The Effect of FracPacks on Sand Stability during Depletion

摘要

Fracpack completions are widely used as the most efficient sand control method in deep GOM fields. For fracpacked wells, rock stability issues are often no longer considered when well drawdown is decided. While fracpacks can prevent sand from entering the wellbore, it is not unusual to witness skin increase and productivity loss over production time. Contradictory to conventional thought that productivity loss results from flow-related fines migration or mechanical failure of completion equipment, this paper documents a field study where it is found the fracpack stimulation and completion surprisingly increases the risk of rock failure around the fracture resulting in severe production decline. The well studied is a fracpacked condensate producer from a high permeability turbidite sandstone reservoir with a strong water drive. After 2-3 years of production, well productivity declines more quickly than expected. Well test analysis attributed the productivity loss to skin at the fracture face and additional permeability damage around the fracture. Extensive rock lab tests have shown that, despite high fluid flow rate, little fines are produced. However, significant permeability reduction occurs when the loading stress increases to a threshold level. The solids produced and collected consist of mainly sand chips and particles. In preparation for geomechanical modeling a rock constitutive model has been developed and calibrated to drilling events, logs, and core data. A detailed geomechanical model with an embedded fracture has been coupled with fluid flow near the wellbore to simulate the fracpack followed by drawdown and long-term depletion investigating rock stability around the fracture. Simulation results indicate that the placement of a propped hydraulic fracture during fracpack operations has two side effects: on one hand, it increases rock strength at most locations around the fracture, which is consistent with whatindustry has believed; on the other hand, there are certain areas, especially surrounding the fracture tip that have been weakened due to elevated shear stress levels. Rock failure occurs at a certain level of drawdown and depletion, either in shear failure mode or compaction failure mode, depending on the location with respect to the fracture, rock strength, and stress path. It is postulated that this depletion induced failure mobilizes the fines which are then transported to the fracture resulting in plugging of the completion. A stress path analysis is used in conjunction with the calibrated cap and cone failure surface to estimate the critical drawdown and depletion causing rock failure. This analysis can be used to manage drawdown as well as investigate variations of the fracpack completion to reduce the risk of fines mobilization.