Paraffin Control in the Northern Michigan Niagaran Reef Trend

Abstract

Summary Wax deposition on downhole equipment and in chokes, flowlines, separators, dehydration and storage equipment is a costly problem in the northern Michigan area called the Niagaran Reef trend. A number of mechanical removal techniques have been used to treat for paraffin. Among these are paraffin cutters, plunger lift, rod scrapers, hot oil or water, plastic coatings, and flowline pigging. Improvements in chemical formulation, testing, and applications have resulted in a number of economically successful chemical programs for paraffin control. Examples of field problems and solutions are presented. Introduction The northern Michigan Niagaran Reef trend consists of a belt approximately 64 km [40 miles] wide extending across the northern part of the state from Ludington in the west to Alpena in the east (Fig. 1). Crude oil is produced from an average depth of 1640 m [5,380 ft] from produced from an average depth of 1640 m [5,380 ft] from the Silurian Niagaran formation. Initial field development began in the late 1960's. Since that time, paraffin-related problems have been a major expense for the producer. problems have been a major expense for the producer. Paraffin problems vary in severity from daily for some Paraffin problems vary in severity from daily for some wells to monthly for other wells. The production of-high-total- dissolved-solids (TDS) produced waters in the range of 100,000 to 400,000 ppm aggravates the paraffin deposition problems, Salt blocks are prevented ppm aggravates the paraffin deposition problems, Salt blocks are prevented through the use of continuous injection freshwater systems or periodic batching to reduce brine weight from 5.0 kg [11.0 periodic batching to reduce brine weight from 5.0 kg [11.0 lbm] down to an average of 4.3 kg [9.5 lbm]. Paraffin-deposition problems occur throughout the production system. Major problems exist with deposition in production system. Major problems exist with deposition in the tubing string, chokes, flowlines, and separators. A wide variety of mechanical, thermal, and chemical methods have been used to control the deposition of wax. Paraffin cutters, mechanical rod scrapers, plastic coatings, Paraffin cutters, mechanical rod scrapers, plastic coatings, flowline pigs, hot oil and water, paraffin solvents, diesel, surfactants, and crystal modifiers have all been used with various levels of economic success. In recent years, improvements in treating efficiency have made a number of chemical treating programs more cost efficient than standard thermal and mechanical methods. A combination of chemical treatments with occasional mechanical/thermal methods has also provided good overall performance. The key to successful chemical treating is a combination of chemical choice and application technique to solve specific field problems. Theory Paraffin Deposition. Typical paraffin deposits are a Paraffin Deposition. Typical paraffin deposits are a mixture of linear and branched-chain hydrocarbons (C 18 H 38 to C70H 142) combined with a wide variety of organic and inorganic materials that add bulk to the deposit. These deposits vary in consistency from a mushy liquid to a firm, hard solid, depending primarily on the amount of oil present. present. Paraffin deposition takes place by three mechanisms that transport both dissolved and precipitated wax crystals laterally. As the oil cools, a temperature gradient is established that leads to the transport and deposition of wax on the colder pipewall by molecular diffusion. Particles of previously precipitated wax crystals carried in the oil are transported laterally by Brownian diffusion and shear dispersion. The cloud point of a crude oil is defined by ASTM as that temperature at which paraffin particles first begin to precipitate from solution. Paraffin deposition can take place only on surfaces below this critical cloud-point place only on surfaces below this critical cloud-point temperature and that of the bulk crude OH. Paraffinic crude oils behave as Newtonian fluids at temperatures above their cloud point. Thixotropic characteristics begin to appear at just below the cloud point of the crude because of precipitated wax crystals and become pronounced as the pour-point or solidification temperature is reached. Paraffin-deposition testing has been conducted by number of authors. Paraffin-deposition testing has been conducted by number of authors. Generally the wax is deposited on a surface referred to as a "cold spot. "When the temperature differential, percent wax, shear rate, or deposition surface material is varied, a number of deposition trends can be analyzed. Testing chemical structures has led to the identification of a number of paraffin inhibitors that provide crystal distortion and reduce paraffin deposition. These compounds are generally added continuously to the crude oil above the cloud-point temperatures. As the crude cools, these inhibitors cocrystallize with the precipitating wax and cause a crystal distortion to take place. The resulting crystals are small, highly branched, dense crystals with greatly reduced agglomeration properties. Crystal modification chemicals are polymers with molecular weights ranging from 1,500 to 100,000. Because of their molecular weight and structure, these chemicals are difficult to use in low winter-time temperatures. SPEPE P. 213

Modeling of Solids Deposition from “Waxy” Mixtures in “Hot Flow” and “Cold Flow” Regimes in a Pipeline Operating under Turbulent Flow

“…The deposition of solids is undesirable as it causes plugging and increases the energy consumption during transportation and processing of the crude oil. Several methods to mitigate wax deposition have been tried over the years, such as chemical treatment, , mechanical methods, and thermal methods . Attempts to control wax deposits have also been made, with limited success, by using bacterial injection, electromagnetic fields, piezoelectric energy, vacuum-insulated tubing, and fused chemical reactions .…”

“…The heat-transfer coefficient in eqs 1 and 2 was estimated using the Dittus−Boelter equation for the turbulent flow forced convection heat transfer in smooth tubes: 45 = Nu R e P r 0.023 0.8 0.3 (6) The available flow area in the pipe changes because of wax deposition. Thus, Nu and Re in eq 6 were calculated on the basis of the actual available local flow diameter (to account for the blockage).…”

Solids Deposition during “Cold Flow” of Wax−Solvent Mixtures in a Flow-loop Apparatus with Heat Transfer

“…Various techniques have been applied to the control or remediation of wax deposition and these can be grouped into: thermal, mechanical, chemical, and biological methods. − However, most of these methods are cost intensive, especially when dealing with long production lines or offshore facilities, and in some cases can lead to increased deposition in process equipment further downstream . “Cold flow” is an alternative approach that has been proposed to control and reduce solids deposition problems during the flow of waxy crude oil.…”

Formation Damage Related to Hot Oiling

“…A viscometer can be used to determine cloud point if the crude samples are too dark to determine an ASTM visual cloud point. 9 Comparison of the results of these tests indicates whether the increase in the paraffin content of the tank sample will cause plugging problems on the basis of the formation temperature.…”