Understanding the effects of lease-line drainage has always been a challenge facing natural gas producers. When approaching this issue, companies generally focus on gas migration caused by reservoir pressure gradients. However, ‘crossflow migration’ in multi-layered reservoirs also needs to be evaluated.

Crossflow migration occurs when wells produce at a flowing pressure higher than the near-wellbore reservoir pressure of a commingled and depleted high permeability zone. This can result in unexpected crossflow of gas from a low-permeability, high-pressured, high-reserves zone to the depleted zone, and, through this ‘thief zone’, migration of the gas to adjacent lands.

This study quantifies reserves and production impacts of crossflow migration, using a model of two neighbouring shallow gas properties in S.E. Alberta, and based on geological characteristics typical of the area around the Alderson, Suffield and Medicine Hat Fields (Figure 1).

The analysis showed that historical lease-line drainage of 1.9 BCF (55 e6m3) occurred during 1960–2007 over a one-mile boundary between two operators, one of which had aggressively developed its property through reduced operating pressure and infill drilling. Of this amount, approximately 45% was through the permeable Medicine Hat A (MHA) formation. Drainage through the MHA was supported by approximately 0.6 BCF (18 e6m3) of crossflow into that formation from other zones completed in the commingled wellbores. If left unaddressed, further drainage of 0.7 BCF (20 e6m3) was predicted to occur over the 30-year forecast period.

Using reservoir simulation, this study also identifies an optimal development strategy to minimize the impact of crossflow migration drainage.

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