This paper describes ways to optimize vertical and lateral spacing of horizontal wells in a stacked bench development up-front and, if necessary, adjust the development spacing based on early time Gas-Oil-Ratio (GOR) behavior, other diagnostic methods, and/or seismic inversion.

In recent years, the Permian Basin has become the leading unconventional resource play due to, among other things, the high quality of the resource and stacked-bench horizontal well developments. Maximizing recovery and profitability in a stacked-bench unconventional play is challenging because operators must optimize both the inter-bench vertical well spacing and the intra-bench lateral well spacing.

Generally, well interference can be detected by comparing well productivities, initial reservoir pressures, Stimulated Rock Volumes, and Expected Ultimate Recoveries against forecasted well performance. However, such analyses do not directly reveal whether any observed interference is due to inter-bench or intra-bench interactions. Without the ability to distinguish inter-bench interference from intra-bench interference in advance, vertical and lateral spacing optimization might be achieved only at the risk of over-capitalizing a development.

This paper presents a case study in which an analysis of GOR behavior was coupled with other analyses to determine the source of interference. Stand-alone (i.e., widely-spaced) wells in each bench were found to display unique GOR characteristics. When horizontal wells in different benches interfered with one another, however, the GOR trends observed in the wells were synchronized.

Wells with synchronized GORs exhibited substantial departure (better or worse) from expected performance, thereby confirming the hypothesis that they were interfering. Among wells that had synchronized GORs, in some cases, production from one well appeared to have been captured by another well, whereas in other cases, productivity of both wells were affected.

It was found that the contrast in the elastic properties of the stacked benches, which was discerned from the inversion of seismic data, played a key role in the vertical interference between wells. When stacked benches have similar elastic properties, much stronger interference was observed than when the elastic properties were dissimilar. As demonstrated by previous laboratory experiments (Thiercelen et al., 1987), mineback studies (Warpinski et al., 1981; Warpinski et al., 1987), and modeling (Thiercelen et al., 1987; Barree et al.; 1998; Smith et al., 2001; Zhang et al., 2007), a large contrast in the elastic properties between benches appears to create flow barriers at the interface.

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