Produced-Water Reinjection in Deep Offshore Miocene Reservoirs, Block 17, Angola

Abstract

In this paper we examine the changes in the injectivity of wells reinjecting produced water in two fields from Block 17, offshore Angola. This analysis suggests that the water quality has a direct impact on well injectivity during matrix injection. Well impairment caused by desulfated seawater treated with membrane technology appears immaterial in comparison with the injectivity declines observed during produced-water reinjection (PWRI). The decline rate is much quicker for the field with the worst quality of treated produced water. Injectivity enhancements observed during seawater-injection tests demonstrate that the matrix decline is partially reversible. However, permanent damage also develops with time; it is not possible to recover the initial injectivity after a long period of injection with produced water. The analysis also shows that fracture injection can effectively mitigate the strong injectivity declines experienced in the field with the worst quality of treated produced water. Fracture injection comes with higher injection pressures, even when operating the wells at low flow rates. As a result, the injectivity index (II), as conventionally defined, displays a strong flow-rate dependency, making it inappropriate for measuring the well performance in fracture condition. Besides the limitations of the high injection pressures, fracture injection was found detrimental to injection conformance in wells with commingled water injection in several reservoir layers. In this situation, most of the injection is thought to take place in the shallowest layer where the fracture is likely to grow, leaving little injection for deeper reservoir layers.