Overcoming Challenges To Resolve Uncertainties in Modelling for Caprock Integrity Assessment

Yang, Baohong (BitCan Geosciences & Engineering Inc.) | Xu, Bin (BitCan Geosciences & Engineering Inc.) | Yuan, Yanguang (BitCan Geosciences & Engineering Inc.)

OnePetro 

SUMMARY: Geomechanical simulations are a powerful tool to forecast caprock deformation and failure behaviour. However, a number of drawbacks associated with simulation are often cited in dissuading their use as a major tool for caprock integrity assessment. This paper will explain that these drawbacks are not inherent in simulation itself. If vigorous efforts are exercised, there are means to overcome these drawbacks. Three approaches are presented: deterministic, probabilistic, and joint inversion. Theoretical principles as well as case histories are given to support observations: (1) forward deterministic simulations are still valid in yielding accurate results that compare well with field observations; (2) probabilistic simulation is a powerful tool to quantify the impact of uncertain material properties and their spatial variability; and (3) the data-intensive nature of a thermal project is an important asset that can be used by a joint mathematical inversion system to make inferences about evolving subsurface processes.

1 INTRODUCTION

Whether or not it contains resources of economic value, every interval of subsurface rock formation is invaluable and their integrity must be safeguarded. If containment integrity of the caprock above a petroleum reservoir is damaged, reservoir fluid can escape into undesirable locations. This is particularly essential for heavy-oil development because it requires reservoir stimulation through injection of steam, solvent, and other chemicals to reduce the oil viscosity. For example, cumulative energy injected into a reservoir and thus stored at the subsurface during a thermal project is too significant to ignore the caprock integrity issues. If not managed properly, this energy source can harm us socially, environmentally, and economically.

Several incidences of steam release, drilling blowout, reservoir fluid escaping into shallow depths including into groundwater aquifers, or reservoir fluid escaping to the ground surface have been reported publically (Smith et al. 2004; ERCB 2010; AER 2013). More common is the significant casing deformation during thermal operation. Moreover, concerns are raised about the significant uneven surface heave that may alter facilities, roads, landscape, and surface/subsurface hydrogeological conditions.

Caprock integrity eventually becomes a geomechanical issue, even though it is first a concern of hydraulic integrity (i.e., no reservoir fluid should escape through the caprock into more shallow areas). Naturally, such hydraulic integrity is already inherently placed in-situ in the geological history because the caprock has prevented any further upward movement of hydrocarbon migration.