Field had a thin oil rim (light oil) overlain with a massive gas cap.  Interestingly, gas composition suggested very high concentration of CO2.  Because of the historic sub-optimal production strategy, the field produced high GOR and most wells were already gassed-out. 

technique studied.  Effects of heterogeneity, mobility, gravity, crossflow and capillary pressure were identified.  Incremental recovery over current strategy was noted and inherent risks were identified. 

Emphasis was placed on the location of the field, challenges of terrain accessibility and availability of transport facilities to the customers.  Some of the processes showed technical promise. 

However, in the end, based on the various scenarios of project economics it was recommended that the field will be best suited for systematic blowdown and gas used for CO2 injection in a nearby field.

Instead of continous CO2 injection, a well can treated with a slug of CO2, shut-in for a soak period and opened up for production.  It has been shown that this process is not only economic i.e. lower CO2 utilization factor but also for the appropriate reservoir (madatory screening required) a successful EOR technique.  For the candidate reservoir a simulation study was instituted to ascertain the influence of slug size, duration of soak period, well spacing, bottomhole pressure, amount of remaining target oil and heterogeneity.  It was found that the swelling of oil, viscosity reduction and reduced water production because of increased oil saturation near the wellbore are the primary mechanisms increase oil production.

Air Injection

Physical Process:  The oil recovery because of High Pressure Air Injection (HPAI) is a combination of repressurization (gas flood) effects and combustion front. Therefore multiphase flow, heat transfer and chemical reactions in porous media govern the process. 

Typically, gasflood is important during the early life of the project whereas combustion dominates recovery towards the latter half.  Combustion process can follow two possible reaction pathways – (a) Bond Scission or (b) Oxygen addition reaction.  During the former oxygen breaks up the hydrocarbon molecules to principally produce CO2 and water. 

However, in the latter oxygen atoms are chemically bound to the molecular structure of oil producing various compounds which further react to form less desirable products promoting trapping of the oil phase.  Clearly, the first pathway is desirable and should be attempted to achieve to maximize recovery.

Causes of Improved Recovery: Depending on the reservoir temperature and pressure following are possible:

• Ameliorated displacement microscopic efficiency

• Rapid reservoir pressurization

• Flue-gas stripping of in situ oil

• Oil swelling

• Injection gas substitution

• Spontaneous oil ignition and complete oxygen utilization

• Near miscibility of the generated flue gas and the oil
  
  

Modeling Effort: Experimental methods include combustion tube study and flue gas miscibility study which determine oxygen utilization and air injection rates.  Analytical methods plot recovery factor (ER) against Gas-Oil Ratio ( R ) to obtain a straight line according to the following equations

ER = m In(R)+n

However, the above equation neglects the recovery due to combustion component and assumes that the recovery is exclusively governed by gasflood mechanisms.  Numerical simulation presents the only comprehensive framework where individual components can be tracked in the midst of complex phase behavior and thermal reaction kinetics.

Case Study: Buffalo field (situated on an anticline parallel to Cedar Creek Anticline) producing from Red River (Ordovician) carbonate reservoirs (~ 8400 ft, 3600 psia, 50% Sw, API 32, 300 psia bubble point, 173 SCF/STB, 2.4 cp, 15 ft thickness, 215 F, 10 mD and 15% porosity). 

Primary recovery resulted 6% of STOIIP.  Three different units were successfully developed using HPAI with the wells responding within 9 months.  For instance, in one of the units over 90 BCF of gas has been injected and 6 MM STB (~16%) of incremental oil produced during 28 years of HPAI.  Producing GOR has consistently shown an initial rise followed by a flattening trend, indicating a stable combustion front which prevents dominating gas flow towards producers. 

Furthermore, high concentration of CO2 (~12%) in the produced gas indicate thermal reaction and maximum utilization of oxygen in the reservoir.