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Summary The rate of penetration (ROP) increases significantly if salt formations are drilled with undersaturated fluids. This is especially true when drilling riserless. Of primary concern is the amount of hole enlargement that will occur. However, if managed, then a step change in drilling performance and costs can be achieved. This paper presents in detail the results of a comprehensive and large-scale laboratory testing program performed on outcrop salt samples that replicate drilling salt formations with undersaturated drilling fluids with flow rates of up to 1,000 gal/min (gpm). The laboratory testing program includes tests performed in a computerized tomographic (CT) scanner to map hole enlargement in real time, as well as variously sized borehole leaching tests with borehole diameters of up to 6 in. Flow rates are scaled to produce comparable levels of turbulence occurring in the field. An analytical hole-enlargement prediction model is presented that incorporates the effects of ROP, pump rate, drill-fluid saturation, dissolving salt drill cuttings, and salt leaching from the borehole wall. This model accurately predicts to within 10% the measured hole enlargements produced in the scaled laboratory tests for a wide range of flow rates and fluid saturations. Predictions of field performance are made, and the implications of the predicted hole geometry are discussed. Provided that high rates of penetration are realized, acceptable hole geometries will result, even when pumping seawater at flow rates of up to 1,600 gpm. A field application is described whereby the historical practice of drilling the last 200 ft of the 20-in. hole into salt using a saturated brine drilling fluid was discontinued in preference to the continued use of seawater. The competing influences of increasing rate of penetration and avoiding the cost of a sacrificial mud system, offset by the increased cost of cement, resulted in cost savings of U.S. $250,000 per well. Implemented in a multiwell subsalt development, cost savings of more than U.S. $1.5 million will be realized in the drilling program. Further applications of this technology are now being sought.

Oil leaks were found in well casings of Caverns 105 and 109 at the Big Hill Strategic Petroleum Reserve site. According to the field observations, two instances of casing damage occurred at the depth of the interface between the caprock and top of salt. This damage could be caused by interface movement induced by cavern volume closure due to salt creep. A three dimensional finite element model, which allows each cavern to be configured individually, was constructed to investigate shear and vertical displacements across each interface. The model contains interfaces between each lithology and a shear zone to examine the interface behavior in a realistic manner. This analysis results indicate that the casings of Caverns 105 and 109 failed by shear stress that exceeded shear strength due to the horizontal movement of the top of salt relative to the caprock, and tensile stress due to the downward movement of the top of salt from the caprock, respectively. The casings of Caverns 101, 110, 111 and 114, located at the far ends of the field, are predicted to be failed by shear stress in the near future. The casings of inmost Caverns 107 and 108 are predicted to be failed by tensile stress in the near future.

This report compiles 3-D finite element analyses performed to evaluate the stability of Strategic Petroleum Reserve (SPR) caverns over multiple leach cycles. When oil is withdrawn from a cavern in salt using freshwater, the cavern enlarges. As a result, the pillar separating caverns in the SPR fields is reduced over time due to usage of the reserve. The enlarged cavern diameters and smaller pillars reduce underground stability. Advances in geomechanics modeling enable the allowable pillar to diameter ratio (P/D) to be defined. Prior to such modeling capabilities, the allowable P/D was established as 1.78 based on some very limited experience in other cavern fields. While appropriate for 1980, the ratio conservatively limits the allowable number of oil drawdowns and hence limits the overall utility and life of the SPR cavern field. Analyses from all four cavern fields are evaluated along with operating experience gained over the past 30 years to define a new P/D for the reserve. A new ratio of 1.0 is recommended. This ratio is applicable only to existing SPR caverns.

