Key problems and solutions in supercritical CO2 fracturing technology

Supercritical CO₂ fracturing is considered to be a new method for efficient exploitation of unconventional reservoirs, such as shale gas, coal bed methane, and tight sand stone gas. Supercritical CO₂ has many special properties including low viscosity, high diffusion coefficient, and lack of surface tension, which brings about great advantages for fracturing. However, these properties also cause several problems, such as difficulty in proppant transportation, high friction loss, and high pump displacement. In this paper, the above problems were analyzed by combining field test with laboratory study and specific solutions to these problems are given. The high frictionloss in the pipeline could be reduced by developing a new drag reducing agent and selecting large-size casing. Besides, for the problem of poor capacity in proppant carrying and sand plug, the methods of adding tackifier into supercritical CO₂, increasing pump displacement and selecting ultra-low density proppants are proposed. Moreover, for the problem of fast leak-off and high requirement for pump displacement, the displacement can be increased or the pad fluid can be injected into the reservoir. After solving the above three problems, the field test of supercritical CO₂ fracturing can be conducted. The research results can promote the industrialization process of supercritical CO₂ fracturing.     

Numerical integration of semidiscrete evolution systems

Methods and algorithms for integrating initial value systems are examined. Of particular interest is efficient and accurate numerical integration of systems of ordinary differential equations that arise on semidiscrete spatial differencing or finite element projection for evolution problems characterized by partial differential equations. Integration schemes for general systems are described. Stiff and oscillatory systems are considered and these motivate selection of specific types of algorithms for certain problem classes. For example, we show that Runge-Kutta methods with extended regions of stability are particularly efficient for moderately stiff dissipative systems derived from parabolic transport equations. The theoretical developments of an earlier paper [1] determine bounds on stiffness and stability and may be used to examine the stiff dissipative or oscillatory nature of the system qualitatively. Order control and stepsize adjustment in variable-order, variable-step algorithms are compared for several integrators applied to stiff and nonstiff initial-value systems arising from representative parabolic evolution problems.     

低渗油藏CO_2驱提高采收率与埋存中的有效设计(英文)

由于具有较低的粘度和注入能力,CO2驱在低渗油田开发中相比衰竭和注水开发有明显的优势,而其开发效果受地质条件、油藏能量、流体特征及工艺技术等多种因素影响．常规的油藏数值模拟方法只能进行单次单因素分析,且不能分析不同因素之间的交互作用,难以满足上述低渗油藏CO2驱中的多因素多水平的研究需要．作者将实验设计方法引入上述CO2驱优化设计的数值模拟研究中,利用该方法可以利用少量具有代表性的CO2方案,最大量地获取需要的敏感性分析信息,从而有效提高数值模拟方法的效率．采用上述方法分析了低渗油藏CO2驱中的关键因素以及它们之间的交互作用,确定了适合低渗油藏条件的CO2驱注采方式、注采井型及其优化组合．研究表明,利用大斜度井连续注CO2,结合生产井生产气油比和油藏平均压力实施开关井控制方法可以增加注入能力、缩短开发周期,并能够同时获得油藏采收率和温室气体埋存的联合优化效果;研究同时发现存在相渗滞后作用的情况下油藏CO2埋存潜力较大．     

Gershgorin theory for stiffness and stability of evolution systems and convection-diffusion

An analysis of stiffness and stability based on Gershgorin theorems for eigenvalues is developed for initial value systems. In particular, semidiscrete formulations for evolution problems are analysed. Common techniques such as semidiscrete finite difference and finite element methods are examined using eigenvalue bounds to characterize stiffness and stability of the associated systems. The analysis is applied to a prototype convection-diffusion problem to demonstrate the arguments and clarify several current questions concerning the qualitative nature of the solution and errors, including effects of: “lumped” versus “consistent” finite element formulations; high- or low-degree bases; mesh refinement, dimensionality and differing material properties. To study general initial value systems such as those arising in consistent finite element formulations, a generalized Gershgorin theory and computable bounds in the chordal metric are utilized.     

Development and Implementation of a Multidimensional Reservoir Souring Module in a Chemical Flooding Simulator

Abstract

Reservoir souring is an after-production phenomenon in reservoirs that are subjected to seawater injection. Souring can damage the reservoir rocks, production facilities, and the environment. In order to predict the onset of reservoir souring, several models have been developed. Study of the published models for the reservoir souring prediction shows that there are many inconsistencies between the simulated results and field data. These differences are due to the capability of the models to consider the effective parameters in the porous media. In this study, we introduce a new model that has more features than previous models in simulation of generation and transportation of H₂S in reservoir for the purpose of field applications.