Intervention in Wells with Buckled Production Tubing

Abstract Sometimes the production tubing in a well can be helically buckled due to problems with the completion design, completion hardware, or installation problems. When coiled tubing (CT), wireline (WL) or jointed pipe (JP) is run through helically buckled production tubing there is a significant risk of getting stuck. This paper discusses the impact of helically buckled production tubing on the forces required to perform an intervention in the well. The theory for these forces is discussed, and a practical example is presented. Introduction Production tubing that is in compression, and is thus helically buckled, is common. Though the production tubing is initially hung in tension, the increase in temperature due to production from the well sometimes causes the production tubing to compress and buckle. In other cases there is not enough tension applied when the well is completed, allowing the bottom portion of the production tubing to buckle. When any through tubing intervention is performed in a well with buckled production tubing, care must be taken to avoid becoming stuck. Since this paper is being written for a CT conference, it will focus on CT intervention, but it is important to note that the same concepts apply to WL and JP. The helical shape of the production tubing, and thus the intervention path, causes increases in the tension or compression due to the capstan or belt friction effect of the helical shape. This effect may cause the tension to reduce while running in the hole (RIH). As long as the CT does not go into compression (in which case it will likely lockup, preventing any further penetration into the well), the operator will simply see a reduced weigh reading on surface. However, upon picking up weight to pull out of the hole (POOH), the increased tension due to the capstan effect may be greater than the breaking strength of the CT. Thus, the CT can be RIH but can become stuck when trying to POOH. Theory The buckled production tubing causes a modified form of the "capstan effect" sometimes known as the "belt effect". The increase in tension or compression due to this effect can be expressed as:Equation 1.1 Equation 1.1 must be included with the other vector forces due to gravity and wellbore curvature in a tubing forces model (TFM) for practical use. The period of the production-tubing helix will vary along the length that is buckled. The TFM must be run for the production tubing to determine the period versus depth in the well.