Research On The Method Of Quantitative Evaluation Of The Water Injection Efficiency Of Perforation Completion Wells In Sandstone Reservoir

Injecting water into oilfields to keep formation pressure is the basic treatmentensuring the long term stable production of oilfields and improving the effectivenessof oilfield development. Currently, studies on water injection efficiency mainlyconcentrate on pipe networks on the ground and there are few researches especiallyfor reservoirs. Besides, it is found that the key point to further enhance the efficiencyof waterflood systems is to decrease the loss caused by restricting flow rate amongwater allocating stations. The solution to this key point depends on the further deeperresearch on techniques of improving the efficiency of waterflood systems extendedfrom grounds to reservoirs. In order to raise completely the efficiency of waterfloodsystems, it is necessary to explore the analysis about water injection efficiency ofreservoirs.Considering that the major completion method is perforation completion and themajority of conventional reservoirs are sandstone reservoirs, this paper completes theresearch on the method of quantitative evaluation of the water injection efficiency ofperforation completion wells in sandstone reservoir and the obtained results includeseveral aspects as following:(1) The concept of water injection efficiency of reservoirs is redefined. The newdefinition is used as the criterion to evaluate the water injection efficiency ofreservoirs. Meanwhile, the improved potential of water injection efficiency is definedas well on the basis of the water injection efficiency.(2) The calculation method for water injection efficiency is proposed in thispaper. The relevant mathematic formulations for calculating water injection efficiencyof water injection wells with perforation completion are mainly developed. Thehydraulic pressure loss in the perforated section can be described with the perforationpseudoskin coefficient. Besides, the perforation pseudoskin coefficient is decomposedinto the perforation hole pseudoskin factor, the pseudoskin factor caused byperforation compacted linear flow and the pseudoskin factor in compacted zone. Theconcrete equations for calculating these three pseudoskin factors in perforation section are presented in this paper, therefore the calculation equation of perforationpseudoskin coefficient can be obtained. Using the horizontal circular tubes tosubstitute perforating holes, the approximate model for pressure loss in perforationholes is established based on the hydromechanics theories to evaluate the pressuredrop in perforation completion.(3) The elements influencing the water injection efficiency are analyzed. Theimpact of perforation parameters on the reservoir water injection efficiency issystematically analyzed in this study. It is commonly believed that the water injectionefficiency increases as the perforation depth, the perforation density and the radius ofthe perforation hole increase. The influence of perforating phase angel on thereservoir water injection efficiency is dependent on the perforation depth.(4) The mathematic formulas for solving regional average formation pressure aredeveloped with the use of material equilibrium method and the theories of flowthrough porous media. Given the pressure loss along the path that injected water flowthrough, the equation to determine the appropriate water injection pressure is derived.Cha39fault block is used as the actual case. The perforation pseudo skin coefficients,limiting water injection efficiency, the actual water injection efficiency and waterinjection efficiency potential of33water injection wells are calculated with theavailable perforation data of33water injection wells in Cha39fault block.Meanwhile, the idealized maximum daily water injection rate, the potential maximumdaily water injection rate, the actual daily water injection rate and the improvedcapability of daily water injection rate are calculated on the condition of maximumpressure difference. Moreover, the production rate in2012is predicted and theappropriate average daily water injection rate of every water injection well in Cha39is35m~3/d which is consistent with the current water flooding situation of this faultblock. Taking the Cha39-15well for example demonstrates that with therequirements of oilfield development it is useful for injection wells with reperforatingpotential to be reperforated to improve the effectiveness of individual injection well.