On the Potential for Induced Seismicity at the Cavone Oilfield: Analysis of Geological and Geophysical Data, and Geomechanical Modeling
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July, 2014 ON THE POTENTIAL FOR INDUCED SEISMICITY AT THE CAVONE OILFIELD: ANALYSIS OF GEOLOGICAL AND GEOPHYSICAL DATA, AND GEOMECHANICAL MODELING BY Luciana Astiz - University of California San Diego James H. Dieterich - University of California Riverside Cliff Frohlich - University of Texas at Austin Bradford H. Hager - Massachusetts Institute of Technology Ruben Juanes - Massachusetts Institute of Technology John H. Shaw –Harvard University 1 July, 2014 TABLE OF CONTENTS EXECUTIVE SUMMARY ……………………………………………………….... 5 INTRODUCTION ………………………………………………………………….9 1. TECTONIC FRAMEWORK OF THE EMILIA-ROMAGNA REGION .................... 11 1.1 SEISMOTECTONIC SETTING ............................................................................................................................ 11 1.1.1 HISTORICAL SEISMICITY IN THE EMILIA‐ROMAGNA REGION .................................................................... 12 1.2 CAVONE STRUCTURE ....................................................................................................................................... 19 1.3 GEOLOGIC EVIDENCE FOR TECTONIC ACTIVITY OF STRUCTURES IN THE FERRARESE‐ROMAGNOLO ARC ..................................................................................................................... 24 1.4 SEISMOTECTONIC ANALYSIS .......................................................................................................................... 26 1.5 GPS CONSTRAINTS ON TECTONICS — PRE‐EARTHQUAKE REGIONAL DEFORMATION RATES ............ 30 1.6 CONCLUSIONS OF SEISMOTECTONIC AND GEODETIC ANALYSES .............................................................. 3 2 2. SEISMICITY IN THE EMILIA-ROMAGNA REGION AND THE PO VALLEY .... 33 2.1 REGIONAL SEISMIC ACTIVITY PRIOR TO MAY 2012 .................................................................................. 34 2.2 THE MAY 2012 EMILIA‐ROMAGNA SEQUENCE: ISC LOCATIONS ........................................................... 43 2.3 REGIONAL SEISMIC ACTIVITY JUNE 2012 – JUNE 2014 .......................................................................... 48 2.4 COSEISMIC DEFORMATION AND ESTIMATES OF EARTHQUAKE SOURCE PROPERTIES .......................... 53 2.5 COULOMB STRESS CHANGES AND TRIGGERED EARTHQUAKES ................................................................. 56 2.6 CONCLUSIONS ABOUT SEISMICITY IN REGION NEAR CAVONE WELL #14 .............................................. 63 3. MECHANISMS FOR INDUCED SEISMICITY AND THEIR APPLICABILITY TO THE CAVONE FIELD .......................................................................................................... 65 3.1 EARTHQUAKES BY FLUID INJECTION ..................................................................................................................... 65 3.1.1 MECHANISM ........................................................................................................................................................ 65 3.1.2 CHARACTERISTICS OF INDUCED AND TRIGGERED SEISMICITY BY FLUID INJECTION ............................. 67 3.1.3 CAVONE PORE FLUID PRESSURES AND INJECTION VOLUMES ..................................................................... 70 3.1.4 DID INJECTION AT CAVONE AFFECT THE COULOMB STRESS TRIGGERING OF THE 29 MAY 2012 EARTHQUAKE? ......................................................................................................................... 74 3.2 INDUCED EARTHQUAKES BY POROELASTIC INTERACTIONS ............................................................................. 77 3.2.1 COULOMB STRESS CHANGES FROM PRODUCTION OF THE CAVONE RESERVOIR ............................... 78 3.3 INDUCED EARTHQUAKES BY ISOSTATIC RECOVERY ................................................................................... 79 3.4 INDUCED SEISMICITY BY THERMO‐ELASTIC STRESSING ............................................................................ 80 3.5 CONCLUSIONS .................................................................................................................................................. 81 3 July, 2014 4 INTERPRETATION OF INJECTION TESTS AND RESERVOIR MODELING ...................................................................................................................................... 83 4.1 INFERENCE OF EFFECTIVE PERMEABILITY OVER A RANGE OF SPATIAL AND TEMPORAL SCALES .... 90 4.2 RESERVOIR FLOW AND GEOMECHANICAL MODELING .............................................................................. 93 4.2.1 CAVONE RESERVOIR STATIC MODEL ........................................................................................................... 