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Abstract

During the past 38 years, CO₂ flood technology for Enhanced Oil Recovery projects evolved from a partially understood process filled with uncertainties to a process based on proven technology and experience. Many questions involved with CO₂ flooding have been thoroughly analyzed and answered. This knowledge is currently being used by a limited number of companies that actually know how to design, implement, and manage a CO₂ flood for long term profit. Unfortunately, this knowledge has not been disseminated to operating companies interested in EOR flooding or to CO₂ Sequestration Communities interested in storing CO₂ in EOR projects.

The primary objective of this report is to target “Conventional WAG Techniques” which have been used in over 90% of all the Enhanced Oil Recovery projects implemented in the Permian Basin in Texas, Colorado, Oklahoma, and Wyoming. Over the years, oil companies have reported a wide range of values of Tertiary Oil Recovery, CO₂ Utilization, and CO₂ Retention, resulting in a wide range of variation and uncertainty. Many of the numbers reported to date are tied to a specific HCPV CO₂ Injected based on some Economic Cut-off. This typically has been in the range of 30% to 80% HCPV Injected. The question becomes “What is life after 80% HCPV?”  And “What effect does life after 80% HCPV have on Tertiary Oil Recovery, CO₂ Utilization and CO₂ Retention in different producing formations?” Results of this study show Tertiary Oil Recovery can be as high as 26% OOIP when slug sizes exceed 190% HCPV injected. 

Carbon Sequestration Options: Five Recovery Methods for Enhanced Oil Recovery (EOR)

Most Enhanced Oil Recovery Projects use one of the following five operating methods: Conventional WAG Recovery, Gravity-stabilized Recovery, Double Displacement, Gas-cycling or Huff-and-Puff. The primary difference between methods depends on the reservoir geology and well pattern configuration. In Conventional CO₂ floods, typical of West Texas, the formations are basically flat (Ramp Sequence), low perm, the fields are developed on pattern spacing (e.g. 5-spot patterns, 9-spot patterns, or Chickenwire patterns), and Conventional WAG Operating schemes are used to control mobility and CO₂ flood response. In conventional WAG operations, the objective is to minimize the amount of CO₂ purchased (CO₂ stored in Sequestration projects), which is typically in the range of range of 30%-40% of the total HCPV CO₂ injected. In un-conventional Gravity-Stabilized and Double Displacement case histories, Flue Gas, CO₂, Lean Gas or N₂ is usually injected in the top of the structure and oil is produced from the bottom. More CO₂ can be sequestered than conventional WAG operations. As much as 80% of the total pore volume can be displaced with CO₂. However, the reservoir must meet certain fluid-dynamic criteria and have structure to make the gravity-stabilized process work.  In Gas-cycling projects, typical of projects operated by Denbury in Mississippi, CO₂ is cycled through the formation. As much as 6 pore-volumes of CO₂ are injected to recover 18% OOIP. In Huff-and-Puff operations, the CO₂ is injected into and produced from the same well. The objective is to mobilize tertiary oil in the near vicinity of the well-bore, and then produce the CO₂ and tertiary oil back. Then repeat the process (typically with 3 cycles). The process technically works. The economic success or failure depends on many factors. The amount of CO₂ sequestered is minimal when compared to the other recovery methods.  

Conventional WAG Recovery

Conventional WAG Operating Methods in the Permian Basin fall into one of four categories:

1. Continuous CO₂ Injection
2. Constant WAG Injection
3. Tapered WAG Injection
4. Simultaneous CO₂ Injection (Limited use)