Thermodynamic modelling of calcium naphthenate formation: model predictions and experimental results

Certain naphthenic acids occur naturally in crude oils and these can, under certain circumstances dissociate and migrate to the oil/water interface where they may react with Na⁺ or Ca²⁺ ion from the formation water to form either sodium naphthenate (NaN) emulsions or solid calcium naphthenate (CaN) deposits. Formation of these metal naphthenates can lead to serious problems in crude oil production. The thermodynamic prediction of CaN deposition is very important in order to anticipate when such problems may arise.In this paper, we present a description of the theory of how calcium naphthenate is formed and a thermodynamic naphthenate prediction model to describe this process. The model is compared directly for the first time with a range of CaN deposition experiments which were specifically designed to test the predictions of the model. Results are presented from calcium naphthenate re-precipitation experiments using naphthenic acid extract from a CaN field deposit. These experiments used extracted naphthenic acid in toluene along with synthetic brine which was pH adjusted. The first set of experiments tested the model predictions of several " successive extractions" of the CaN from an original; naphthenic acid solution in toluene. Experiments were then performed for a range of naphthenic acid concentrations at variable water/oil volume ratios (V_w/V_o) in two different types of experiment; viz. (a) where (V_w/V_o) is varied with V_o fixed and (b) where (V_w/V_o) is varied but with V_w+V_o=constant.The thermodynamic naphthenate model predicted that (i) in a (V_w/V_o)=1 deposition experiment, only a relatively small % of the naphthenic acid appears in the deposit, and hence successive extractions showed similar amounts of deposit (until total depletion of the acid occurs); (ii) that as a deposit experiment is performed at increasing (V_w/V_o)=0.25, 0.50, 0.75, 1, 2, 5 and 10 or % watercut, then a progressive increase in the mass of calcium naphthenate was observed up to around 65% watercut and then this amount started decreasing. The experimental results confirmed these predictions and there is excellent qualitative agreement between the experimental and modelling results. © 2010 Elsevier B.V.