The Feasibility of CO2 Injection for IOR in Shale Reservoirs

摘要

Improved oil recovery(IOR) methods for shale-oil reservoirs are considered relatively new concepts compared with IOR for conventional oil reservoirs.Different IOR methods-including CO2,surfactant,natural gas,and water injection-have been investigated for unconventional reservoirs using laboratory experiments,numerical simulation studies,and limited pilot tests.For a variety of reasons,CO2 injection is the most-investigated option.In this paper,numerical simulation methods of compositional models were incorporated with logarithmically spaced,locally refined,and dual-permeability reservoir models and local grid refinement(LGR) of hydraulic-fracture conditions to investigate the feasibility of CO2 injection in shale oil reservoirs.Advancements in horizontal drilling and hydraulic fracturing enabled unconventional liquids-rich reservoirs(ULRs),such as shale and source-rock formations and very tight reservoirs,to change the oil industry.ULRs are characterized by pore throats of micro-to nanomillimeters and an ultralow permeability.Although different studies reported that these ULRs contain billions of recoverable oil barrels in place,it is estimated that less than 7% of the original oil in place can be recovered during the primary depletion stage.Production sustainability is the main problem behind the low oil recovery in these unconventional reservoirs.Oil wells in ULRs typically start with a high production rate,but show a steep decline rate in the first 3-5 years of production life because of the rapid depletion in the natural fractures combined with a slow recharge from the rock matrix.