Summary This paper describes successful practices applied during polymer flooding at Daqing that will be of considerable value to future chemical floods, both in China and elsewhere. On the basis of laboratory findings, new concepts have been developed that expand conventional ideas concerning favorable conditions for mobility improvement by polymer flooding. Particular advances integrate reservoir-engineering approaches and technology that is basic for successful application of polymer flooding. These include the following:Proper consideration must be given to the permeability contrast among the oil zones and to interwell continuity, involving the optimum combination of oil strata during flooding and well-pattern design, respectively;Higher polymer molecular weights, a broader range of polymer molecular weights, and higher polymer concentrations are desirable in the injected slugs;The entire polymer-flooding process should be characterized in five stages--with its dynamic behavior distinguished by water-cut changes; -Additional techniques should be considered, such as dynamic monitoring using well logging, well testing, and tracers; effective techniques are also needed for surface mixing, injection facilities, oil production, and produced-water treatment; andContinuous innovation must be a priority during polymer flooding. Introduction China's Daqing oil field entered its ultrahigh-water-cut period after 30 years of exploitation. Just before large-scale polymer-flooding application, the average water-cut was more than 90%. The Daqing oil-field is a large river-delta/lacustrine facies, multilayered with complex geologic conditions and heterogeneous sandstone in an inland basin. After 30 years of waterflooding, many channels and high-permeability streaks were identified in this oil field (Wang and Qian 2002). Laboratory research began in the 1960s, investigating the potential of enhanced-oil-recovery (EOR) processes in the Daqing oil field. After a single-injector polymer flood with a small well spacing of 75 m in 1972, polymer flooding was set on pilot test. During the late 1980s, a pilot project in central Daqing was expanded to a multiwell pattern with larger well spacing. Favorable results from these tests--along with extensive research and engineering from the mid-1980s through the 1990s--confirmed that polymer flooding was the preferred method to improve areal- and vertical-sweep efficiency at Daqing and to provide mobility control (Wang et al. 2002, Wang and Liu 2004). Consequently, the world's largest polymer flood was implemented at Daqing, beginning in 1996. By 2007, 22.3% of total production from the Daqing oil field was attributed to polymer flooding. Polymer flooding boosted the ultimate recovery for the field to more than 50% of original oil in place (OOIP)--10 to 12% OOIP more than from waterflooding. At the end of 2007, oil production from polymer flooding at the Daqing oil field was more than 10 million tons (73 million bbl) per year (sustained for 6 years). The focus of this paper is on polymer flooding, in which sweep efficiency is improved by reducing the water/oil mobility ratio in the reservoir. This paper is not concerned with the use of chemical gel treatments, which attempt to block water flow through fractures and high-permeability strata. Applications of chemical gel treatments in China have been covered elsewhere (Liu et al. 2006).

This paper describes successful experiences employed during polymer flooding at Daqing that will be of considerable value to future chemical floods, both in China and elsewhere. Based on laboratory findings, new thoughts have been developed that expand conventional ideas concerning favorable conditions for mobility improvement by polymer flooding. Particular advances integrate reservoir engineering approaches and technology which is elementary for successful application of polymer flooding. These include: (1) Considering permeability differential among the oil zones and interwell continuity, optimizing the oil strata combination and well pattern design. (2) The injection procedures and injection formula are the key points when designing a polymer flood project. These points include: profile modification is needed before polymer injection and zone isolation is of value during polymer injection, higher molecular weight of the polymer used in the injected slugs, large polymer bank size, higher polymer concentrations and injection rate based on the well spacing and injection pressure.(3) Characterizing the entire polymer flooding process in five stages, with its dynamic behavior distinguished by the water cut changes. Additional techniques involved with reservoir engineering should also be considered, such as dynamic monitoring using well logging, well testing, and tracers. Effective techniques are also needed for surface mixing, injection facilities, oil production, and produced water treatment.Continuous innovation and effective response to new challenges must be a priority during polymer flooding. New directions and opportunities at Daqing will (1) explore the feasibility of polymer flood application in poorer ("third-class") strata, (2) to identify new polymers to suit portions of the reservoir with higher temperatures and higher water salinities, and (3) continually see improvements in our approach to polymer flooding.

Using the principles of seepage mechanics, this paper analyzes the interrelationship between formation pressure and injection-production ratio and the influential factors. On the basis of a mechanical study, it quantitatively analyzes the pressure history of each area by means of a mathematical statistic method and the controlling factors, and the effect of every factor on formation pressure. It furnishes a scientific basis for the effective and rational control of the Southern development area of the Saertu Reservoir and each sub-area. Introduction Injection-production ratio is a composite index, which reflects the balance condition of injection and production, characterizing the relation between fluid-withdrawal rate, water injection rate and formation pressure during waterflooding in an oilfield. It is also an important index to program and design the water injection rate. Reasonable injection production ratio can maintain formation pressure, so that the oilfield can have vigorous ability to produce fluid and oil, reduce ineffective energy consumption and improve oil recovery. Therefore, injection-production ratio should be regulated in accordance with the geological features of the oilfield and the development conditions, such as the active control of the formation pressure level, which is important in the optimization of the whole injection-production system, and is of great significance to the scientific development of the oilfield. The development principle of waterflooding at the early stage is to maintain formation pressure. However, with the development of deep reservoirs and taking various measures to tap oil potentials, especially successive development of different types of reservoirs, injection-production ratio of the oilfield has greatly changed. The previous procedure of keeping the injection-production ratio around 1.0 has been unable to maintain the desired formation pressure. The control of the injection-production ratio of the oilfield, and the relation between the injection-production ratio and formation pressure has problems to be resolved during further production in southern development area of Saertu Reservoir. Practical performance knowledge of the field, and history matching by means of a mathematical statistical method have shown the quantitative relations between the influential factors of the Southern development area of Saertu Reservoir and each sub-area. Establishment Of Mathematical Model Between The Formation Pressure And Injection-Production Ratio Of the Oil Wells The formation pressure usually used in the material balance equation is the average formation pressure of the whole reservoir system. It is different from the oil well formation pressure usually used in the development performance analysis.