Recovery Mechanisms in Fractured Reservoirs and Field Performance

Abstract

Abstract Fractured petroleum reservoirs provide considerable challenge in studyingnatural depletion, immiscible gas injection, miscible gas injection, and waterinjection. In this overview, certain key aspects of two-phase flow in relationto gas injection and water injection in fractured reservoirs are reviewed. Onemain conclusion from the review is that the field performance can be veryefficient by water injection in some weakly water-wet fractured eservoirsdespite the poor recovery in the laboratory by the conventional imbibitiontests. Introduction Fractured hydrocarbon reservoirs provide over 20 % of the world oil reserves andproduction. Examples of the prolific fractured petroleum reservoirs are:the Asmari limestone reservoirs in Iran,the vugular carbonate reservoirs in Mexico, andthe group of chalk reservoirs of the North Sea. These prolificreservoirs produce more than five million barrels of oil a day; their commonfeature is a long life span, which could last several decades. There are alarge number of other fractured hydrocarbons reservoirs that may have featuresvery different from the above reservoirs. Examples of such reservoirs are the Austin chalk field and the Keystone (Ellenberger) field in Texas, and the Tempa Rossa field in Italy. In the Keystone field, the average matrix porosity isaround 2.5% the Austin chalk and Tempa Rossa also have very low porosity. Onthe other hand, the average matrix porosity of the Ekofisk chalk field in the North Sea is around 35%. Fractured reservoirs can be classified into three different groups. For groupone, the bulk of the hydrocarbon resides in the matrix and fracture pore volume(PV) is very small in comparison to the matrix PV. The Ekofisk field in the North Sea is an example of this group(1). In group two, most of thehydrocarbon is in the matrix, but fracture PV could be as high as 10 to 20%.The Asmari limestone reservoirs are an example of the secondgroup(2). For group three, more than half of the hydrocarbon residesin the fracture; in some cases, the contribution of the matrix can benegligible. The Keystone (Ellenberger) field in Texas is an example of afractured reservoir where most of the hydrocarbon is from thefractures(3). There are very few reports of the productionperformance of group three in the literature. For all three groups, the matrixpermeability is often low-of the order of several md to less than 0.01 md. Theeffective permeability due to fractures increases from one to several orders ofmagnitude. In some of the reservoirs of group three, the productive life variesfrom less than one year to several years. The ultimate recovery from fracturedreservoirs varies widely- from less than 10 % to over 60%. The recovery factorin group three could vary from 10% to over 60% the recovery factor of 10 % ismostly from the fracture and rock compressibility, and the recovery of 60 % ismainly from gravity drainage. Later we will study the key factors that affectrecovery performance of fractured reservoirs.