Well completions, either cased hole gravel packed or cased hole frac packed, often suffer significant productivity decline during hydrocarbon production. In these completions, the perforations are intended to be packed with gravel or proppant, which provides the necessary support to prevent perforation failure and sand production. Ideally, the entire perforation volume is packed with proppant. However, there are cases of poor proppant packing jobs, which may leave perforations partially or completely unsupported. This increases the chances of perforation failure, sand production, and productivity decline. The effect of proppant packing efficiency, i.e., the fraction of perforation volume filled, on productivity decline was investigated in hollow cylinder sand production tests on two different types of sandstone. The experimental results show that lower packing efficiency may lead to earlier perforation failure but does not necessarily cause a significantly larger productivity decline. Different productivity decline mechanisms are also demonstrated with varying packing efficiencies and sandstone types.
Perforated casing is a common type of petroleum wellbore completion which entails placing production casing along the producing reservoir section, cementing it in place, and perforating through the casing, cement, and into the reservoir rock. Hydrocarbons are then produced through the perforations and into the wellbore. If there is a risk of sand production, gravel or proppant may be pumped into the perforations, and held in place by a screen. The high permeability proppant serves as a sand control mechanism by providing mechanical support to the perforations and trapping produced solids above a certain size.
Sand production is a combined mechanical and hydrodynamic phenomenon, in which sand grains and debris are transported by the flowing hydrocarbons. Hydrodynamic forces in permeable sands are generally too low to dislodge cemented grains, even for very weakly consolidated sands (Fjær et al., 2008). The rock must therefore fail mechanically before sand production can commence. In a cylindrical or near-cylindrical cavity, such as a perforation, the tangential (hoop) stress will generally be the largest stress, and the radial stress will be the minimum stress. Unsupported perforations may therefore fail in shear, and sand production from the perforation may continue unabated for some time. If, on the other hand, the perforation is filled with proppant, it may support the cavity by increasing the radial stress upon borehole closure and holding in place pieces of material resulting from breakouts or spalling.