Drainholes have been drilled in several areas of the world and there is a need to understand these drainage systems for accurate well test analysis. This study presents an analytical solution for the transient pressure response of a uniform flux horizontal drainhole in an anisotropic reservoir of finite thickness. The solution also applies for a reservoir with multiple drainholes in a vertical array. The solution shows that there are two possible types of transient pressure behavior depending on the length of the drainhole relative to the height of the reservoir. If the drainhole is short, initial radial flow perpendicular to the drainhole axis is followed by a transition to a pseudo-radial flow period. If the drainhole length is long relative to the reservoir height, the initial radial flow period ends rapidly and transient pressure behavior becomes identical to that of a uniform flux vertical fracture.

The pressure transient response for multiple drainholes is identical to the single drainhole solution if dimensionless variables are defined relative to the number of drainholes. Consequently, the pressure response of a uniform flux vertical fracture can also be approximated by a vertical array of drainholes. The pressure response for infinite conductivity drainholes is also suggested by analogy to the infinite conductivity vertical fracture solution.

Log-log type curves are presented for various drainhole radii and can be used in the conventional manner to determine reservoir characteristics including directional permeability or drainhole half length. Short and long time approximations are presented along with appropriate time limits. Finally, conditions for greater productivity than with vertical wells or hydraulic fractures are presented.

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