ABSTRACT:

Reservoir sands experience pressure depletion from the production of fluids that are contained within them. These producing sands can be so depleted that they hinder future developments of deeper producible sands because of the corresponding reduction in stresses across such depleted sands that may present mud weight constraints for in-fill drills. Our experience in fields in the Gulf of Mexico has shown that issues related to depleted drilling (such as lost circulation) typically occur in the shales above or below depleted unconsolidated sands. In this paper, evidence is provided to show that the pressure in shales can reduce and can be a contributing mechanism to the issues encountered in these shales. For this study, a 1-D pressure diffusion model was utilized to simulate pressure changes in bounding shales of multiple depleting reservoirs in a field in the Gulf of Mexico. The modeled shale pressure depletion was then converted to strain using assumptions on compressibility. The strain estimates were subsequently compared with the strain measurements data acquired from Formation Compaction Monitoring logging across various sands in several wells in the field. The study shows that the strain associated with pressure depletion can account for the strain experienced in these bounding shales, and thus provides a means to explain the stress reduction associated with the shales and the associated lost circulation events. This will enable better informed decisions in designing wells and can save millions of dollars in cost due to downtime from lost circulation and borehole instability during drilling through depleted zones.

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