Care and Feeding of Steam-Injection EOR Projects
Authors: Jones J.E. 
Published: June 2014

Steam-enhanced recovery methods have been the overwhelming EOR champions since their inception in the mid-1960's. About 6 of every 10 EOR barrels produced worldwide are the result of some steam process. Historically most well known in the San Joaquin Valley of California, large steam projects exist in countries around the world, including Venezuela, Canada, Colombia, Indonesia, China, and the CIS. Therefore, one would think that most of the basic problems, such as effective steam-distribution piping systems and accurate methods of metering steam into wells, would have been solved years ago. Unfortunately, this is not the case. In fact, this technology has been woefully lacking compared with its relative importance. This paper summarizes current efforts by the industry to improve metering and distribution of quality steam.

Defining the Problem

To understand why progress has been so slow, one must review the thermodynamics of the phase change of liquid water to steam vapor. The best steam for EOR methods turns out to be "quality steam," or water that is converted 40% to 90% to vapor on a weight basis. Reservoirs respond best when the small liquid fraction associated with quality steam is part of the injectant. Projects that are run with all hot water or all superheated vapor, both easily distributed and metered, have usually performed poorly by comparison.

Two major problems arise with quality steam. First, distributing the liquid and vapor phases through a large piping network with many offtakes to individual wells from main distribution headers often results in a disproportionate ratio of the phases arriving at the wells. Second, an investigation of the steam tables1 reveals that the commonly used physical parameters of temperature and pressure are dependent in the phase change region. Thus, they cannot be used in combination to determine the state of the injectant. Because the vapor phase is a compressible fluid, positive-displacement meters are useless and velocity-type meters require information on an independent physical property, such as bulk density, that is very difficult to get.

Uniform Phase Distribution in Piping Systems

Every time steam enters a flow splitting device, such as a common piping tee, the potential for uneven phase distribution exists. Fig. 1 shows the worst-case orientation of a side-branched piping tee. It is well documented that this piping orientation, although highly attractive on a capital-cost basis, results in some wells receiving all water and some wells all vapor. It is normal to find all wells receiving off-specification steam quality in these systems and, therefore, is no longer accepted practice to use them. The graph in this figure indicates the unstable nature of flow in this split.

Fig. 2 is a solution proposed by Chevron Production Technology Co. in the early 1980's that consists of introducing steam into the "dead-end" stem of the piping tee. 2 The graph in this figure shows that distribution is good when the volume is about equal leaving each branch of the tee but degrades rapidly in the typical situation of nonequal injection rates in various wells. This is a good low-cost solution if equal injection rates are maintained in all wells and low installation cost is an important consideration, especially in mature projects that cannot absorb the economic burden of an expensive retrofit.

Figs. 3 and 4 show two other inexpensive solutions for new or retrofit installation that were recently introduced by Santa Fe Energy Resources Inc.3 and Chevron U.S.A. Inc.,4 respectively. The former device is a common pipe tee with an internal baffle or "stratifier" and upstream static mixer that combine to yield very good distribution splits over a wide range of inlet quality. It has fixed chronic problems in many projects. Note that it can be installed in a dead-end or side-branch tee orientation, which further reduces installation cost. The only critical consideration is that nominal flow velocity must be &gt;25 ft/sec to avoid phase separation, which often requires reducing the size of the pipe at the split compared with the rest of the system. The design in Fig. 4 is a pair of baffles that also fit in a standard tee. It may have higher pressure drop of the two designs, but performance is reported to be good. The company also reports that good results occur if steam velocity is held at &lt;20 ft/sec, but this requires larger, more-expensive piping.

Fig. 5 is a device used by Texaco Inc.5 in the Midway Sunset field in California, consisting of a large physical separating drum that receives the effluent from a cogeneration plant and separates the liquid and vapor phases by gravity segregation. These phases are distributed in separate, parallel piping systems then recombined through appropriate gas and liquid meters before introduction into the well. This system works but has limitations. The company states that it is very difficult to adjust the rate into individual wells manually and that gas and liquid rates must be adjusted separately at each location. Addition of automated valve actuators and a centralized computer is an interesting possibility. The device is also expensive to install and to maintain so only major splits on new installations, such as Texaco used on the outlet of a cogeneration plant, seem practical. Fig. 6 shows another Texaco6 split design that again depends on physical separation, metering, and recombination of the phases. Narrow control range is claimed, limiting use to major splits with consistent flow.

Cal Resources7 uses another innovative approach (Fig. 7). It too has a large pressure vessel as a key component but uses internal baffles to divert homogenous steam entering past a static mixer at the top to a multiplicity of radial exit lines surrounding the midriff of the vessel. This passive device has the advantage of low maintenance cost but is relatively expensive to install and does not lend itself to retrofit into existing systems.