In this work, the interaction between two immiscible fluids with a finite mobility is investigated numerically in a Hele-Shaw cell, simulating conditions found during the injection and lateral spreading of supercritical CO 2 in a deep subsurface aquifer.A two-phase numerical method is presented that uses a direct boundary element approach to compute the normal velocity at the interface between two fluids in a 2-D Hele-Shaw cell, through the evaluation of a hypersingular integral. The resulting second kind Fredholm equation is solved numerically using a truncated convergent Neumann series.Utilising cubic B-Spline surface geometry and function interpolation, the numerical scheme exhibits 6th order spatial convergence and a computational cost that scales with O(N 2 ). This allows the long term non-linear dynamics of a growing CO 2 -brine interface to be explored accurately and efficiently, revealing large differences with previous single-phase models and interface capturing techniques.