Factors Affecting Injectivity Decline

Abstract

SPE Member Abstract Reservoir pressure support for a large portion of a giant field is maintained by peripheral seawater injection. The temperature of the injected sea water is significantly lower than the reservoir temperature and also lower than the previously injected water obtained from aquifers. Therefore, a decline in injectivity is expected as a result of higher water viscosity due to lower injection water temperature. This paper presents actual data collected from power water injection wells in two areas of the field, both before and after the start of seawater injection. This data has shown a significant decline in injectivity after conversion to seawater in only one area. The quality of the injected water and other related factors will be reviewed to determine the reasons for this behavior that apparently contradicts what was expected. The paper also discusses the actual effects that cold seawater injection has on pressure falloff behavior. Introduction In a water injection pressure support program, water temperature plays an important role in the efficiency of the injection process. This is because the viscosity of the injected water is directly related to its temperature. The higher the viscosity, the higher the pressure differential required to achieve the same injection rate. As a result, a decline in injectivity is expected upon conversion to a colder water for the same pressure differential. Water injection in the field was first started using warm temperature (aquifer) water. After several years, injection of colder temperature water (seawater) began. In this paper, data collected before and after seawater injection are reviewed to show the actual effect of cold water on injectivity. Only one of the two areas has shown a clear trend of decline in injectivity. Several other factors such as the quality of the injected water, the static bottom hole reservoir pressures, and temperatures will be reviewed in an attempt to explain the observed behavior of the injectivity. HISTORY OF WATER INJECTION This study will be focused on two of the seven areas that constitute the giant field, Area-i and Area-2 This field was discovered in late 1 940s. Oil production from this field commenced in 1951 while water injection was started in 1966 in Area-i using Aquifer-A water and in 1976 in Area-2 using Aquifer-B water. Later, the injection system was converted to seawater to conserve aquifer water. Area-i was converted in 1978, while Area-2 was converted in 1992. P. 593