Analysis of the source parameters behaviour during the Val D'Agri test-injection (Italy)

Abstract

Recent advances in understanding induced earthquake mechanics are due to the new and

high-quality observations from seismic signals, natural faults and laboratory experiments. The synergy of different approaches allows the acquisition of useful observations to understand the earthquake source process. In this study we investigate the source parameters behaviour of induced earthquakes recorded during daily injection tests in a wastewater disposal well in Val D’Agri (VDA, Italy). We choose this area because of its complexity and particularity. VDA is a Quaternary extensional basin located in the Southern Apennines characterized by a complex tectonic evolution, high seismic hazard and significant hydrocarbon exploitation. The basin hosts the largest onshore oilfield in Europe that

produces oil and associated gas from low- porosity, fractured Cretaceous limestone.

Wastewater associated to hydrocarbon production is reinjected into the Costa Molina 2 high-rate well (CM2) into an unproductive marginal portion of the carbonate reservoir at 2890-3096 m depth (b.s.l.). Initial injection tests were performed from 2 to 12 June 2006 with variable duration (from 4 to 32 hr) and hourly injection rate of 38 m3/hr. During the tests a maximum well-head pressure of 101 bar was measured. These tests induced a swarm microseismicity (69 events, ML comprised by 0.3 and 1.8) recorded by a high-performance local network run by INGV (Mc 0.4, 23 stations) that constitutes our testing data-set. We investigate the full moment tensors decomposing them in Double-Couple (DC), Isotropic (ISO) and Compensated Linear Vector Dipole (CLVD) components by using HybridMT technique. We obtain significant contribution of %ISO and %CLVD components that tend to increase with time. Also, we compute the moment magnitudes (that we compare with ML) and the static stress drops (by spectral analysis). Finally, we discuss each analysed source parameter as a function of the injection pressure curve in the geological/tectonic contest.