Abstract

This paper presents the results of experiments conducted to evaluate the solubility and dissolution rate of paraffin in selected chemical solvents. The causes of formation damage from paraffin deposition and the methods used to control or mitigate the damage are discussed. A thermodynamic model using regular solution theory was developed to predict phase equlibrium of solvents and paraffin precipitation. Reasonable agreement was obtained between the model prediction and laboratory results when the solvent compositions are known. The kinetic approach to the dissolution rate suggests the use of pseudo-first order reaction form between the paraffin and the solvent.

Introduction

Many crude oils deposit waxy materials called paraffin during production and transportation when subjected to changes in temperature and pressure. The extent of the deposition can be manifested as damaged zones in the formation, in plugged tubing and flow lines, and in sludge deposited in tank bottoms. These paraffin-related problems cost the petroleum industry billions of dollars annually, in terms of cost of treatment chemicals, reduced production, well shut-in, inefficient use of production capacity, choking of flowlines, equipment failure, premature abandonment and increased manpower attention.1 Figure 1 shows a map of the United States and the states where organic deposition problems have been identified. Areas where petroleum production operations are conducted, both onshore and offshore locations, almost always manifest paraffin-related problems to one degree or another. This map illustrates the geographical extent of the problem area.2

The definition of paraffin (wax) problems refers to the deposit of carbonaceous material which is not soluble or dispersible by the crude oil under prevailing conditions. This carbonaceous material normally consists of high-molecular-weight paraffin hydrocarbons including either straight-chain (normal), branched or cyclic alkanes ranging from C18H38 to C70H142.3 They are generally very inert and resistant to attack by acids, bases and oxidizing agents. Previous research indicated that n-paraffins are predominately responsible for the deposition problem.4,5 Compounds other than n-paraffins, especially asphaltenes and resins, occluded oil and water, and possibly sand and silt, have profound effects on solubility of n-paraffins.

Formation damage resulting from crystallization and deposition of paraffin within the reservoir is a recurrent production problem. The occurrence of these problems are highly dependent upon the temperature, pressure and flowing conditions near the wellbore and the reservoir crude compositions. Frequently, formation damage is caused by improper well operations such as hot oiling, non-isothermal/cold fluid injections and incompatible fluid chemistry. As fluids flow through the reservoir at pressure and temperatures below the cloud point of the fluid, precipitated paraffin particles are deposited within the pores of the reservoir. Subsequently, the absolute permeability of the region of the reservoir in which deposition has occurred is reduced, which results in a decrease in reservoir flow, mostly near the wellbore.

This paper presents results of laboratory tests and analytical approaches to the evaluation of solubility and dissolution rate of paraffin in the selected commercial solvents. The research effort is to evaluate the effectiveness of these solvents for the removal of paraffin-related formation damage.

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