A New Pressure Decay Technique Measurement Of Live Heavy Oil Mobility In Unconsolidated Oil Sands Cores
Authors: Guo F. , Wong R.C.K. 
Published: March 2010

Abstract
The pressure decay test is an efficient and fast method for determining the mobility of fluid in porous media. Previous test procedures by other investigators require the use of upstream and downstream reservoirs. To facilitate data analysis, some conditions are imposed on the upstream or downstream reservoirs. This paper presents an alternative method to conduct pressure decay tests without using the upstream and downstream reservoirs. Mobility is calculated from the pressure decay in the sample using the finite difference method. The proposed testing method eliminates restrictions imposed on the reservoirs, shortens the testing period, and has the capability to analyse poorly controlled upstream and downstream pressures. It is validated by comparing the mobilities measured by the new pressure decay method with that measured under the steady state condition.

Introduction
The usual method of measuring mobility in the laboratory is to establish steady state flow through the sample. The mobility is calculated from the measured flow rate and pore pressure gradient. If the mobility is low, steady state takes a long time to establish. Therefore, the steady-state-flow method is generally impractical for the measurements of very low mobility.
Brace et al,(l) introduced a transient flow method to measure the low permeability of Westerly Granite. Their experimental set up consists of a cylindrical rock sample that is connected to two fluid reservoirs. At the start of the experiment, the fluid pressure in the upstream reservoir is suddenly increased. As this pressure decays, fluid flows from the upstream reservoir, across the sample, to the downstream reservoir. They found that the mobility can be estimated from a semi-log plot of the observed pressure difference across the sample versus time when the compressive storage of the rock sample is negligible(2,3).
Taking the compressive storage of the sample into consideration, Lin(4) interpreted the mobility from the pressure decay using a numerical method. Lin's analysis, however, assumed that the specific storage was known. To determine the specific storage, other independent tests are required, as Hsieh et al.(5) commented.
Hsieh et al.(5,6) presented an analytical solution using the pore pressure gradients at both ends of the sample as boundary conditions. The pressure responses at the upstream and the downstream ends can be expressed as a series. A graphical method, utilizing type curves, was developed to calculate both the mobility and the specific storage of the rock sample.
Bourbie and Walls(7) developed another analytical solution for the pulse decay test. The constant upstream pressure was used as one of the boundary conditions in their solution. The downstream pressure can be expressed as an explicit function of mobility. However, the mobility cannot be solved explicitly, so a computer program was written to find the mobility from the pulse decay. To maintain a constant upstream pressure the upstream reservoir volume must be much larger than the total volume of the sample pore and the downstream reservoir.
Chen and Stagg(8) presented an analytical solution using the boundary conditions similar to those in Bourbie and Walls' solution.