Abstract

The selection of horizontal wells as barefoot and multilateral well candidates requires consideration of the well integrity over the producing life of the field. The evaluation of the production lifetime of a well requires that both the mechanical and chemical changes which the well experiences be modeled and predicted during the well design. The mechanical changes which the well will experience during production is primarily a function of stress and strength changes; the stress changes which result from depletion, the stress-depletion response, are addressed in this paper. Stress-depletion responses from reservoirs in the Gulf Coast region and in the North Sea have been collated and show a range of stress reductions which fields undergo accompanying the field-wide pore pressure depletion. The stress-depletion response of a field only addresses the minimum stress changes, consequently, the maximum horizontal stress change accompanying depletion can only be estimated using models.

The ability to predict the stress-depletion response of a field requires an understanding of what processes control the initial stress magnitudes, and the boundary conditions which the reservoir experiences during the production lifetime. The factors which define and influence the stress-depletion response of reservoirs are addressed, specifically the geological boundary conditions. The ability to model and predict the stress-depletion response of the reservoirs is then discussed, specifically focusing on the role and influence of the overburden surrounding the reservoir on the predicted response. Analytical and numerical models illustrate the influence of the material properties in the reservoir and overburden, and the reservoir size on the stressdepletion response. The ability to predict stress changes during depletion is erratic primarily due to insufficient data.

Uncertainty in the pre-production estimation of the stress- depletion response of the reservoir has a considerable impact on the predicted stability of wells in the reservoir interval. The ability to determine an accurate stress-depletion response is a principal consideration for reliable stability predictions and design recommendations for horizontal and multilateral wells.

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