# Dimensionless Wellbore Storage Constant (CD)

The wellbore storage constant in dimensionless form is defined as:

A large value of dimensionless wellbore storage is associated with a large wellbore volume (Vw). In practice, a range of values from 500 to 10,000 has been observed. Occasionally, a value much greater than 10,000 is observed when matching actual test data against a type curve. In such cases, it is considered that some part of the formation (high permeability or fracture) is in communication with the wellbore, and is acting as an extension to it.

In a horizontal or slant well, the dimensionless wellbore storage constant (CD) is sometimes expressed in terms of the effective wellbore half-length (Le/ 2). In such a case, the value for wellbore radius (rw) in the above equation is replaced by the effective wellbore half-length, which makes CD a very small number.

Wellbore storage is normally assumed to be constant during a test, and in practice, this assumption is often reasonable. However, there are numerous situations where wellbore storage is not constant. This changing wellbore storage may be caused by a changing wellbore fluid compressibility, by phase redistribution, or by a change in the type of storage from a changing liquid level to a liquid filled wellbore. Changing wellbore storage is accounted for using a modified form of the dimensionless wellbore storage definition, which includes additional dimensionless parameters, apparent dimensionless storage (CaD), and a storage pressure parameter (CpD).

The wellbore storage constant and dimensionless wellbore storage constant are calculated from the early time data, by performing an afterflow or wellbore storage analysis.