Geomechanical Behavior of Caprock and Cement: Plasticity

in Hydrodynamic Seals

Bill Carey, Hiroko Mori, Diana Brown, Rajesh Pawar

Earth and Environmental Sciences Division Los Alamos, NM

October 6, 2014 International Conference on Greenhouse Gas Technologies (GHGT-12) Austin, Texas

LA-UR-20408

Abstract The buoyancy of supercritical CO2 requires that caprock and wellbore systems provide a seal that prevents upward migration of CO2 to the surface or to underground drinking water sources. The geomechanical behavior of caprock and wellbore systems will determine the robustness of the CO2 storage system to stress, pressure and temperature (e.g., Verdon et al. 2013). Injection of CO2 will result in thermal- and pressure-generated stresses in the injection wellbore, in the storage reservoir, and at the caprock-reservoir interface that have the potential to compromise the integrity of the storage reservoir. In particular, recent concerns have focused on the potential of injection-triggered seismicity to damage the caprock seal (Zoback and Gorelick 2012). In this study, we focus on the hydrologic consequences of geomechanical damage to caprock and wellbore systems. There are few experimental or field data available that provide measures of the effective permeability of damaged rock or cement. Typical caprock (e.g., shale or evaporite) and cement are not especially brittle materials and may not be capable of supporting large fracture apertures with high permeability. In fact, the practice of using proppants in the hydraulic fracturing industry demonstrates the potential for limited as well as short-term transmissivity of fractured shale. These considerations emphasize the need to quantitatively evaluate fracture permeability in caprock and cement systems. We have conducted triaxial coreflood experiments to directly measure permeability of water and supercritical CO2 in caprock and cement. The experimental system allows simultaneous application of independent confining and pore pressure to 35 MPa and axial load to 480 MPa; measurement of permeability with controlled pressure drop; and quantification of sample deformation. We use x-ray tomography to characterize initial sample heterogeneity and subsequent fracture geometry with a resolution of 10-25 m. In the experiments, undamaged, impermeable samples are subject to an axial load that produces either compressive shear or pure shear fractures with an applied confining pressure and injection pressure of CO2 or water. Sample deformation and permeability are recorded as the axial load increases. After sample failure, the system is returned to hydrostatic conditions for additional permeability characterization. We studied the behavior of shale (Utica from Pennsylvania and Ohio), anhydrite (Springerville, Arizona), Portland cement, and shale-cement-steel composites (synthetic wellbore systems). Experimental conditions ranged from 20-45 oC with a relatively low confining pressure between 2 and 12 MPa. All three materials showed evidence of significant ductile behavior. The cement samples reached their ultimate strength and then either had a small fracture event followed by continuous deformation or simply plastically deformed until the end of the experiment. The shale samples had a greater tendency to fracture at ultimate strength, but recovered and plastically deformed until the end of the experiment. The anhydrite went through a period of extensive deformation before fracturing and recovering slightly. The behavior of the synthetic wellbore systems was most similar to that of shale. Prior to deformation, the samples generally had no measurable permeability (

Motivation

Mechanical damage to cement and caprock creates potential CO2 leakage pathways

In principle, we can calculate the magnitude of damaging stresses

There is little basis for assessing the consequences: permeability of damaged seals

What role does plastic deformation have in limiting permeability?

Triaxial Multiphase Coreflood with Tomography

Portable for use in different facilities Max operating conditions: 100 oC, 350 bar

confining/pore, 4,800 bar axial load Samples: 1x3" Applications:

CO2 sequestration (multiphase flow, caprock integrity, wellbore integrity)

Shale gas (fracture generation and behavior, extraction efficiency, multiphase flow behavior)

Geothermal (fracture patterns, flow behavior) Nuclear waste disposal site (material stability)

Triaxial Coreflood Experiments

Triaxial coreflood experiments with x-ray tomography Conventional compression studies Pure shear configuration

Materials Shale and anhydrite caprock Type G oilwell cement Wellbore Composites

Shale-Cement-Steel Temperature: 45 or 22 oC Confining pressure: 120 or 35 bars Procedure

Measure elastic properties Bring to hydrostatic conditions Apply steady axial strain as increasing axial pressure Monitor/measure permeability continuously until sample fails Measure permeability as function of confining and injection pressure at

hydrostatic conditions Pre- and post-experiment x-ray tomography

Thanks to Chesapeake Energy for Shale Samples!

Triaxial Coreholder: Self-supported triaxial stress with permeability measurement

Triaxial Coreflood: Confining pressure Axial load Multiphase fluid injection

Strain measurement Piston displacement Acoustic velocity Fluid pressure Temperature Fluid samples

In Situ X-ray or Neutron Tomography with triaxial coreflood

Type G Oilwell (Neat) Cement in Compression

Portland (Oilwell) Cement: Strain-Stress-Permeability

Permeability not Measurable at < 1 D

Utica Shale Fracture Patterns in Compression

Experiment Utica-1-2 Experiment Utica-7

Utica Shale: Permeability-Strain

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Strain-Permeability Relations: Utica 2: 9/19/2013

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Shale-Cement-Steel Wellbore System

Optical X-ray Tomography

Synthetic Wellbore System

Composite Wellbore: Stress-Strain

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Strain-Permeability Relations: Synthetic Wellbore 5778-1: 8/13/2013

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Pure Shear in Portland Cement

Cement Pure Shear Permeability

Pure Shear of Utica Shale Perpendicular Layers

Strain-Stress-Permeability for Shale

Permeability and Confining Pressure

Impact of Layer Orientation in Shale Parallel Layers

Permeability and Confining Pressure

Non-Darcy Flow?

Conclusions Portland cement shows strong plastic behavior

at confining pressure of just 120 bars Compression experiments ambiguous but

permeability is negligible in shale and cement Permeability in pure shear experiments

Portland cement < 90 mD Shale perpendicular to layering < 30 mD Shale parallel to layering < 500 mD

Increasing confining pressure shows limited reduction in permeability of shale in pure shear

Factor of 2 4 with increase of 100 bars Extensive deformation (1 7%) required to

generate permeable pathways Stress shadow and cement plasticity may

protect cement in wellbore system

Los Alamos National Laboratory Multi-Disciplinary Work: Fundamental to Field Deployment

Posters Integrity of pre-existing wellbores (S. Kelkar ) CO2 Leakage into Shallow Aquifers (M. Porter)

Monday Plasticity in hydrodynamic seals (B. Carey) Source sink matching in China (P. Stauffer)

Tuesday EOR uncertainty with CO2 (Z. Dai) Shallow aquifer monitoring (E. Keating) NRAP results (R. Pawar) CO2 enhanced shale gas (R. Middleton)

Wednesday CO2-PENS water module (J. Sullivan) Wellbore leakage with thief zones (D. Harp)

(Funding Acknowledgement: DOE-FE and DOE-IA)