Cyclic loading (fatigue) tests were performed on thick-wall hollow cylinders of reservoir rock material. The number of cycles to failure follows a log relationship with the peak cyclic load, for cycles less than 100 and peak loads less than 87% of the base hole collapse strength. For peak loads between 75% and 87% of the base hole collapse strength, the samples do not fail within 100 cycles, but they are weakened. These findings are applied to injection well design using a coupled 3D non-linear finite element model that includes the wellbore, casing, cement and perforations, and which drives a more detailed perforation submodel. The amount of failed and plastically-strained material surrounding each perforation of interest is compared to the amount revealed in a finite element simulation of the hollow cylinder tests. The non-linear elasto-plastic behavior of the rock is obtained from the results of triaxial compression tests. The analyses show that certain well and perforation orientations will be vulnerable to sanding, but that other well and perforation orientations are expected to remain stable even after repeated injection and shut-down cycles.
1. INTRODUCTION
Wells used for water injection in oil reservoirs are often cased and perforated. If the rock is not too weak, the injection wells can be left with open perforations and with no sand control installed. When under injection, the stress level on the perforations may be insufficient to fail the rock. This is because the wellbore pressure is greater than the reservoir pressure (the drawdown is negative).
However, injection wells are often shut down, either for routine maintenance or due to unexpected circumstances. This can result in flow into the well (a positive drawdown), which increases the effective stress loading on the perforations. Flow into the well can occur due to dynamic effects immediately after shut-down, and can also occur due to crossflow (flow from a higher-pressured formation to a lower-pressure formation, using the wellbore as a flow conduit). In addition, multiple cycles of injection and shut-down may occur during a well's lifetime.
Many materials are known to weaken with cyclic loading. We aimed to answer these questions: Does the rock next to a perforation weaken due to cyclic loading? How can we determine this? And how do we apply the findings to injection well design for a particular field of interest?
One of the best methods for simulating the behavior and stability of perforations in the laboratory, without using an actual shaped-charge perforator, is to use thick-wall hollow cylinders loaded on the outer surfaces [1-3]. Although such test samples are subjected to axisymmetric loading, which is usually an unrealistic approximation, they do include the correct geometry and they also include the effects of non-linear behavior and plastic deformation that could occur around perforations in-situ. For our hollow cylinder tests we use a hole size of 0.5- inch and an outer diameter of 1.5 inches, and a specified loading rate [3]. We refer to these tests as 'hole collapse' tests because we do not measure deformations in the hole but simply measure the collapse pressure of the sample.
