Formation Damage Related to Hot Oiling

Abstract

Summary Hot oil has been used to remove paraffin deposits almost as long as oil has been produced. It is still the most widely used procedure for paraffin removal in use today because of its relative simplicity of application, immediate results, and low cost per application. These apparent benefits have obscured the damage that hot oil can cause when used to clean downhole production equipment. Formation damage caused by hot oiling is related to the physical characteristics of the oil used, the source of the oil, the formation temperature, and the hot-oil process. Potential problems are discussed and suggestions made to minimize or to eliminate them. Laboratory tests are presented for determining whether a crude will cause formation damage during hot oiling. Case histories of successful cleaning of hot-oil formation damage are also given. Introduction Many different types of formation damage are caused by oilfield operations.1 Formation damage from hot oiling results primarily from three potential plugging agents carried into the casing annulus with the oil: high-molecular-weight paraffins, asphaltenes, and inorganic solids. The paraffin or high-molecular-weight alkane components of the oil, greater than C20 chain length, are the most common cause of formation damage during hot oiling. This paper focuses on these oil components because the systems that are hot oiled are usually those with paraffin problems. Inorganic solids - such as iron sulfide, iron oxide, clay, scale, or sand - can be damaging agents when they are oil-wet. These solids may be oil-wet because of their absorption of asphaltic components, the deposition of paraffin, or the chemicals used in the system. No further discussion of solids is included here, but they occur frequently in combination with paraffin. Asphaltenes, aromatic-based hydrocarbons of amorphous structure, are present in crude oils as colloidally dispersed particles.2,3 These hydrocarbons are generally found in greater quantity in lower API-gravity crude. Asphaltenes are peptized or suspended in the oil by lower-molecular-weight neutral resins (maltenes) and aromatic hydrocarbons. Only a small percentage of the formation damage caused by hot oiling is related to asphaltenes. This damage would be of two possible types: asphaltene particle plugging or oil wetting of water-wet formations. Asphaltene particle plugging can occur if the hot oil contains precipitated asphaltenes or if the hot-oil fluid used causes precipitation of asphaltenes in the formation. The use of diesel, kerosene, or condensate as hot oil in formations producing <38° API [<0.835-g/cm3] black crudes can plug the formation by precipitating asphaltenes. The use of black oil on condensate or light-oil wells may also precipitate asphaltenes. Even cold treatments in these combinations can cause precipitation of asphaltenes. This occurs because the mixing of a high ratio of low-surface-tension organic fluid - i.e., below 24 dynes/cm at 77°F [24 mN/m at 25°C), such as condensates - removes the layers of peptizing maltenes. This can be particularly troublesome because asphaltene damage cannot be removed by heat or aromatic solvents if asphaltenes are adsorbed on the formation minerals. Specially formulated chemicals are needed to repeptize the asphaltenes from the surfaces of formation minerals. Paraffin and asphaltene damage that can occur during hot oiling is of particular importance. Less is understood concerning these hydrocarbons and their ability to cause formation damage than other phases of oil production. Problems With Hot Oiling Hot oiling is the process of using heat to melt paraffin deposits for removal from a well. A truck is used to pick up a load of oil, heat it to 150 to 300°F [65 to 149°C), and pump it into a well. Hot oiling is done either directly down the tubing or by pumping into the annular space. This paper deals with hot oiling down the annular space, which is the most common practice. Note that formation damage that results from hot oiling down the tubing can be the more severe of the two practices because the paraffin being removed is pushed downhole. If more than one tubing volume of hot oil is used, the paraffin can be pushed directly into the formation. Hot oiling down the annulus is a thermal transfer process. When hot oil is pumped down the annular space, the tubing is surrounded by oil that is hot enough to melt the paraffin in the tubing. As the depth of paraffin deposits in the tubing increases, the amount of oil needed or the temperature of the oil used must increase because of heat loss to the outer casing walls and to the oil being produced up the tubing. The hot oil never contacts the paraffin it is removing from the tubing; it continues to fall to the bottom of the well, losing heat, and is produced back over the next several hours or days.4 The hot oil will reach the bottom of the well near the temperature of the surrounding formation. The instantaneous removal of the paraffin from the walls of the tubing and its dilution into the oil in the tubing by this thermal transfer can be verified by catching oil samples from the flowline (see Table 1). Within minutes, large quantities of paraffin dissolved in the produced oil will increase its pour point and cloud point to extremely high levels (see Table 2). This high-paraffin-content oil will eventually be produced through the system into the produced-oil tank. The hot-oil process has three major disadvantages that can lead to formation damage. Source of Oil. In any production system during any phase of its production, the best oil (having the lowest cloud point,5 lowest pour point,6 and highest API gravity7) at a given moment is the oil in the formation. This is the oil that would have the lowest temperature of crystallization of paraffin.8 Owing to high-pressure and -temperature conditions, the oil in the formation is capable of holding more paraffin in solution. The worst oil from this standpoint is the oil that reaches the sales tank. This oil, because of the loss of low-molecular-weight hydrocarbons and the lower-temperature environment, has lost much of its ability to hold the paraffins in solution. The source of the oil for hot oiling is the sales tank. As production problems with paraffin have worsened, so have the tank-bottom problems. Any paraffin removed from the system ends up in the sales tank. The lower the production or the larger the tank, the longer the precipitated paraffin crystals have to settle to the bottom of the tank. Because almost all sales tanks are set up with bottom draw-off lines, the oil picked up in the hot-oil truck is the worst oil in a system. Source of Oil. In any production system during any phase of its production, the best oil (having the lowest cloud point,5 lowest pour point,6 and highest API gravity7) at a given moment is the oil in the formation. This is the oil that would have the lowest temperature of crystallization of paraffin.8 Owing to high-pressure and -temperature conditions, the oil in the formation is capable of holding more paraffin in solution. The worst oil from this standpoint is the oil that reaches the sales tank. This oil, because of the loss of low-molecular-weight hydrocarbons and the lower-temperature environment, has lost much of its ability to hold the paraffins in solution. The source of the oil for hot oiling is the sales tank. As production problems with paraffin have worsened, so have the tank-bottom problems. Any paraffin removed from the system ends up in the sales tank. The lower the production or the larger the tank, the longer the precipitated paraffin crystals have to settle to the bottom of the tank. Because almost all sales tanks are set up with bottom draw-off lines, the oil picked up in the hot-oil truck is the worst oil in a system.

