Coupled 3D Simulator Models Wastewater-Injection-Induced Seismicity
Authors: Feder Judy 
Published: November 2019

This article, written by JPT Technology Editor Judy Feder, contains highlights of paper SPE 191670, “Wastewater Injection and Slip Triggering: Results From a 3D Coupled Reservoir/Rate-and-State Model,” by Mohsen Babazadeh, SPE, and Jon•Olson, SPE, The University of Texas at Austin, prepared for the 2018 SPE Annual Technical Conference and Exhibition, Dallas, 24–26 September. The paper has not been peer•reviewed.

This paper presents a coupled 3D fluid-flow and geomechanics simulator developed to model induced seismicity resulting from wastewater injection. The simulator modeled several cases of induced earthquakes with the hope of providing a better understanding of such earthquakes and their dominant causal factors, along with primary mitigation controls. Implementation of rate-and-state friction to model friction weakening and strengthening during fault slip to accurately model earthquake occurrence, and an embedded discrete fracture model to efficiently model fluid flow inside the fault, are among the essential features of the simulator. The complete paper presents results from a combined model that brings together injection physics, reservoir dynamics, and fault physics to explain better the primary controls on induced seismicity.

Introduction

Since 2009, a substantial increase in the number of earthquakes in the central and eastern United States has occurred. Oklahoma has been one of the most affected regions, with several earthquakes of M•5+, including the Prague earthquake in 2011 and the Pawnee earthquake in 2016. This has prompted efforts to find and understand any correlation between oil and gas activity—mainly wastewater disposal—and the occurrence of the earthquakes. Induced or triggered seismicity associated with wastewater injection, mining, oil and gas extraction, and geothermal operations has been identified since the earthquakes at the Rocky Mountain Arsenal in 1960s. Fluid injection into subsurface formations can increase pore pressure, reduce the effective stress, and induce slip on faults. Laboratory studies show that the sliding displacement may enhance fracture transmissivity and create a hydraulic pathway through the formation. In an unconventional reservoir, the enhancement could lead to improved hydrocarbon production by using the slipped natural fractures. But in some formations, such as water-disposal aquifers, seismicity might be induced when faults are activated within the igneous basement.

Seismicity induced by fluid injection is controlled by several groups of parameters (injection, reservoir, and frictional). A fundamental understanding of which factors are the most important in triggering slip in areas of active wastewater injection and disposal has been hampered by interrelationships between the various parameters, leading to suggestions of injection volume, rate, or pressure being the most important. However, necessary reservoir characteristics, such as size and permeability, are not well characterized at the well or in the subsurface, and remain the main challenge for deterministic models. Additionally, rupture nucleation on faults near a region of injection depends on rate-and-state and related physics.

The literature contains many relevant seismicity studies, which are identified in the reference section of the complete paper. A 2D simulation of fluid flow inside the fault zone suggested that post-shut-in earthquakes are likely to occur nucleating at the fault edges. Using a similar numerical scheme, other researchers investigated the 2011 Prague earthquake sequence to model the delayed triggering mechanism between the M 4.8 fore-shock and M 5.6 main shock. The result of the study contributed to defining constraints on values of fault transmissivity, fault compliance, and rate-and-state frictional properties. Although expensive, these types of full-physics models are capable of characterizing reservoir and fault properties. A 3D simulation in 2015 modeled fault activation under direct injection into the fault during shale-gas hydraulic fracturing. It found that for brittle faults, the moment magnitude can be higher.