Abstract

Butterfly modeling is a technique used in Fractional Dimension Rate Transient Analysis (FD-RTA) that reduces a system with multiple fractures to an equivalent system with a single fracture. With this approach, numerical simulations run in seconds, enabling the automatic calibration of multiphase models. Currently, it is limited to single well models of infinite conductivity fractures. We present a modification of this technique that allows modeling wells with finite-conductivity fractures as well as groups of wells with independent schedules. This includes combinations of producers and injectors as required in Enhanced Geothermal Systems (EGS). We present comparisons with the numerical resolution of fractures swarm models, and a multiwell example.

Introduction

The technique known as butterfly modeling (Acuna, 2018) is used to represent an unconventional well with an equivalent system of a single fracture. For parallel fractures, this is done by partitioning the permeable medium along no-flow boundaries located at half-distance between fractures and reorganizing the fragments into a different object with a single fracture. The wider fragments go at the center so the variation of reservoir area with distance from the fracture can be expressed analytically. The new object has the same variation of reservoir volume with distance from the fractures.

Butterfly models can be treated analytically for single-phase systems and as a first iteration for multiphase systems (Acuna et al., 2024). For multiphase flow it is better to rely on numerical formulations of the butterfly model that can account for realistic fluid properties variation without sacrificing too much in terms of computational time. The new formulation presented here is strictly for use with numerical formulations.

If the original system is a fracture swarm model (Acuna, 2020), the fragments of the permeable medium consist of pairs of equal size fragments created by splitting the distance between two fractures as shown in Figure 1. Fractures at the side of these elements are combined into one fracture and the tips are connected to other fractures. The dots in Figure 1 represent the position of the tips on the fractures in the fracture swarm model and the corresponding position in the butterfly model. The butterfly model and the fracture swarm model have the same thickness h.

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