Multilateral wells (MLW) are attracting increasing attention as a cost effective approach for an economic exploitation of smaller oil and gas fields or reserves located in more marginal assets. The cost reduction is especially pronounced, if the laterals can be completed as open holes, i.e. without a cemented liner. The most critical part of such a completion is the junction where the lateral branches off from the main bore. The ideal objectives of a stability analysis for a MLW junction is; in these cases, to determine the overall stability, estimate the amount of collapse to be expected for a certain drawdown/depletion scenario and in this way see if the utility of the junction/completion is compromised. This paper presents the application of a refined numerical model that meets these objectives. The model enables the simulation of post-peak strength strain localization that allows the geometry and distribution of shear zones and breakouts to be analyzed. In order to verify the results of the numerical model, a refined laboratory test, simulating a cross section through the MLW junction is demonstrated. It is shown that the results from the laboratory test corroborate the numerical simulations and it is thus argued that this type of refined numerical model can with confidence be used to assess hole-junction integrity and eventual degree of collapse thereby assisting the completion design process.
1. INTRODUCTION
Multilateral wells (MLW) are attracting increasing attention as a cost effective approach for an economic exploitation of smaller oil and gas fields or reserves located in more marginal assets. The use of MLW increases the reservoir drainage for a given well slot and top hole completion. The cost reduction relative to more conventional well designs can be especially pronounced if the lateral well bores and the junction can be completed as open holes; i.e. without a cemented liner, commonly referred to as bare foot completions. The most critical part of such a completion is the junction which, due to its larger hole size, is more susceptible to failure and collapse. Only if the junction is located in rock with rather high strength can it be expected that the junction is stable without hoop stresses exceeding the rock strength locally. In many cases where the junction is located within the reservoir, rock will fail at least locally resulting in some sand falling in to the well bore and junction void. Such a failure can either be tolerated - this assumes that the failed material is removed during well cleaning before production starts - or the junction can be protected by the completion. In the latter case the junction can either be cased and cemented or completed with a liner or stand-alone screen.
To date, completion design for MLW junctions has been conservative in cases where there has been a perceived risk of hole collapse. This arises from the fact that most methods for hole stability analysis are incapable of accurately predicting the volume of failed rock. Rather these methods predict the onset of rock failure and thus the completion design errs on the side of safety.
