Introduction
Pore Volume Compressibility testing is performed to:
- provide reliable compressibility values for reserves estimation
- reservoir compaction, permeability change, and subsidence predictions
- and production forecasting and history matching
Modification of the reservoir effective stress due to production causes volumetric changes in pore space in a reservoir. The engineering parameters quantifying these volumetric variations are compressibilities. Reliable compressibility values are essential for resource estimation, reservoir maintenance and drive assessments, as well as subsidence evaluations. Production forecasting is intimately related to a total system compressibility combining the compressibilities of liquid and gaseous phases in the pore space, the grain compressibility of the solid portions and the pore volume compressibility, often referred to as formation compressibility.
Testing Equipment and Setup
Uniaxial compaction testing requires triaxial stress conditions. Triaxial stresses are provided by applying an all-around confining pressure, as well as an additional load in the axial direction, to a cylindrical core sample installed in a pressure cell. Confining pressure is applied using pressurized fluid and the additional axial force is applied with an independent actuator. Each test sample is placed between polished hardened steel endcaps and jacketed with polyurethane, polyolefin or Teflon®. After jacketing, the test sample is instrumented with axial and radial extensometers for measuring displacements. The instrumented test sample is then placed inside a pressure vessel equipped with an internal load cell. The pressure vessel is then filled with the confining fluid, which is used to apply confining pressure. Confining pressure, axial stress, axial strain, and radial strain (in two perpendicular directions) are measured and/or controlled during each test. As indicated, axial strain and radial strain (along two perpendicular directions at the midpoint of the sample) are measured using strain gauge extensometers. Axial stress is calculated from measurements of force on the internal load cell.
Tests are performed either drained to atmosphere, with controlled pore pressure or undrained, as requested. Elevated temperature, to simulate actual in-situ conditions, can also be applied. The maximum temperature available is approximately 400°F.
Sample Preparation
- On receipt, the core is stored under appropriate environmental conditions.
- Samples for laboratory testing are then subsequently drilled from the original cores using appropriate drilling fluids. The most common laboratory sample sizes are one to one and a half inch in diameter by two to three inches long.
- After trimming the samples to their nominal length, their end surfaces are made flat and parallel to within a tolerance of 0.001inch, in accordance with the ISRM’s (International Society of Rock mechanics) recommendations.
Testing
Various testing procedures for simulation of in-situ compressibility and loading are used. The most common are outlined below.
Uniaxial Strain Compressibility Testing Procedures
- Jacket the sample and install displacement measuring devices and end-caps with pore fluid ports.
- Install the sample in a triaxial pressure vessel.
- Apply nominally 100 psi hydrostatic pressure, at a rate of 1 psi/s. If the sample is frozen, allow it to thaw and strains to equilibrate. Pre-saturate sample as required.
- Increase the hydrostatic confining pressure to 200 psi at a rate of 0.5 psi/s.
- Switch to uniaxial strain boundary conditions. Increase the axial stress at a relatively slow rate, (for example, 0.1 psi/s). If the test is drained to ambient conditions (alpha =1 assumed), fluid expelled is measured.
- Unload after the appropriate total axial stress level is reached; this stress level is estimated from the depths of the samples, local experience and anticipated abandonment or water flooding pressures. Compressibility is calculated from the measured stresses and strains.
Drawdown or Depletion Compressibility Under Uniaxial Strain Conditions
- Jacket the sample and install displacement measuring devices and end-caps with pore fluid ports.
- Install the sample in a triaxial pressure vessel.
- Apply nominally 100 psi hydrostatic pressure, at a rate of 1 psi/s. If the sample is frozen, allow it to thaw and strains to equilibrate. Pre-saturate as required.
- Increase the hydrostatic confining pressure to 200 psi, at a rate of 1 psi/s.
- Increase the confining pressure, at a controlled rate (e.g. 1 psi/s). Increase the pore pressure at the same rate. Increase the confining stress to the level of the anticipated horizontal stress conditions. Increase the pore pressure to the anticipated initial reservoir pressure. Grain compressibility can be determined from deformations measured in this phase.
- Increase the total axial stress at a controlled rate (1 psi/s) to the target in-situ stress level (or less if it is desired to minimize the initial effective mean stress), while maintaining confining stress and pore pressure constant (Young’s modulus and Poisson’s ratio can be determined in this stage).
- Hold all of the stresses to allow equilibration. The sample is now nominally at virgin in-situ conditions.
- Maintain the total axial stress constant. Establish and maintain uniaxial strain boundary conditions to prevent additional radial displacement. Reduce the pore pressure at a controlled rate (e.g. -0.1 psi/s), in order to simulate drawdown.
- Unload the sample after an appropriate pore pressure level is reached; this stress level is estimated from the depth of the sample, local experience and anticipated abandonment or waterflooding pressures.