Understanding the Chemistry of Carbonate Rock Formations During a Phosphonate Based Scale Inhibitor Squeeze Treatment

Carbonate rock formations are present in a high proportion of the remaining active oil/gas reservoirs, globally. The interaction between the rock and scale inhibitors during squeeze treatments to prevent inorganic scale formation is still not fully understood. Using a combination of core floods and bottle tests, this work furthers the understanding of the kinetic relationship between the dissolution of the rock and precipitation of inhibitor/cation complexes. These findings can then be used in the optimisation of field squeeze treatments in carbonate formations.
Core flooding experiments were carried out using an Indiana limestone rock substrate with di-ethylene tetra-amine penta(methylene-phosphonic acid) (DETPMP) as the phosphonate based scale inhibitor. The input levels of divalent cations, namely Ca²⁺ and Mg²⁺, were controlled by manipulation of the seawater composition. As well as cation concentrations, the injection rate was varied to understand the influence of this on the kinetics of the system. pH data were correlated with the levels of divalent ions/scale inhibitor present in the effluent characterised by ICP analysis.
Mass balance calculations from the main treatment stages of the core flood experiments have shown agreement when compared to the mass balance values from previously performed static bottle tests, when an appropriate flowrate is selected. This suggests that the core flood experiments can approach the thermodynamic equilibrium on the timescale of the experiment, depending on the flowrate used. Similar mass balance calculations for the post-flush after shut-in produced some interesting results when an intermediate flowrate was used, where an increase in scale inhibitor concentration was observed over ca. 400 pore volumes, before declining. This phenomenon is proposed to be due to ongoing precipitation/dissolution cycles of the DETPMP/Ca complexes, as the post flush progresses and more rock substrate is dissolved in the post flush brine.
While DETPMP has been extensively studied in the literature for its potential to prevent scale formation and its interaction with sandstone, more information is required to understand its behaviour in carbonate rock formations. This work has begun to untangle the various kinetic interactions of DETPMP scale inhibitor with carbonate rock substrates and seawater cations.
Our conclusions suggest that understanding these findings will have a significant impact on squeeze operations, by both reducing the time required for treatment application and also optimisation of the treatment volumes (main treatment and over flush) required. This in turn would reduce the associated greenhouse gas emissions of the squeeze process. In addition, by optimising/extending the squeeze lifetimes the number of required interventions are reduced, with a net positive impact on the carbon intensity of the entire extraction operation.