页岩气开发中的几个关键现代力学问题

Several key problems of modern mechanics in shale gas exploitation

伴随着现代信息技术的发展,现代力学的研究进入了一个新的阶段.本文将钱学森先生介绍的现代力学概念赋予了新的内涵,概括了现代力学的研究主体、研究方法和主要研究方向.从拉格朗日方程出发,按照表征元的物理变量和积分区域的几何表述,对计算力学的基本方法进行了分类,提出了通过实验和数值模拟共同获得介质的力学参数、研究介质力学特性和行为的思路和方法,在此基础上建立了表征元(计算单元)的积分——微分方程,体现了现代力学基本理论框架的特点.表征元的材料特性、力学行为以及由表征元构成的宏观介质全场运动规律均是现代力学的重要研究内容.现有材料实验目的是获得抽象为连续介质的材料应力应变关系和材料强度,而现代力学可以突破这个限制,通过多变量测量并结合数值模拟获得材料的特征和演化规律.在现代力学中全场解可以由数值计算获得,并由监测结果校核.通过与丰富的监测结果比较,数值模拟不可信的问题会逐步化解.页岩气开发的关键问题是研究地下页岩在各种条件下的破裂演化规律.页岩气开发新方案的技术路线应该是现代力学先行,以避免盲目在工程尺度上进行方案论证;借助现代力学可以打破水力压裂技术的局限性,探索新的技术方案.主要理论研究包括以下几个方面:(1)在认识表征元破裂度、渗透性演化的基础上,寻求关联性的理论表述方法;(2)研究体破裂度的渗流场、破裂场以及颗粒的相互作用规律;(3)时空全尺度模拟需要从理论上研究新的力学模型,其中包括借用岩体表征元破裂度和灾变破裂度的概念,实现破裂场与材料渗透性关联;(4)由流量、井口压力曲线计算出的岩体破裂参量可以被页岩气的产量校核,从而为提出新的工程方案、指导工程施工提供可参考的技术指标.

Investigation of modern mechanics has come into a new stage with the development of modern information technology.In this paper,modern mechanics introduced by Hsue-Shen Tsien has been endowed with new connotation,and the principal research subject,methods and direction of modern mechanics are summarized.Based on the Lagrange equation,the fundamental methods of computational mechanics are classified according to the geometrical description for the physical variants and integration domain of the representative element.The method to obtain mechanical parameters,properties and behaviors of materials with the combined effect of experiments and numerical simulations are proposed.And the integral and differential equations of the representative element are established with this method.The material properties,mechanical behavior of the representative element and the macro mechanical characteristics of the media composed of the representative elements are the key research topics in modern mechanics.The aim of the current material tests is to obtain the stress-strain relationship and the strength for the abstract continuous media.However,modern mechanics could do more.Properties and the evolution process of the material are determined through multi-variable measurement and numerical simulation.The whole field resolution are calculated with numerical methods and verified with monitoring data.Comparing with abundant monitoring data,questions for simulation can be gradually eliminated.The key problem in shale gas exploitation is to investigate the fracture evolution law of the shale under various situations.The technical route for the new method in shale gas exploitation should use modern mechanics to avoid blind argument in engineering scale.With modern mechanics,the limitation of hydraulic fracturing technology could be broken through and new techniques can be explored.The key issues includes：（1） To understand the fracture degree and permeability evolution of the representative element,and search for the relevant theoretical expression.（2） Investigate the problems of seepage-fracture coupling and interaction of particles.（3） New mechanical model for the full scale simulation should be developed,which could establish the relationship between fracture and permeability referring to the concept of fracture degree of the representative element.（4） Validate the fracture parameters calculated by flow and pressure at the wellhead,to provide technical indicators for the engineering design and construction.