture is created, the interface between the fracture and the formation may encounter some permeability reduction either intentionally (leak-off additives) or unintentionally (e.g., relative permeability effects, non-breaking gel). This is modeled by assuming the presence of a skin on the fracture face as shown below.
The skin on the fracture face causes an additional pressure drop that tends to negate some of the benefits of the fracture. Sometimes this skin on the fracture face will improve with production or time (clean up or temperature effects). Cinco-Ley and Samaniego (1987 and 1981) have defined sf as:
Even a small skin on the fracture face, sf < 1, can cause a substantial change in the shape of the derivative curve. Typically, sf ranges from 0 to 1.
References
1. "Effect of Wellbore Storage on the Transient Pressure Behavior of Vertically Fractured Wells’, H. Cinco-Ley and F. Samaniego-V., Paper SPE 6752 presented at 1977 AFTCE, Denver, CO, October 9 - 12.
2. "Transient Pressure Analysis: Finite-Conductivity Fracture Case Versus Damaged Fracture Case", H. Cinco-Ley and F. Samaniego-V., Paper SPE 10179 presented at 1981 AFTCE, San Antonio, TX, October 5 - 7.
3. "Optimization of the Productivity Index and the Fracture Geometry of a Stimulated Well with Fracture Face and Choke Skins", D.J. Romero, P.P. Valko and M.J. Economides, SPEPF (February 2003) 57 - 64.
Total Skin (s')
Total skin also known as effective or apparent skin is a summation of the following skin components:
- Skin due to damage (sd)
- Skin due to partial penetration (spp) for a partially penetrated well only
- Skin due to inclination (sinc)
- Skin due to turbulence (sturb) or non-Darcy flow (for gas wells only)
- Two-phase skin (s2p) for gas-condensate wells only
Usually, radial analysis provides the total skin (s') of the system. The value of s' can be positive, negative, or zero. Sometimes it is important to know what skin components are contributing to the total skin. To determine this, the relationship between s' and its various contributing components can be expressed as:
Except for skin due to damage (sd), all other skin components mentioned above are always non-negative (i.e., are either zero or positive). Note that in the case of a partially-penetrated well, the skin due to damage (sd) is magnified by a factor of h / hp in the total skin value. Also note that the skin due to turbulence (sturb) in a gas well is rate sensitive.
Keep in mind that because total skin is a summation of various other skin components, a positive or negative value may not indicate whether a well should or should not be stimulated. For example, for an unstimulated well, a positive value of total skin does not necessarily mean that the well is damaged. It could just be that the sum total of the skin components results in a positive value. Similarly, for a stimulated well, a zero or positive value of total skin can result, even if the skin due to damage (sd) is negative. Therefore, the effectiveness of the stimulation is really represented by the skin due to damage (sd) rather than the total skin.
Two-Phase Skin (s2p)
This type of skin applies only to a gas condensate system. Although a gas phase exists in the majority of a gas condensate reservoir, when the reservoir pressure is above the dew point pressure, a condensate bank can develop around the wellbore when the well pressure is below the dew point pressure. This results in two-phase flow of gas and condensate in the vicinity of the wellbore. Being the dominant phase in the system, gas experiences an extra pressure drop while flowing through the condensate bank. In other words, due to the presence of condensate around the wellbore, the gas flowing towards the wellbore from the reservoir finds it harder to flow for a lower relative permeability.
The skin due to this extra pressure drop of the gas phase is referred to as the two-phase skin. The method proposed by Raghavan et al. (1995) is used to calculate the s2p value, when pressure, volume, temperature (PVT) properties and relative-permeability data of the gas condensate system are available. This method allows you to separate the two-phase skin from the total or effective skin (s'). The value of s2p can either be zero (when the well pressure is above the dew point pressure, and no condensate bank exists), or a positive number (when the well pressure is below the dew point pressure, and a condensate bank exists). If the PVT and relative permeability data are not available, it is not possible to calculate s2p exclusively. In this case, only the total or effective skin (s') (which includes s2p) can be calculated. The details of the process of calculating s2p are outlined in the single phase pseudo-pressure procedure for a gas condensate reservoir.
Typically, two-phase skin (s2p) ranges from 0 to 30. Note that by rearranging the total skin (s') equation, the skin due to damage (sd) can be determined when two-phase skin (s2p) and the other skin components are known.
References
"Practical Consideration in the Analysis of Gas Condensate Well Tests", R. Raghavan, W., C. Chu, and J.R. Jones, Paper SPE 30576 presented at 1995 ATCE, Dallas, TX, October 22 - 25.