A Systematic Approach to Evaluate the Sanding Potential Caused by Formation Shear Failure in Unconsolidated Oil and Gas Reservoirs

This article will address two issues related to sand production in unconsolidated reservoirs. First, it will examine the relationship between formation compressibility (Cf), elasto-plastic hysteresis and the shear failure of the formation macroscopically (when the fluid and formation pressure together cannot support the overburden stress), as well as the methodology to predict this failure pressure. Second, it will explore the means to recognize which formations are more friable and likely to produce sand grains – microscopic shear failure. The two effects are only tangentially related but can occur simultaneously. Logs and petrophysical data should be methodolically used to qualitatively and quantitatively assess the sanding potential of a well or reservoir. The first method is evaluating the compressibility of formation rocks as they first demonstrate elasto-plasticity, then have catastrophic shear failure. The other method evaluates the sanding potential based on the friability of the formation. The most effective way to manage/mitigate catastrophic/macroscopic shear failure is to observe the dynamic behavior of the reservoir. By plotting the build-up permeability vs. skin-less FBHP, the failure pressure of the formation can be determined. Good operating practices then dictate that the well should not be flowed at pressures below the value plus a safety factor. The approach to managing potential sand grain failure (microscopic shear failure) is to design the completion (frac-pack, gravel pack, etc.) to collect the sand grains in the pack and screens, then perform periodic pump-in stimulation treatments to push the fines away from the screens/pack. Two examples each from the Gulf of Mexico and the Louisiana Gulf Coast will be presented to demonstrate the methodology for both macroscopic and microscopic shear failure. It should be noted that it is important to differentiate the cause of sand production/fines migration as one of the two (macro/micro) causes. This can be determined by tracking the accretion of skin due to fines. If this occurs coincident with a decrease in permeability or mobility thickness, it should be assumed that the cause is macroscopic shear failure. If the permeability remains constant as skin due to fines increases, it is due to microscopic shear failure. Technically, both mechanisms can occur simultaneously, but it is best to approach the issue conservatively and assume that any increase in skin due to fines that occurs with a decrease in mobility thickness is due to macroscopic shear failure. Applying the sanding potential systematically to formation evaluation can improve the completion design; predicting the macroscopic shear failure pressure of the formation contributes to better overall reservoir management.