Real-time Wellbore Stability Analysis: An Observation from Cavings at Shale Shakers
Kumar, Devendra1; Ansari, Sajjad A.2; Wang, ShunChang3; Ahmed, Sajjad4; Tichelaar, Bart5
1DCS, Schlumberger, Navi Mumbai, India.
2DCS, Schlumberger, Calgary, AB, Canada.
3SIS, Schlumberger, Montpellier, France.
4DCS, Schlumberger, Perth, WA, Australia.
5Geophysics, Shell, Cairo, Egypt.
Drill cuttings are considered to be representative of the lithology being drilled in a wellbore. However, the fragments that are two to three times larger and/or having odd shapes compared to the regular cuttings are commonly understood as cavings from the wall of the borehole and they are seldom of any help in compilation of a lithology. In fact, cutting description manual recommends ignoring cutting sizes greater than half an inch. Nevertheless, these cavings carry critical information that needs expert decoding with regards to impending or happening wellbore instability, formation overpressure and overall well behavior evaluation.
In this article, several real-time cases are presented as road signs along with few exception and practical complication in real-time interpretation. Cavings can be produced due to several mechanisms, such as underbalance drilling, stress relief, pre-existing planes of weakness or simply by mechanical action of the drilling process and/or drilling tools. The use of cavings to understand borehole instability and its mechanism entails correct description coupled with proper interpretation, which can be tricky and require a good amount of experience as well as overall understanding of the geology, geomechanics and drilling system and process. Therefore, a simplified approach is made to describe the cavings morphology and its interpretation in terms of wellbore stability. The relative amount of cavings in the bulk sample is also an indication of the degree of instability of the borehole walls.
Cavings are the first and foremost indicator of wellbore deterioration, and the correct interpretation or knowledge of cavings can help save millions of dollars by using appropriate prevention / remedial actions. The most noticeable and predictive cavings for wellbore stability and formation pressure are those of clay and shale. Collectively size, shape, appearance and relative percentage of the cavings compared to the total load of what is coming at the Shale-Shaker versus time are necessary to keep track of the health of the wellbore. Hence, listening to the wellbore by monitoring continuously of what is coming at the shaker by an expert set of eyes during any drilling fluid circulation pre-drilling, syn-drilling or post-drilling operation is essential.
AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012