Stability of a Multilateral Junction: Experimental Results and Numerical Modeling
- Panos C. Papanastasiou (University of Cyprus) | Malek Sibai (Laboratoire de Mecanique de Lill) | Juliane C. Heiland (Schlumberger) | J.F. Shao (University of Lille) | John M. Cook (Schlumberger) | Dominique M. Fourmaintraux (Total) | Atef Onaisi (Total) | Benjamin P. Jeffryes (Schlumberger) | Philippe Charlez (Total S.A.)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- March 2006
- Document Type
- Journal Paper
- 4 - 11
- 2006. Society of Petroleum Engineers
- 3.3.2 Borehole Imaging and Wellbore Seismic, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 1.14 Casing and Cementing, 2 Well Completion, 2.4.3 Sand/Solids Control, 1.6 Drilling Operations, 1.6.3 Drilling Optimisation, 1.10.4 Onshore Drilling Units, 5.1.10 Reservoir Geomechanics, 1.2.2 Geomechanics
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We studied the stability of multilateral (ML) junctions in a combined experimental and numerical modeling program. The experiments were carried out in a true triaxial machine on large cubic blocks (40 cm) of weak triassic sandstone with two holes intersecting. Six tests have been performed with two different geometrical configurations and three different stress states. The experimental results are presented and compared with numerical modeling obtained with finite-element software developed for assessing the integrity of rock surrounding an ML junction.
Drilling inclined wells through producing strata can greatly improve reservoir drainage and hydrocarbon recovery. The horizontal sections are accessed through multiple inclined wells drilled from a relatively small footprint in many or all directions, something that allows better exploitation of offshore platforms and land rigs that are under economic and environmental restrictions. Drilling inclined and horizontal wells, though, is more difficult and more expensive, because of wellbore instabilities. A particular area of concern is the integrity of the rock near an ML junction. The junction is the region in which a second wellbore (lateral) takes off from the main wellbore (parent). In the terminology used for ML junctions, different levels are defined according to whether there is mechanical and/or hydraulic integration between lateral and parent holes. By the ML Levels 1 and 2 definitions, the rock at the junction is not supported mechanically with cemented casing, so the integrity of the rock around the area of two intersecting tubes becomes very important in terms of stability.
We performed physical tests on large cubic blocks (40 cm) with two holes intersecting, in the true triaxial cell of the U. of Lille. The tested rock is weak Triassic sandstone called "Grés des Vosges.?? Six tests have been performed with two different geometrical configurations (lateral differently oriented with respect to the main bore) and three different stress states (two blocks with a hydrostatic stress state and three blocks with anisotropic stress state). The blocks were loaded to generate breakouts of the borehole wall in various directions. The deformation of the borehole walls and the development of breakouts are monitored in real time with a video camera placed in the main bore. The image is then analyzed by image-processing software. Graphs of the relative diametric decrease (convergence of the borehole wall) in various directions can then be plotted against loading. After testing, the blocks were cut in cross sections perpendicular to the parent-hole axis at different distances from the junction. The experimental technique and results will be presented in the Rock Characterization and Experimental Procedure and the Experimental Results sections, later in this paper.
We compared the experimental results with numerical modeling obtained with software developed for assessing the integrity of rock surrounding an ML junction. The tool was developed using finite-element analysis and a graphical user interface for providing the input data and visualizing the results. The analysis is based on a generalized plane-strain formulation that is carried out in cross sections in succession, perpendicular to the parent-hole axis. We compared the load level at which breakouts are initiated. Results are presented for two different junction geometries and three different loading paths (in isotropic loading, all three applied stresses on the block were the same, and in anisotropic loading, two of the applied stresses were the same, and the third stress had a different magnitude). The results, based on the elastic/brittle analysis, qualitatively reproduce those obtained during the experimental tests.
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