Radius of Investigation

Introduction

Radius of investigation (r_inv) represents the distance that transient effects have travelled into the reservoir for a given time. Conceptually, radius of investigation is can be illustrated as:

Radius of investigation is an approximate calculation, and not theoretically robust. In reality, as soon as pressure transient or disturbance occurs (I.e., a rate change at a well), it has an immediate effect throughout the entire reservoir. However, due to limited numerical precision, the disturbance at some points, and times, does is negligible for practical purposes.

Radial Flow

In dimensionless terms:

In field units:

Notice that the radius of investigation is only a function of the reservoir properties, and does not depend on flow rate. Increasing the flow rate results in a greater drawdown, but radius of investigation remains the same.

Estimating Drainage Area

Assuming that R_inv is a suitable, it can be used to estimate drainage area from:

In practice, this may be a minimum value

References:

• T. A. Blasingame, Analysis of Performance, v20230723, Texas A&M.