Gas EOR Processes in Tight Unconventionals
- Stian Mydland (NTNU, Whitson AS) | Ilina Yusra (Whitson AS) | Curtis Hays Whitson (NTNU, Whitson AS) | Mohamad Majzoub Dahouk (Whitson AS) | Mathias Lia Carlsen (Whitson AS)
- Document ID
- Society of Petroleum Engineers
- SPE Improved Oil Recovery Conference, 31 August - 4 September, Tulsa, Oklahoma, USA
- Publication Date
- Document Type
- Conference Paper
- 2020. Society of Petroleum Engineers
- 5.2 Fluid Characterization, 3 Production and Well Operations, 5.4 Improved and Enhanced Recovery, 2 Well completion, 4.3.4 Scale, 5 Reservoir Desciption & Dynamics, 5.4 Improved and Enhanced Recovery, 5.2.1 Phase Behavior and PVT Measurements, 2.4 Hydraulic Fracturing
- Tight Unconventionals, Gas Injection, EOR, Huff and Puff, Miscbility
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The purpose of this paper is to (1) describe the mechanisms of gas-based enhanced oil recovery (EOR) in tight unconventionals, and (2) emphasize the need for single-porosity model tuning of the dual-porosity model when it is used to model EOR for unconventionals on well or field scale. We study two different gas-based EOR methods that inject and produce cyclically through the same well: The Huff-n-Puff (HnP) method, and a method we will refer to as the Fracture-to-Fracture (F2F) in which every other hydraulic fracture is used for injection and production in each cycle. We show that the recovery mechanisms and EOR target volume for HnP and F2F are fundamentally different.
We argue that the target volume for HnP is a rubblized ("shattered") rock volume adjacent to the hydraulic fracture. To accurately predict the performance of this rubblized region, we use a compositional reservoir simulator that includes molecular diffusion to model the EOR performance of rubble-rock pieces of varying size.
Gridding of numerical models is given considerable attention for both HnP and F2F to show its importance when modeling miscible EOR processes. Coarse gridding may result in significant numerical dispersion, which can falsely yield artificially optimistic recoveries for the HnP process.
Results from this paper show that the primary recovery mechanism for HnP stems from a target EOR volume represented by a rubblized rock volume. The size of the rubble, and in particular its minimum dimension, will control the amount of gas that enters, mixes, and recovers oil from the rubble pore space through a process of Darcy flow, molecular diffusion, and phase behavior that involves swelling, vaporization, and first-contact miscibility conditions. The F2F method is not particularly affected by the rubblized region, but instead targets recovery from the entire rock volume between hydraulic fractures; this EOR process is akin to a conventional miscible-displacement mechanism with a much larger EOR target than HnP.
The F2F method is presented in this paper as an alternative to the HnP method to show that HnP is not necessarily the best or the only EOR strategy in tight unconventionals. The EOR target volume for F2F is potentially much larger than for HnP, as everything between the fractures may be swept with a piston-like efficiency. However, the response time (i.e. the time before uplift in production is observed) can be much longer for F2F than HnP, depending mainly on the fracture spacing and matrix permeability.
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