Importance of Fracture Closure to Cuttings Injection Efficiency

The major benefit of adequate fracture closure is to release injection pressure and restore it to its initial magnitude during the shut-in period to prevent gradual pressure build-up over the injection batches that follow. This paper examines a number of injection cases where the benefits of adequate fracture closure and the detrimental impacts of insufficient fracture closure are respectively revealed. In-depth examination of fracture closure impact can be set for short durations with relatively fewer injection batches, as well as for long periods with numerous batch injections. The former scenario focuses on determining the physics of the individual fracture closure, while the latter situation emphasizes the general trend with respect to peak pressure at the end of injection and terminal pressure at the end of the shut-in period. In addition, this paper addresses the added complexities during fracture closing after shut-in that can be identified from the relationship between injection pressure and the G function superposition derivative. Straight-line behavior usually indicates that the formation is homogeneous and leak-off from the fracture into the formation is linear. Other responses such as concave or convex shapes of the G superposition derivative relationship may indicate the formation is either naturally fractured or tight (i.e., low permeability). Or, the pressure decline shapes may imply fracture tip extension, or fracture height recession. Direct examination of the pressure decline curves may reveal the relationship between the fracture responses and formation characteristics.