Test Objectives
The purpose of performing an isochronal test is to determine the rate dependent skin. Skin can be separated into two terms: Mechanical Skin and non-Darcy skin. S represents the mechanical skin, that is, the damage to the near-wellbore area caused by drilling, completing and producing the well. Non-Darcy skin (or rate dependent skin) is a way of measuring the effect of fluid compression and non-radial flow paths due to high flow rates through porous media near the well. It is a linear function of rate and can be represented by the non-Darcy flow coefficient, D, multiplied by the rate. S and D combine to form the total skin, ST via the following equation: ST = S + D*Q
Since ST is dimensionless, the units of D and Q must cancel each other out.
Generally, nothing can be done to change the value of D for a well; however, stimulating the well can reduce the value of S. Thus, it is highly recommended that this type of test be run on a well prior to a chemical stimulation job. If the well has high non-Darcy skin, but low Mechanical Skin, stimulation will have little effect on the well's production.
Procedure
Install the SPIDR on a well that has been flowing steadily for several days. Check for leaks in the system after installation. The SPIDR must be recording (check the box for the SPIDR wake-up time) for at least 15 minutes prior to shut in. Shut the well in manually at the wing. Do not shut in using the automatic chokeÖthey have a tendency to leak. After the well has been shut in, check for leaks again. When the build-up is over, begin cycles of a short flow period followed by a short build-up period. At the end of the last short build-up, begin a long flow period that should last for the same duration as the short cycles combined.
The length of these flow and shut in periods is a function of the fluid and rock properties (k, Cg, porosity and viscosity) at reservoir temperature and pressure. They should be of the following relative lengths:
- Shut in well (long) = 12 t
- First short flow = t
- First short buildup = t
- Second short flow = t
- Second short buildup = t
- Third short flow = t
- Third short buildup = t
- Long flow = 6 t
As a rule of thumb, if the perm is greater than 5 mD, t = 2 hours.
Advantages
The purpose in performing this type of test is that it gives engineers a tool in screening potential workover or acidizing candidates. If a well has a large mechanical skin, it is a stimulation candidate. If the well has a large non-Darcy skin, but a low mechanical skin, it is a poor stimulation candidate. In addition, this technique can be applied to calculated pressure drop in the near wellbore area as a function of rate.
Disadvantages
The only disadvantage is the loss of production during the test. However, if the reservoir has moderate permeability or better, the entire test can be completed in 2 days.
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