Abstract
The study concerns an onshore oil field in Southern Europe where wells are facing productivity decline. Both asphaltene and inorganic scale risks have been predicted at project phase. Since the Water Cut is close to 1%, no scale deposition was anticipated and the suspected cause for productivity reduction was either linked to the poor performance of Electrical Submersible Pumps (ESPs) and/or the poor efficiency of the asphaltene inhibitor.
The objective of the study is to investigate the cause of the productivity reduction. Deposit analysis collected on ESPs showed that it is composed of 95% of aragonite and 5% of organic material. Two hypotheses have been proposed: (1) Erosion of the carbonate reservoir, resulting in deposition on the ESPs or (2) Precipitation of Aragonite on the ESP due to thermodynamic variations and turbulent flow regime. Since aragonite is a metastable calcium carbonate mineral, not present in the reservoir, hypothesis 2 was selected. In the context of a very low water cut (around 1%), the challenge was to develop a novel chemical product able to prevent scale deposition. Two products have been tested in the laboratory and on site: a scale treater (STA) and a multifunction product (MFB), to prevent both asphaltene and scale. The scale treater was designed to modify the adhesion of scale with the formation of a protective film on the metallic surface. The second chemical (MFB) has been developed by combining the STA with an asphaltene inhibitor. This multifunction product has allowed to prevent incompatibility issues between chemicals and helped in the reduction of OPEX. Laboratory and field tests with STA and MFB have shown good results; both products were able to prevent scale deposition on the ESPs and improve productivity. STA and MFB products helped to improve productivity where a scale inhibitor could not be used because of the very low basic sediment and water (BSW). As soon as the BSW reaches 5%, the mitigation strategy could be to switch to two separate products: scale and asphaltene inhibitors; unless further tests show that MFB performs also for higher BSW.