Three dimensional finite element calculations were performed to investigate the effect field size, in terms of the number of caverns, on the performance of SPR oil storage caverns leached in domal salt (interms of surface subsidence, storage losses, and cavern integrity). The calculations were performed for cavern fields containing 1, 7, 19, and an infinite number of caverns. The magnitude and volume of subsidence was significantly affected by increasing the number of caverns (nearly an order of magnitude increase was predicted for each increase in field size), while the extent of subsidence (approximately 2000 m fromthe center of the field) and storage loss were not. Furthermore, the percentage of storage loss volume manifested as surface subsidence increased as the cavern field was enlarged. This was attributed to elasticvolumetric dilatation of overlying strata. The multiple cavern calculations demonstrate that storage losses are greater for caverns farther from the center of the caverns field. Based on an accumulated strain stability criteria, the larger cavern fields are predicted to have a shorter life. This criteria also indicates that caverns on the periphery of a field may show signs of instability before the inner caverns. The West Hackberry site (containing 22 caverns) subsidence datamore » closely agrees with the 19 cavern model subsidence predictions, providing confidence in the calculations. Even a 19 cavern field, substantially large by SPR standards, does not approach the behavior predicted by infinite cavern models (which are frequently used because they are economical). This demonstrates that 3D modeling is required to accurately investigate the performance of a multi-cavern array. Although based on a typical SPR cavern design, the results of this study describe mechanics common to all multi-cavern fields and should, in general, be useful tocavern engineers and architects.« less

This report summarizes a series of three-dimensional simulations for the Bayou Choctaw Strategic Petroleum Reserve. The U.S. Department of Energy plans to leach two new caverns and convert one of the existing caverns within the Bayou Choctaw salt dome to expand its petroleum reserve storage capacity. An existing finite element mesh from previous analyses is modified by changing the locations of two caverns. The structural integrity of the three expansion caverns and the interaction between all the caverns in the dome are investigated. The impacts of the expansion on underground creep closure, surface subsidence, infrastructure, and well integrity are quantified. Two scenarios were used for the duration and timing of workover conditions where wellhead pressures are temporarily reduced to atmospheric pressure. The three expansion caverns are predicted to be structurally stable against tensile failure for both scenarios. Dilatant failure is not expected within the vicinity of the expansion caverns. Damage to surface structures is not predicted and there is not a marked increase in surface strains due to the presence of the three expansion caverns. The wells into the caverns should not undergo yield. The results show that from a structural viewpoint, the locations of the two newly proposed expansion caverns are acceptable, and all three expansion caverns can be safely constructed and operated.

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.

This report presents computational analyses that simulate the structural response of caverns at the Strategic Petroleum Reserve Bryan Mound site. The cavern field comprises 20 caverns. Five caverns (1, 2, 4, and 5; 3 was later plugged and abandoned) were acquired from industry and have unusual shapes and a history dating back to 1946. The other 16 caverns (101-116) 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 3-D geomechanics model of the site that predicts deformations, strains, and stresses. Future leaching scenarios due 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 result in this report is relevant to caverns 1, 2, and 5. The caverns have non-cylindrical shapes and have potential regions where the surrounding salt may be damaged during workover procedures. During a workover the normal cavern operating pressure is lowered more » to service a well. At this point the wellhead pressures are atmospheric. When the workover is complete, the cavern is repressurized. The resulting elastic stresses are sufficient to cause tension and large deviatoric stresses at several locations. With time, these stresses relax to a compressive state due to salt creep. However, the potential for salt damage and fracturing exists. The analyses predict tensile stresses at locations with sharp-edges in the wall geometry, or in the case of cavern 5, in the neck region between the upper and lower lobes of the cavern. The effects do not appear to be large-scale, however, so the only major impact is the potential for stress-induced salt falls in cavern 5, potentially leading to hanging string damage. Caverns 1 and 2 have no significant issues regarding leachings due to drawdowns; cavern 5 may require a targeted leaching of the neck region to improve cavern stability and lessen hanging string failure potential. The remaining caverns have no significant issues regarding cavern stability and may be safely enlarged during subsequent oil drawdowns. Well strains are significant and consequently future remedial actions may be necessary. Well strains certainly suggest the need for appropriate monitoring through a well-logging program. Subsidence is currently being monitored; there are no issues identified regarding damage from surface subsidence or horizontal strain to surface facilities. « less

A sensitivity study was performed utilizing a three dimensional finite element model to assess allowable cavern field sizes in strategic petroleum reserve salt domes. A potential exists for tensile fracturing and dilatancy damage to salt that can compromise the integrity of a cavern field in situations where high extraction ratios exist. The effects of salt creep rate, depth of salt dome top, dome size, caprock thickness, elastic moduli of caprock and surrounding rock, lateral stress ratio of surrounding rock, cavern size, depth of cavern, and number of caverns are examined numerically. As a result, a correlation table between the parameters and the impact on the performance of a storage field was established. In general, slower salt creep rates, deeper depth of salt dome top, larger elastic moduli of caprock and surrounding rock, and a smaller radius of cavern are better for structural performance of the salt dome.

