Poroelastic Models for Fault Reactivation in Response to Injection and Production: Application to an Earthquake Sequence near Venus, Johnson County, Texas

Abstract

Poroelastic stress changes in response to production can result in fault reactivation. The combined effects of production from unconventional reservoirs and saltwater disposal in adjacent layers are not captured by flow models alone, requiring geomechanical models that fully couple fluid flow and poroelastic response. To achieve this coupling, we conducted poroelastic finite element simulations in Abaqus, in which Biot’s theory of poroelasticity governs the poroelastic deformation and the post-processed Coulomb failure stress can be introduced as a fault stability criterion. Previous studies on the viability of this technique for the fault reactivation prediction in various disposal-production scenarios reveal that production can often stabilize the fault in the disposal layer and increases fault reactivation potential at the interface between the production layer and the overburden. We initiated an integrated seismic-geologic-geomechanic case study for an earthquake sequence in Johnson County near Venus, Texas. The simulation integrated the following features into a poroelastic model: 2 basement-rooted normal faults from seismic reflection data and earthquake epicenters, horizon interpretations, saltwater disposal volumes into the Ellenburger, and hydrocarbon production volumes from the Barnett Shale. Modeling involved: 1) coordinate system rotation toward in-situ principal stresses; 2) inclusion of two nonplanar and nonparallel faults and five disposal wells surrounding the earthquake sequence; and 3) development of novel modeling methodology. Through this methodology, the model includes complex conduit-barrier fault permeability structures, non-parallel and non-horizontal stratigraphy, arbitrary disposal well locations, simplified stimulated reservoir volumes for producing wells, and eight distinctive facies within the Ellenburger. To meet modeling challenges, we established our computational domain on the basis of tetrahedral elements, in contrast to the hexahedral elements commonly used in geometrically simple conceptual models. Preliminary results for this field study demonstrate that favorably oriented fault, which dips at 63°, gets reactivated earlier in the basement than in the Ellenburger and Barnett, based on Coulomb failure stress criterion after 13 years of injection. After approximately half of this time period, the other fault gets reactivated in the vicinity of the basement top where the fault dip angle changes from 63° in the sediments to 47° in the basement. These simulations provide input for optimized disposal strategies that minimize fault reactivation in areas with concurrent production and disposal.

AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019