Author’s Accepted Manuscript A review of oilfield mineral scale deposits management technology for oil and gas production Abass A. Olajire www.elsevier.com/locate/petrol PII: S0920-4105(15)30106-6 DOI: http://dx.doi.org/10.1016/j.petrol.2015.09.011 Reference: PETROL3173 To appear in: Journal of Petroleum Science and Engineering Received date: 4 June 2015 Revised date: 1 August 2015 Accepted date: 9 September 2015 Cite this article as: Abass A. Olajire, A review of oilfield mineral scale deposits management technology for oil and gas production, Journal of Petroleum Science and Engineering, http://dx.doi.org/10.1016/j.petrol.2015.09.011 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. A review of oilfield mineral scale deposits management technology for oil and gas production Abass A. Olajire Industrial and Environmental Chemistry Unit, Department of Pure and Applied Chemistry, Ladoke Akintola University of Technology, P. M. B 4000, Ogbomoso, Oyo State, Nigeria Abstract The presence of formation water and the treatment methods (both water flooding and chemical treatments) employed during exploration and production operations have great potential for mineral scale formation. Scale deposition poses a lot of serious threat in field production and it is a menace to production flow assurance, which in turn reduces the production flow resulting in production losses. Although oilfield scale deposit is a long standing problem, oil and gas industry are facing new challenges in managing scale deposits created during offshore exploration activities in ultra-deepwater and other harsh environments. An ideal management program maximizes hydrocarbon production and minimizes the cost of scale deposits control, thereby maintaining the economic viability of the operations. This paper reviews various types of mineral scale deposits as well as the thermodynamics and kinetics prediction of mineral scale formation potentials. Also, the mitigation strategies of oilfield mineral scale deposits and chemical stimulation techniques used in oil industry to improve well productivity are discussed. Keywords: Scale deposits, inhibitors, thermodynamics, kinetics, management, prediction, Correspondence author: [email protected] (Prof. Olajire A.A) 1. Introduction Mineral scale formation in surface and subsurface oil and gas production equipment has been recognized to be a major operational problem and cause of formation damage either in injection or producing wells. Scale contributes to equipment wear and corrosion and flow restriction, thus resulting in a decrease in oil and gas production. Scale deposition results from the chemical treatment operations which are applied to maintain sustainable hydrocarbon production at oil, gas or gas condensate fields. Hydrocarbons coming out of a reservoir consist of hundreds of different components (N2, CO2, H2S, C1-C6, C7+, etc.) and may be present in a liquid (C5+) or gaseous form (C1-nC5). In addition water is usually associated to hydrocarbons coming out of reservoir, bringing within itself to the surface various dissolved compounds. The resulting composition of producing fluid will experience drop in pressure or a change in temperature, and some compounds may become more stable at solid state and will start to precipitate (Time, 2011). The precipitation of these solids occurs as a result of changes in the ionic composition, pH, pressure and temperature of the brine. When water flooding method is applied for enhance oil recovery, then the problem of scale may occur right from water injection facilities to the producing well, and generally scale can occur during and after injection operation in the injector wellbore, near the injection well bottom hole, in the reservoir between the injector and producer, at the skin of producer well, in the producer wellbore, oil well casing, oil pipelines and other production facilities (Liu et al. 2012; Dickson et al. 2011). The composition of scaling samples is basically made up of organic, inorganic and crystal water (Jiecheng et al. 2011). A scale may occur as single mineral phases, 2 but more often as a combination of different elements, which can occur when a solution becomes saturated, mostly due to changes in temperature during injection operations, changes in pH values or if two different chemicals that will precipitate are brought together. Scale deposition products in oilfield are mainly consisted of calcium carbonate, calcium sulfate, barium sulfate and strontium sulphate and carbonates, iron, silicon sediment and other insoluble solids (Jinling et al. 2009; Senthilmurugan et al. 2011; Dickinson et al. 2012). The effect of scale can be dramatic and immediate, the production can fall to zero in a few hours and the treatment cost can be massive (Al Salami and Monem, 2010). Scale in the formation pores restricts fluid flow through the formation or reduces the diameter of the production tubing by the formation of a thick layer in the wellbore tubular, which consequently chokes the production from the reservoir (Fig. 1). This can lead to a drastic increase in pressure drop and thus a decrease in the production. Scale precipitation could also cause various damages including blockage of pipeline and equipment, energy leak, accelerate corrosion, and severe accidents, which can influence the safety of production and the economic benefit of petroleum industry (El-Said et al. 2009) and thus should be avoided completely in the oilfield industry. This paper reviewed and assessed some of the management and mitigation strategies of oilfield scale for oil and gas production. Also discussed are thermodynamics and kinetics models of scale prediction. 2. Types of oilfield scales Common oilfield scales can be classified into “pH independent” and “pH sensitive” scales. The scaling tendency of sulphates (calcium sulphate, barite and celestite) and halite scales are not a strong function of brine pH. The carbonates 3 (calcite, dolomite and siderite) and sulphide scales are acid soluble and their scaling tendencies are strongly influenced by the brine pH. The most common oilfield scales are listed in Table 1 (Kelland, 2009; Bin Merdah, 2008). These scales include sulfates such as calcium sulphate (anhydrite, gypsum), barium sulphate (barite), and strontium sulphate (celestite) and calcium carbonate. Other less common scales which have also been reported include iron oxides, iron sulfides and iron carbonate and calcium naphthenate scale from acidic crudes. 2.1 pH independent scale These are sulphate compounds of barium, strontium, or calcium. The sulphate 2 2 2 2 ion (SO4 ) normally found in the seawater reacts with Ba , Sr , and /or Ca ions, which are naturally found in the formation water depending on the geological history of the oilfields. 2 2 2 2 Ba (Sr or Ca ) SO4 BaSO4 (SrSO4 or CaSO4 ) (1) These compounds are also mildly soluble in water and as a result they subsequently precipitate out and form solid solutions. 2.2 pH dependent scale The carbonates (calcite, dolomite and siderite) and sulphide scales are acid soluble. Examples of formation of this class of scales include: Ca 2 (Fe 2 ) CO 2 CaCO (FeCO ) (2a) 3 3 3 Fe 2 (Zn2 or Pb 2 ) S 2 FeS (ZnS or PbS) (2b) 2.3 Metal naphthenate scale Naphthenic acids are mixtures of alkyl-substituted acyclic and cyclic structures with the general chemical formula Cn H2mzO2 , where m indicates the carbon number and z is zero, in the case of fatty acids, or negative, depending on the number of 4 condensed and/or aromatic rings. Naphthenic acids refer to all the organic acids present in crude oil (Brientet al. 1995; Seifert, 1975). Figure 2 displays some examples of possible monoprotic naphthenic acid structures for different z values. The naphthenic acids which are generated from in-reservoir biodegradation of petroleum hydrocarbons (Jaffe and Gallardo, 1993; Behar and Albrecht 1984; Meredith et al. 2000) may react with metal cations in the water to form naphthenates which agglomerate in the oil phase. Metal naphthenate deposition may lead to costly shutdowns and causing serious problems to the oil industry (Simon et al. 2008; Dyer et al. 2003). The reaction of naphthenic acids and divalent cations at the o/w interface involves two reaction steps where the acid monomers sequentially bind to one cation, according to the following reaction scheme (Scheme 1). k1 k2 2 M 2RCOO [RCOOM ] RCOO (RCOO)2 M k1 Scheme 1: Reaction between naphthenic acids (RCOO–) and divalent cation (M2+), where {k1, k2} and {k–1} represent the reaction rate constants of the forward and the reverse steps, respectively. 3. Mechanisms of scale formation Scale formation can occur through homogeneous and heterogeneous nucleation. Homogeneously formed scale particles do not necessarily deposit or grow onto a surface and hence, could flow through the system without causing too many depositional issues. However, the scale that forms by heterogeneous nucleation builds up on solid surfaces causing problems such as pressure increases and restriction of fluid flow in the formation, pipelines, surface facilities and can potentially prevent production