In this paper we describe how we have used finely gridded single porosity reference models to simulate the behavior of gravity drainage in fractured secondary gas caps/gas invaded zones with and without block to block reinfiltration.

These models were used as reference solutions in the development of the Subdomain-Dual Permeability Dual Continua model (SD-DK model). This extension of the classic Subdomain model 1  can simulate gravity drainage flow due to both direct matrix flow (capillary continuous) and matrix to matrix reinfiltration from fractures without the use of pseudo functions.

The behavior of the SD-DK model compares well with the fine grid single porosity reference models in situations where vertical oil drainage with capillary continuity between vertical matrix blocks is assumed or where vertical oil drainage is assumed to occur entirely due to reinfiltration.

In addition the model has been extended (heuristically) to allow it to model both drainage mechanisms occurring simultaneously (capillary continuous gravity drainage and reinfiltration).

The paper includes the details of the formulation as well as examples comparing the SD-DK technique to reference solutions and existing models.

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