Identification of SSC (Sulphide Stress Cracking)-Susceptible Wells and Risk Prediction
- Tapantosh Chakrabarty | Richard James Smith (Imperial Oil Resources)
- Document ID
- Society of Petroleum Engineers
- SPE Heavy Oil Conference Canada, 12-14 June, Calgary, Alberta, Canada
- Publication Date
- Document Type
- Conference Paper
- 2012. Society of Petroleum Engineers
- 4.1.2 Separation and Treating, 5.4.6 Thermal Methods, 4.2.3 Materials and Corrosion, 2.4.6 Frac and Pack, 7.2.4 Statistical Techniques for Managing Risk, 4.6 Natural Gas, 2.2.2 Perforating, 5.2.1 Phase Behavior and PVT Measurements, 4.1.5 Processing Equipment, 5.3.9 Steam Assisted Gravity Drainage, 5.8.5 Oil Sand, Oil Shale, Bitumen, 4.3.4 Scale
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Hydrogen sulfide (H2S) generated by aquathermolysis—a high-temperature reaction of condensed steam (water) with sulfur-bearing bitumen in the reservoir rock—may increase the risk of sulfide stress cracking (SSC) in cyclically steam stimulated (CSS) wells. In a given field, H2S levels and wellbore conditions vary significantly among wells and so do their SSC-susceptibility. Identifying the SSC-susceptible wells is important in terms of reducing SSC risk by allocating resources and implementing pro-active intervention measures to the SSC-susceptible wells. A comprehensive research program, with a dedicated instrumented CSS well as the centerpiece, has been undertaken by Imperial Oil Resources with the objectives of characterizing H2S evolution in the wellbore and developing a tool for identifying the SSC-susceptible wells. The research includes laboratory and field tests, and statistical, phase behaviour and kinetic modelling. The SSC-susceptible zone for Cold Lake CSS has been established from Cyclic Slow Strain Rate (CSSR) laboratory tests incorporating CSS fluid chemistry, stress-strain environments, casing metallurgy, and variable temperature and H2S partial pressure. A statistical logistic model matches the experimental CSSR data well. The instrumented well data validate the phase behavior model, which in turn explains the measured H2S profile in the wellbore. An aquathermolysis kinetic model has been developed for the instrumented well and validated with data from nine other CSS wells. The research has led to the development of an engineering tool for identifying the wells at the risk of falling into the SSC-susceptible zone.
Imperial Oil is using Cyclic Steam Stimulation (CSS) process commercially to recover bitumen from the Clearwater Formation at the Cold Lake field in Alberta, Canada. In this process, the reservoir is stimulated by injecting high-pressure (~12000 kPaa) and high-temperature (~325 ?C) steam to reduce the bitumen viscosity and produce the thinned bitumen through the same well in a cyclical manner. One cycle in CSS consists of three stages: steam injection, soaking of the reservoir with steam, and bitumen and condensed steam production. Over 4000 wells produce about 24000 m3/d of bitumen at Cold Lake. In a given well, steam injection is through the casing, liquid (bitumen and condensed steam) production is through the tubing, and gas production is mainly through the casing annulus. It is the casing that is susceptible to sulfide stress cracking (SSC), as shown in a small casing section retrieved from a Cold Lake CSS well (Fig. 1).
SSC in a CSS well is induced by H2S that is generated by the reaction between condensed steam and sulfur (S)-bearing bitumen (Clark and Hyne, 1984; Clark et. al., 1988; Lamoureux-Var and Lorant, 2005; Monin and Audlbert, 1988; and Hoffmann et al., 1995). Termed aquathermolysis (AQT), the reaction can be represented by:
H2O + S-Bearing Bitumen = H2S + Altered Bitumen …………………………. (1)
AQT also occurs in Steam-Assisted Gravity Drainage (SAGD) process (Thimm, 2001)—another bitumen recovery process in which steam is injected continuously through an injection well, while bitumen, condensed steam and gases are produced continuously through a production well.
In addition to H2S, pH of condensed casing water, metallurgy of the casing material, cyclical variation in casing temperature and tensile stress are some of the factors responsible for SSC in a CSS well.
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