A Literature Analysis of the WAG Injectivity Abnormalities in the CO_2 Process

摘要

As a result of research in the 1950s and 1960s, carbon dioxide(CO_2) flooding has been implemented at the pilot and projectstage in the petroleum industry since the early 1970s. A plethoraof articles have been produced on the problems and successesof using CO_2 as an enhanced oil recovery process. Anumber of the operational problems regarding full-scale implementationhave been settled to some degree. The WAG(water alternating with gas) technique to improve mobilityefficiency of the higher mobile CO_2 gas over the lower mobilereservoir fluids was an evolutionary step in the technical andeconomic implementation of CO_2 as a tertiary recovery process.This combination of two improved oil recovery processes(waterflooding and gas injection) resulted in a problem thathas perplexed the industry since implementing the pilot studiesin the early 1970s. A recent survey conducted by NewMexico Petroleum Recovery Research Center (NMPRRC) onCO_2 flooding indicated that injectivity abnormalities duringWAG cycles has been a crucial limiting factor in many projects.Additionally the WAG process has been expanded toinclude most gas injection processes of enhancing hydrocarbonrecovery – immiscible and miscible processes.Based on the fluid flow properties of CO_2 and other IORgases, one would intuitively expect that gas injectivity wouldbe greater than the waterflood brine injectivity. However, inpractice this behavior is not always observed. In addition, waterinjectivity may be higher or lower than the waterfloodbrine injectivity. What is more perplexing is that some reservoirsmay lose injectivity and others may increase injectivityafter the first slug of gas (CO_2) is injected. In addition, thisphenomenon may occur on a local scale. Injection wells in thesame field and reservoir may have significantly different behavior.A number of researchers have studied and proposedreasons for this phenomenon over the past 20 years.This paper summarizes the hypothesis and theories as tothe causes and expectations of injectivity behavior in variousCO_2 and gas flooded reservoirs. The intent of the paper is to:1. Provide a concise compendium to the current understandingof the WAG mechanism and predictability,2. Provide a comprehensive single source review of thecauses and conditions of injectivity abnormalities inCO_2/gas flood EOR projects,3. Aid in formulating the direction of research, and4. Help operators develop operational and design strategiesfor current and future projects as well as input parametersfor simulating current and future projects.