Optimum salinity profile in surfactant/polymer flooding

Abstract

An optimum salinity profile in surfactant/polymer flooding from formation water to post-flush drive that leads to the highest oil recovery factor is shown. The optimum salinity determined from core-flooding experiments is preferably used in the surfactant slug. The surfactant slug is protected from deterioration by the injection of cushion slugs immediately before and after the injection of the surfactant slug in a reservoir wherein the cushion slugs have the same salinity or about the same salinity as the surfactant slug. According to embodiments, a salinity lower than the lowest salinity of Type III, C sel , is used in the post-flush drive, while formation water could be of any salinity.

1 . A method of recovering oil from a reservoir using surfactant flooding, said method comprising:
 injecting a fluid slug in said reservoir;   injecting, after said first injection, a first cushion slug in said reservoir,   injecting a surfactant slug following said first cushion slug;   injecting a second cushion slug following said surfactant slug, wherein said first and second cushion slugs have salinities selected from the list consisting of: the salinity of said surfactant slug and about the salinity of said surfactant slug; and   injecting a fluid slug of lower salinity than said surfactant slug, after said second cushion slug, wherein said injected fluid slug, of lower salinity than said surfactant slug, has a salinity lower than the lowest salinity at which a three phase microemulsion system exists at equilibrium.   
   
   
       2 . The method of  claim 1  wherein said salinities of said first and second cushion slugs are the same as said surfactant slug. 
   
   
       3 . The method of  claim 1  wherein said surfactant slug contains a co-surfactant. 
   
   
       4 . The method of  claim 1  wherein said surfactant slug includes a polymer. 
   
   
       5 . The method of  claim 1  wherein said first and second cushion slugs are selected from the list consisting of:
 water, brine and mobility control agents.   
   
   
       6 . (canceled) 
   
   
       7 . The method of  claim 1  wherein said first injected fluid slug is selected from the list consisting of:
 water, brine and mobility control agents.   
   
   
       8 . The method of  claim 1  wherein said first injected fluid slug has a salinity different from said surfactant slug. 
   
   
       9 . (canceled) 
   
   
       10 . (canceled) 
   
   
       11 . The method of  claim 1  further comprising:
 injecting a fluid slug of lower salinity than said surfactant slug, after said second cushion slug.   
   
   
       12 . The method of  claim 11  wherein said fluid slug is selected from the list comprising:
 water, brine and surfactant solution mobility control agents.   
   
   
       13 . The method of  claim 1  wherein the salinity of said surfactant slug is an actual optimum salinity determined by running experiments, wherein said actual optimum salinity determined from said experiments is a function of IFT and parameters other than IFT. 
   
   
       14 . The method of  claim 13  wherein said experiments include running core flood experiments. 
   
   
       15 . The method of  claim 14  wherein said core flood experiments are run first to determine the actual optimum system type and then to determine an actual optimum salinity in said actual optimum system type. 
   
   
       16 . The method of  claim 14  wherein said core flood experiments are run in at least two surfactant system types, said surfactant system types selected from the list consisting of: Type II(−), Type III, and Type II(+) system. 
   
   
       17 . A method of recovering oil from a reservoir, said method comprising:
 selecting an optimum salinity by running at least one core flood experiment to measure oil recovery applicable to each of at least two surfactant system types, said surfactant system types selected from the list consisting of: Type II(−), Type III, and Type II(+) system;   injecting a first cushion slug in said reservoir,   then injecting a surfactant slug; and   then injecting a second cushion slug, wherein said first and second cushion slugs and said surfactant slug is or about at the optimum salinity.   
   
   
       18 . The method of  claim 17  further comprising:
 injecting a fluid slug prior to said first cushion slug.   
   
   
       19 . The method of  claim 17  further comprising:
 injecting a fluid slug of lower salinity than said optimum salinity, after said second cushion slug.   
   
   
       20 . The method of  claim 17  wherein said first and second cushion slugs are selected from the list consisting of:
 water, brine and mobility control agents.   
   
   
       21 . A method of recovering oil from a reservoir using surfactant flooding, said method comprising:
 injecting a fluid slug in said reservoir;   injecting a first cushion slug;   injecting a surfactant slug following said first cushion slug, said surfactant slug comprising a polymer and a co-surfactant;   injecting a second cushion slug following said surfactant slug, wherein said first and second cushion slugs comprise brine and a mobility control agent and said first and second cushion slugs have salinities selected from the list consisting of: the salinity of said surfactant slug and about the salinity of said surfactant slug; and   injecting a fluid slug of lower salinity than said surfactant slug, after said second cushion slug, wherein said injected fluid slug, of lower salinity than said surfactant slug, has a salinity lower than the lowest salinity at which a three phase microemulsion system exists at equilibrium.   
   
   
       22 . The method of  claim 21  wherein said surfactant salinity is an actual optimum salinity determined by running experiments and said actual optimum salinity determined from said experiments is a function of IFT and parameters other than IFT. 
   
   
       23 . The method of  claim 22  wherein said experiments include running core flood experiments. 
   
   
       24 . The method of  claim 23  wherein said core flood experiments are run first to determine the actual optimum system type and then to determine an actual optimum salinity in said actual optimum system type. 
   
   
       25 . The method of  claim 14  wherein said core flood experiments are run in all of the following surfactant system types: Type II(−), Type III, and Type II(+) system.