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Background: This presentation summarizes our ongoing hybrid sociological-geological field research into the May 12, 2008 Wenchuan earthquake. In this extreme geo-disaster, mortality was 69,226, with 274,643 injuries, and 17,923 missing. The human toll was accompanied by significant destruction of the natural environment and the economy, estimated at US$ 176 billion. A 300 km long surface rupture occurred in the Longmen Mountains along its margin with the Sichuan Basin. Discussion: This disaster was caused by the relationships among (1) towns built in on or in proximity to fault lines, (2) the low earthquake-resistance of residences, schools and hospitals, and other buildings, and (3) the concentration of population distributed along rivers lying below steep-sloped mountains. Mortality and devastation were compounded by post-earthquake landslides. The Chinese central government started a national-level response within 2 hours, upgrading it to the highest national emergency level within 10 hours. Most lives were saved by local people. Military rescue units were activated within minutes of the earthquake, and regional militia, local and provincial units such as the Sichuan Seismological Bureau self-activated immediately. By day-two, 20,000 rescue and engineering soldiers had been deployed. Over 15 large medical treatment, epidemic prevention, and psychological intervention teams responded and more than 10 million volunteers took part in relief activities. In spite of mobilization of the nation's resources, emergency relief was frustrated by formidable obstacles such as cloud cover, a destroyed ground transportation network, loss of communication, and continued geo-hazards in the form of landslidedammed rivers which threatened large downstream urban centers. Expert national planning for recovery began five days post-earthquake; the plan was promulgated by national law in September, 2008. By the second anniversary of the Wenchuan earthquake, most school and residential construction was completed in earthquake-resistant areas.

In investigations of tectonic stress fields, natural fractures are often found in low-permeability formations. Fracturing wells in naturally fractured gas fields can differ dramatically from those in conventional isotropic formations. Fluid leak-off is the pri-mary difference. To take account of fracturing fluid leaking into the formation through natural fractures and the matrixes, this paper presents a new model that describes fluid leak-off in naturally fractured gas fields and its exact solution is given by or-thogonal transformation. We studied the effects of some sensitive parameters to fluid leak-off. From the cases studied, fluid leak-off in naturally fractured gas fields should not use the classic leak-off theory which states that leak-off velocity is inversely proportional to the exponent 0.5 of leak-off time. Based on numerous calculations, the equation V = C.t ∧ b (b < 0) is presented to describe fluid leak-off in naturally fractured gas fields, and the constant C is defined as the equivalent leak-off coefficient. If b is equal to -0.5, the new model can be simplified according to the classic model. The model has been applied to investigate fracture propagation in naturally fractured gas fields. Introduction Fluid loss can be described as leakage of the fracturing fluid leaking out of the main fracture. The rate of fluid leaking into the formation is one of the critical factors involved in the treatment design and in determining the fracture geometry. Moreover, in naturally fractured reservoirs, fluid loss is the key factor affecting the treatment success. Therefore, many investigators have studied the simulation of fluid loss and a number of mathematic models were developed to accurately model fluid loss. Classic theory(1, 2) considers the combined effect of the phenomena as a material property, so that leak-off velocity is given by the Carter equations. To date, it is still extensively used in fracturing simulation and fracturing design. In fact, the net fracturing pressure changes with an increase in pumping time, which has an important effect on fluid loss. Therefore, the pressure-dependent leak-off model was derived(3, 4), which is used for studying hydraulic fracture propagation(5). More recently, van den Hoek(6) developed a nonlinear fluid leak-off model for high-permeability fracturing. But the effects of natural fractures on fluid loss are not considered in the existing models. However, the fluid leak-off rate in naturally fractured reservoirs is typically excessive and completely dominated by the natural fractures(7). Vinod et al.(8) studied dynamic fluid loss in low-permeability formations with natural fractures by room experiments, which has some useful implications for fracturing design in naturally fractured reservoirs. Dietzel and Koehler(9) and Rodgerson(10) offered some key considerations in the hydraulic fracturing of naturally fractured reservoirs. Whereas, the leak-off model with natural fractures has not been reported, the objective of this paper is to set up a new theoretical leak-off model for natural fractures and analyze the effect on fracture propagation. Leak-off Model for Natural Fractures Basic Assumptions Figure 1 illustrates the leak-off of fracturing fluid in naturally fractured gas fields. The following are the basic assumptions for deriving the leak-off model: