Partitioning Thresholds in Hybrid Implicit‐Explicit Representations of Naturally Fractured Reservoirs. Issue 2 (28th April 2020) - Wong, Daniel Lorng Yon
Abstract: Fractures can have variable effects on fluid flow in a porous rock. Moderately conductive fractures may enhance the rock's overall effective permeability, while highly conductive fractures may completely dominate fluid transport. Fluid flow modeling is important to quantify the impact of fractures on the performance of a reservoir. However, simulating fluid flow is computationally intensive due to the heterogeneities introduced by the fracture network. In this work, complex fracture patterns are simplified using hybrid implicit‐explicit representations to yield a computationally tractable model. Hybrid modeling requires the selection of a partitioning size to group fractures by size. Small fractures are upscaled with the rock matrix; large fractures are explicitly represented. Our study shows that, given a naturally fractured reservoir, an upper limit exists for the partitioning size and that this threshold partitioning size can be determined without trial and error. Using artificial and realistic fracture patterns, we created hybrid models using different partitioning sizes and subjected them to pressure drawdowns. Simulated production rates were compared against reference results obtained from simulations on the original fracture patterns. Beyond a threshold partitioning size unique to each fracture pattern, hybrid model results deviate significantly from reference solutions. The threshold is identified from the relationship between upscaled permeabilities and partitioning sizes and corresponds to the point where the effective permeability of small fractures begins to increase rapidly. The permeability‐size relationship is obtained using numerical flow‐based upscaling. For uniformly distributed fractures with no abutment relationships, the effective medium theory is shown to generate accurate permeability‐size relationships. Plain Language Summary: Naturally fractured reservoirs are exploited in several industries as they (1) may contain oil and gas, (2) can also be used to extract heat from underground, (3) form pathways for groundwater to flow, and (4) can be used to store CO 2 and mitigate climate change. As such, it is important to predict how fluids will move through such reservoirs. This is difficult because fractured reservoirs contain fractures that differ greatly in size. One approach that alleviates this problem is to represent the rock and smaller fractures by an equivalent porous medium that has a comparable flow behavior. Our study finds that this approach works, but there is a limit to what can be considered small. When this simplification is applied to fractures that are larger than the limit, the resulting equivalent representation fails to capture the correct flow behavior. We show that the limit can be predetermined by studying how the equivalent rock properties change with the size range of small fractures. We also show that this procedure can be carried out efficiently using a technique based on the effective medium theory. The findings in this study will allow industry practitioners to systematically simplify their fractured reservoir flow modeling problems. Key Points: Small fractures up to a threshold size can be lumped with the rock matrix and upscaled into an equivalent porous medium We determine the threshold size from the relationship between upscaled permeability and the size range of small fractures Where applicable, the upscaled permeabilities are efficiently established using the effective medium theory compared to numerical upscaling