Reliability Of Laboratory Porosity And Pore Compressibility Obtained On Unconsolidated Deep-Off Shore And Heavy-Oil Reservoirs.
Authors: Pauget L. , Specia F. 
Published: December 2007

Reliability in laboratory porosity and pore compressibility data obtained on unconsolidated (non cemented) sands is a critical issue in heavy oil and deep-offshore reservoirs as initial pore volume and rock behavior during depletion could have a tremendous impact on production strategy and reserves. Experimental work addresses the issue of sample integrity after the coring process: two types of core damage are investigated by dedicated laboratory tests:relaxation of stress at the core bit androck swelling during trip out due to gas expansion in heavy oil reservoirs.

Experimental work then tackles the visco-plastic behavior of unconsolidated reservoir sands and how this specificity can be faced.

Introduction

The quantity of reserves in sand reservoirs, which represent most deep water and heavy oil targets, has imparted to petrophysical measurements on sands an importance it did not formerly have. Yet those kinds of measurements are regarded skeptically. And indeed there is some doubt in the ability of the sand sample to represent the reservoir grain structure at the in-situ initial state. This problem should not be ignored. The aim of this work is to tackle this issue with a systematic approach considering the damage that can affect the quality of sand core and its consequences.

Procedures and equipment

The entire work is based on laboratory measurements under stress. The tests are performed on reservoir sand samples. Samples are cylinders 40mm in diameter and about 65 to 80mm in length. They are set in rubber sleeves and cleaned by solvent flooding. Sample selection is based on sedimentological descriptions and analyses of CT-Scan images of the core and subsequently of the sample. Tests are performed using tri-axial cells that are used to apply independent pore pressure, axial and radial confining stresses. Measurements of axial and horizontal strain, pore volume variation, axial and vertical velocity, and formation factor are performed continuously during the test. Brine permeability measurements are performed during stabilization phases.

Investigations and results

The issue of the preservation of formation integrity can be tackled by thoroughly examining what happens to the formation during the coring process.

Tool rotation

At the core bit, the tool turns whereas the fiberglass sleeve is presumed to remain steady. The rotation tensor at the core bottom section compensates the friction between the tool and the fiberglass. In non-cemented formations, tool rotation can stir the grains and the torsion tensor can twist the soft core incessantly. Figure 1 invalidates this assumption. It represents a CT Scan image of a damaged ductile sand core. The orientation of visible lamination does not vary along the core. Locally, grain-to-grain contacts are not significantly affected by the tool rotation. If any core twist does exist, it has a low rotation frequency that is not observable at meter scale and, even less so, at sample scale.

Stress Relaxation

The coring process is also characterized by the stress relaxation: the unloading stress path during coring is neither smooth nor homogeneous and is indeed quite violent and disturbing for poorly or non-cemented samples.

It corresponds to a sharp variation and inversion of the deviatoric stress which crosses the critical friction slope (in Mohr-Coulomb or Cam-Clay P'/Q representation*). The stress path represented in Figure 3 and the diagram in Figure 2 show the corresponding location of an element of the formation related to the tool. The critical phase occurs just in front of the tool where the overburden no longer weighs on the sand and horizontal stress is not yet fully relaxed.

Tool rotation

At the core bit, the tool turns whereas the fiberglass sleeve is presumed to remain steady. The rotation tensor at the core bottom section compensates the friction between the tool and the fiberglass. In non-cemented formations, tool rotation can stir the grains and the torsion tensor can twist the soft core incessantly. Figure 1 invalidates this assumption. It represents a CT Scan image of a damaged ductile sand core. The orientation of visible lamination does not vary along the core. Locally, grain-to-grain contacts are not significantly affected by the tool rotation. If any core twist does exist, it has a low rotation frequency that is not observable at meter scale and, even less so, at sample scale.

Stress Relaxation

The coring process is also characterized by the stress relaxation: the unloading stress path during coring is neither smooth nor homogeneous and is indeed quite violent and disturbing for poorly or non-cemented samples.

It corresponds to a sharp variation and inversion of the deviatoric stress which crosses the critical friction slope (in Mohr-Coulomb or Cam-Clay P'/Q representation*). The stress path represented in Figure 3 and the diagram in Figure 2 show the corresponding location of an element of the formation related to the tool. The critical phase occurs just in front of the tool where the overburden no longer weighs on the sand and horizontal stress is not yet fully relaxed.