| Ordovician Reservoir, in the6-7block of Tahe Oilfield, is fractured vuggycarbonate reservoir. With complex fracture cave combination, fractured vuggyreservoirs vary widely, resulting strong heterogeneity of the Ordovician carbonatereservoir in the6-7block of Tahe Oilfield. Two sets of interlayer developed verticallymakes vertical heterogeneity very strong. as a result, fracture cave units previouslydivided by the method based on area are not compatible with the actual production ofthe field. Therefore this work has redivided the fracture cave units by the methodbased on three-dimensional space. Crude oil from most of wells in Tahe oilfield isheavy oil and even thickened oil, with high density, high viscosity, and smallcoefficient of compressibility, which leads to small reservoir elastic energy releases ata unit pressure drawdown. At the beginning production period the reservoirs wereexploited mainly depending on elastic energy, at mid-to-late production period, edgeor bottom water drive began when the reservoir water began to be produced andsubsequently the injected water drive emerged. Because the production of thereservoir decreases rapidly in the elastic energy period and slowly in the water driveperiod, the production of the fracture cave units and blocks declines with two trends.This work studied the effects of this multiple trends on the movable oil reservesprediction of oil wells and reservoirs.This work reviewed9empirical production decline method, including Arpsdecline method, and modern production decline curves by Fetkovich. This workintroduced the application of these methods and deduced the theoretical bases of someof them. The advantages and disadvantages as well as the using problem and error ofthese methods are figured out in this work. The applying ranges of main declinemethod are expounded. The production performances of4different scale fracturecave units and115wells are analyzed to find that the productions of carbonatefracture cave units and the oil wells influenced by water cut increasing are stronglyvolatile. Thus this greatly affected the accuracy of decline curve fitting anduniqueness of the fitting result. This work proposed a combined type curves chartbased on a great deal of previous research, which improves the fitting precision greatly and reduces the fitting uncertainty. The combined type curves chart takes theadvantage of the stability of the cumulative production changes over time.Combining the pseudosteady-state (or boundary-dominated) flow theory andmaterial balance theory and by introducing of T.A. Blasingame material balance time,the decline curves of fracture cave units and wells meeting the material balance infinite outer boundary and being pseudosteady-state flow can be transformed intoharmonic decline type. According to this, it is possible to diagnose if the fracture caveunits and wells meet production condition that is meeting the material balance andpseudosteady-state flow in finite outer boundary and at constant bottom-hole pressure.Through the analysis, it is known that the match point of the type curve match will notchange before and after the conversion. Afer conversed to harmonic decline over thematerial balance time, the production changes stablely over the material balance time.This can also make the match more precisely. And a method of calculating movableoil reserves and reserves is presented in this work.According to the analysis results, the decline type of fracture cave units andwells are diverse and changeable. Decline types are different at different stages ofproduction. This work analyzed influence of the rise of water cut, reservoir spacetypes, structural characteristics, changes in energy and the time wells began toproduce on the oil production. The main factors affecting each fracture cave unit areanalyzed and quantified in detail. According to the production affecting factors, thecauses of4fracture cave units are figured out. That provides a guide for the effectivedevelopment of fractured vuggy unit. |
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