Study On Improved Oil Recovery By Urea Assisted Steam Injection Technique In Heavy Oil Reservoirs

Biqian10block Ⅳ9layer in Henan oil field, which has been developed for21years, isin the final period of steam huff and puff and shows the characteristics of low pressure,serious steam channeling, low individual well producing rate, high water net injection percent,low period oil/steam ratio and low degree of reserve recovery. The difficulty of developmentis becoming greater. To find ways to make pressurization, decrease viscosity and expandswept area, urea was proposed which can decompose to form CO₂and NH₃, and the methodof steam flooding was used to achieve the connection between different wells. Doing in-depthresearch into the mechanism of the interaction between urea and heavy oil, as well asimproving urea assisted steam flooding（UASF） are thus very important in the development ofheavy oil reservoir.The mechanism of the interaction between urea and heavy oil is the base of themechanism study of urea assisted steam flooding. By using HPHT reactor, PVT device,HPHT falling ball viscometer, oil-water interfacial tension meter and displacement devicetogether with analyzing the rule of urea decomposition, CO₂-heavy solubility test andassessment, NH₃-heavy oil emulsification analysis, collaborative visbreaking effectassessment of CO₂and NH₃and urea assisted steam flooding analysis, the interaction rule ofurea and heavy oil and the mechanism of EOR were revealed.Based on the results of in lab test, a urea assisted steam flooding model was built byconventional thermal simulator, including conventional steam flooding model and interactionmodel of urea and heavy oil. Steam flooding model takes into consideration reservoir fluidphase property model, viscosity-temperature relationship model, rock-fluid and well borethermal loss model, relative permeability-temperature relationship model and so on.Interaction model of urea and heavy oil includes decomposition model and decomposed gasvisbreaking model. Decomposition model could realize the quick decomposition of urea atcritical temperature of150℃; decomposed gas visbreaking model calculated the dissolving and visbreaking effect of CO₂on heavy oil using phase modeling software. Moreover, itrepresented the emulsification ability with the decrease of NH₃-heavy oil interfacial tension.According to the property of the target reservoir, a geological concept model was built,the characteristic of urea assisted steam flooding was analyzed, and the mechanism of EORand the constitution of the recovery were revealed. The results showed that urea assistedsteam flooding achieved the goal of expanding steam swept area and further visbreaking, byusing CO₂pressurization and visbreaking, slight emulsification effect of NH₃and gas gravitysegregation effect. CO₂is the main contributing factor to oil recovery.Utilizing fine geological modeling software, the fine3D reservoir geological model ofthe block at east of Ⅳ9layer abnormal fault was built, with techniques of sub zoneclassification, property modeling, and fault modeling. It also performed producing historymatching. Using numerical reservoir simulation and production performance monitoring, thedistribution of the residual oil in target block was studied, the plane affecting factors ofresidual oil are the poor reservoir properties and the maldistribution of sand bodies wererevealed. Residual oil mainly concentrates at the unwept interwell areas; vertically, it isinfluenced by the ability difference of steam injection. In combination of the characteristics ofresidual oil distribution and field producing data together with the results of numericalmodeling, the thermochemical flooding pilot experimental well groups was optimized.Using the geological concept model of target block, the sensitive reservoir parameters ofthermochemical flooding were analyzed. And the thermochemical flooding developmentparameters based on the geological concept model of the pilot experimental well groups wasoptimized, and four projects were designed: steam flooding, urea assisted steam flooding,urea assisted steam flooding+foam profile control, urea assisted steam flooding and foamblockage and huff and puff inducing and revealed the EOR mechanisms of each project. Thefinal degree of reserve recovery is more than20%.