Injection of Aquifer Water and Gas/Oil Separation Plant Disposal Water Into Tight Carbonate Reservoirs

Abstract

Summary Disposal of produced water from oil fields is a major concern for environmental and economic reasons. One way to dispose of this water is to mix it with aquifer water and inject it back into the formation. One of the carbonate reservoirs in Saudi Arabia produces wet oil where the total dissolved solids (TDS) of the produced water is high (up to 238,000 mg/L). The produced water also contains dissolved gasses (H2S and CO2) and suspended material (oil and corrosion products). Thus, disposal water from a gas/oil separation plant (GOSP) not only has CaCO3 scaling potential, but can also precipitate sulfides because it contains nearly 750 mg/L of H2S. In contrast, the aquifer waters from the area contain approximately 2 mg/L of total iron. Therefore, if these two waters were to be mixed, the possibility of damaging the formation from iron sulfide precipitation could be signficant. The objective of this study is to assess potential formation damage that can result when the two waters are mixed and injected into the tight carbonate reservoir. The current study included a detailed analysis of field waters, determination of scaling potential of various waters, and extensive coreflood testing using reservoir cores. A unique feature of this study was that the cores were examined after the injection of the mixed waters by computerized tomography (CT) and scanning electron microscopy (SEM) to determine both macro and porelevel effects on core properties. The study revealed that the injection of mixed water into reservoir cores created wormholes, which increased core permeability. This new finding indicates that disposal water is not always damaging. On the other hand, the precipitated iron sulfide particles seem to damage the cores, but the damage was constrained by the core initial permeability. For example, the iron sulfide particles (0.25 μm) caused damage to cores with permeability of less than 20 md after injecting 1,000 pore volumes (PV) of the mixed waters. However, no damage was observed in cores with permeability greater than 60 md, even after injecting 600 PV of the same mixed water. The study identified various types of scale resulting from mixing produced and aquifer waters, and determined conditions under which these waters could be injected. Also, the study highlights an unexpected benefit of injecting H2S-containing waters, which is to create wormholes and thus significantly increase permeability of tight carbonate reservoirs.