Scale and Scale Inhibition Challenges for an Alkaline Surfactant Polymer Flood in a Seawater Flooded Reservoir

Abstract

Abstract Scale control during Alkaline Surfactant Polymer (ASP) flooding is a recognised challenge especially following ASP breakthrough at the production wells, due to continual mixing of the high pH ASP waters with reservoir brine. The effective performance of scale inhibitors (SIs) is a problem, given the high oversaturation expected due to mixing of injected alkali and formation water. Previous work (SPE 141551) described this challenge for a conventional system whereby ASP was injected into a reservoir containing low salinity / low divalent ion Formation Water (FW). This work showed that chemical performance (SI/ brine compatibility and performance) could be achieved using conventional inhibitors and that treatment of the production wells via downhole squeeze treatments was achievable. It also demonstrated that the production of high pH ASP fluids had a considerable impact on the retention and release properties of the chemicals. This paper progresses significantly from the previous work by examining a field case study. Although the formation water is a low salinity / low divalent cation brine similar to that in the earlier example, this field represents a considerably more severe scaling challenge since it has already been flooded with Sea Water (SW). The SW contains considerably higher divalent cation concentrations (c.f. Ca2+ ~ 450mg/l vs. ~40 mg/l for the formation water) and results in a considerably more severe challenge in terms of SI/Brine compatibility and chemical performance when examining a range of conventional SI's. The paper describes extensive modeling of the in situ conditions for this field following ASP flooding, extensive scale inhibitor performance tests across a wide range of conditions and mixtures of ASP/FW/SW together with a series of core flood tests to assess the potential for squeeze treatments in this new pilot ASP system. For the pilot, reservoir modelling was essential to simulate the expected mixture of ASP/FW/SW which would be produced, such that chemicals could be targeted for the expected produced mixtures (Note: increased FW:SW ratio results in more achievable inhibition) allowing chemical selection for squeeze. The pilot ASP flood in this field is now planned for 2014.