# Skin Model

Mechanical/Geometrical Skin

If a reliable skin value is available from transient well testing, then this value should be directly entered by selecting the “Enter by hand” option. In addition, PROSPER provides 3 methods of estimating skin factor using input parameters such as perforation geometry, depth of damage etc.  The required input parameters are often difficult to accurately define, therefore the absolute value of the calculated skin cannot be precisely predicted.  The power of these techniques is their ability to assess the relative importance of completion options on the overall value of well skin.

If reliable total skin data is available from well tests, PROSPER can be used to estimate the value of the skin pressure drop across the completion and the proportion of the total pressure drop attributable to the various completion elements.

The skin estimation models provided in PROSPER are those of:

Locke

McLeod

Karakas and Tariq

Karakas and Tariq has been found to give good results in many field applications and is explained here.

The following input data is required:

Perforation parameters modelling

The two parameters Perforation diameter and Perforation Length can be entered by the user or calculated by using the API RP43 perforation calculation.

A sketch outlining the main geometric variables is shown below.

Deviation/Partial Penetration Skin

Two models of this type are provided in PROSPER :

 Cinco/Martin-Bronz
 Wong-Clifford

The first (Cinco/Martin-Bronz) requires the following data:

Deviation angle of well

Partial penetration fraction

Formation vertical permeability

Penetration is the proportion of the total reservoir thickness that is completed.  (e.g. a 200 ft thick reservoir with 100 ft of perforations would have a Penetration of 0.5)  Deviation skin is calculated using Cinco-Ley’s method, and is therefore valid up to 75 degrees deviation.

The second (Wong-Clifford) can compute a skin for multiple completions. This model does not have a separate calculation for the deviation & partial penetration skin – it is a point source solution that calculates a skin that combines all of the skin effects in one value. This total skin is placed in the Deviation skin column and the partial penetration skin is set to zero.

It requires the following data entered:

Reservoir parameters:

Formation vertical thickness

Drainage area

Dietz shape factor

Formation vertical permeability ratio

Local vertical permeability ratio

Horizontal distance from well to reservoir edge

Depth of top of reservoir

Completion parameters – the following for each completion:

Completion start measured depth

Completion end measured depth

Completion start true depth

Completion end true depth

The calculation is based upon the paper by Cinco-Ley, H., Ramey, Jr., H.J. and Miller, F.G.: “Pseudo-Skin Factors for Partially-Penetrating Directionally-Drilled Wells”,  SPE 5589 presented at 50th Annual Fall Meeting of SPE of AIME,   Dallas, TX, September 28 – October 1, 1975.

Plotting Skin Pressure Drop

Enter the requested data and click on Calculate to display an IPR plot.  The plot shows the pressure drop resulting from the total skin as well a breakdown of the individual factors contributing to the total skin as per the following example:

This plot is useful to assess the efficiency of a particular perforating program by allowing the user to instantly assess the completion pressure loss resulting from different perforation options.

Note on Skin

Locke’s technique is valid for shots per foot of 1,2,4,6,8,10,12,and 16.