Processes of Screenout Development and Avoidance

Abstract

In SPE 18955, published in 1989, a strong case was made suggesting that observed rapid increases in treating pressure were not caused by fracture "tip screenouts". These conclusions were further supported by extensive laboratory studies of proppant bridging in slot-flow geometries. These studies showed that conventional "bridging criteria" predicting slot widths of 3-6 particle diameters would cause permanent proppant bridging or packing, were invalid. These results were published in SPE 67298 in 2001. Yet, more than 30 years after the first cited publication the terms "tip screenout" and "tip screenout design" are still in common usage. This paper presents a physics-based description of the mechanisms that cause actual wellbore screenout and the associated rapid pressure rise, and easy to apply methods to prevent or avoid developing screenouts while pumping. Field cases and numerical simulation studies will be presented to show that screenouts occur in the very near-wellbore region, particularly at the borehole sand-face. The proximity to the wellbore allows changes to be made to pump rate and other parameters, without the need to adjust fluid composition, stability, or prepad/pad volumes in future stages. A numerical simulator has been used to simulate observed screenout conditions, predict their occurrence, and demonstrate that screenout progression can be halted or controlled. The paper presents the underlying physical processes that control screenouts and explains why the proposed (and field tested) avoidance protocols work. The numerical formulation presented in the paper models screenout development and shows how to avert progression of the associated treating pressure rise. The method has been tested and verified in the field, and examples are provided. The work shows conclusively that screenouts occur at the wellbore surface or entry to the fracture intersections with the borehole, and not in the body or tip (perimeter) of the propagating fracture. The information presented in the paper invalidates decades of conventional fracture design theory including pad volume requirements based on fluid efficiency, design of fracturing fluid rheological stability at bottomhole temperature for the duration of pumping time, methods for proppant concentration scheduling (ramping), and virtually all conventional "design" practices.