~~Observation of Productivity Loss in Large Oil Wells due to Scale Formationwithout Apparent Production of Formation BrineR.H. Hausler Brent W.A. Sherar and R.M. KrishnamurthyBlade Energy Partners Ltd.16285 Park Row Suite 600Houston TX 77084United StatesProductivity loss of up to 90% had been observed in large offshore oil wells in some cases within less than a year of operation. Extensive investigation into possible formation damage and/or pore plugging due to migration of formation fines (clay particles) mechanisms had been conducted. Scale formation was originally ruled out because no formation brine was observed being coproduced with the oil. Finally however it was demonstrated that shallow acidizing procedures could restore the original productivity. Modeling studies were subsequently initiated with the aim of resolving this possible discrepancy using the little known phenomenon that the water solubility in the oil is pressure dependent.Brine samples were available from an aquifer below the oil bearing strata. It was assumed that the composition of aquifer brine was likely the same as that of the residual brine saturation in the oil bearing formation. The scaling index of these brine samples was essentially 1 (saturation equilibrium) under downhole conditions. Equally the oil phase was in equilibrium with the accompanying brine phase in all respects including the water content. Modeling indicated that at a given temperature the water content in the oil was shown to be function of pressure – the higher the pressure the lower the water content in the oil. As the oil was being produced and a pressure gradient was established between the formation and the wellbore these equilibria became disturbed. Consequently as the pressure decreased the oil became richer in water at the expense of the brine. The brine in turn became more concentrated hence oversaturated in minerals whereupon scale deposition started.As the water rich oil (with about 1% water under reservoir conditions) is being produced the temperature decreases and the oil will shed water as it comes to the surface. Additionally due to the high gas/oil ratio (GOR) gas will break out of the oil below the bubble point pressure and tend to become saturated in water. If however due to the specific high GOR not enough water is present in the oil to completely saturate the gas phase no liquid water can remain in the production stream.These intertwined equilibrium conditions were quantitatively modeled and it was shown that indeed with the prevailing production rates it is entirely possible to plug the formation around to perforations to a relatively small depth but sufficiently so as to be responsible for the productivity impairment.Keywords: Scaling Calcium Carbonate Brine Composition Formation Damage Water content in Oil Pressure Effect Temperature Effect Scaling Index