Studies On The Distribution Of Fluids Of Ordovician Reservoir In Tahe Oilfield And The Strategies Of Its Water Control

Located at the southwest slope of Akekule heave to the south of Shaya uplift in northern Tarim basin, Tahe oilfield is discovered in 1990. With a well-controlled oil-bearing area of 2800 km~2, it is the first marine Paleozoic oilfield with an oil reserves more than 100 million tons on Chinese continent. The primary reservoir is feathered by a fracture-cave type in the Ordovician carbonate formation.Vertically, the hydrochemical profile is characterized by reverse variation and multi-cyclicity. Three typical units could be defined downwards along the profiles: ① a freshening zone from ground to 5300m due to meteoric water, with relative low salinity, infiltration.②a concentrating zone from 5300 to 5800m resulted from cross-formational flow and evaporation, with salinity of 150-250 g/L. ③a freshening zone below 5800m due to water released from compacted mudstone where salinity drops from 150 g/L to 80 g/L. Horizontally, a freshening area due to meteoric water can be found in the north of southern slope of Akekule heave. A marginal freshening area due to water released from compacted mudstone can be found at the bottom of the southern slope, which is located in northern edge of Manjiaer depression. Located in the cross-formational flow discharge area formed by the centripetal flow and the centrifugal flow, the primary Tahe oilfield, featured by high salinity and high concentration of Cl~- and K~+ + Na~+, is favorable for accumulation of hydrocarbon. After accumulation through late Hercynian period, Indosinian period, Yanshan period and Himalayan period, the Ordovician hydrocarbon reservoirs in Tahe oilfield was formed after the infiltration and metamorphose of the highly matured hydrocarbon produced during YanShan-Himalayan period.Significant "speedway" effect was observed in the migration of hydrocarbon in Tahe oilfield. Oil and gas enter reservoirs through macropores (with smaller caterpillary pressure) first and then forced into smaller pores, resulting in the driven-out of the residual water there. Due to heterogeneity and complicate pore structure of carbonate reservoirs, it is possible that some corners remained un-displaced after the hydrocarbon filling-in. The residual water increases gradually toward south from the main part of Tahe oilfield.Based upon the characteristics of pore structure, oil-gas displacement process and chemical-dynamic response of produced water during the development of Tahe oilfield, three types of water were identified in this dissertation: 1) residual water in bottom of cave after oil and gas displacement, 2) residual water in fracture/pore around cave after oil and gas displacement, and 3) interlayer water below reservoirs. Due to strong heterogeneity of Ordovician carbonate reservoir in Tahe oilfield, pore structure and oil-water distribution are very complicated. Because of the differencesin the development of background reservoirs, displacement percentage of oil and gas, and the size of reservoirs, variations in the corresponding oil-water distribution, development performance and water cut behaviors are observed. Based upon the well performance and reservoir development, the water cut trend curve can be divided to: 1) slowly increasing curve, 2) stepped increasing curve, 3) rapidly increasing curve, and 4) fluctuating curve. From the variations in salinity, Cl~- and K~+ + Na~+ concentrations in produced water, four dynamic types are categorized: stationary, fluctuant, rising, and descending types. Based upon the analysis of development performance and oil/water characteristics, groundwater could be categorized into two groups with discrimination model: 1) A-type groundwater, usually found at edge of tight reservoirs and in background reservoirs, is the primary residual water in bottom and around cave after oil/gas filling-in. Salinity, Cl~- and K~+ + Na~+ concentrations remain steady, or minor fluctuation/increasing during development of oilfield. 2) B-type groundwater, usually separated from reservoirs and found in aquifer below reservoirs, is mainly interlayer water under reservoirs. Salinity, Cl~- and K~+ + Na~+ concentrations decline rapidly during development of oilfield.The main oil-bearing space of Ordovician reservoir in Tahe oilfield includes pore, cave and fracture in carbonate karst reservoirs. Fracture and cave are effective storage and percolation space. Fracture-cavity units are the basic units of storage room, each of them can be looked as an independent hydrocarbon reservoir. The hydrochemical characteristics are related with the units' connectivity. The wells in units with good connectivity share similar hydrochemical characteristics and those in units with poor connectivity have different hydrochemical characteristics.The cave system consists of main cave, branch cave and depressions between caves. With thick oil layer and high displacement of oil/gas, groundwater evaporate and concentrate at the center of main cave during cross-formational flow, resulting in high salinity and Br~-concentration areas, the distribution of which corresponds well with the scope of fracture-cavity unit. From the center to the edge of main cave, displacement and enrichment of oil/gas decrease and residual water increase. Meanwhile, salinity and Br~-concentration decrease along with the descending concentration. The salinity and concentration of Cl~-, Na~+ + K~+ in main cave generally appears to be stationary, fluctuant, or rising. The produced water of oil well in center is almost A-type, and those in edge are mostly A-type and sometimes B-type. The salinity and concentration of Cl~-, Na~+ + K~+ in branch cave usually appear to be stationary or fluctuant. Groundwater is mostly A-type and sometimes B-type. The salinity and concentration of Cl~-, Na~+ + K~+ in depressions between caves generally fluctuant or descending. Groundwater is characterized as A-type and B-type present alternately or mainly B-type.Based upon reservoir characteristics and hydrochemical-hydrodynamiccharacteristics of oil pool, following recommendation for stable production and water control in Tahe oilfield are made: ①Design various development scenarios to efficiently control the water yield by clarifying the different types of water and their chemical-dynamic characteristics. ②Adjust distribution of fluids within reservoirs and improve displacement efficiency at various development stages by injecting water. ③Deploy intense wells to reach the usually "hard-to-reach" reservoirs ④Set the output rate of single wells appropriately to extend the stable-production period. ⑤ Adjust the development of oilfield based upon the reservoir situation.⑥Employ acid fracturing to improve reservoir permeability and conductivity. ⑦Employ physical/chemical technology to plug water and control profile.