Fracture Initiation From Inclined Wellbores in Anisotropic Formations
- See Hong Ong (Petronas Research & Scientific Services) | Jean-Claude Roegiers (The University of Oklahoma)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- July 1996
- Document Type
- Journal Paper
- 612 - 619
- 1996. Society of Petroleum Engineers
- 1.2.3 Rock properties, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 2.5.1 Fracture design and containment, 4.1.5 Processing Equipment, 4.1.2 Separation and Treating, 1.6 Drilling Operations, 2 Well Completion
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This paper presents a wellbore stability design code that can be used tooptimally drill, complete and stimulate inclined wells. The design code wasdeveloped for the most general case where the borehole, in-situ stress, androck property coordinate frames are totally independent of each other. Itessentially consists of a three-dimensional linear elastic stress analysis inanisotropic formations. Although this particular paper is only limited totensile failure, based on the minimum normal stress theory, the numerical modelis also able to consider shear failure by involving a generalizedthree-dimensional anisotropic yield criterion. The design code enables theselection of the most optimum borehole orientation and wellbore fluid gradientto avoid fracture initiation. It is also useful in conducting sensitivitystudies, particularly when the data is scanty. A remarkable finding byexercising such code was that fracture initiation is strongly dependent on theformation an isotropy; a contradiction to the current industry thinking.
Sedimentary rocks, because of their depositional environments, have alaminated structure with directional properties which are best described astransversely isotropic. Rahn showed that the moduli of elasticity of a foliatedrock can vary by about 50%, if measured normally and parallel to the beddingplane. Shale, a common sedimentary rock which accounts for 75% of the drilledsections in the world and causes 90% of the wellbore stability problems, is atransversely isotropic material. Coal, another sedimentary r6ck commonlyencountered during drilling of shallow formations, is an orthotropic materialwhich has also been known to cause considerable borehole instabilityproblems.
Fracture initiation occurs when the stress at the borehole wall, or itsvicinity, goes into tension and exceeds the rock's tensile strength. Thisusually happens when there is an excessive wellbore pressurization such asduring hydraulic fracturing stimulations or during highly overbalanced drillingoperations. Traditionally, fracturing due to tensile splitting of the rock waspredicated using the linear elastic stress distribution around a wellbore inhomogenous and isotropic media. However, in view of the inadequacy of simpleisotropic models to describe the behavior of real rocks, a more complex modeldescribing more realistically the rock response, needed to be developed.
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