Steven R. Sobolik

Analysis of Cavern Shapes for the Strategic Petroleum Reserve

This report presents computational analyses to determine the structural integrity of different salt cavern shapes. Three characteristic shapes for increasing cavern volumes are evaluated and compared to the baseline shape of a cylindrical cavern. Caverns with enlarged tops, bottoms, and mid-sections are modeled. The results address pillar to diameter ratios of some existing caverns in the system and will represent the final shape of other caverns if they are repeatedly drawn down. This deliverable is performed in support of the U.S. Strategic Petroleum Reserve. Several three-dimensional models using a close-packed arrangement of 19 caverns have been built and analyzed using a simplified symmetry involving a 30-degree wedge portion of the model. This approach has been used previously for West Hackberry (Ehgartner and Sobolik, 2002) and Big Hill (Park et al., 2005) analyses. A stratigraphy based on the Big Hill site has been incorporated into the model. The caverns are modeled without wells and casing to simplify the calculations. These calculations have been made using the power law creep model. The four cavern shapes were evaluated at several different cavern radii against four design factors. These factors included the dilatant damage safety factor in salt, the cavern volume closure, axial well strain in the caprock, and surface subsidence. The relative performance of each of the cavern shapes varies for the different design factors, although it is apparent that the enlarged bottom design provides the worst overall performance. The results of the calculations are put in the context of the history of cavern analyses assuming cylindrical caverns, and how these results affect previous understanding of cavern behavior in a salt dome.

Three dimensional simulation for Big Hill Strategic Petroleum Reserve (SPR).

3-D finite element analyses were performed to evaluate the structural integrity of caverns located at the Strategic Petroleum Reserves Big Hill site. State-of-art analyses simulated the current site configuration and considered additional caverns. The addition of 5 caverns to account for a full site and a full dome containing 31 caverns were modeled. Operations including both normal and cavern workover pressures and cavern enlargement due to leaching were modeled to account for as many as 5 future oil drawdowns. Under the modeled conditions, caverns were placed very close to the edge of the salt dome. The web of salt separating the caverns and the web of salt between the caverns and edge of the salt dome were reduced due to leaching. The impacts on cavern stability, underground creep closure, surface subsidence and infrastructure, and well integrity were quantified. The analyses included recently derived damage criterion obtained from testing of Big Hill salt cores. The results show that from a structural view point, many additional caverns can be safely added to Big Hill.

3-D CAVERN ENLARGEMENT ANALYSES

Three-dimensional finite element analyses simulate the mechanical response of enlarging existing caverns at the Strategic Petroleum Reserve (SPR). The caverns are located in Gulf Coast salt domes and are enlarged by leaching during oil drawdowns as fresh water is injected to displace the crude oil from the caverns. The current criteria adopted by the SPR limits cavern usage to 5 drawdowns (leaches). As a base case, 5 leaches were modeled over a 25 year period to roughly double the volume of a 19 cavern field. Thirteen additional leaches where then simulated until caverns approached coalescence. The cavern field approximated the geometries and geologic properties found at the West Hackberry site. This enabled comparisons are data collected over nearly 20 years to analysis predictions. The analyses closely predicted the measured surface subsidence and cavern closure rates as inferred from historic well head pressures. This provided the necessary assurance that the model displacements, strains, and stresses are accurate. However, the cavern field has not yet experienced the large scale drawdowns being simulated. Should they occur in the future, code predictions should be validated with actual field behavior at that time. The simulations were performed using JAS3D, a three dimensional finite element analysis code for nonlinear quasi-static solids. The results examine the impacts of leaching and cavern workovers, where internal cavern pressures are reduced, on surface subsidence, well integrity, and cavern stability. The results suggest that the current limit of 5 oil drawdowns may be extended with some mitigative action required on the wells and later on to surface structure due to subsidence strains. The predicted stress state in the salt shows damage to start occurring after 15 drawdowns with significant failure occurring at the 16th drawdown, well beyond the current limit of 5 drawdowns.

