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”.
For a well geometry with uneven fracture spacing, the flow regime is 1) infinite acting until the boundary between the fractures with the smallest spacing is observed, 2) thereafter it is transitional flow until the boundary between the fractures with the largest spacing is observed, and 3) after that 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).
Want to learn more about fractional RTA?
Jorge Acuna will present on Fractional RTA in the upcoming whitson webinar 8 June 2022. Registration is open here: https://lnkd.in/ghvndfd4.
whitson will also present a paper on the topic for the upcoming URTeC 2022 (paper nr. 3718584).
Fractional RTA functionality will be in our software, whitson+, in the near future (June and August 2022 release).
Learn more about our software products
Curtis Hays Whitson
Mathias Lia Carlsen
whitson supports energy companies, oil services companies, investors and government organizations with expertise and expansive analysis within PVT, gas condensate reservoirs and gas-based EOR. Our coverage ranges from R&D based industry studies to detailed due diligence, transaction or court case projects. We help our clients find the best possible answers to complex questions and assist them in the successful decision-making on technical challenges. We do this through a continuous, transparent dialog with our clients – before, during and after our engagement. The company was founded by Dr. Curtis Hays Whitson in 1988 and is a Norwegian corporation located in Trondheim, Norway, with local presence in USA, Middle East, India and Indonesia.