Paraffin Precipitation During Fracture Stimulation
Authors: Sutton G.D. , Roberts L.D. 
Published: December 2007

Experiments show that after certain formations have been fractured, paraffin may precipitate and damage the permeability if a reservoir crude paraffin may precipitate and damage the permeability if a reservoir crude oil is cooled below its cloud point. The extent of such damage depends on formation permeability, the amount of paraffin precipitated, and the bottom-hole temperature.

Introduction

Crude oil in an untapped reservoir exists in a state of chemical and physical equilibrium. As the oil is produced through the formation, this equilibrium no produced through the formation, this equilibrium no longer exists, and the fluid undergoes physical and chemical changes. The volatile liquid constituents are continuously lost from the crude oil after it enters the fracture and the wellbore, since the pressure there is less than the pressure driving the fluid through the reservoir. Also, because of the pressure differentials that exist in the well, the crude oil begins to cool below formation temperature. A loss of light ends and a decrease in temperature combine to cause the solution to become saturated with paraffin. Then the paraffin begins to precipitate, and often it collects paraffin begins to precipitate, and often it collects on the tubing, in flow lines, etc. The equilibrium balance of the crude oil can also be disrupted within the formation by the injection of cold fluids such as those used in fracturing. This is particularly true when the surface fluid temperature particularly true when the surface fluid temperature is much colder than the formation temperature. If the fluid in the formation is cooled to a temperature below the cloud point, paraffin precipitates and may deposit in the formation pores, partially blocking or plugging the fluid flow channels and thus restricting plugging the fluid flow channels and thus restricting the flow. The precipitation of paraffin is almost irreversible in that the wax, once removed from solution, is very difficult to redissolve in the same fluid, even after original formation temperatures are restored. Obviously, formations having temperatures higher than the melting point of the precipitated paraffin would not be affected. Paraffin precipitation in the pores of the formation often causes serious problems and is difficult to detect. The damage resulting from such wax accumulation near the fracture faces may manifest itself in decreased production, slow cleanup of wells after fracturing, or failure to attain predicted production increases. Because such symptoms are often thought to result from poor fracturing treatments, very little attention has been given to the idea of paraffin precipitation in a formation. Our investigations deal with theoretical considerations of reservoir cooling during fracturing and with formation damage that can occur if the crude oil is cooled below the cloud point. From well and reservoir fluid information, the effect on production can be calculated. Experimental results of paraffin damage in cores cooled below the crude-oil cloud point will be discussed.

Method of Calculation Wellbore and Fracture Temperature

Hydraulic fracturing is commonly used to increase oil or gas production in formations with low permeability. The process involves applying enough permeability. The process involves applying enough hydraulic pressure to overcome the stresses in the formation. When this hydraulic pressure is applied, the formation ruptures perpendicular to the least principal stress and the created fracture will continue principal stress and the created fracture will continue to propagate until the end of the treatment if the pressure is maintained. The least principal stress is pressure is maintained. The least principal stress is generally accepted to be in the horizontal plane; therefore, most fractures would be vertical.

JPT

P. 997