Preliminary thermomechanical results of a heater test in welded tuff

Abstract The Yucca Mountain Project is conducting a Single Heater Test (SHT) in the Exploratory Studies Facility at Yucca Mountain, NV to evaluate coupled thermal-mechanical-hydrologic-chemical processes in situ . This paper describes the thermomechanical aspects of the SHT and presents preliminary results from the test. The 5-m long heater deployed in the SHT was energized on 26 August 1996 with the heater power set at 4 kW. Thermomechanical instrumentation was installed within and on the rock mass surrounding the SHT. The thermomechanical instrumentation includes temperature measurements using thermocouples, RTDs, and thermistors; displacement measurements using multiple-point borehole extensometers (MPBXs), tape extensometers, and surface-mounted wire extensometers; load measurements from load cells on rock bolts installed both within thermally perturbed and ambient regions; and rock mass modulus measurements using the NX borehole jack. Pretest analyses of the thermomechanical response were conducted using the thermohydrologic code TOUGH2 and the thermomechanical structural code JAC3D. The test design, preliminary data, and comparisons between pretest predictions and early data from selected thermal and mechanical sensors are presented.

Hydromechanical response of jointed host granitic rock during excavation of the FEBEX tunnel.

Abstract The FEBEX experiment is a coupled T-H-M test conducted at the Grimsel Test Site in Switzerland designed to study various processes occurring near a radioactive waste storage repository. The experiment uses cylindrical heaters placed along the axis of a drift to simulate the presence of radioactive waste canisters. An engineered barrier made of bentonite blocks is placed in the surrounding annular space. Several research teams performed blind predictions of changes in water pressure and flow rate induced by the excavation of the FEBEX tunnel. This report outlines two numerical approaches to these predictions. One, the compliant joint model, captures the average response of a jointed rock mass by distributing the response of the individual joints throughout it. The other, a modified Crack tensor theory, treats the non-linear mechanical and hydraulic behavior of fractures. The difference of results between the models are discussed and compared to the actual test data.

Analysis of salt and casing fracture mechanisms during cavern integrity testing for SPR salt caverns.

This report presents computational analyses to evaluate the risk of Cavern Certification Testing on casing damage and salt fracturing. This deliverable is performed in support of the U.S. Strategic Petroleum Reserve. Several models have been built utilizing either a 1-cavern or 19-cavern, 30-degree wedge of symmetry as has been used on previous West Hackberry (Ehgartner and Sobolik, 2002) and Big Hill (Park et al., 2005) analyses. Various stages of complexity have been added to each model: mesh generation for both 1-cavern and 19-cavern fields; the inclusion of the small-diameter well going from the surface to the top of the caverns; and the addition of steel casing and cement liners to the well surfaces. A stratigraphy based on the Bayou Choctaw site has been chosen for modeling, and the caverns are modeled as equivalent cylinders to simplify the calculations. Salt dilation, as defined by two separate dilation criteria, has been generated during simulated workover routines and cavern integrity tests, but only for the models that include the steel casing and cement liners. The addition of these features generates the tensile and shear stress conditions in the model necessary to cause stress states that exceed dilation criteria. These predictions have been made using the power law creep model. The results show that the salt can be damaged during high pressure change conditions in the wells and caverns.

Geomechanical Model Calibration Using Field Measurements for a Petroleum Reserve

A finite element numerical analysis model has been constructed that consists of a mesh that effectively captures the geometries of Bayou Choctaw (BC) Strategic Petroleum Reserve (SPR) site and multimechanism deformation (M-D) salt constitutive model using the daily data of actual wellhead pressure and oil–brine interface location. The salt creep rate is not uniform in the salt dome, and the creep test data for BC salt are limited. Therefore, the model calibration is necessary to simulate the geomechanical behavior of the salt dome. The cavern volumetric closures of SPR caverns calculated from CAVEMAN are used as the field baseline measurement. The structure factor, A2, and transient strain limit factor, K0, in the M-D constitutive model are used for the calibration. The value of A2, obtained experimentally from BC salt, and the value of K0, obtained from Waste Isolation Pilot Plant salt, are used for the baseline values. To adjust the magnitude of A2 and K0, multiplication factors A2F and K0F are defined, respectively. The A2F and K0F values of the salt dome and salt drawdown skins surrounding each SPR cavern have been determined through a number of back analyses. The cavern volumetric closures calculated from this model correspond to the predictions from CAVEMAN for six SPR caverns. Therefore, this model is able to predict behaviors of the salt dome, caverns, caprock, and interbed layers. The geotechnical concerns associated with the BC site from this analysis will be explained in a follow-up paper.

