Water-alternating-gas (WAG) is an enhanced oil recovery process which is being widely practiced in the oil fields to increase the extent of the reservoir contacted by the displacing fluids, the gas is generally injected intermittently with water. WAG process improves the recovery from the reservoir and hence increases the production performance; however, the process of alternating water with gas, if not properly studied and designed, would lead to several flow assurance issues including hydrate formation risk, unsuccessful fluid displacement, bottom hole pressure exceeding fracture pressure and vacuum/low pressure condition at the surface etc. All these factors can impact the success of the WAG operation.

Dynamic modelling of the water alternative gas changeover process is essential to address the potential challenges during changeover operation and also to formulate the operational guidelines. Each type of changeover (i.e. water to gas / gas to water) have unique challenges which is discussed in detail in this paper.

This paper presents the results of the modelling and simulations of WAG operation carried out on a horizontal well. Of particular interest is the the period of change over interval (i.e. gas to water and water to gas) where there are significant pressure, flow and temperature transients leading to possibility of various flow assurance related risks. These risks were evaluated in detail for the subject well and the WAG changeover operation was designed to avoid any potential problems using transient multiphase flow simulation.