SAND INJECTION STRUCTURES – CRITICAL DISRUPTORS TO MODELLED GEOFLUID FLOW AND PROJECT SUCCESS

Sand injection structures disrupt bedding-dominated flow of geofluids (water, gasses, oil), are difficult to detect, generally hinder accurate modeling and prediction of geofluid flow and may critically hinder projects.

Injection structures have both seismic and aseismic origins including liquefaction, and release of fluid pressure enhanced by methanogenesis or other fluid generation or trapping process.

Direct detection of injection structures is difficult because they are hidden (subsurface); physical/direct sampling by borehole intercept is haphazard; and they are often scaled below the resolution of tools such as seismic or radar. Small and/or widespread injection structures are difficult to detect from fluid pressure monitoring (e.g., via piezometers, etc.) under ambient conditions because pore pressure gradients are primarily subvertical and imprecisely measured (e.g., large vertical sampling intervals in many wells), and have innate uncertainty attributable to interpolation between widely-spaced wells. The use of site-specific and/or empiric factors in models may increase model accuracy but mask data anomalies that might otherwise evidence cross-bed flow through injection structures.

Detection of geofluid flow that is altering by injection structures – and subsequent modification of flow models to acknowledge and potentially manage flow through such structures – usually occurs after critical decisions and capitol has been committed to projects. A wide variety of projects can be affected including groundwater remediation, groundwater withdrawal, groundwater recharge or re-injection, or hydrocarbon withdrawal, enhanced oil recovery or gas storage projects. Failure to identify the existence and effects of injection structures may lead to critical divergence between predicted and actual flow, failure to meet public safety and resource protection goals and other project performance metrics.

Injection structures in California and their effects on flow of both groundwater and hydrocarbon geofluids are discussed.