Active Distributed Temperature Sensing for high-resolution fluid-flow monitoring in boreholes

如果我们要评估和成功管理饮用水、地热能源系统、页岩气矿床和煤气化项目的含水层，测量地下流体流速的能力是至关重要的。在这里，了解流体流动、渗透率和地质构造之间的相互联系是很重要的。这可以通过在钻孔中进行观察来尝试。钻孔安装后，不同深度的岩石之间会自然地产生向上或向下的流动。上述任何一个工程的附近作业都可能扰乱岩石中的流体，破坏井内的流动。如果井眼在这些工程应用中用于提取流体，那么井眼内流动的任何变化都表明最具渗透性的深度在哪里。目前的钻孔内流量测量方法通常是在单个位置进行的。为了确定整个钻孔发生了什么，必须反复降低探头，并进行另一次测量。这个过程很乏味，而且当流随着时间的推移而变化时，就不可能充分确定这是如何在所有深度发生的。另一方面，新的分布式传感器允许在连续的空间覆盖范围内进行测量。分布式温度传感（DTS）可以沿着光纤电缆连续测量温度。光纤电缆就像一个长（几百米到几千米）的温度计，每12厘米就可以测量一次温度。安装在井眼中的这种电缆可以在几秒钟内提供整个长度的非常详细的温度记录。这本身是有用的，但仅仅通过被动测量温度来精确量化流量通常是不可能的。我们相信一种使用加热光纤电缆和DTS的新方法将能够测量流量。采用该方法，通过将电流通过光纤周围的保护材料，将集中安装并通往钻孔底部的电缆均匀加热。使用DTS测量电缆的温度会升高，温度的升高应该取决于流体流过电缆的速度。更快的流动应该更有效地去除热量，降低电缆温度。这样的系统可能能够每12厘米测量一次流量，并且能够检测到每隔几秒钟流量发生的变化。该方法将在实验室用PVC管建造的井眼中以受控的方式进行测试。这将允许进入内部，并允许我们在测试期间直观地检查流程（使用染料）和设备。一个加热“主动”DTS （A-DTS）系统的原型将被安装在管道中。从储水罐中，水将以不同的速率通过管道，模拟钻孔内的流动。这将允许我们确定电缆的温度在不同的流动条件下是如何变化的。然后我们将调整电缆的加热功率，因为当使用更高或更低的功率时，由于不同流量而引起的温度变化可能更容易检测到。最后，将使用人工井眼中心的流入/流出口来研究流入/流出位置的温度效应（如岩石破裂时发生的情况）。精确的设置和底层物理将使用先进的数值模型技术进行测试。

The ability to measure the flow rates of fluids in the subsurface is critical if we are to assess and successfully manage aquifers for drinking water, geothermal energy systems, shale gas deposits, and coal gasification projects. Here, it is important to understand the interlinked relationship between fluid flows, permeabilities, and geological structure. This can be attempted through observations made in boreholes. When a borehole is installed, flow up or down the borehole may occur naturally between rock at different depths. The nearby operation of any of the above projects may disturb the fluids in the rock, disrupting the flow in the borehole. If the borehole is used to extract fluids in these engineering applications, then any variability in the flow inside the borehole indicates where the most permeable depths are. Current methods of flow measurement inside boreholes are usually made at a single location. In order to establish what is happening along the entire borehole, a probe must be repeatedly lowered, and another measurement made. This process is tedious, and when the flow is changing over time, it can be impossible to adequately determine how this is happening at all depths. On the other hand, new distributed sensors allow measurements to be made with continuous spatial coverage. Distributed Temperature Sensing (DTS) gives continuous measurements of temperature along fibre optic cables. A fibre optic cable acts as a long (100s of metres to kilometres) thermometer from which temperature measurements can be obtained up to every 12 cm. Such a cable installed in a borehole can give a highly detailed log of temperature along its entire length in just a few seconds. This is useful in itself, but exact quantification of the flows by just passively measuring the temperature is not usually possible. We believe a new method, using heated fibre optic cables and DTS, will be able to measure flow rates. With the proposed method, a cable installed centrally and running to the base of a borehole is heated uniformly by passing a current through the protective materials surrounding the optical fibre. The temperature of the cable, measured using DTS, will increase, and the increase in temperature should depend on how fast the fluid is flowing past it. Faster flows should remove heat more efficiently, lowering the cable temperature. Such a system would potentially be able to measure flows every 12 cm, and be able to detect changes occurring in the flow every few seconds.The method will be tested in a controlled way using a borehole constructed in a lab from PVC tubing. This would allow access inside and allow us to visually inspect the flow (using dyes) and equipment during testing. A prototype heated 'Active' DTS (A-DTS) system is to be installed in the tube. From a storage tank, water will be pumped through the tube at varying rates, mimicking flow inside a borehole. This will allow is to determine how the temperature of the cable changes in different flow conditions. We will then adjust the heating power of the cable, as the temperature changes due to different flows may be more readily detectable when using higher or lower powers. Finally, the temperature effect at inflow/outflow locations (as would happen where a rock is fractured) will be investigated using inflow/outflow ports in the centre of the artificial borehole. The exact set-ups and the underlying physics will be tested using advanced numerical model techniques.