Modified Isochronal Testing
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.
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 choke?ñthey 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.
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.
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.