Taking Advantage of Multiphase Flow Reversals Enhances Downhole Gas and Solids Separation for Artificial Lift
Authors: Saponja Jeff , Hari Rob , Brignac Justen , David Sean , Jaszan Dayley , Coyes Corbin , Nagoo Anand , Penner Brandon 
Published: August 2022

Abstract
Downhole separation of gas and solids for sucker rod pumping (SRP) and electrical submersible pumping (ESP) continues to be a significant challenge, particularly for horizontal wells. A major advancement in downhole separation has been achieved by realizing there was an opportunity to intentionally take advantage of transient multiphase flow conditions where liquids phase reversals or fallback exists. Multiple case studies demonstrate that designing of a downhole separator that takes advantage of liquid phase flow reversals can enhance downhole separation performance and capacity, while at the same time lower operational risk.
Improving downhole separation without undesirably increasing operational risk and cost has been challenging. A separator design that requires a packer or annular seal, such as a cup, is inherently more operationally risky from an installation and retrieval perspective. Further, a separator design that imparts pressure drops or increases flow turbulence faces the reliability risks of scale deposition and erosion. Flow turbulence can increase the entrained gas foaming tendency in the liquid and reduce gas bubble size in the liquid, consequently lowering separation efficiency and increasing pump gas interference.
It is generally understood that separation capacity, in terms of gravity separation principles, has been physically limited by a separator's cross-sectional area for separation. It is less understood that separation capacity has also been limited by the location and orientation of a separator's intake, as well as the shape of the conduit in a separator's separation region, and that it has been limited by a common mechanical design practice of a concentric or centralized pump intake dip tube or mandrel.
Technical literature, industry research and transient multiphase flow simulations have revealed, under certain conditions, that liquid phase flow reversals are not only present, but also occur at high frequencies. Such reversals or liquid fallback also occurs at much higher velocities than gas bubbles can rise, which suggested there is an opportunity to improve downhole separation. Industry research also disclosed that gas-liquid separation in an eccentric annulus is more efficient than in a concentric annulus and that separation efficiency is greater in a conduit shaped as an open tube versus an annulus. Such gains in separation efficiency are primarily due increased liquid hold-up, meaning increased liquid phase flow reversals or liquid fallback.
It was hypothesized that downhole separation could be significantly improved by a separator engineered to take advantage of liquid phase flow reversals, thereby avoiding the limitations of downhole separators that are governed by gas bubble rise velocity. A separator was then designed, built, extensively flow loop tested and successfully field implemented.
This paper describes the design process and results of the field implementation of an enhanced downhole separator. Flow loop testing results and comprehensive analytical transient multiphase flow simulation will be shared. A set of case studies, in multiple basins, reviews the field installations and presents the results of improved downhole separation performance and lowered operational risks, resulting in lowered operating expense and increased production.