Abstract: A method can include providing a system of equations with associated variables that describe physical phenomena associated with a geologic formation; decoupling the system of equations to provide a system of pressure equations with associated pressure variables; solving the system of pressure equations for values of the pressure variables; and, based at least in part on the values of the pressure variables, solving the system of equations for values of the associated variables where the solving the system of equations includes applying a block approximate inverse preconditioner technique to at least blocks of mass conservation terms of the system of equations. Various other apparatuses, systems, methods, etc., are also disclosed.