A New Conceptual Model For Enhancement And Inhibition Of Low-Magnitude Microseismic Events Related To Fluid Injection

Abstract

Summary Microseismic events have been observed during many fluid injection projects and are generally related to hydraulic fracturing or re-activation of pre-existing, critically stressed weakness zones. However, microseismic events have also been recorded in several cases where injection pressure is quite low and below the estimated fracture pressure. The primarily assumed mechanism for such events is that the rock was critically stressed and brought to failure by small stress changes, related to the load changes and/or pore pressure changes. This paper explains a new and additional possibility to enhance or inhibit the generation of small microseismic events through an interdisciplinary approach based on the Lugeon test analysis. We analyze the successive evolution of injection pressure versus injection rate of CO2. Based on the type of flow patterns, fluid flow is classified into characteristic groups proposed by Houlsby (1976) . Different flow patterns such as turbulent, laminar, and washout were observed throughout the CO2 injection at e.g. In Salah. Mobilization (wash-out) of fines and clogging of pores affect the pore pressure and may destabilize rock blocks. Effective stress acting on fractures also changes upon injection shut-in/restart, possibly causing small displacements of a rock block and result in a low-magnitude microseismic event.