Evaluating Crossflow Migration Drainage

Abstract

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.

Introduction

Flow across a lease line occurs when reservoir pressure on one side of the lease line is lower than the reservoir pressure on the other side.  This can be due to one of, or a combination of, low operating pressures and high well density.  However, what is not widely recognized is that a high permeability zone in a multi-zone reservoir can greatly increase the volume of lease-line flow if wells on the high pressure side are commingled and have a higher bottomhole flowing pressure than the pressure in the thief zone.  In other words, drainage benefits to the aggressive operator may be significantly larger than expected.

In order to quantify the total lease-line flow and look at alternatives for reducing future flow, we have chosen to model a six-section region in southwestern Alberta.  All data used was from the public domain.  We further limited our investigation to three sections on each side of a hypothetical lease-line.  The modeling implicitly presumed no flow across any of the external model boundaries.  This and other assumptions may have some impact on the results relative to the actual performance.  Due to a lack of operator data, results obtained in this study may not be representative of a specific property but may be considered as a possible outcome.