Abstract
In northwest Germany, oil reservoirs are characterized by high-salinity brines with up to 23% TDS. For such salinity conditions, fatty alcohol derivatives with 4.5 ethene oxide (EO) units were found to lower the interfacial tension (IFT) drastically and to mobilize residual oil almost completely. Intensive flood experiments under reservoir conditions with the use of sand packs 2 m in length allowed optimizing the low-tension process for an oil field that was considered a possible candidate. A combination of surfactant slug followed by a tailored mobility buffer showed best results in terms of additional oil recovery and process duration. A preflush of low-concentration aqueous polymer solution brought a decisive further increase in additional oil recovery. Results obtained for the slug process indicated that variables such as IFT, surfactant concentration, flooding velocity, and pressure gradient influence the low-tension process in a combined manner. Oil produced in the oil bank showed alteration in properties, compared with the oil used to saturate the pore space.
Introduction
This paper summarizes the concept, development, and results of a low-tension flood process for the high-salinity reservoirs in northwestern Germany with the use of surfactants. The objectives were:to design an appropriate surfactant flooding process for mobilizing residual oil in reservoirs in northwest Germany such that a pronounced lowering of IFT between the oil and aqueous phase is achieved,to conduct investigations on the main parameters influencing the process, andto perform practically oriented laboratory flooding experiments for optimizing the process for real oil reservoirs.
Most oil reservoirs in the Federal Republic of Germany (FRG) are characterized by extremely unfavorable conditions of salinity. Besides the high sodium chloride content, the reservoir brines have remarkably high concentrations of calcium and magnesium salts. Typical values are 50 to 250 g/dm3 NaCl and 4 to 20 g/dm3 Ca++. No surfactants suitable for such saline environments were known before 1975.
Reserve Situation in the FRG
Because it is not likely that new oil fields will be discovered in the FRG, the domestic oil industry is striving to develop new technologies for enhancing oil recovery after termination of primary and secondary production phases. For highly saline conditions such as those prevailing in northwestern German oil fields, the development of effective surfactants was necessary. In addition to being soluble in original reservoir water, these surfactants must lower the IFT drastically and must completely mobilize the residual oil remaining in the porous medium after previous water flooding. A modification of original reservoir brines by eventual conditioning or by preflushing in-situ formation water was not intended. Such measures had proved effective in the laboratory but not in the field. Oil production statistics from 1978 show that about 22% OOIP in the FRG already has been produced. According to these statistics, it is expected that a further 10% OOIP will be produced with conventional recovery processes (e.g., waterflooding). Of the OIP remaining (target for tertiary recovery, about 68 % OOIP), approximately 15% should be recoverable by EOR processes currently known or to be developed. Thus, the total recovery would be increased to 47%. The remaining 54% will not be recoverable, according to current estimates.
SPEJ
P. 831^
Optimisation of Smart Water to Enhance Oil Recovery Efficiency in a Part of Oil Field of Upper Assam Basin, India