Importance of the Adsorption Method Used for Obtaining the Nanoparticle Dosage for Asphaltene-Related Treatments

“…Asphaltenes form the solubility class that is considered the heaviest, most aromatic, and most surface-active fraction of crude oil. − These compounds are insoluble in low molecular weight paraffins such as n -heptane and n -pentane and soluble in light aromatic hydrocarbons such as toluene, benzene, and pyridine. , In addition, because of their complex chemical structures, polarizability, and amphiphilic behavior, asphaltenes exhibit a self-associating feature that promotes aggregation and subsequently increases the viscosity of crude oil. , The aggregation behavior of asphaltenes is highly dependent on the chemical nature of the crude oil and its temperature, pressure, concentration, etc. , Due to these characteristics, asphaltenes can cause different problems at different stages of crude oil production, any of which cause a loss of productivity. − Therefore, asphaltene aggregation behavior has typically been studied to describe the stability of asphaltenes and their interactions with each other and with solid surfaces. Understanding the adsorption of asphaltenes onto solid surfaces has practical significance for monitoring the fluid-property variations that are common during oil production.…”

A New Model for Describing the Adsorption of Asphaltenes on Porous Media at a High Pressure and Temperature under Flow Conditions

“…3 The adsorption and/or deposition of asphaltenes onto formation grains is one of the primary causes of formation damage in oil reservoirs, which has been one of the major unresolved flow assurance problems in the oil industry. [4][5][6][7] Asphaltenes, which are the heaviest and most polar compounds in crude oil, are typically defined as the solubility class obtained from crude oil by fractionation using solvents. 8 Asphaltenes may also contain nonmetallic heteroatoms, such as nitrogen, oxygen, and sulfur, as well as metals, such as vanadium, iron, and nickel.…”

Biodegradation of paraffin wax by crude Aspergillus enzyme preparations for potential use in removing paraffin deposits

“…As traditional methods of wax removal (mechanical, thermal, and chemical) are problematic [6][7][8][9], microbial method has been considered an alternative [4,6,10]. Microbial remediation utilizes microbes or their metabolic byproducts (e.g., surfactants and paraffin solvents) to prevent and remove paraffin damage [11].…”