Disposal of produced water from oil fields is a major concern to oil companies for environmental and economic reasons. One way to dispose off this water is to mix it with injection water. A carbonate reservoir in Saudi Arabia produces wet oil where the salinity of the produced water is high (TDS up to 230,000 mg/L). The produced water contained dissolved gasses (H2S and CO2), and suspended solids (oil, corrosion products). GOSP disposal water has CaCO3 scaling potential. It contains 750 mg/L of H2S. The aquifer water contains 2 mg/L of iron. Iron sulfide scale forms once the disposal water is mixed with the aquifer water. 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 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 is that the cores were examined after the injection of the mixed waters by CT and SEM techniques.
Injection of mixed water into reservoir cores created wormholes which increased core permeability. This new finding indicates that disposal water is not always damaging. The effect of iron sulfide particles was found to be a function of the initial core permeability. Iron sulfide particles (0.25 micron) caused damage to cores with permeability < 20 mD after injecting 1,000 PV of the mixed water. No damage was noted in high permeable cores (> 100 mD) even after the injection of 600 PV of the same water.
The results of the study identifies various types of scale related to mixing GOSP disposal water and aquifer water, and determine conditions where these waters can be injected. Also, the study highlights an unexpected benefit from injecting H2S containing waters into carbonate reservoirs.
As part of expansion and development program of a field in eastern Saudi Arabia to start production, various water injection scenarios were evaluated. For the first five years of injection, the main source of injection water will be the aquifer water, disposal water, and proposed aquifer-disposal co-mingled waters. However, if aquifer water and the disposal waters can be commingled, assuming no adverse effects on the reservoir performance, a joint pipeline system can be used instead of a segregated system. This can have significant cost avoidance.
The formation waters have high TDS (> 200,000 mg/L) and very high H2S content (> 750 mg/L). Aquifer waters contain dissolved iron up to 4–5 mg/L1. Mixing these two waters can precipitate iron sulfide, Equation 1:
Iron sulfide can cause formation plugging and injectivity decline2. Filtration of the mixed waters is extremely difficult, particularly in an oily system. Therefore, compatibility studies for these waters are very important.
The main objective of this study is to assess the feasibility of mixing aquifer water with disposal waters.
Representative formation water from the field is not available as the field has been mothballed. The chemical compositions of the formation brine from an observation well that was mothballed (Q-47), disposal water from a nearby field-A and aquifer water (Q-859) are given in Table 1. The iron content of aquifer water was 2 mg/L. Q-47 formation water is also not representative of the field. Disposal water from the field-A does not contain significant H2S (Table 1). Since the Field-Q contains about 15 mol% of H2S in the gas phase, (nearby Field-B has about 8 mol% H2S, and measured H2S levels are about 750 mg/L), H2S saturated disposal water was used for evaluating the chemicals and coreflood tests. The sulfide content of this water was measured using Zn-acetate method, and was found to be in the range of 1,200 - 1,300 mg/L.
The OKSCALE program was used to evaluate the scaling potential and saturation indices, and the data are presented in Table 1. Predicted scaling potentials for individual waters and their compatibilities is discussed below.