Poroelastic Effects on Borehole Ballooning in Naturally Fractured Formations
- Ole A. Helstrup (U. of New South Wales) | Khalil Rahman (U. of Western Australia) | Zhixi Chen (U. of New South Wales) | Skeik. S. Rahman (U. of New South Wales)
- Document ID
- Society of Petroleum Engineers
- SPE/IADC Drilling Conference, 19-21 February, Amsterdam, Netherlands
- Publication Date
- Document Type
- Conference Paper
- 2003. SPE/IADC Drilling Conference
- 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 1.2.3 Rock properties, 1.11 Drilling Fluids and Materials, 1.6 Drilling Operations, 4.3.4 Scale
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This paper presents a study on wellbore ballooning and (in)stability in naturally fractured formations. A model containing a single fracture is analysed using a poroelastic numerical approach for a number of in-situ stress ratios and rock matrix permeabilities. It was found from this study that the stress response for this system depends not only on the in-situ stress, but also on rock matrix permeability and time. The results are compared with that from an analytical linear elastic solution approach, and it is shown that the results are significantly different.
Maintaining wellbore stability remains a major issue when drilling wells, both for safety and economic reasons. Borehole ballooning effects are a recognised phenomena in drilling operations that causes complications for the driller in correctly identifying and dealing with lost circulation or well kicks. Borehole ballooning refers to a situation where the wellbore returns more/less drilling mud than what would be expected compared to the intact and rigid wellbore volume. The exact causes for ballooning are however still somewhat uncertain, but is generally accepted as being due to one or more of thermal effects on the drilling fluid, elastic deformation of the wellbore and fracture charging. Kårstad1 did a thorough study on time dependant thermal effects, while Helstrup et al.2 presented a general solution for elastic deformation together with a preliminary study on fracture charging, based on linear elastic fracture mechanics theory.
Various solutions exist to determine potential failure of intact wellbores, usually based on linear elastic theory (e.g. Aadnøy and Chenevert3 or Tan and Willoughby4). However, in naturally fractured formations, borehole failure analysis becomes much more complex, as the material in question is no longer continuous or homogenous. Atkinson and Thiercelin5 presented linear elastic analytical solutions for the interaction between the wellbore and a pre-exisiting, arbitrary fracture. For multifracture situations solutions get more complex still, making analytical solutions very unlikely; however, the interaction of wellbore fluid pressures, rock stress concentrations and the crack displacements (opening/closure as well as shear displacements) can be investigated by solving a coupled problem in a suitable numerical simulator. Santarelli et al.6 presented field case data from a naturally fractured formation and used a discrete element model (DEM) in an attempt to simulate and explain why the classical responses of increasing drilling mud density and viscosity not only failed to improve wellbore stability, but actually made matters even worse. Zhang et al.7 conducted wellbore stability studies in fractured formations using the same DEM code, investigating the stress/strain response and potential wellbore failure, as well as fluid flow in the fractures for a range of different in-situ stresses and for different fracture patterns. Chen et al.8, again using the same DEM code, concentrated their studies on the effects of fracture friction angle reduction due to mud infiltration in the fracture system, and what impact this would have on wellbore stability. Recently, Jing et al.9 presented a discontinous deformation analysis (DDA) method, developed with the aim of studying similar types of scenarios as those mentioned above.
In common for all these is that they consider a steady state (or, in other words, time = 0 or time = 8), linear elastic situation, with fluid only allowed to flow inside the fractures (i.e. impermeable solid). Further, they all consider a two way coupling between mechanical and hydraulic behaviour, with the fracture conductivity depending on deformation and displacement, which again is dependant on the fluid pressure.
This paper presents the initial steps in formulating a different approach to investigate the wellbore response in naturally fractured formations using finite element modeling (FEM), with the aim to include
Time dependency (i.e., 0 < t < 8)
Finite length fractures
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