Step 1: Empirical Reservoir Characteristics:
The first step in our assessment of an oil reservoir for CO2 miscible, or immiscible, flooding potential is comparison to published empirical rough screening criteria. (7)(8)(9)(10)(11) Multiple publications were researched and combined to summarize the reservoir conditions favorable to CO2 flooding as stated below:
- Reservoirs with good waterflood response are the best candidates for CO2 flooding.
- Ideal recovery of OOIP from waterflooding is greater than 20% OOIP but less than 50% OOIP.
- Reservoir depth must be greater than 2500 ft to reach MMP depending on bottom hole temperature, API oil gravity and oil composition.
- An oil gravity greater than 27 degrees API is preferred with an oil viscosity less than 10 cp at reservoir conditions.
- Formation porosity greater than 12% with an effective permeability to oil of greater than 10 md is ideal.
Several empirical quick rules of thumb can be applied to assess CO2 flooding feasibility as stated below:
- Recovery factor of original oil in place in the best reservoirs range from 8-11% OOIP for miscible CO2 floods.
- Immiscible CO2 floods are usually 50% or less of miscible CO2 results.
- In order to achieve CO2 miscible flooding the minimum miscibility pressure (MMP) is roughly equal to initial bubble point pressure.
- CO2 requirement is an initial 7-8 Mcf CO2 per barrel of oil with an incremental 3-5 Mcf of CO2 per barrel required to be re-compressed and re-injected.
- Water and gas injection (WAG) is an alternative to reduce high CO2 injection concentrations and increase ultimate oil recovery. However, CO2 recovery percentage does decrease if total CO2 concentrations below a total of 10 Mcf per barrel are injected even if WAG is used.
- Water injection after primary production is required to fill gas voidage and increase reservoir pressure to original pressure conditions prior to CO2 injection.
- Top down CO2 inject can be applied to highly fractured or thick reservoirs. Higher recovery factors are achieved but at the expense of higher CO2 volume purchases required.
Step 2: Analogy to Successful CO2 Floods
The second step for screening oil reservoirs for CO2 flooding potential is utilization of a dimensionless analogy model. A dimensionless analogy model is a graph of the cumulative percentage of the reservoir barrels of CO2, and or water, injected divided by the original hydrocarbon pore volume in reservoir barrels vs. the cumulative percentage of the actual oil production in stock tank barrels divided by the original oil in place in stock tank barrels. The theory is that reservoirs with similar initial reservoir characteristics will respond a similar manner to water or CO2 injection. Therefore when injection volumes are normalized on a dimensionless basis they can be used to estimate oil recovery on a dimensionless basis for any reservoir with similar reservoir characteristics even though the size of the reservoir may be different. These plots are usually created from oil production and water or CO2 injection history using Excel graphs. The shapes and slopes of dimensionless analogy model graphs can vary radically for different reservoir characteristics.
One pre-programmed Excel dimensionless analogy model is available at no cost from the Texas Center for Energy and Economic Development (CEED). (12)(13) This Excel model was initially created by Shell Oil Company, updated by Kinder Morgan, and is based upon a dimensionless curve from the Denver Unit in the San Andres formation in West Texas. Pre-programmed dimensionless analogy models, such as the Shell Kinder Morgan program, do give meaningful results but they must be used with some caution since they are based upon specific CO2 project field results. Applying the models to other non-San Andres formation producing fields will introduce error because reservoir variations such as fractures, natural water influx, and reservoir heterogeneity cannot be accounted for.
Step 3: Reservoir Simulation Studies and Economic Analysis
The third step used in CO2 application reservoir screening is simulation modeling. Rough simplified simulation screening can be accessed using the DOE CO2 Prophet simulation model available in the public domain. (14) The preferred low cost simulation CO2 screening simulation program by the authors is IFLO. (15) High-cost reservoir simulation programs, such as Schlumberger’s ECLIPSE, Computer Modeling Group’s GEM or Landmark’s VIP compositional computer models are usually used at the end of a detailed feasibility study both before, during and after full scale project initiation has begun.
High-cost reservoir simulation computer modeling input requires complete core and laboratory analysis of rock and reservoir fluids to generate accurate predictions. Laboratory analyses of liquids and gases are used to create pressure-volume-temperature (PVT) tables. Core analysis is used to more accurately define the reservoir parameters for modeling in the reservoir simulation program. Full core CO2 flood tests are also performed, and simulated, to verify the accuracy of the simulation predictions. Low-cost simulation can be performed, prior to conducting complete laboratory analysis, by using empirical PVT data or PVT data from reservoirs with fluid or rock characteristics. Economic analysis of the reservoir simulation production response is performed to assess the feasibility of full-scale field development.
Step 4: Field Pilot
If economic analysis indicates application of CO2 flooding would meet economic goals the next step would be implementation of a small field pilot such as those conducted by Gulf in 1983 in Little Knife North in the Madison formation. (1)(2)(3)