In order to raise 80% quality steam at 15 MPa from the produced water at Cold Lake, the water muse be deoiled, desilicized and softened. In this paper we outline our efforts to deoil the produced water by lowering the pH to around 4.5. A mini pilot running on 4L/min produced water was constructed to measure the effects of acidification on the reduction of suspended oil, dissolved organic matter and hydrogen sulfide as indicated in our previous laboratory experiments. It is shown that acidification is a viable and practicable alternative to using a reverse demulsifying agent. The implications of the process are outlined.
It is expected that up to 2500 m3/h of produced water will be treated when the Cold Lake commercial plant comes on line in 1986. This treatment is necessary for reuse because a) of the need to conserve fresh water, and the difficulty of disposing of very large volumes of produced water, b) the heat value of the produced fluids can be recovered for use as feed-water to steam generators.
A produced water clean-up program suitable for reuse in high pressure steam generation is necessarily comprised of deoiling, desilicizing and softening stages. Since the former operation must of necessity precede the latter, effective means of demulsification become of prime importance.
Demulsification of petroleum from aqueous media by pH adjustment dates back at least forty years. One of the early patents (1) describes the process as being successful at aqueous phase pH range of 4.8 to 5.5 "by adding a suitable reagent." Although demulsification is usually attained through the use of surfactants, heat, and electric fields, the use of acids and bases can be cheaper especially when large surfactant concentrations are needed to break an emulsion or when the emulsion is "tighter" than normally encountered.
Although a demulsifier chemical was found to work satisfactorily on Cold Lake produced water by destabilizing the oil/water emulsion, it was decided to look for an alternative process for two main reasons:
as a back-up technology especially if the amount of dissolved organic matter becomes critical in raising high pressure steam
it would assist in reducing hydrogen sulphide concentration in the water.
It must always be borne in mind that. aside from suspended oil, the produced water contains dissolved oil and organic matter which may cause deposition problems in the generator if not reduced to sufficiently low levels.
Generally speaking, acidification can be conceived as increasing the ionic strength and compressing the electrical double layer around the oil droplets thus allowing coagulation and facilitating coalescence. It also displaces the ionization equilibrium of carboxylic acids and phenols towards their molecular forms which are less hydrophilic than their ionized states. Acidic species of high molecular weight thus lose their tendency to accumulate at the interface where they strongly stabilize the emulsion.
Heavy crudes and bitumens are more susceptible to these effects than paraffinic types for three main reasons: