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Abstract
Depleted hydrocarbon reservoirs are attractive targets for gas storage and CO2 disposal because of proven storage capacity and seal integrity, existing infrastructure, etc. Optimum well completion and injection design in depleted reservoirs would require understanding of important rock mechanics issues such as: 1) drillability and completion of new wells, 2) maximum sustainable storage pressures avoiding fracturing and fault reactivations considering rock-fluid interaction effects. Building a field specific geomechanical model calibrated with well and production data is a pre-requisite for addressing these issues. Through a case study from a North Sea field, this paper demonstrates a systematic approach for geomechanical risk assessments for CO2 storage in depleted reservoirs.

A depleted gas reservoir at 4,265ft depth with current pressure of 45psi is considered in this study for CO2 sequestration. Historical well and production data are used for geomechanical modellings and defining the change of earth stresses associated with depletion and injection. Analyses show that because of the low fracture gradient within the depleted sandstone reservoir and the presence of non-depleted overburden shale, the inclination angle for new injectors should be kept below 50° to avoid hole failure or mud losses. Field data and analytical sanding evaluations indicate no sand control installation would be needed for injectors. Fracturing and faulting assessments confirm that the critical pressures for fault reactivation and fracturing of intact rocks are, far beyond the planned CO2 injection and storage pressures up to the original pressure 1,962 psi; hence no leakage due to faulting or fracturing is expected over the life of CO2 storage.

The methodology and overall workflow presented in this paper is expected to assist well engineers and geoscientists with geomechanical assessments for optimum well completion and injection design for gas and CO2 storage in depleted reservoirs.

Introduction
Carbon Dioxide (CO2) emissions have elevated the concentration of greenhouse gases in the atmosphere. It has been reported that the atmospheric CO2 concentration had leaped from 315 ppm in the year 1958 to 387 ppm in 2009 at an accelerating rate (Tans, 2010), significantly higher than values at any time in the past 650,000 years (Brook, 2005). With a growing concern about the dangerous anthropogenic interference with the climate system, endeavors have been urged to mitigate the build up of the CO2, which accounts for more than 60% of greenhouse gases (Mavor et al, 2002).