Well Interference Effects for Multiwell Configurations in Unconventional Reservoirs
Authors: Siddiqui Shameem , Kumar Amit 
Published: November 2016

Abstract
This paper aims to develop a deeper understanding of well interference effects by evaluating various multiwell configurations for their sensitivity to key reservoir parameters. Reduction in well spacing, over the development of shale plays, has rendered single-well reservoir models to be of limited utility as a planning tool. Presently, well interference effects need to be accounted for in well placement planning and completion optimization, especially for the various multiwell configurations in current use. Often, such configurations ("stack and frac") have gone through expensive field trials but not rigorous analysis because of a lack of proper simulation tools to model these scenarios.
This study is based on reservoir simulation with unstructured grids. Horizontal wells with multiple hydraulic fractures were modeled in "stack-and-frac" configurations, including stacked, co-planar, and W-shaped configurations, in a conceptual reservoir. The sensitivity parameters used were matrix permeability, vertical anisotropy of matrix permeability, fracture heights, and well spacing. Typical values from the liquids-rich part of the Eagle Ford were used as the base case, but the conclusions of this study should have broader applicability because of the wide range of sensitivity parameters. Cumulative productions, producing gas-oil ratios (GOR), and decline rates were reported as the main metrics to compare simulation results. The efficiency of different multiwell configurations was also evaluated under varying combinations of parameter values.
As laterals get closer, the onset of well interference marks the transition from adding reserves through infill drilling to accelerating production. Well spacing, relative to fracture lengths, is a key factor for well interference, and this effect becomes more pronounced at higher matrix permeability. This is particularly important for the co-planar configuration but is also true for the W-shaped configuration. Vertical anisotropy in permeability was found to be more important for the co-planar configuration than the stacked and W-shaped configurations. Fracture heights, which should be considered relative to the vertical distance between the laterals in these configurations, contribute to well interference, especially at high vertical anisotropy. Not all factors affect oil and gas production in the same proportion. The stacked triple was determined to be the least efficient in terms of cumulative oil production per unit of lateral length. The efficiency of the W-configuration was between that of the stacked triple and co-planar triple in the cases evaluated. The operator should perform a thorough sensitivity study before a field trial of multiwell configurations because the complex interaction of factors that impact well productivity cannot be satisfactorily resolved exclusively through field trials. Reservoir simulations of multiwell configurations for multistage, hydraulically fractured wells are not reported with unstructured grids. The pressure-volume-temperature (PVT) model used in this study is new and generates the data required for an equation of state (EOS) model, when given readily-available inputs.