News – What is Fractional RTA

This month we wanted to highlight an important topic in Rate Transient Analysis (RTA) for tight unconventionals—how uneven fracture spacing impacts flow behavior and model accuracy—and share a foundational paper that explores this in depth.

An inherent assumption in most industry RTA is equally spaced fractures. However, as shown in several field studies (Raterman 2017, Gale 2018), the distance between individual fractures tends to be unevenly spaced along the wellbore (e.g., “fracture swarms”). Fractional RTA extends industry standard RTA workflows to account for uneven fracture spacing.

Flow Regimes 1.01

To understand why it is important to account for uneven fracture spacing, we repeat the three relevant flow regimes in tight unconventionals.

1. Infinite acting flow, often referred to as transient flow, is the flow regime that ends as the pressure transient reaches one reservoir boundary.
2. Transitional flow is the flow regime starts as the pressure transient reaches one reservoir boundary and ends when the pressure propagation reaches all reservoir boundaries.
3. Boundary dominated flow, also called pseudo-steady state, is the flow regime that starts as the pressure propagation reaches all reservoir boundaries. It occurs when all outer boundaries of the reservoir are no-flow boundaries. These boundaries can be both sealing faults and nearby producing wells or fractures. During this period, the change in pressure at any place in the reservoir decreases at the same, constant rate. The reservoir is said to behave as a “tank”.

In wells with uneven fracture spacing:

• flow regime is initially infinite acting until the boundary between the fractures with the smallest spacing is observed,
• thereafter it is transitional flow until the boundary between the fractures with the largest spacing is observed,
• then ultimately it is in full boundary dominated flow.

When is fractional RTA important?

Traditional RTA methods leverage a so-called “symmetry element” model, in which the underlying model is representing one-quarter of a fracture. This is highlighted by the red-lined box in Fig. 1. The model is “1 dimensional”, as there are no flow contributions beyond the fractips or beyond the frac height. Hence, there is only one no-flow boundary, resulting in only two dominant flow regimes over time; (1) infinite acting (IA) linear flow, followed by (2) boundary dominated (BD) flow.

The problem with this model when applied to real field cases is that all observed boundary effects are “forced” into one boundary (which is observed at one point in time). However, as shown in several field studies (Raterman 2017, Gale 2018), the distance between individual fractures tends to be unevenly spaced along the wellbore, resulting in boundaries being observed at different points in time. The resulting well performance is a compound result of observing different reservoir boundaries (here fractures) at different points in time. Hence, characterizing and accounting for transitional flow becomes important. Transitional flow can in many cases be the dominant flow regime for unconventional wells, especially if the fracture spacing is highly uneven (high heterogeneity).

Featured Paper: URTeC 2118 – “Rate Transient Analysis of Fracture Swarm Fractal Networks” by Jorge Acuna

To explore this concept further, we highly recommend URTeC 2118 by Jorge Acuna. The paper is a few years old but remains one of the most insightful studies on how RTA can be performed on wells with complex fracture networks.

Fractal techniques are used to create networks with fracture swarm geometry that resembles that observed in exploratory cores in the literature. These networks have desired total pore volume, maximum and minimum fracture spacing, and fractional dimension, and their hydraulic behavior is controlled by these properties and fracture conductivity. Numerical simulation of individual fragments and total production is shown to be consistent with simulations of the entire network when fracture conductivity is high, leading to sub-linear flow (pressure derivative slope between 0.5 and 1). When fracture conductivity is low, the network exhibits sub-radial flow (pressure derivative slope between 0 and 0.5) at early times, with a transition to sub-linear or boundary-dominated flow (BDF) at later times. The duration of sub-radial flow can be extended by reducing fracture conductivity. These types of flow behavior cover the entire range seen in unconventional wells and demonstrate how the power-law behavior frequently observed in diagnostic plots can be produced by the combined effect of matrix fragments that individually can only show linear, bi-linear, or BDF flow. The relatively simple geometry of fracture swarms also allows for the calculation of properties for sub-radial flow, providing new insights into the production mechanisms of unconventional wells.

Want to learn more about fractional RTA?

• Watch Jorge Acuna presenting on Fractional RTA in our whitson webinar series, June 2022
• Dive deeper in our whitson+ user manual here

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