2013 strategic petroleum reserve big hill well integrity grading report.

This report summarizes the work performed in developing a framework for the prioritization of cavern access wells for remediation and monitoring at the Big Hill Strategic Petroleum Reserve site. This framework was then applied to all 28 wells at the Big Hill site with each well receiving a grade for remediation and monitoring. Numerous factors affecting well integrity were incorporated into the grading framework including casing survey results, cavern pressure history, results from geomechanical simulations, and site geologic factors. The framework was developed in a way as to be applicable to all four of the Strategic Petroleum Reserve sites.

U.S. strategic petroleum reserve Big Hill 114 leak analysis 2012.

This report addresses recent well integrity issues related to cavern 114 at the Big Hill Strategic Petroleum Reserve site. DM Petroleum Operations, M&O contractor for the U.S. Strategic Petroleum Reserve, recognized an apparent leak in Big Hill cavern well 114A in late summer, 2012, and provided written notice to the State of Texas as required by law. DM has since isolated the leak in well A with a temporary plug, and is planning on remediating both 114 A- and B-wells with liners. In this report Sandia provides an analysis of the apparent leak that includes: (i) estimated leak volume, (ii) recommendation for operating pressure to maintain in the cavern between temporary and permanent fixes for the well integrity issues, and (iii) identification of other caverns or wells at Big Hill that should be monitored closely in light of the sequence of failures there in the last several years.

Analysis of Cavern Stability at the West Hackberry SPR Site

This report presents computational analyses that simulate the structural response of caverns at the Strategic Petroleum Reserve (SPR) West Hackberry site. The cavern field comprises 22 caverns. Five caverns (6, 7, 8, 9, 11) were acquired from industry and have unusual shapes and a history dating back to 1946. The other 17 caverns (101-117) were leached according to SPR standards in the mid-1980s and have tall cylindrical shapes. The history of the caverns and their shapes are simulated in a three-dimensional geomechanics model of the site that predicts deformations, strains, and stresses. Future leaching scenarios corresponding to oil drawdowns using fresh water are also simulated by increasing the volume of the caverns. Cavern pressures are varied in the model to capture operational practices in the field. The results of the finite element model are interpreted to provide information on the current and future status of subsidence, well integrity, and cavern stability. The most significant results in this report are relevant to Cavern 6. The cavern is shaped like a bowl with a large ceiling span and is in close proximity to Cavern 9. The analyses predict tensile stresses at the edge of the ceiling during repressuization of Cavern 6 following workover conditions. During a workover the cavern is at low pressure to service a well. The wellhead pressures are atmospheric. When the workover is complete, the cavern is repressurized. The resulting elastic stresses are sufficient to cause tension around the edge of the large ceiling span. With time, these stresses relax to a compressive state because of salt creep. However, the potential for salt fracture and propagation exists, particularly towards Cavern 9. With only 200 ft of salt between the caverns, the operational consequences must be examined if the two caverns become connected. A critical time may be during a workover of Cavern 9 in part because of the operational vulnerabilities, but also because dilatant damage is predicted under the ledge that forms the lower lobe in the cavern. The remaining caverns have no significant issues regarding cavern stability and may be safely enlarged during subsequent oil drawdowns. Predicted well strains and subsidence are significant and consequently future remedial actions may be necessary. These predicted well strains certainly suggest appropriate monitoring through a well-logging program. Subsidence is currently being monitored.