Abstract

We studied the stability of multilateral junctions in a combined experimental and numerical modeling program. The experiments were carried out in a true triaxial machine on large size cubical 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 a multilateral junction.

1. Introduction

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 area 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, due to wellbore instabilities. A particular area of concern is the integrity of the rock near a multilateral (M-L) junction. The junction is the region where a second wellbore (lateral) takes off from the main wellbore (parent). In M-L levels 1 and 2 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. In this paper we studied the stability of multilateral junctions in a combined experimental program and numerical modeling.

We performed physical tests on large cubical blocks (40 cm) with two holes intersecting, in the true triaxial cell of the University 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, 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 analysed by image processing software. Graphs of the relative diametric decrease (convergence of the borehole wall) in various directions can then be plotted versus 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 sections 2 and 3.

We compared the experimental results with numerical modeling obtained with software developed for assessing the integrity of rock surrounding a multilateral 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 (isotropic and anisotropic loading). The results, based on elastic/brittle analysis, reproduce qualitatively those obtained during the experimental tests.

The developed tool will be useful to drilling engineers to decide in which formation, in which azimuth and which deviation to drill a stable lateral. The completion engineer can use the tool to decide, a) where to place a stable junction, e.g. in the reservoir or in the overburden, b) what level of M-L junction is needed (mechanically supported or not) and c) if the junction is drilled in the reservoir section, at which draw-down and depletion pressures it may become unstable.

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