Abstract
AbstractTight naturally fractured carbonates are "hybrids" between conventional and unconventional reservoirs and are becoming increasingly important in an operator’s portfolio. Wells produce if natural fractures are present and not damaged. Otherwise, large stimulation jobs are required. The steep initial production declines in these reservoirs are influenced by matrix-fracture transients, near-wellbore damage / production impairment, and factors that that cause the natural fracture conductivity to change (decrease) during the production. In this paper we focused on the simulation of the matrix-fracture transient.Our work focused on reservoir simulation of naturally fractured carbonates with matrix permeabilities in the 0.0001 to 0.1mD range. We used our existing knowledge from simulating these reservoirs to build conceptual models for modelling transient behavior in these reservoirs. We considered dynamic models with both structural and unstructral grids to understand the impact of these in modelling production transient effects. The structural grid simulations used 100m by 100m gridcells with dual porosity, while the unstrucutral grids were built using a single porosity (SP) and complex gridding with variations in grid size to capture natural fractures.History matching production data using conventional simulation models was challenging because families of natural fractures are typically not uniform as we move away from the wellbore and large structural grids will have difficult capturing this discontinuity. In these scenarios we have seen that dual porosity structural grids will provide too much pressure support causing the well’s simulated bottom hole pressure to be overestimated. Conversely, SP models with complex unstructured grids gave much better results. They were effective in matching the data after the initial production decline because of greater accuracy in accounting for the matrix contribution. They also provide a more accurate representation of discrete fracture network models as opposed to an upscaled approximation.Production decline rates in tight naturally fractured reservoirs can be highly variable and have a strong impact on EUR and project economics. Therefore, understanding the mechanisms that influence these declines and modeling them accurately through fit-for-purpose dynamic simulations as shown in this work, are an important step for optimizing the development of these reservoirs.