Liu Jishan, Polak Amir, Elsworth Derek, Grader Avrami
School of Oil and Gas Engineering, The University of Western Australia, 39 Stirling Highway, Crawley WA 6009, Australia.
J Contam Hydrol. 2005 Jun;78(1-2):53-70. doi: 10.1016/j.jconhyd.2005.03.001.
Flow in a rock fracture is surprisingly sensitive to the evolution of flow paths that develop as a result of dissolution. Net dissolution may either increase or decrease permeability uniformly within the fracture, or may form a preferential flow path through which most of the injected fluid flows, depending on the prevailing ambient mechanical and chemical conditions. A flow-through test was completed on an artificial fracture in limestone at room temperature under ambient confining stress of 3.5 MPa. The sample was sequentially circulated by water of two different compositions through the 1500 h duration of the experiment; the first 935 h by tap groundwater, followed by 555 h of distilled water. Measurements of differential pressures between the inlet and the outlet, fluid and dissolved mass fluxes, and concurrent X-ray CT imaging and sectioning were used to characterize the evolution of flow paths within the limestone fracture. During the initial circulation of groundwater, the differential pressure increased almost threefold, and was interpreted as a net reduction in permeability as the contacting asperities across the fracture are removed, and the fracture closes. With the circulation of distilled water, permeability initially reduces threefold, and ultimately increases by two orders of magnitude. This spontaneous switch from net decrease in permeability, to net increase occurred with no change in flow rate or applied effective stress, and is attributed to the evolving localization of flow path as evidenced by CT images. Based on the X-ray CT characterizations, a flow path-dependent flow model was developed to simulate the evolution of flow paths within the fracture and its influence on the overall flow behaviors of the injected fluid in the fracture.
岩石裂隙中的流动对因溶解作用而形成的流动路径的演变出奇地敏感。净溶解作用可能会使裂隙内的渗透率均匀增加或降低，也可能会形成一条优先流动路径，大部分注入流体都会通过该路径流动，这取决于当时的环境力学和化学条件。在室温下、3.5 MPa的围压环境下，对石灰岩中的一条人工裂隙进行了流通试验。在1500小时的实验过程中，用两种不同成分的水依次循环通过样品；前935小时用自来水，随后555小时用蒸馏水。通过测量进出口之间的压差、流体和溶解物质通量，并同时进行X射线CT成像和切片，来表征石灰岩裂隙内流动路径的演变。在地下水初始循环期间，压差增加了近三倍，这被解释为随着裂隙中相互接触的粗糙面被去除以及裂隙闭合，渗透率出现净降低。随着蒸馏水的循环，渗透率最初降低了三倍，最终增加了两个数量级。这种从渗透率净降低到净增加的自发转变，是在流速或施加的有效应力没有变化的情况下发生的，这归因于CT图像所显示的流动路径的逐渐局部化。基于X射线CT表征，建立了一个与流动路径相关的流动模型，以模拟裂隙内流动路径的演变及其对注入流体在裂隙中整体流动行为的影响。
