While well interference is a known phenomenon in shale gas plays, it is often overlooked or only considered when its effects are readily apparent. Spacing tests are often performed when starting to appraise or develop a new area where a qualitative evaluation of interference between drilled wells will determine the appropriate well spacing.
While numerical reservoir modelling and build-up analysis can be applied in the Marcellus shale play they are not appropriate for determining optimal spacing due to very low, nano-darcy reservoir permeabilities and uncertainty over hydraulic fracture geometry. The challenge was to develop a new approach to measure and estimate the impact of interference on gas recovery and optimize well spacing.
The impact of interference was initially evaluated by assessing the change in the productivity index of wells due to offset wells being added, put on production or shut/in. In order to have a more meaningful way of estimating its impacts on economics, interference was also quantified based on projected future five year cumulative production using Arps decline curve, Rate Transient Analysis and Pressure Normalized Rate methods. Results were compared and a statistical workflow was used to estimate optimal spacing.
By relating the degree of interference with overlap between wells, the nature of interference was also investigated. Interference can be due to a Stimulated Rock Volume (SRV) overlap. This is characterized by a proportionality increasing interference with SRV overlap. The impact of natural features such as faults or high permeability streaks that can act as conductivity highways across many wells is usually not proportional to overlap.
This study in Marcellus shale play demonstrated that measurable interference occurred at wide spacing and that 1,000 ft spacing should result in an increased FYFCP (Five Year Forecasted Cumulative Production) of 10% over existing spacing assumptions, key at low gas prices. Therefore, 1,000 ft spacing has been recommended for future well placement. The workflow outlined in this paper is currently being used to evaluate well spacing for other assets and can be used by Reservoir Engineers to evaluate spacing in tight or shale gas/oil plays.