To minimize fluid loss and the associated formation damage, foam is a preferred fluid to perform cleanout operations and reestablish communication with an open completion interval. Because of their high viscosity and structure, foams are suitable cleanout fluids when underbalanced well-cleanout operations are applied. Although several studies have been conducted to better understand foam-flow behavior and hydraulics, investigations performed on foam stability are very limited. Specifically, very little is known regarding the impact of wellbore inclination on the stability of foams. Unstable foams do not possess high viscosity, and as a result, they are not effective in cleanout operations, especially in inclined wellbores. Predicting the downhole instability of foam could reduce the number of drilling problems associated with excessive liquid drainage, such as temporary overbalance, formation damage, and wellbore instability. The objectives of this study are to investigate the effects of wellbore inclination on the stability of various types of foams and develop a method to account for the effect of inclination on foam stability in inclined wells.

In this study, foam-drainage experiments were performed using a flow loop that consists of a foam-drainage-measurement section and pipe viscometers. To verify proper foam generation, foam viscosity was measured using pipe viscometers and compared with previous measurements. Drainage experiments were performed with aqueous, polymer-based, and oil-based foams in concentric annulus and pipe under pressurized conditions. Tests were also conducted in vertical and inclined orientations to examine the effect of wellbore inclination on the stability of foams. The foam-bubble structure was examined and monitored in real time using a microscopic camera to study bubble coarsening. The foam quality (i.e., gas volume fraction) was varied from 40 to 80%.

Results show that the drainage rates in the pipe and annular section were approximately the same, indicating a minor effect of column geometry. More importantly, the drainage rate of foam in an inclined configuration was significantly higher than that observed in a vertical orientation. The inclination exacerbated foam drainage and instability substantially. The mechanisms of foam drainage are different in an inclined configuration. In inclined wellbores, drainage occurs not only axially but also laterally. As a result, the drained liquid quickly reaches a wellbore wall before reaching the bottom of foam column. Then, a layer of liquid forms on the low side of the wellbore. The liquid layer flows downward because of gravity and reaches the bottom of the test section without facing the major hydraulic resistance of the foam network. This phenomenon aggravates the drainage process considerably.

Although foam-drainage experiments have been reported in the literature, there exists only limited information on the effects of geometry and inclination on foam drainage and stability. The information provided in this paper will help to account for the effect of inclination on foam stability and subsequently improve the performance of oilfield operations involving foam as the working fluid.

为了使流体损失和相关的地层损害最小化，泡沫是执行清除操作并以开放完井间隔重新建立连通的优选流体。由于泡沫的高粘度和结构，当进行欠平衡的井清理作业时，泡沫是合适的清理流体。尽管已经进行了一些研究以更好地理解泡沫的流动特性和水力学，但是对泡沫稳定性的研究却非常有限。具体而言，关于井眼倾角对泡沫稳定性的影响知之甚少。不稳定的泡沫不具有高粘度，因此，它们在清理操作中尤其是在倾斜的井眼中无效。预测泡沫的井下不稳定性可以减少与过度排液有关的钻井问题，例如暂时的不平衡，地层破坏和井眼不稳定性。这项研究的目的是研究井眼倾角对各种类型泡沫稳定性的影响，并开发一种方法来解释倾角对斜井中泡沫稳定性的影响。

在这项研究中，使用由泡沫排水测量部分和管道粘度计组成的流动回路进行了泡沫排水实验。为了验证产生适当的泡沫，使用管道粘度计测量泡沫粘度并将其与先前的测量结果进行比较。在同心环带和管道中，在加压条件下，使用水性，聚合物基和油基泡沫进行了排水实验。还以垂直和倾斜方向进行了测试，以检查井眼倾角对泡沫稳定性的影响。使用显微镜照相机实时检查和监测泡沫-气泡结构，以研究气泡变粗。泡沫质量（即气体体积分数）在40％至80％之间变化。

结果表明，管段和环形段的排水速率大致相同，表明塔几何形状的影响较小。更重要的是，倾斜状态下的泡沫排水速率明显高于垂直方向上的排水速率。倾斜严重加剧了泡沫的排水和不稳定性。泡沫排出的机制在倾斜状态下是不同的。在倾斜的井眼中，排水不仅发生在轴向，而且发生在横向。结果，排出的液体在到达泡沫塔的底部之前迅速到达井眼壁。然后，在井眼的下侧形成一层液体。液体层由于重力而向下流动，并到达测试部分的底部，而不会遇到泡沫网络的主要水力阻力。

尽管文献中已经报道了泡沫排水实验，但是关于几何形状和倾斜度对泡沫排水和稳定性的影响只有有限的信息。本文提供的信息将有助于说明倾斜度对泡沫稳定性的影响，并随后改善以泡沫为工作流体的油田作业的性能。