Methods of treatment of fluids produced by an oil or gas well following a stimulation operation, allowing separation from the fluids and re-injection of oil and gas hydrocarbons in a production pipeline under pressure, and allowing achievement of suitable quality for the residual fluids compatible with their rejection, for example into the sea, including the following three elements; neutralization of the fluids by mixing with a high pH chemical, until the resulting pH reaches a level compatible with the equipment and pipes; use of optimized emulsion breakers in a phase separator, selected for best results with the fluids produced by the well, to accelerate the separation of oil from the fluids, and to lower the residual oil content in the fluids to levels compatible with environmental regulations; and use of a multi-phase pump to pump the oil and gas hydrocarbons produced and reinject them in a pipeline under pressure.