Effect of formation strength of lithified deposits on stability of wellbore

Abstract

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 [1] costing the industry, accordingly to conservative estimates, $700 million annually [5]. As it was proven in several studies [6] 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.