Feasibility Evaluation of a Downhole Steam Generator
Authors: Wright David E. , Binsley Robert L. 
Published: December 2007

Downhole steam generation has been recognized for several years as potentially beneficial because of reduced heat loss and improved steam quality compared with conventional steam generators. This paper describes Rocketdyne Div.'s development of a downhole steam generator (DHSG); benefits and features are described. The work was done under the management of Sandia Laboratories as part of the U.S. DOE's Project Deep Steam. The effort began with a comparison of alternate ways of generating steam downhole. The most economical was found to be the indirect system, where heat from low-pressure [approximately 5-atm (500-kPa)] combustion of fuel oil and air is transferred to high-pressure water. A feasibility test unit was constructed and tested extensively; results are described. Comparative economic analyses indicate that the DHSG is competitive with conventional systems even at moderate depths. Below 2,500 ft (762 m) it is a clear choice. The system is ready for a prototype demonstration in a downhole environment.

Introduction

The DHSG is a logical extension of existing surface steam generators; it delivers high-quality steam at the sand face with high thermal efficiency (low heat losses). As deeper reservoirs are probed and higher reservoir pressures encountered, the problem of heat loss and consequent steam quality loss becomes severe. The technical details of this situation have been covered earlier by Fox.1 The DHSG shows great promise as an alternative - if it can be made practical.

In 1978 Project Deep Steam set out to determine DHSG feasibility under the management and direction of Sandia Laboratories. At that time, Rockwell Intl.'s Rocketdyne Div. was chosen to evaluate the feasibility of a DHSG. This paper describes some key results of that study, from its beginning, in Sept. 1978, to Sept. 1981. Additional details on that contractual effort are found in the final report2 and design summary report.3

Initial studies performed by Rocketdyne in early 1978 developed conceptual designs for several direct and indirect DHSG configurations. The first phase of the contract was a more detailed comparison of possible designs for generating steam downhole. In the proposal, five concepts were presented. By the time the contract began, a total of 13, plus some variants, had been defined. Following the economic comparison studies, the indirect air/fuel-oil design was selected for further contractual efforts, and experimental equipment was designed and tested. Additional system and component design analyses also were performed. Those efforts are described further.

Requirements

The preliminary requirements for a DHSG, as determined by Sandia, appear in Table 1. The energy outputs, steam flows, pressures, etc. were based on a survey of potential users. The requirement that the unit be installed in a 7-in. (17.8-cm) well casing was relaxed later when it became apparent that the costs of a larger-diameter injection well were not prohibitive.

System Comparison

Economics was the primary criterion for comparison of all the systems considered. For those systems compared in detail, operating and capital costs were combined to give a total cost of steam generated. Since approximately three-fourths of the total cost of a system was fuel cost, it was possible to rule out some of the systems on the basis of fuel cost alone.