Shale Preservation and Testing Techniques for Borehole-Stability Studies
- M.E. Chenevert (U. of Texas) | M. Amanullah (U. of Texas)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- September 2001
- Document Type
- Journal Paper
- 146 - 149
- 2001. Society of Petroleum Engineers
- 1.6 Drilling Operations, 4.3.1 Hydrates, 4.1.5 Processing Equipment, 1.11 Drilling Fluids and Materials, 5.3.4 Integration of geomechanics in models, 4.1.3 Dehydration, 1.8 Formation Damage, 4.1.2 Separation and Treating, 1.6.9 Coring, Fishing
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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, but results are limited because of the scarcity of test samples. Rapid deterioration of cuttings and cores has made data collection 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 result in false test results owing to the incorporation of capillary pressure artifacts.
This paper discusses results obtained with a specially preserved, highly reactive shale core obtained at a depth of 4,500 ft 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 measurements are to be made.
Swelling data show that shales that 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 here.
If a shale becomes unsaturated during retrieval or handling, the rock's mechanical parameters, such as the strength, elastic constants, and physiochemical response (swelling) can be significantly altered through the creation of artifacts.1 The intake of water into the capillaries of unsaturated shale containing air compresses the air and can lead to significant damage of the rock mass skeleton. Any level of desaturation during handling or storage can cause an irreversible change in the shale matrix and thus the ultimate behavior of the shale. The following highlights the importance of maintaining the in-situ saturation status of subsurface cores before laboratory testing.
Unsaturated shale is expected to show high compaction resulting from the highly compressible pore fluid. The short drainage paths associated with the displacement of pore water from the intergrain contact areas to the adjacent voids produce a rapid compaction response.
In the case of unsaturated shales, fluid entry into the shale is rapid and vigorous, owing to the combined action of physiochemical attraction and capillary suction. The available area of interactions is higher because of 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.2
The capillary forces present in an unsaturated shale matrix keep the mineral particles of the petrofabric in tight contact and a good packing state. When water enters the pore spaces, the capillary force decreases. As the sample becomes fully saturated, the capillary forces vanish completely. This causes a loosening effect with a decreasing packing state and may cause swelling known as capillary swelling.3,4 It indicates that the swelling of unsaturated shales is the combined result of capillary and physiochemical action. On the other hand, swelling in saturated shale is the result of physiochemical action alone. This is reflected by a significantly different swelling behavior in the same rock under saturated and unsaturated conditions.2
Unsaturated conditions indicate the presence of interfacial tension and tight packing conditions in the matrix under the action of capillary tension. This effect produces effective contact between particles with an increase in angle of internal friction and interparticle cohesion. The interfacial tension and strength of the pore matrix decrease, while the degree of saturation increases. This is reflected by the higher strength properties of the same shale material tested at an unsaturated condition.5
Owing to the unsaturated nature of altered shales, experiment results based on the differences of axial stress and pore pressure are not sufficient to predict the true behavior of the shales, because the difference between air and water pressure must be incorporated into the analysis.6
It is often impossible to extract reliable wellbore-stability information from unsaturated shale because of the detrimental action of capillary forces. To avoid the unsaturation artifacts associated with storage, handling, and transportation of shale cores, special care was taken with the North Sea shale. This paper evaluates the effectiveness of the coring, preservation, and storage method by assessing the saturation status of the North Sea shale. The possibility of restoring an altered shale to its saturation level by transferring it in a water-vapor environment that has an activity similar to the shale is also discussed. A comparative assessment of the swelling response of both the preserved and restored samples is presented.
Shale Coring and Handling
An inhibited palm-oil ester-type mud (trade name Petrofree) was selected to core the shale to minimize its alteration during coring. It contained about 60% ester and 20% emulsified internal phase of 0.84 activity CaCl2 water, which was mixed with 95% purity CaCl2 and weighted to 10.2 lbm/gal with barite. The emulsifiers were fatty acid calcium soaps and polyamides.
When the core reached the surface, it was immediately placed in plastic tubes; each tube's annulus was filled with a pure synthetic oil, and the ends of the tubes were sealed. This procedure ensured that the core would not come in contact with air during shipping and storage, which could cause drying, or dehydration, of the shale. Another advantage of this technique is that a puncture to the tube could be easily observed (by leaking oil); this is not so when cores are sealed in wax.
The preserved shale cores were then sent to various laboratories for testing. Upon arrival at the test laboratories, the oil level in the tubes was checked to ensure that no loss had occurred.
Once all test equipment was in place and tests could begin immediately, a tube was opened, the core was removed, and 6- to 8-in. pieces of shale were placed in 1-gal. cans, covered with synthetic oil, and sealed.
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