Paraffin Deposition From Crude Oils: Comparison of Laboratory Results With Field Data

Abstract

Summary In this study, we evaluate the characteristics of four reservoir fluids sampled from two offshore fields in the Gulf of Mexico as well as their propensities towards paraffins deposition due to temperature and/or pressure variations. State-of-the-art techniques, namely a cross polarized microscopy and a laser based solids detection system, have been used to measure the onsets of paraffin crystallization temperatures, also known as wax appearance temperatures (WAT) and/or cloud points. The results are presented, discussed and compared to reported field data. Explanations for the observed differences and/or similarities between laboratory and field WAT are proposed. Introduction In recent years, paraffin (wax) deposition has created diverse production problems in many of the world's oil-producing regions. Often the problem is more severe for offshore fields where solutions to rectify wax precipitation can be costly. Due to the relative difficulty and expense associated with operating waxy reservoirs, especially offshore, it has become necessary to understand the natures of petroleum waxes and the fundamental variables that affect their depositions during production and/or processing. Waxes are essentially mixtures of long-chain hydrocarbons (n-paraffins) with carbon chain lengths ranging from C15 to C75+.1–3 They are crystalline in nature and tend to crystallize/precipitate from crude oils at and below their cloud points. Crystallization is the process whereby an ordered solid structure is produced from a disordered phase, such as a melt or dilute solution (e.g., crude oil). It usually involves two distinct stages, namely nucleation and growth which may be considered separately.4,5 As the temperature of the liquid solution or melt is lowered to its WAT, the energy of molecular motion becomes increasingly hindered, and the randomly tangled molecules in the melt tend to move closer together and form clusters of adjacently aligned chains. The paraffin molecules continue to attach and detach to and from these ordered sites until the clusters reach a critical size and become stable. This process is termed nucleation and the clusters are called nuclei. These nuclei are only stable below the melting/dissolution temperature of the wax as they are disrupted by thermal motion above this temperature. Once the nuclei are formed and the temperature is kept at or below the WAT, additional molecules are laid down successively on the nucleation sites and become part of the growing lamellar structure. This mechanism is called the growth process. Nucleation can be either homogeneous, meaning that the sample is pure and the nucleation sites are time dependent or heterogeneous, which implies that all nucleation sites are activated instantaneously. The latter type is the most common in crude oils where impurities such as asphaltenes, formation fines, clay and corrosion products act as nucleating materials for wax crystals.2 Although wax appearance temperatures (WAT) and pour point temperatures are specific thermodynamic properties for waxy crude oils, their relative positions/boundaries within pressure-temperature (P-T) diagrams as well as the rate (and amount) of wax deposition and accumulation between them are dependent upon a number of factors which includeoil composition,temperature/cooling rate,pressure,paraffin concentration,molecular mass of paraffin molecules,occurrence of nucleating materials such as asphaltenes, formation fines, and corrosion products,water-oil ratio, andshear environment. Recognizing that the onset of wax deposition is sensitive primarily to the waxy crude composition, it is important to review the properties and characteristics of the four major oil fractions namely, the paraffins, the naphthenes, the aromatics, and the polars (i.e., resins and/or asphaltenes). Examples of each of these classes of chemicals, their structures, and their boiling and melting points have been reported elsewhere.6,7 Briefly, both n-paraffins and iso-paraffins are flexible hydrocarbon molecules and, hence, tend to cluster together and precipitate from crude oil as wax solids. Being branched molecules, however, the iso-paraffins tend to delay the formation of wax nuclei and usually form unstable wax solids. Aromatics, on the other hand, are known to be good solvents for paraffinic waxes. Naphthenes, also known as cyclo-paraffins, are stiff and bulky in nature; they tend to disturb and/or disrupt the wax nucleation and growth processes. Unlike in the case of asphaltenes precipitation, resins do not have a direct effect on wax deposition. Finally, the presence of impurities and/or other amorphous solids (such as asphaltenes) in the oil usually induce a wax nucleation process2,8,9 as they tend to lower the energy barrier for forming the critical wax nucleus.4,5 There have been reports,10,11 however, that the presence of asphaltene solids which are amorphous in nature help remediate paraffin deposition from crude oil, i.e., asphaltenes act as inhibitors. It appears that whether asphaltenes contribute to or prevent paraffin wax deposition depends on their relative concentrations, sizes and characters. Further investigations need to be undertaken to clarify this issue.

Cloud Points: Can We Measure or Model Them?

“…The measurement of cloud point depends on a number of factors including oil composition, thermal history, pressure, shear environment, measurement technique, and cooling rate (Hammami and Raines, 1999). An understanding of the key parameters affecting wax precipitation is critical to the interpretation of both laboratory results and model predictions.…”

Liquid-Solid Phase Equilibria of Paraffinic Systems by DSC Measurements

“…As it was mentioned above, there are many experimental works in literature [3,14,16,[20][21][22][23][28][29][30] reporting the use of DSC to study the paraffins crystallization process. In this section, it is described the DSC methodology to characterize the liquid-solid phase equilibria of paraffins in model systems and crude oil samples.…”

“…In order to develop solutions to the wax deposition problem is necessary to get a deep understanding of the crystallization phenomena in which the crude oil composition, particularly the content of high molecular weight paraffins and asphaltenes have a significant impact [29]. Comparison of experimental methods for measurement of wax precipitation in crude oils have been reported in literature [5,21,[24][25][26][27]29], where wax detection limits vary depending on the measurement technique, oil composition, thermal history, time of measurement and fluid properties related to crystal nucleation and growth [27]. In this section is presented a characterization of the wax precipitation phenomena in crude oils by using DSC measurements.…”

“…Some of these industrial sectors are: petroleum production, petroleum refining and products, chemical, energy and consumer products [1]. However, as it has been widely reported in literature [2][3][4][5][6], one of the most affected industrial sectors by the paraffin crystallization phenomena is the petroleum industry. Crude oils contain heavy paraffins that may form solid wax phases at low temperature in the pipelines and hydrocarbon production facilities.…”

“…black crude oils) [20]. Hence, for the study and measurement of paraffin crystallization process, Differential scanning calorimetry (DSC) is an experimental method widely used due to its simplicity, accuracy and fast response to monitor the phase transitions during cooling and heating that gives related thermodynamic quantities such as heat capacity and enthalpies of transition [3,14,16,[20][21][22][23]. DSC has been usually used for the determination of wax appearance and/or dissolution temperatures (WAT or WDT) in petroleum products [3,20].…”