This paper presents the results of laboratory and nuclear magnetic resonances researches of seventy-seven crude oil samples from the Devonian, Carboniferous and Permian formations of Tatarstan oil fields with a wide range of viscosity varying from several cps to 60,000 cps. The results can be used to estimate the viscosity of the oil, including in situ viscosity. It has been found that the NMR relaxation characteristics of the tested samples presented in the form of T2 time distribution spectra confirm that crude oil is multicomponent. Numerical characteristics resulting from the analysis of the spectrum of spin-spin relaxation time T2 have been determined, which show good correlation with the viscosity of the hydrocarbon fluid. The paper presents the obtained experimental correlations between the rheological properties and NMR characteristics for oil samples studied.

The existing methods of reservoir management based on bottomhole pressure control imply setting of limited bottomhole pressures that are determined either through equations or based on pressure transient analysis (PTA). However use of project profitability metrics commonly applied in oil industry–cumulative oil production and net present value (NPV)–yields more valid results. In doing so, all basic aspects must be taken into consideration: geological setting, current reservoir performance, reservoir fluid properties, well interference, and others. This can be achieved through application of 3D flow simulation and inverse modeling methods. To this end, we have developed an algorithm and created a few pilot models with different reservoir characteristics and fluid properties identical to conditions of actual reservoirs operated by the Company. The calculation results were analyzed considering specific reservoir conditions. We have revealed some principles and tendencies that can be applied to improve performance of oil and gas fields. The research results show that the choice of optimal bottomhole pressure is largely dictated by presence or absence of gas-cut fluid flow. In case the bubble point pressure is close to the initial reservoir pressure, optimal bottomhole pressures differ significantly from limiting bottomhole pressures and, thus, should be calculated. In case the bubble point pressure is lower than the original in-situ pressure, limiting bottomhole pressure is the right choice for production wells. We also found that in fractured-porous reservoirs the right time of putting injection wells into operation is of critical importance. The revealed principles and recommendations can be applied to improve the effectiveness of reservoir management operations and increase final recovery.

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Oil industry knows dozens of hundreds of different EOR/IOR methods to improve reservoir recovery efficiency. Among today's priorities are assessment of various EOR/IOR and bottomhole treatment technologies and selection of the most effective ones that will meet the specific reservoir conditions. For assessment of stimulation efficiency, different techniques can be used: decline curve analysis (DCA), production rates analysis before and after stimulation, analysis of reservoir properties in the near-wellbore zone and in the reservoir using pressure build-up (PUB) curves. Each technique has advantages and disadvantages. Thus, comparison of production performance ignores bottomhole pressure changes before and after stimulation, pressure buildup curves are not infrequently of a rather low quality, DCA is based on empirical relationships liable to misinterpretation because of subjective estimate. Devoid of these drawbacks is the rate transient analysis (RTA). The advantage of this method is that it makes allowance for change of production rates always occurring following stimulation. This is achieved through use of diffusion equations. Practice has shown that RTA provides a comparative analysis of production rates and cumulative oil production through time, porosity and permeability before and after stimulation, being, thus, a comprehensive tool for efficiency evaluation. Variation in oil production is the most reliable parameter, because it accounts for changes in bottomhole pressure and water cut before and after stimulation. To determine this parameter, an algorithm based on the pressure drop change is offered. RTA allows production forecast by two scenarios, the scenario involving stimulation, and the scenario without any production enhancement operations with a view to assess cumulative incremental production. In conclusion, it can be said that rate/pressure transient analysis allows assessment of efficiency of a large variety of EOR/IOR projects and a long-term production forecast. The offered approach may serve a good alternative to the decline curve analysis and comparison of production rates and PUB curves before and after stimulation.