Production of CO2 From a Reservoir - A New Concept
Authors: Weeter Robert F. , Halstead Louis N. 
Published: December 2007

Very large reservoirs of approximately 985 CO2 have been discovered in southwestern Colorado. These reservoirs have pressures up to 2,700 psi (18.70 Mpa) and temperatures to 171°F (77°C). Wells are capable of flowing more than 10 MMcf/D (0.28×106 m3/d) CO2. The CO2 flows to the surface at pressures and temperatures below the critical and, therefore, in two-phase flow. For convenient handling, the CO2 must be heated to form a single-phase gas before it is transported, dehydrated, and compressed. This paper discusses a patented procedure for downhole pressuring followed by the production and processing of CO2 under continuous supercritical conditions. It also describes a field test of the procedure and the test results.

The process is dependent on the properties of CO2 under continuous high pressure and even at high temperature, which approach those of a liquid. Thus, the CO2 can be produced by conventional pumping operations.

The process appears applicable to production from any CO2 reservoir containing a fluid above its critical pressure. The results of the field test were very close to those anticipated by calculation.

Introduction

For at least the past 15 years, interest in CO2 as a tertiary recovery technique has grown. To date, most of the CO2 has been salvaged off-gas from sweetening operations in gasoline plants. This gas generally contained 10% or more extraneous gases, including nitrogen, hydrogen sulfide, and methane. The principal advantage of CO2 in tertiary recovery is its miscibility, under certain conditions of pressure and temperature, with the reservoir liquids.

The first reservoir where large volumes of CO2 have been injected is the Canyon Reef reservoir of the SACROC unit in Scurry County, TX. The procedure and results of that operation are well documented.1–4 It was established that CO2 of less than approximately 87.5% pure would not be miscible with the reservoir liquids under the Canyon Reef conditions. The purity required varies from field to field because of differences in temperature, pressure, and reservoir liquids characteristics.

Many fields in west Texas are reported candidates for CO2 flooding; however, some are at lower pressures than the Canyon Reef, and some can be expected to require higher purity of CO2 than 87.5%. Higher purity CO2 is miscible with reservoir fluids at considerably lower pressure. Fig. 1 shows the approximate conditions for miscibility of 87.5% pure CO2 with produced oil from various reservoirs as they existed in 1966. Reservoir pressures have decreased in some of these reservoirs because of fluid withdrawals; others have increased because of pressure maintenance. Therefore, it is fortunate that approximately 98% pure CO2 has become available in large volumes in the McElmo field in southwestern Colorado and in other areas.5

The peculiarities of CO2, particularly the characteristic that pure CO2 exists as a two-phase fluid at ambient temperature and pressures up to 1,080 psi (7.44 MPa), make it difficult to handle at the flowing conditions of the McElmo field. Consequently, the process of pumping the supercritical CO2 with a multistage centrifugal pump was conceived.6 The procedure has been tested in the field and has proved successful.