Chemical Effects on Crude Oil Pipeline Pressure Problems

Abstract

Summary In the surface transportation of crude oils, high flow line pressures are encountered for a number of reasons. Basically these are a function of the rheological and depositional properties of the crude oil under the temperature-profile and shear-rate conditions developed in the system. These problems can be categorized into these areas: paraffin deposition, asphaltene deposition, thixotropic crude oil, turbulent flow transmission, and low-gravity asphaltic-based crude oils. Various laboratory and field tests are used to identify the key features of these problem crudes for identification and chemical treatment purposes. Introduction Pressure problems during the production of crude oil result in considerable trouble and expense to the producer. Ten years' investigation has resulted in substantial progress in identification and chemical treatment of these problem areas. Proper identification of the mechanism causing the pressure increase has resulted in the elimination of many mechanical and chemical misapplications. The net result has been a reduction in costs to operate these problem systems. Each problem area has key features differentiating it from the other causes of high pressures. The tests used to identify these key factors include cloud point, pour point, viscosity, yield value, solubility parameters, pumping studies, and various deposition and removal tests. Comparative testing of chemical additives under simulated field conditions yields valuable data on the relative treating efficiency of various chemical structures. Subsequent field trials clearly demonstrated the value of preliminary laboratory and onsite testing prior to chemical applications. Paraffin Deposition Paraffin deposits consist of a mixture of linear and branched chained hydrocarbons in the range Of C 18 H 38 to C60 H122, generally mixed with other organic and inorganic materials such as crude oil, gums, resins, asphaltic material, salt, sand, and water. The accumulation of paraffin waxes on pipewalls leads to constriction, which effectively reduces the useful diameter of the line. This causes an increase in pumping pressure and/or a decrease in volume throughput. The solubility of paraffin in crude oil depends on the chemical composition of the crude, temperature, and pressure. Paraffin precipitates from the crude oil at an equilibrium temperature and pressure defined as the cloud point. Paraffin deposition takes place by three mechanisms that transport both dissolved and precipitated waxy crystals laterally. When the oil is cooled, a concentration gradient leads to the transport, precipitation, and deposition of wax at the wall by molecular diffusion. Additionally, small particles of previously precipitated wax can be transported laterally by Brownian diffusion and shear dispersion. Wax Deposition Equipment. Paraffin deposition equipment has been described by a number of authors. All make use of a deposition surface cooled under controlled conditions to a temperature below the cloud point and crude oil solution temperature. We use two types of deposition equipment. The first method is a "static cold finger" similar to that outlined by Jorda. This method allows control over the variables of oil-solution temperature, deposition surface, temperature, temperature differential, T, and time. A second method is a "rotating disk apparatus," as described by Eaton and Weeter, which also controls the velocity variable. Fig. 1 is a diagram of the apparatus. This unit consists of four probes, each of which can be set electronically for a specific temperature. Work has been conducted on a variety of synthetically prepared crude oils and stabilized field crude samples. Paraffin Inhibitors. Paraffin inhibitors are polymers capable of crystal distortion or modification during the deposition process. Because of this cocrystallization mechanism, it is necessary to have the chemical in solution above the cloud-point temperature. This prevents or interferes with the molecular diffusion mechanism of deposition. It also modifies the crystal structure of waxes precipitated into small, highly branched structures with low cohesive properties. Three popular crystal modifiers are copolymers in these groups:Group A-copolymer of ethylene vinyl acetate,Group B-copolymer of C18 through C22 methacrylates, andGroup C-copolymer of olefin/maleic anhydride esters. Laboratory Deposition Testing With Rotating Disk Apparatus. A yellow Wasatch crude oil from the Altamont, UT, area was tested for paraffin inhibition. This crude had a pour point of 38 degrees C [100 degrees F] and a cloud point of 66 degrees C [151 degrees F]. Test conditions are shown in Table 1. Additives 1 and 2, both in Group C, were the most active paraffin inhibitors on this crude. JPT P. 779^