Screening Criteria for CO2 Huff 'n' Puff Operations

Abstract

Abstract This paper reviews design and performance data on sixteen (16) CO2 huff 'n puff projects conducted in different wells in the Forest Reserve oilfield of Trinidad and Tobago over the last twenty (20) years. Specific inferences on conditions under which these projects succeeded in increasing oil production were generalized taking into account published results of similar projects elsewhere. With a variety of technical, operational and economic variables, a strong correlation or a definitive conclusion is difficult. However, by correlating various performance attributes with different parameters, certain inferences are drawn and tested which could be used to:Determine if a candidate well could benefit from CO2 huff 'n puff operations andIdentify optimal design and operational configurations in specific situations, based on field experience and engineering considerations. Successful projects were conducted in reservoirs with crude oil gravities from 11- 38ºAPI and in-situ viscosities from 0.5 to 3000 cp (mPa.s) porosities of 11 to 32%, depths from 1150 to 12870 feet (345 to 3900 m), thicknesses from 6 to 220 feet (2 to 67 m) and permeabilities ranging from 10 to 2500 mD. Huff 'n puff operations benefited from high oil saturations, mild pressure support to production, soak intervals of 2 to 4 weeks and production against back-pressures to discourage excessive water or gas production. Successful operations had CO2 utilization ranging from 0.3 Mcf/barrel to 10 Mcf/barrel for light oil reservoirs and 5 Mcf/barrel to 22 Mcf/barrel for some heavy oil reservoirs. CO2 huff 'n puff operations are essentially near wellbore stimulation techniques which can lead to increased oil recovery via removal of some productivity damage, reduced oil viscosity, increased dissolved gas content, oil swelling and vaporization of lighter components of oil. They can also suppress water production. They have significantly boosted short-term oil production and generated quick payouts, especially at attractive oil prices. In certain cases, they have also provided strategic information on injectivity and pressure communication with adjacent wells, and helped determine if a drive process is indicated. This paper attempts to demonstrate that by properly understanding relevant reservoir mechanisms, one can screen specific prospects and design appropriate operations. Introduction The first huff ‘n’ puff experimental project in Trinidad and Tobago was conducted in 1984 in the Forest Reserve oilfield. CO2 was easily accessed from nearby immiscible floods, which sourced its supply from an ammonia plant 25 miles away via pipeline. Selected wells were either inactive or poor medium to heavy oil producers with potentials of less than 5 bopd. Details on immiscible CO2 floods in the Forest Reserve oilfield of Trinidad and Tobago are provided elsewhere1. The mechanisms contributing to increases in oil recovery are:Oil viscosity reductionOil swelling due to dissolution of CO2 in crude oilRemoval of near well-bore damageSolution gas drive aided by gravity drainageImproved drainage of reduced viscosity oil by encroaching waterVaporization of lighter components of oil by CO2Reduction of water relative permeability due to trapped gas and reduction of water saturationReduction of relative permeability to water and gas during the puff phase due to hysteresis andReduction of interfacial tensions. For the Forest Reserve reservoirs with medium to heavy crude oils, viscosity reduction and oil swelling aided by gravity drainage and water encroachment are believed to be the most significant.