Interpretation of pressure recovery data from packer inflation

Abstract

A packer test conducted in porous or fractured medium includes a stage prior to the actual test in which packers are inflated. If the packers are inflated quickly, the pressure within the injection interval rises in a near‐instantaneous fashion and decays with time as it comes into equilibrium with the surrounding formation. These pressure recovery data are commonly not collected and never analyzed. However, the pressure transient resulting from packer inflation is akin to a pressurized slug test conducted within a packed off interval. Here we interpret 18 records of such pressure recovery data obtained prior to a cross‐hole pneumatic injection test conducted in unsaturated fractured tuff at the Apache Leap Research Site (ALRS) in central Arizona. We analyze these records by modifying the M. J. Hvorslev's method to account for airflow. Results from the analysis are compared against permeability (k) estimates obtained from a pneumatic slug test as well as several single‐ and cross‐hole pneumatic injection tests at the ALRS. This comparison shows that the k determined from the pressure recovery compares well with the result from the pneumatic slug test, but the estimates are noticeably smaller than the values determined from single‐ and cross‐hole tests, suggesting a k scale effect. In addition, the basic time lag (tB) obtained from the analysis of the pressure recovery data using M. J. Hvorslev's method is smaller than what was previously determined from the type curve interpretation of a cross‐hole pneumatic injection test. These results collectively suggest that the pressure transients from single‐ and cross‐hole tests conducted over a longer period at higher flow rates sample a much larger volume of the rock compared to the pressure recovery data from packer inflation at the ALRS.