Method of maintaining oil reservoir pressure

Abstract

The method of maintaining oil reservoir pressure involves injecting seawater having a concentration of about 1 wt % of a polyamino carboxylic acid or salt thereof as a chelating agent into the water zone of an oil reservoir, which reduces the formation of scale, thereby maintaining oil reservoir pressure. The polyamino carboxylic acid may be ethylenediaminetetraacetic acid (EDTA), acid (HEDTA), or hydroxyethyliminodiacetic acid (HEIDA). When used in larger concentrations, e.g., between 5 to 10 wt %, flooding the core with seawater containing the polyamino carboxylic acids not only prevents scale formation, but also improves core permeability. Instead of seawater, low salinity water may be used for the latter purpose.

We claim: 
     
         1 . A method of maintaining oil reservoir pressure, comprising the step of injecting untreated seawater containing an effective amount of a polyamino carboxylic acid or salt thereof as a chelating agent into the water zone of an oil reservoir to prevent scaling caused by precipitation of sulfates. 
     
     
         2 . The method of maintaining oil reservoir pressure according to  claim 1 , wherein said effective amount of a polyamino carboxylic acid or salt thereof comprises about 1 wt % ethylenediaminetetraacetic acid (EDTA) or salt thereof. 
     
     
         3 . The method of maintaining oil reservoir pressure according to  claim 2 , wherein said solution of EDTA in untreated seawater has a pH of about 11. 
     
     
         4 . The method of maintaining oil reservoir pressure according to  claim 1 , wherein said effective amount of a polyamino carboxylic acid or salt thereof comprises about 1 wt % hydroxyethyl ethylenediaminetriacetic acid (HEDTA) or salt thereof. 
     
     
         5 . The method of maintaining oil reservoir pressure according to  claim 4 , wherein said solution of HEDTA in untreated seawater has a pH of about 11. 
     
     
         6 . The method of maintaining oil reservoir pressure according to  claim 1 , wherein said effective amount of a polyamino carboxylic acid or salt thereof comprises about 5 wt % hydroxyethyliminodiacetic acid (HEIDA) or salt thereof. 
     
     
         7 . The method of maintaining oil reservoir pressure according to  claim 2 , wherein said solution of HEIDA in untreated seawater has a pH of about 11. 
     
     
         8 . The method of maintaining oil reservoir pressure according to  claim 1 , wherein said effective amount of a polyamino carboxylic acid or salt thereof comprises between 5 wt % and 10 wt % ethylenediaminetetraacetic acid (EDTA) or salt thereof, whereby the oil reservoir core permeability is increased. 
     
     
         9 . The method of maintaining oil reservoir pressure according to  claim 8 , wherein said solution of EDTA in untreated seawater has a pH of about 11. 
     
     
         10 . The method of maintaining oil reservoir pressure according to  claim 1 , wherein said effective amount of a polyamino carboxylic acid or salt thereof comprises between 5 wt % and 10 wt % hydroxyethyl ethylenediaminetriacetic acid (HEDTA) or salt thereof, whereby the oil reservoir core permeability is increased. 
     
     
         11 . The method of maintaining oil reservoir pressure according to  claim 10 , wherein said solution of HEDTA in untreated seawater has a pH of about 11. 
     
     
         12 . The method of maintaining oil reservoir pressure according to  claim 1 , wherein said effective amount of a polyamino carboxylic acid or salt thereof comprises about 10 wt % hydroxyethyliminodiacetic acid (HEIDA) or salt thereof, whereby the oil reservoir core permeability is increased. 
     
     
         13 . The method of maintaining oil reservoir pressure according to  claim 12 , wherein said solution of HEIDA in untreated seawater has a pH of about 11. 
     
     
         14 . A method of increasing core permeability of an oil reservoir, comprising the step of flooding the core with untreated seawater containing an effective amount of a polyamino carboxylic acid or salt thereof as a chelating agent to chelate metals in mineral formations in the core. 
     
     
         15 . The method of increasing core permeability of an oil reservoir according to  claim 14 , wherein said effective amount of a polyamino carboxylic acid or salt thereof comprises between 5 wt % and 10 wt % ethylenediaminetetraacetic acid (EDTA) or salt thereof. 
     
     
         16 . The method of increasing core permeability of an oil reservoir according to  claim 15 , wherein said solution of EDTA in untreated seawater has a pH of about 11. 
     
     
         17 . The method of increasing core permeability of an oil reservoir according to  claim 14 , wherein said effective amount of a polyamino carboxylic acid or salt thereof comprises between 5 wt % and 10 wt % hydroxyethyl ethylenediaminetriacetic acid (HEDTA) or salt thereof. 
     
     
         18 . The method of increasing core permeability of an oil reservoir according to  claim 17 , wherein said solution of HEDTA in untreated seawater has a pH of about 11. 
     
     
         19 . The method of increasing core permeability of an oil reservoir according to  claim 14 , wherein said effective amount of a polyamino carboxylic acid or salt thereof comprises about 10 wt % hydroxyethyliminodiacetic acid (HEIDA) or salt thereof. 
     
     
         20 . The method of increasing core permeability of an oil reservoir according to  claim 19 , wherein said solution of HEIDA in untreated seawater has a pH of about 11.