Screening Of Gas Injection Techniques For IOR In Low Permeability Reservoirs

The targeted oil reservoir?Q131 block?is in Liaohe oilfield,Northeast of China,which is featured with light oil,very low permeability and sensitive to formation water,but with thick oil layers and large dip angle.The original reservoir pressure is of 33.4 MPa,reservoir temperature of 98-112 ?,and the OOIP is 2.53x106 m3.The viscosity of crude at reservoir condition is of 0.5mPa.s.Low primary production and failed water flooding experience have shown that the sweep efficiency and hence oil recovery factor in the block is very low due to lack of reservoir energy and poor water injectivity as a result of the low permeability and high heterogeneity of the reservoir block.Gravity Stabilized gas injection method has been optioned as an alternative tertiary improved oil recovery?IOR?technique to increase and/or maintain reservoir pressure,increase sweeping and displacement efficiencies to improve oil recovery.For this purpose,three different gas injection techniques,namely Air injection,Carbon dioxide?CO₂?injection and Nitrogen?N₂?injection are carefully selected in this study to be investigated and screened,as possible IOR methods for application in the low permeable Q131 light oil block.Real-time field data from this oil block were reviewed and summarized to evaluate feasibility of gas injection IOR technologies.The feasibility study of using these 3 gas injection methods for IOR was combined with literature review,theoretical analysis,knowledge of mechanisms of gas injection techniques,field application experiences,laboratory experimental studies,PVT analysis,history matching of the field's production data,building a generic reservoir static model and reservoir numerical simulation of the 3 different gas injection techniques using the compositional CMG-STARS simulation software.Further work is carried out on screening the corresponding effects of the injected gases on increasing reservoir pressure,improving oil recovery,project economics,and their associated potential safety,corrosion and risk factorsFor the air injection process,kinetic models of low temperature oxidation?LTO?reactions for the air injection process were designed through laboratory experiments and used in the reservoir simulation study.The displacement experiments of air flooding indicate that air injection can achieve relatively higher oil recovery than water injection in the block and the oil recovery increased proportionally with increasing dip of the reservoir.The LTO experimental results over a temperature range of 130-170°C of a typical light oil indicate that LTO reaction is effective to completely consume O₂ and have a high CO₂ conversion rate of about 58%.The results of the reservoir numerical simulation study show that the cumulative produced oil increases as the gas injection rate increases,up to an optimum level and no oxygen breakthrough was observed at the production wells.For a 30 years period of injection using a base case 30000m3/day of gas injection rate?4 injectors?,an incremental oil recovery factors of 35%,36.5%,35.5% of OOIP were achieved in air,CO₂ and N₂ injection simulations respectively,with CO₂ having the best production performance.Early gas break-through and high GOR in producers are important factors to influence the gas injection performance.Sensitivity study indicates that shut-in of wells with high GOR can effectively reduce gas production,and a better reservoir performance of incremental oil recovery factors of about 33.5%,36%,and 34% OOIP can be achieved for the respective gas injections in just an average period of 20 years.This is very favorable to the project economics.Preliminary economic and safety screening analyses,also confirmed that the 3 injection techniques are feasible and they all are profitable,i.e.positive net profit value,with air injection proving to be the most attractive of the three methods in terms of net profit due to its low cost of operation.Safety operations and corrosion controls of the different gas injection techniques are discussed in details and some recommendations are also provided for better IOR process in low permeability oil reservoirs.