There are many cases when operators are forced to deviate from the conventional bottom-up approach for the development of multiple stacked reservoirs, thereby being exposed to the challenges of drilling through depleted reservoirs. The field under study, called Field X in this paper, presents similar challenges with shallow, fault-bounded depleted reservoirs (depletion of several MPa) underlain by (overpressured) reservoirs at virgin pressure. Reservoir pressure changes (ΔPp) are accompanied by changes in the total principal stresses (Sv = vertical stress, Shmin = lesser compressive horizontal stress, Shmax = greater compressive horizontal stress), characterized by the three stress path coefficients (γv = ΔSv / ΔPp, γshmax= ΔShmax / ΔPp and γshmin= ΔShmin / ΔPp). From drilling perspective, the two horizontal stress path coefficients (γshmax and γshmin) are the key for accurate borehole collapse pressure and fracture pressure prediction in the depleted reservoirs. However, field measurements and theoretical descriptions have focused only on the change in the minimum horizontal stress. Constrained by opportunities for direct measurement and calibration, the changes in the total stresses in the horizontal plane are often assumed symmetric, i.e. γshmax = γshmin. This paper explores the stress path computed for the depleted reservoirs in Field X from full-field numerical simulations and its implications on future drilling activity. The major findings are: The numerical model suggests significant anisotropy in the two horizontal stress path coefficients. The model also predicts variations in the two horizontal stress path coefficients with time at different stages of depletion despite a linear-elastic reservoir constitutive model. The onset of production is defined by a relatively higher γshmin compared to γshmax. This is seen to reverse with a higher γshmax compared to γshmin at the later stages of reservoir development. These insights on the evolution of horizontal stresses in Field X favorably affect future development options in Field X. The paper discusses the causes and implications of the temporal variations in the horizontal stress path coefficients in Field X, which are also applicable to other small fault bounded reservoirs.

Drilling overpressured and deep reservoirs is a challenge in itself, but can be complicated by the need to drill through depleted (depressurized) shallower reservoirs. The field under study consists of multiple stacked clastic reservoirs bounded by steeply dipping sealing faults. The deeper reservoirs fall in the high pressure high temperature (HPHT) category and account for one third of the in-place volumes. Ideally, field development for such stacked reservoirs is recommended through the "bottom-up" strategy to prevent late-in-life drilling through depleted zones with reduced drilling window and increased risk of fluid losses and well failure. Here, this would imply drilling and developing the deeper HPHT reservoirs before the shallow, normally pressured reservoirs. From a technical and financial perspective, it is tempting to develop and produce the shallow, normally pressured reservoirs (that contain 70% of the volumes and also have better flow properties) first, and bring the deeper HPHT reservoirs on-stream later. But, is such phased development of the reservoirs possible? Or would producing from the shallower reservoirs first permanently damage our ability to drill and produce the deeper HPHT reservoirs at a later stage in field life? These were the questions we tried to answer in this work. We built a full-field finite element model to simulate the geomechanical response of the reservoirs to pressure depletion i.e. quantify the displacement, strains and total stress changes in and around the reservoirs as a function of production. Such geomechanical models can serve as a predictive tool to help answer the questions above. In this paper, we show the construction and application of such a geomechanical model in field development planning. Our paper highlights how our geomechanical model results were applied, together with other work, to develop this field safely and efficiently, emphasizing field life cycle value.

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