This paper has proposed an emerging methodology for testing Sustained Casing Pressure (SCP) by using helium tracer. The model of helium tracer dispersion in wellbore is built and an iterative calculation procedure is applied to calculate the depth of the leakage point of production string. Based on the model, a helium tracer diagnostic system is manufactured and applied in one experimental well with SCP. Also the method can be applied to gas lift surveillance. Compared with down-hole logging, the advantage of tracer technology is the ability of testing from the surface and no need to shut-in or choke the well.
Sustained Casing Pressure (SCP) is a threat to the safety of oil and gas production. High casing pressure even may collapse the inner string of a well (Bradford, et al.,2002; Williamson, et al.,2003). In addition, SCP is a common problem during oil and gas production. In the Gulf of Mexico (GOM), SCP exist in over 11,000 casings in over 8,000 wells. The US Minerals Management Service (MMS) developed regulations (30 CFR 250.517) to implement in operation on a well with SCP (Bourgoyne, et al.,1999). And API RP 90 proposed the management of wells with SCP, which is applicable to the offshore oil and gas production activities (API,2006). Both of the practices recommended that wells with significant SCP problems must be periodically tested so that they can be controled.
Recently, many methods have been illustrated for testing and evaluating SCP. In the study of Bourgoyne, et al.,(2000), a casing venting system was connected to casing/tubing head to diagnose SCP. It is recommended that a well with SCP should be tested periodically, if the SCP cannot bleed down to zero through a 0.5-in. needle valve in less than 24 hours. In order to predict SCP from the early buildup data, researchers have proposed many theoretical models. Somei,(1999) built a model of gas migration through annulus cement to the surface. The model analyzed effects of cement porosity, temperature, and gas gravity on SCP. However, the model is not suited for wells whose casings are not cemented to the surface. Xu, et al.,(2001) developed a model of SCP buildup in annulus with gas-free mud. Further, Xu, et al.,(2003) considered gas migration in annulus mud as a dispersed two-phase flow to improve the SCP buildup model. While the model is under the assumption of constant temperature profile of the well and thermodynamic equilibrium. T., et al.,(1959) developed a model of heat transfer between wellbore and formation in injection wells. Ramy,(1962) presented a model to predict transient temperature behavior in the formation at all times. The solution lead to unrealistic predictions at small times. Hasan, et al.,(1991) refined the temperature prediction model by consideration of the appropriate boundary conditions that heat transfer at the formation/wellbore interface is represented by the Fourier law of heat conduction. Considering deformation of casing, Jin, et al.,(2013) established different models for the interactions among the casing-cement-formation system. In addition to oil or gas production wells, Zhu, et al.,(2012) presented a model of prediction of SCP in production casing annulus of CO2 injection wells. The risk of wells with SCP can be rapidly estimated on the basis of pressure prediction model (Rocha-Valadez, et al.,2014). However, the source of the SCP and the leakage point location have not been diagnosed, especially SCP in production casing annulus which is more frequent than in other annulus and considered as the source of the most serious problems (Bourgoyne, et al.,1999). Usually, temperature logging, flow logging and noise logging are used to determine the leak point in production string which leads to SCP (Hull, et al.,2010; Johns, et al.,2006; Julian, et al.,2007). However, the detection is complicated and costly, because logging tools are inevitable required to be run in the well. Therefore, it is necessary to develop a less costly leak detection and location technique without downhole logging.