93 4.2.1.1 STRATIGRAPHIC MODEL ................................................................................................................................. 93 4.2.1.2 GEOMECHANICAL GRID .................................................................................................................................. 95 4.2.2 CAVONE RESERVOIR DYNAMIC MODEL ....................................................................................................... 97 4.2.2.1 COUPLED FLOW AND GEOMECHANICAL MODELING ................................................................................. 97 4.2.2.1.1 INTRODUCTION ..................................................................................................................................... 97 4.2.2.1.2 MATHEMATICAL FORMULATION OF COUPLED MULTIPHASE POROMECHANICS ....................... 99 4.2.2.1.3 POROMECHANICS OF FAULTS .......................................................................................................... 104 4.2.2.1.4 FAULT PRESSURE IN THE FAILURE CRITERION ............................................................................ 106 4.2.2.1.5 IMPLEMENTATION INTO A SIMULATION SOFTWARE ................................................................... 107 4.2.2.2 COUPLED FLOW‐GEOMECHANICAL SIMULATION OF THE CAVONE RESERVOIR ................................ 109 4.3 SYNTHESIS AND CONCLUSIONS FROM THE DYNAMIC MODELING ........................................................ 121 REFERENCES ..................................................................................................................... 125 4 July, 2014 EXECUTIVE SUMMARY Production of substantial volumes of oil within tectonically active areas such as Southern California and Northern Italy has been ongoing since the early 20th century with no documented instances of triggered seismicity. However, the recent increase in seismic activity in more stable plate interior regions such as the central United States where new, largely unconventional oil and gas resources are being developed, has caused concern that human activity could trigger earthquakes. Although almost all documented cases of triggered seismicity are associated with injection of large volumes of fluids, not net production, it is natural and prudent to ask the question of the extent to which production of oil might trigger earthquakes. In particular, the occurrence of a sequence of highly damaging earthquakes during May, 2012, near the Cavone oil field raised the question of whether these earthquakes might have been triggered, or, if not, if future activities might trigger other damaging events. The purpose of this report is to attempt to answer these questions given our current state of knowledge and state-of-the-art techniques in seismology, structural geology, tectonic geodesy, reservoir flow simulation, and geomechanics. Because the likelihood of triggering earthquakes depends upon the current tectonic stress and rate of loading by ongoing deformation of the crust, we begin our study by examining the seismotectonic setting of the field. We analyze seismic data and previous studies to determine the rate, pattern, and style of deformation recorded in the geological record of folding and faulting. We also examine the rate of ongoing deformation observed using GPS, as well as the historical rate at which earthquakes comparable to the damaging May 2012 sequence occur. All three approaches give consistent results, showing deformation rates of ~ 1 mm/yr over distances of tens of kilometers, elastic stress accumulation rates of order 0.02 bars/yr, and the release of this accumulated elastic stress by earthquakes with typical rates of occurrence of about one per century somewhere in the region for magnitude Mw = 6 events. To compare the tectonic rates of stress accumulation to the loading rates from production of oil and the associated lesser amount of injection of waste water in the Cavone field, we use reservoir models that include not only the pressure changes associated with the flow of pore fluids, but also the regional stress changes resulting from volumetric strain of the reservoir. We also identify the geometry and tectonic style of potentially seismogenic faults. There are good records of the volumes of oil and water produced from and injected into all of the wells in the Cavone field since production began in 1980. In order to estimate the resulting fluid pressure changes, along with the changes in stress on the faults in the region, it 5 July, 2014 is necessary to determine the properties of the reservoir. Most important is the permeability structure; the porosity structure and compressibility structure are also important. These properties can be estimated from measurements on cores, from injection tests of wells, and from the history of pressure variations in wells on time scales of weeks to decades. The variation of