Effect of formation strength of lithified deposits on stability of wellbore
Wellbore stability is one of the major problems while drilling oil and gas wells. Shale, in turn, makes up about 75% of drilled formations, and over 70% of the borehole problems are related to shale instability  costing the industry, accordingly to conservative estimates, $700 million annually . As it was proven in several studies  the shale stability problem is related mainly to penetration of drilling fluid into laminations / fractures (between bedding planes and through the microfractures of shale) causing the mechanical failure and thus the loading of the well with cavings. In other words, the fluid invasion clearly weakens the shale making the shale stability very sensitive to the drilling fluid design. If the drilling fluid weight is too low, the risk of wellbore collapse increases. If drilling fluid weight is too high, drilling fluid invasion into these fractures and subsequent shale weakening makes the wellbore prone to instability. As a way to prevent or minimize the loss of wellbore stability the drilling fluid system includes proper bridging agents together with right drilling practices were proposed. At the same time it was noticed that shale in the same interval and formation reacts differently to drilling process. Some of them showing significant washouts and signs of instability and other stay stable and have in gauge hole. It has been suggested that stability of shale formations affected also by formation strength. And there are boundary values after which formations collapse and significant washouts occur. To prove this assumption the analysis of formation strength of shale was performed. The objective of this paper is to show the impact of formation strength to the stability of the wellbore in shale formations. The paper also provides the relation between thickness of layers and depth of washouts, and the boundary values of formation strength when shale loses the stability.
- Lal M. Stabil'nost' slancev: Vzaimodejstvie burovogo fljuida i prochnost' slancev // Zhurnal neftjanyh tehnologij. 1999.№4. 10 s. [in russian]
- Anvar H., Braun T. Modelirovanie mehanicheskih svojstv geologicheskoj sredy kak sredstvo rasshifrovki naprjazhenij v gornyh porodah // Neftegazovoe obozrenie. 2005. T.9. N1. S.20. [in russian].
- Teale R. Koncepcija udel'noj mehanicheskoj jenergii v burenii // Mezhdunarodnyj zhurnal geomehaniki i gornyh nauk. 1965. №2. S. 57-73 [in english].
- GOST 21153.2-84. Metod opredelenija predela prochnosti pri ob#emnom szhatii. Gosudarstvennyj komitet SSSR po standartam. M: Izdatel'stvo standartov, 1984. 17 s [in russian].
- Ruzhnikov A.G. Stabil'nost' stvola skvazhiny pri burenii na mestorozhdenijah juzhnogo Iraka // Neftegazovoe delo: jelektron. nauch. zhurn. 2013. №6. S. 58-80. URL: http://www.ogbus.ru/eng/authors/
- Udaj A., Mese A., Modi F. Interpretacija i primenenie akusticheskih dannyh dlja povyshenija prognozirovanija stabil'nosti slancev // Zhurnal neftjanyh tehnologij. 2000. №10. S.9. [in english].
- Garsia R., Saavedra N. Razrabotka jeksperimental'nyh korreljacii mezhdu laboratornymi parametrami i dannymi POS na obrazcah slancev // Nauka, tehnologija i budushhee. 2008. T. 3, №4. S. 61-81. [in english].
boundary value.; lithified deposits; shale; stability; strength; граничное значение.; литифицированные отложения; предел прочности; сланцы; стабильность DOI 10.17122/ogbus-2014-1-1-13 References to this article (GOST) Ruzhnikov A.G. Effect of formation strength of lithified deposits on stability of wellbore // Electronic scientific journal "Oil and Gas Business". 2014. №1. P.1-13. URL: http://ogbus.ru/authors/RuzhnikovAG/RuzhnikovAG_2.pdf