The Commercial Viability And Comparative Economics Of Downhole Steam Generators In Alberta
Authors: Singh S. , Nguyen D. , Wong S. 
Published: March 2010

Downhole steam generation is an emerging technology being developed as an alternative to conventional surface steam generation. Its major advantage is that it eliminates surface and wellbore heat losses. In addition, the reinjection of combustion gases with steam in some downhole steam generators (DSG) could create potential advantages of a steam and additive process.

Over a dozen downhole steam generator designs have been developed. This paper reviews past laboratory/field experience and technical criteria for application in heavy oil and oil sands reservoirs. It includes an analysis of equipment costs, and develops economic criteria for the use of DSGs in Alberta.

Introduction

Among the many processes being applied for the recovery of low-gravity crude oil, thermal methods have proven to be the most successful. These methods employ steam injection, in situ combustion, or a combination of both. The use of steam, however, appears to be the most popular because it may increase oil production rate and ultimate oil recovery. The two basic injection techniques are steam stimulation and steamflood.

Steam stimulation (or ‘soak’, ‘cyclic’, ‘huff-n-puff’) is a single well operation (Fig. la). Steam is injected into the well and is allowed to soak before the oil is produced. Oil viscosity is the chief variable involved, because the oil recovery depends primarily on a reduction in the viscosity(1). The production of oil is a discontinuous operation, but relatively fast. The ultimate oil recovery is low.

Steamflood (or ‘drive’) requires a set of injector wells into which the steam is injected, and a set of producer wells from which the oil is produced (Fig. lb). In this operation steam distillation may be of prime importance in increasing total recovery efficiencies(l). As shown in Figure 2, a number of other variables are also involved. The production of oil is a continuous operation, but relatively slow. The ultimate oil recovery is high.

Steam is conventionally produced in oil field steam generators. They differ from industrial boilers in their ability to produce low quality steam from saline water with minimal treatment(2). They can endure rapid load changes and provide unattended operation. They may be either skid-mounted or completely portable. Finally, they can utilize a wide range of liquid and gaseous fuels. In Alberta, because of its low cost, natural gas is the primary fuel source of steam generators. These generators create less pollution problems than the Californian counterparts which burn high-sulphur liquid fuels.

Steam generated in conventional surface boilers suffers its largest drawback in the form of heat losses as it flows through the surface piping and in the wellbore. The biggest surface loss isup the exhaust gas stack. It is possible to recover part of this heat, but it is not economical because the acids condensing at the lower temperature are extremely corrosive. Heat lost in the wellbore could be significant in the case of deep wells which require higher steam injection pressures and temperatures.

The development of downhole steam generators (DSG) was natural because it may eliminate surface and wellbore heat losses.