Warm-Air Coking - A New Completion Method for Unconsolidated Sands
Authors: Terwilliger P.L. , Smith F.M. , Goodwin R.J. 
Published: December 2007

A new well completion technique has been used to prevent the production of sand into wells producing oil from an unconsolidated sand formation. The laboratory development of this method of sand consolidation, known as "warm-air coking", is described. Warm air injected into an unconsolidated sand saturated with a heavy crude oil causes the oil to be oxidized. Continued oxidation of this very viscous oil forms an insoluble coke of resin which cements the sand grains together. Ignition of the coke-forming residue is prevented by proper control of the injection rate and of the temperature-increase rate of the injected air. Reservoir oils having cokable heavy ends provide the most satisfactory source of coke, but specially compounded oils or suitable crude oils not native to the formation could be injected to replace the reservoir oil. Usually crude oils having gravities of less than 20 deg. API contain a sufficient amount of heavy ends. Several wells producing from unconsolidated reservoirs containing heavy crudes have been satisfactorily treated with this method. These wells have produced sand-free oil over an extended period of time and have high productivity indices.

Introduction

The prevention of sand production concurrent with the oil from unconsolidated reservoirs has been a major problem in some areas. In many cases, the sand control problem has been solved by the use of mechanical devices such as screens, perforated liners and gravel packs, or by consolidating the sand around the wellbore with a plastic which is injected into the formation. The advantages and disadvantages of these methods and their success in combating sand production have been discussed by several authors. As pointed out by Hower and Brown, mechanical devices in the wellbore do not leave the casing clear for multiple completions, while sand consolidation with plastics has been only partially successful.

In reservoirs producing low-gravity, viscous oil, the conventional methods are only moderately successful because they sometimes do not prevent sand production and frequently require remedial work during the life of the well to maintain the productivity at a satisfactory level. This is more pronounced when the formation contains a wide range of particle sizes, including clays and silts.

This paper presents a new sand-control method known as the warm-air coking sand consolidation process. It was developed to prevent sand production from unconsolidated sand reservoirs producing low-gravity crude oils. Sand consolidation is accomplished by the injection of heated air into the formation around the wellbore until the crude oil becomes oxidized into an insoluble resin or coke. Details of the laboratory development and a description of the equipment are given. The results of several field tests of the method are summarized.

Two obvious requirements of any sand-control technique are that sand production be prevented and that the oil productivity be maintained. Other desirable features are that the completion lasts the life of the reservoir without remedial work and that no obstruction be left in the casing. It was believed that these requirements might be satisfied if the sand surrounding the wellbore could be cemented with coke. The coked sand would need sufficient permeability to provide the required productivity and enough strength to withstand the pressure gradients created by the flowing fluids and the weight of the overburden. It is well known that crude oils, especially those having a low gravity, form coke when heated to high temperatures. When an unconsolidated sand saturated with an oil containing heavy ends is heated to the coking temperature the coke formed binds the sand grains together. Several methods of heating to form coke in sand-oil mixtures were tried in the laboratory and the most satisfactory appeared to be the injection of heated air.

The use of air as opposed to an inert gas enhances the formation of an insoluble binder in two ways.

JPT

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