Application of Geological Modeling and Reservoir Simulation to the West Saertu Area of the Daqing Oil Field

Abstract

Summary An integrated geological and engineering study was conducted for arepresentative section of the Saertu and Putaohua reservoirs in the West Saertuarea of Daqing, the largest oil field in the People's Republic of China. Thispaper discusses various aspects of the study related to reservoir geology, field development, geological and reservoir simulation modeling, productionhistory matching, and reservoir performance predictions. Geological andreservoir simulation models developed during this study are being usedroutinely in Daqing as part of a plan to improve reservoir management of the West Saertu portion of the field. Introduction The Daqing oil field in the Songliao basin about 1,000 km [620 miles]northeast of Beijing (Fig. 1) was discovered in 1959 and has been on productionsince 1960. Oil is present in the Saertu, Putaohua, and Gaotaizi formations, which will be referred to as Reservoirs S, P, and G, respectively. Theseformations, encountered at depths from 700 to 1,200 m [2300 to 3940 ft], arepart of a Lower Cretaceous, non-marine, fluvial/deltaic sedimentary sequencethat contains up to 100 individual sand layers with thicknesses from 0.2 to 20m [0.66 to 66 ft]. From a structural viewpoint, the Daqing oil field can bebroadly described as a large composite anticline (140×20 km [87×13 miles]) thatis elongated north/south and includes seven subsidiary "highs" (Fig.1). The Saertu pool is the second "high" from the north end. Thewestern portion of this pool (West Saertu) was one of the earliest sections of the field to be developed, with most of the production thus far coming fromcommingled wells completed in Reservoirs S and P. As part of a plan to improvereservoir management of this portion of the field, an integrated geological andengineering study was carried out by a team of geologists and engineers fromthe Daqing Petroleum Administrative Bureau and Exxon Production Research Co. Inthis study, a detailed reservoir description was developed and used to build asimulation model of a representative segment of Reservoirs S and P in WestSaertu. (Because underlying Reservoir G was only recently brought on productionthrough dedicated wells and facilities, this reservoir was not included in thestudy.) The geological and simulation models resulting from this effort wereexpected not only to provide a better understanding of the complex geologicalnature of Reservoirs S and P, but also to be of considerable assistance inmaking operational decisions and in selecting the most attractive strategy fordepleting this portion of Daqing. In addition, a reservoir simulation modelmatched against almost 27 years of production history should result in areliable tool for effective management of these reservoirs during theirhigh-water-cut production phase. This paper describes the approach used todevelop the geological and reservoir simulation models and illustrates theirapplicability by comparing reservoir performance predicted by the simulationmodel for two distinct operational strategies. Reservoir Geology A small, but geologically representative, 89-well segment of West Saertu wasselected as the study area for the reservoir description phase (Fig. 2). This89-well segment is located on the west flank of the Saertu pool (Fig. 1), covers an area of approximately 4.5 km2 [1.74 sq miles], and has an originaloil in place (OOIP) in Reservoirs S and P of about 52 × 106 stock-tank m3 [327million STB]. The study area is bounded on the northeast and southwest flanksby sealing faults, with average throws ranging from 20 to 130 m [66 to 426 ft]. Structural dips are gentle, varying from 5 to 10. Within the study area, the Reservoir S section averages 130 m [426 ft] in thickness, of which about 30% isnet sandstone distributed among 15 to 25 individual beds. The Reservoir Psection averages 80 m [262 ft] in thickness, of which about 45 % is netsandstone distributed among 10 to 20 beds. The reservoir section included inthis study was subdivided into zones and subzones by well log correlation-threezones and nine subzones in Reservoir S and two zones and six subzones in Reservoir P. Fig. 3 shows these subdivisions on a typical well profile. Thesubzone markers were used to provide the stratigraphic control required forgeologic modeling. (As explained later, these markers also defined most, butnot all, of the simulation model layers.) Displayed as stippled bars in thecenter track of Fig. 3 are the individual sand beds encountered in this well;their log-estimated porosity and permeability are plotted in the left and righttracks, respectively. Black bars in the center track are indicative of sandintervals interpreted from logs as having been waterflooded at the time thewell was drilled. Porosity ranges from 20 to 30%, with an average of 25 %;permeability varies from 20 to 1600 md, with an average of 230 md. Field Development Reservoirs S and P in the 89-well study area have been on production sincemid-1960. Pressure maintenance by water injection started almost immediatelyfollowing the inception of oil production. Injection was initially accomplishedthrough a widely spaced 3:1 line-drive pattern, with water-injection wellslocated along the southeastern and northwestern ends of the study area. Unfortunately, the injection water channeled along the high-permeability sandsat the top of Reservoir P and reached the production wells in less than 3years. In the early 1970's, additional drilling for oil production wasaccompanied by a corresponding expansion of water injection, in which theoriginal linedrive system was supplemented with wells drilled in aconfiguration resembling a nine-spot pattern. These infill wells, selectivelycompleted in intervals other than the top subzones of Reservoir P, havecontributed significantly to the observed increase in the overall waterfloodsweep efficiency. At present, essentially all production wells are onartificial lift; the average water cut of these wells exceeds 89 %. The moststriking feature of Daqing operations relates to the field-wide use of surfaceand subsurface equipment for effective segregation of production and injectionin individual wells by groups of completion intervals. This segregation isaccomplished by the intensive use of packers and special wireline-operatedtools for running eccentric mandrels with adjustable flow chokes for eachseparate packed-off interval. These special wireline-operated tools are alsoused for such well services as stimulation, perforation plugging, pressurerecording, flow testing, and flow metering. Up to 14 packers per well (allowingfor separate operation and control of 15 groups of completion intervals) havebeen used in Daqing. For wells in the study area, however, only three to fivepackers are commonly used. SPERE P. 99^