Xie Lingzhi

Novel idea of the theory and applicationof 3D volume fracturing for stimulation of shale gas reservoirs

Shale gas has become a preferably alternative energy resource and national strategic goal for many countries for its noticeable advantages, including huge reserves and potentials, green, and broad distribution. Successful exploitation and utilization of shale gas has profoundly impacted global economy, regional politics, and even military strategies around the world. China holds the largest amount of recoverable shale gas reserves in the world. The shale gas recoverable reserves of China rank firstly in the world. However, most of the shale gas formations in China are low-permeability deposit and in conditions of geological complex, tectonic active, and lack of water, which makes the exploitation of shale gas difficult using the current methods. It is imperative to renovate the methods and techniques for stimulation of shale gas formations and to establish theory and technology for achieving high-efficiency exploitation and utilization of shale gas in China. This paper briefly reviews the adopted methods, principles, intractable issues and challenges in reservoir fracturing. A promising reservoir stimulation idea, i.e., 3D volume fracturing, and its principles and technology, are proposed. The volume fracturing of shale rock is defined as a three-dimensional fracturing process from the mechanics point of view, taking into account the interaction, growth, bifurcation between internal pores, inherent joints or cracks, and artificial fractures of reservoir rock under complex geostresses. The volume fracturing of shale rock is a failure process featured by large scale, multi cracking, high strength, and intensive energy release. The paper preliminarily frames the 3D volume fracturing theory from the aspects of 3D fracture modes, growth mechanism, interaction between artificial fractures and structural cracks, crack networks and development, loading type, modes and media for realizing large-scale volume fracturing, visualization of 3D fracturing process and computing programs, and stimulation implementation. Based on the early laboratory and field tests, we discussed the key scientific issues and technical challenges in realizing this promising stimulation technology for shale gas reservoirs.

Anisotropic mechanism and characteristics of deformation and failure of Longmaxi shale

Although spectacular advances in shale gas reservoir stimulation, i.e., hydraulic fracturing, the deformation behavior and failure mechanism of shale are not understood well enough. The basic reason is that shale contains stratified structure named bedding plane and its complicated petrophysical properties. The mechanical behavior of shale as the loading direction is parallel to the bedding plane is significantly different from that as the loading direction is perpendicular to the bedding plane. This work is devoted to characterization of the Longmaxi shale deformation and failure mechanism through experiments, from which 4 different deformation stages are obtained and volumetric strain deformation behavior is analyzed systematically. Then the effects of confinement as well as bedding layer orientation, which is defined as the angle between the loading direction and normal direction of bedding plane, on the elastic modulus, peak strength, and cohesion of shale are investigated. The analytical results illustrate that the volumetric strain dilatation is a constant, which is independent on the confining pressure and bedding layer orientation; its brittleness is high (0.55−0.85, B 1) and porosity is small (4.7%−5.2%). It is also found that the anisotropic degree of shale is better to be described by Young’s modulus than to be characterized by peak strength. As the confining pressure increases from 0.1 MPa to 60 MPa, the anisotropy degree decreases from 1.36 to 1.16, which declare that the Longmaxi shale belongs to low anisotropy rock according to the standard introduced by Ismael. These results not only enrich experimental database of shale, but also provide references for reservoir hydraulic fracturing design. Moreover, these results will help us to further understand shale laboratory mechanical properties.

Analysis and design procedure of hybrid long-span cable-stayed bridge using advanced composite material

An innovative solution is proposed in this paper by introducing a hybrid-type cable stayed bridge as a competent system to safely bridge very long spans. The new system leads into huge reduction in deck weight and its critical stresses in the pylon zones by using hybrid advanced composite deck. It also reduces the stiffness losses of the stay-cables due to the catenary’s action by using carbon fiber reinforced polymer cables. The paper proposes a proper consistent and systematic analysis and design procedure that optimize and precisely simulate the proposed bridge system. It recognizes the changes in structural behavior of the cable-stayed system, accordingly it clearly defines the ultimate and serviceability limit states for such new structural system in consistence with the limit state design philosophy. The design steps of the ADP represent a multi-scale design technique while the analysis steps characterize a multi-scale modeling technique. They represent the material design in the micro/macro-level, and accurate homogenization of the advanced composite components properties and evaluation of the resulting anisotropic characteristics and then, three-dimensionally simulate the hybrid bridge system involving all its nonlinearities. This paper investigates the performance of a new hybrid long-span cable-stayed bridge, which engages the use of advanced composite materials for the deck and the stay-cables, applying the analysis and design procedure. This design is scaled to match the general geometrical shapes, structural and aerodynamic characteristics of three of the world-longest cable-stayed bridges. What’s more, four advanced composite deck section models are proposed in this research. In order to study the performance of the referenced bridges, such as the natural frequencies, the deck top surface maximum vertical displacement, maximum Tsai-Hill failure function and the critical flutter velocity, three sets of parameters are investigated. The parameters include: (i) the micro level parameters-fiber fraction, (ii) the macro level parameters-Laminas Dominant Fiber Alignment and Laminate Thickness and (iii) the structure level parameters- cable radius.