A more realistic sand production prediction using pore collapse theory

Sand production has alluded production engineers over a century since oil production began. Starting from the '80s, rock mechanics specialists have attempted to explain, and predict the onset of sanding in a hydrocarbon well. Two common shortcomings in the traditional approaches are: (1) use of Mohr-Coulomb and similar failure criteria, which do not seem to explain the large, episodic manner of sand production from the reservoir rock; and (2) limited scope of laboratory rock mechanics tests e.g., triaxial tests at low confining pressures. Both aspects address the brittle failure of rock, which does not seem to account for the 'tons' of sand commonly produced from a deep well. We have studied the development of a ductile failure zone around a well under high effective mean stress that leads to pore collapse and growth of a sand bank. This mechanism can explain the production of large amount of sand produced from deep wells. Using the classical Kirsch equations for stresses around a circular hole, and a cap model to calibrate the constitutive behavior of rock, we have demonstrated that pore collapse indeed takes place around a borehole or perforation tunnel, leading to the growth of a sand bank. We have developed a graphical calibration procedure to validate the sanding prediction model. A case history of sand production prediction from a deep gas-condensate sandstone reservoir (Lower Devonian) is presented.