Shale Preservation and Testing Techniques for Borehole Stability Studies
Authors: Chenevert M.E. , Amanullah M. 
Published: December 2007

SPE Members

Shale rocks are known to be the primary troublesome formation that has plagued drillers for many years. Attempts have been made to investigate the characteristics of shales, however, results are very limited because of the scarcity of test samples. Rapid deterioration of cuttings and cores have made data collection very limited at best, and a complete failure in many cases. It now appears that the shales become unsaturated as they dry out because of poor storage conditions, which in turn results in false test results due to the incorporation of capillary pressure artifacts.

This paper discusses results obtained using a specially preserved highly reactive shale core obtained at a depth of 4500 feet in the North Sea. A previously developed method was used for the evaluation of shale cores to determine their level of saturation. This study shows that shales must be preserved at their native water content if accurate physical measurement are to be made.

Swelling data show that shales which are altered during handling (hydrated or dehydrated) do not respond properly even when restored to their native hydration conditions. They tend to experience excessive swelling compared to cores kept at their native water content.

Results also showed that the North Sea core used in this study was maintained in a fully saturated condition during coring, retrieval and storage. The technique used is discussed.

Introduction

Should a shale become unsaturated during retrieval or handling, the rock's mechanical parameters such as the strength and elastic constants and physio-chemical response, (swelling) can be significantly altered due to the creation of artifacts (Chenevert et. al.). The intake of water into the capillaries of unsaturated shale which contains air compresses the air and can lead to significant damage to the rock mass skeleton. Any level of desaturation during handling or storage can cause irreversible change of the shale matrix and thus the ultimate behavior of the shale. The following citations highlight the importance of maintaining the in-situ saturation status of subsurface cores before laboratory testing.

Compactional Behavior: Unsaturated shale is expected to show high compacting due to the compressibility of the highly compressible poro. The short drainage paths associated with the displacement of pore water from the inter-grain contact areas to the adjacent voids produce a rapid compaction response.

Rock-Fluid Interactions: In the case of unsaturated shales, fluid entry into the shale is rapid and vigorous due to the combined action of physio-chemical attraction and capillary suction. The available area of interactions is higher due to the creation of new surfaces of easy entrance under the busting and fissuring effect of capillary suction. The high intensity of interactions within an unsaturated shale matrix is likely to influence the stress-strain behavior of the shale (Amanullah, 1993).

Swelling Behavior: The capillary forces present in an unsaturated shale matrix keep the mineral particles of the petrofabric in tight contact and in a good packing state. When water enters into the pore spaces, the capillary force decreases. As the sample becomes fully saturated, the capillary forces totally vanish. This causes a loosening effect with decreasing packing state and may cause swelling known as capillary swelling (Terzaghi, Seed et al.). It indicates that the swelling of unsaturated shales is the combine result of capillary and physio-chemical action. On the other hand swelling of saturated shale is the result of physio-chemical action alone. This is reflected by a significantly differing swelling behavior of the same rock under saturated and unsaturated conditions (Amanullah 1993).

Strength Behavior: Unsaturated conditions indicate the presence of interfacial tension and tight packing conditions of the matrix under the action of capillary tension. P. 863^