In the Huntington field, bilinear flow with a clear one-quarter unit slope in the pressure derivative has been observed during intermediate-times in horizontal well tests, instead of the usual linear flow with a half unit slope expected in a homogeneous reservoir. This paper investigates various analytical methods with numerical methods to explain this behaviour.
A number of plausible explanations of this pressure behavior were investigated in this study: rate history or production-time effects; transition between early and late time derivative radial flow stabilizations which could yield a quarter slope straight line on the derivative for specific combinations of horizontal and vertical permeabilities, well length and reservoir thickness; and geological conditions such as high-permeability streaks, the presence of shale barriers, and layering with varying permeability.
It was concluded from numerical simulations and geological evidence that the most likely explanation, which yields the best match with well test data, was uniform horizontal permeability and non-uniform vertical permeability increasing from the bottom to the top of the reservoir. This is consistent with the geology of the Huntington field, where the depositional sequence is dominated by high-density turbidity currents.
At each sedimentation pulse, heavier pebble/gravel lags were deposited first at the base of channels followed by finer particles which may have formed thin shale drapes. In addition, diagenesis might have led to cementation of pebble/gravel lags. The non-uniform and non-laterally extensive occurrence of the cemented pebble/gravel lags and thin shale drapes act as local baffles to vertical flow but not to horizontal flow resulting in bilinear flow behaviour in the reservoir. This paper offers a unique insight into an integrated effort at combining analytical, numerical well test models and geological evidence to explain the reservoir behavior observed in the Huntington field.