Stability of Hydraulic Fracture Growth under Simultaneous Injection from Multiple Initiation Sites along a Wellbore

[摘要] Emerging applications in unconventional gas production, geothermal power generation, and mining are driving an intensifying focus on the effective creation of arrays of hydraulic fractures from a single wellbore. Relative to creating each of the hydraulic fractures in the array individually, it is often cost effective to simultaneously create more than one hydraulic fracture. However, simultaneous injection to more than one entry/initiation point raises the issue of the uniformity with which the injected fluid will naturally partition among the hydraulic fractures. In particular, if localization of the fluid supply and hence hydraulic fracture growth to a single entry point is energetically favorable relative to uniform, simultaneous growth of hydraulic fractures from all entry points, then simultaneous injection is not an effective method for creating a hydraulic fracture array. This issue, then, is fundamentally tied to the stability of uniformly growing array with respect to perturbations in the partitioning of the fluid among the entry points.As a first step, this report presents an energy balance for the hydraulic fracture array. The stability of the array with respect to perturbations in the fluid partitioning is examined by determining the derivative of the dominant term in the energy balance with respect to changes in the number of hydraulic fractures in the array. Uniform growth of the array is thus considered unstable when a reduction in the number of growing hydraulic fractures results in a lower energy input requirement for sustained growth.Through this analysis it is shown that the stability of the array is intimately tied to the relative importance of the stability-promoting dissipation due to viscous fluid flow in the hydraulic fracture. The stability is also fundamentally linked to the chosen geometric idealization, with plane strain (2D) arrays universally unstable while arrays of radially symmetric or finger-shaped (so-called ``PKN'') hydraulic fractures are stable provided that viscous dissipation is sufficient. Furthermore, in cases where the array is initially stable, a transition to an unstable regime occurs due to mechanical interaction among the growing hydraulic fractures. It is shown that this transition will take place when the length is on the order of the spacing, and as such, the spacing between the hydraulic fractures provides an order of magnitude estimate of the maximum length the array will attain before symmetry breaking, such as localization to a dominant hydraulic fracture, occurs.This report presents the first developments in a CSIRO Petroleum and Geothermal Portfolio funded effort to understand the self-organizing geometries that emerge when fluid is simultaneously injected from multiple entry points to create an array of hydraulic fractures.

[发布日期] 2011-11-21 [发布机构] CSIRO

[学科分类] 地球科学(综合)