Formation Damage Characterization of Horizontal Well in Extra-Heavy Oil Reservoir and Methods of Remedy

Primary production using horizontal wells has been successfully applied in extra-heavy oil reservoirs, heavy oil belt, Orinoco, Venezuela. During drilling period, formation damage due to drilling fluid invasion lowers the effective permeability around the well, leads to higher pressure drop and has detrimental impact on well productivity in such pressure depleted reservoirs. In this study, laboratory experiments of drilling fluid invasion are conducted to identify damage mechanism. The oil sample used in the test is taken from Block J, Orinoco Belt and the formation temperature is restored in the lab to mimic the process of drilling fluid mixing with crude oil in horizontal drilling. The stable emulsion is formed by mixing the crude oil with drilling fluid in different proportions. Furthermore, damage level is quantified by well test interpretation. Skin factor, wellbore storage and other formation parameters are estimated by deconvolution method based on three build-ups. Finally, acid washing and well bottomhole electric heating, two methods of remedy are applied in oilfield. Experiment results show that low productivity of the horizontal wells is due to the presence of highly viscous emulsion system stabilized by large amount of calcium carbonate in drilling fluid. The emulsion system restricts reservoir fluid flowing from the formation to the slotted liner. In Block J, constrained by the operational problems, the effective shut-in time is commonly short. Conventional well test method can hardly give the reliable results. Compared with the conventional method, deconvolution method can solve the data limitation, minimize the initial distortion caused by wellbore storage and get more reliable results. The production can be improved temporarily after the acid washing while declines rapidly. Electric heating can obviously improve the oil mobility near wellbore and maintain stable production. This paper combines both lab studies and well test interpretation to characterize the formation damage, provides guidance for the remedial operations to improve well productivity.