Hydraulic Impedance Testing (HIT)

Introduction

Hydraulic Impedance Testing (HIT) is used to:

  1. detect hydraulic (or natural) fractures; intersecting a wellbore,

  2. determine the fracture closure pressure (indicative of the total minimum in-situ stress); and,

  3. to estimate fracture dimensions.

The test is performed by sending a pressure pulse down the wellbore (from the wellhead), and then analyzing the frequency response of the consequent wave train. HIT has been successfully used to determine fractures in numerous injection wells (Paige, et al., 1992)1 and the method is reported to work in both poorly consolidated and more consolidated formations (Paige et al., 1993).2

"HIT uses the transient response of the fluid in the wellbore and fracture which results from the introduction of a pressure pulse into the well to provide information about the fracture."2

"The field procedure for generating free oscillations consists of initiating a pressure wave that propagates to the bottom of the well where it is reflected upward. Upon returning to the wellhead it is again reflected, this time downward, and the process continues until the wave is fully damped by friction along the wellbore and losses into the fractured zone at the bottom of the well. The characteristics of the oscillations, which are typically recorded at the wellhead, are determined by the initial impulse, the geometry of the tubular goods in the well, the properties of the fluid in the well, the geometry of the hydraulic fracture, and the physical properties of the rock surrounding the fracture. A basic assumption in our analysis is that each of these characteristics, except fracture geometry, is known and can be used to construct a hydraulic model of the well. This model is then used to predict oscillatory behavior at any point in the well for a specified impulse. What then remains to be done is to tune the model by varying fracture dimensions until a match is found between the field observations and the predictions of the model. The modelled fracture dimensions that yield the best fit to the observed behavior are taken as an estimate of the dimensions of the actual fracture."3

What Can Be Determined?

  1. Whether a fracture is present.

  2. Whether the residual aperture is small - is the fracture nominally closed.

  3. Fracture height can be estimated from the magnitude of the reflection at the fracture mouth.

  4. Fracture length can be determined from the time that the pulse takes to traverse the fracture.

  5. "The use of the distinct reflections from fracture mouth and tip distinguishes this method developed by BP from that adopted by Holzhausen."2


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