Risk Assessment for SAGD Well Blowouts

Abstract

A comparative risk assessment was conducted to evaluate the risk associated with a Steam Assisted Gravity Drainage (SAGD) well blowout. The main comparison was between an isolated (double barrier) completion and an open (single barrier) completion used in conjunction with an effective blowout response plan. The target application was a SAGD pilot project in the Orinoco Belt in Venezuela.

The overall approach for the risk assessment included the investigation of the blowout flowing potential of the SAGD wells pair through reservoir modelling, the estimation of the probability of a blowout using fault-tree analysis, and the evaluation of the possible consequences (life safety, environmental and economic) of such blowout using various quantitative consequence models. Details of this approach are discussed in this paper, along with the results specific to the target application.

Results of this work can provide guidance for similar operations where decisions regarding completion options and blowout response plans are required. One key result was that, for this specific SAGD pilot project, a blowout response plan must be able to reduce the blowout duration substantially (from 3 days to 1 day for environmental risk, and from 3 days to 2 hours for economic risk) for the environmental and economical risks associated with a open completion to be comparable to those of an isolated completion.

Introduction

A Steam Assisted Gravity Drainage (SAGD) pilot project is currently being considered for a heavy oil development in the Orinoco Belt in Venezuela. Under the expected operating conditions of the pilot, it is anticipated that both the injection and the production wells will be able to flow unassisted to surface should a loss of well control incident occur. Industry practice regarding the design of such flowing wells dictates that the well completion include double barriers in the production tubing string, e.g. the flow tee plus a downhole safety valve (DHSV), and in the production casing annulus, e.g. the tubing head plus a downhole packer. Unfortunately, downhole equipment suitable for such a high temperature application does not currently exist. A possible alternative to the double barrier completion is to use a single barrier completion in conjunction with an effective blowout response plan. To evaluate the risks associated with this alternative, a comparative risk assessment was conducted.

The main purpose of this paper is to present the overall approach used in the risk assessment, which includes the investigation of the blowout flowing potential of the SAGD well pair through reservoir modelling, the estimation of the probability of a blowout using fault-tree analysis, and the evaluation of the consequences of such blowout using various quantitative consequence models. Results from each of these segments for the specific pilot project are presented as an example of applying the approach.

As the pilot project is set in Venezeula, it is important to note that some characteristics of the application are different from the common SAGD projects in Canada, such as a higher permeability (~ 30 D) and a higher initial reservoir temperature (~ 48°C).

Author: Worth, D. J., Crepin, S., Alhanati, F. J. S., & Lastiwka, M.

Publisher: International Thermal Operations and Heavy Oil Symposium, 20-23 October, Calgary, Alberta, Canada

Year: 2008

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