Three dimensional finite element analyses were performed to evaluate the structural integrity of the caverns located at the Bayou Choctaw (BC) site which is considered a candidate for expansion. Fifteen active and nine abandoned caverns exist at BC, with a total cavern volume of some 164 MMB. A 3D model allowing control of each cavern individually was constructed because the location and depth of caverns and the date of excavation are irregular. The total cavern volume has practical interest, as this void space affects total creep closure in the BC salt mass. Operations including both cavern workover, where wellhead pressures are temporarily reduced to atmospheric, and cavern enlargement due to leaching during oil drawdowns that use water to displace the oil from the caverns, were modeled to account for as many as the five future oil drawdowns in the six SPR caverns. The impacts on cavern stability, underground creep closure, surface subsidence, infrastructure, and well integrity were quantified.

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.

3-D finite element analyses were performed to evaluate the structural integrity of caverns located at the Strategic Petroleum Reserve's 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.

This Final Report on the Monitoring Phase of the former Weeks Island Strategic Petroleum Reserve crude oil storage facility details the results of five years of monitoring of various surface accessible quantities at the decommissioned facility. The Weeks Island mine was authorized by the State of Louisiana as a Strategic Petroleum Reserve oil storage facility from 1979 until decommissioning of the facility in 1999. Discovery of a sinkhole over the facility in 1992 with freshwater inflow to the facility threatened the integrity of the oil storage and led to the decision to remove the oil, fill the chambers with brine, and decommission the facility. Thereafter, a monitoring phase, by agreement between the Department of Energy and the State, addressed facility stability and environmental concerns. Monitoring of the surface ground water and the brine of the underground chambers from the East Fill Hole produced no evidence of hydrocarbon contamination, which suggests that any unrecovered oil remaining in the underground chambers has been contained. Ever diminishing progression of the initial major sinkhole, and a subsequent minor sinkhole, with time was verification of the response of sinkholes to filling of the facility with brine. Brine filling of the facility ostensively eliminates any furthermore » growth or new formation from freshwater inflow. Continued monitoring of sinkhole response, together with continued surface surveillance for environmental problems, confirmed the intended results of brine pressurization. Surface subsidence measurements over the mine continued throughout the monitoring phase. And finally, the outward flow of brine was monitored as a measure of the creep closure of the mine chambers. Results of each of these monitoring activities are presented, with their correlation toward assuring the stability and environmental security of the decommissioned facility. The results suggest that the decommissioning was successful and no contamination of the surface environment by crude oil has been found.« less

To establish strength criteria of Big Hill salt, a series of quasi-static triaxial compression tests have been completed. This report summarizes the test methods, set-up, relevant observations, and results. The triaxial compression tests established dilatant damage criteria for Big Hill salt in terms of stress invariants (I{sub 1} and J{sub 2}) and principal stresses ({sigma}{sub a,d} and {sigma}{sub 3}), respectively: {radical}J{sub 2}(psi) = 1746-1320.5 exp{sup -0.00034I{sub 1}(psi)}; {sigma}{sub a,d}(psi) = 2248 + 1.25 {sigma}{sub 3} (psi). For the confining pressure of 1,000 psi, the dilatant damage strength of Big Hill salt is identical to the typical salt strength ({radical}J{sub 2} = 0.27 I{sub 1}). However, for higher confining pressure, the typical strength criterion overestimates the damage strength of Big Hill salt.

Rate of penetration (ROP) increases significantly if salt formations are drilled using undersaturated fluids. This is especially true when drilling riserless. Of primary concern is the amount of hole enlargement that will occur. However, if managed, then a step-change in drilling performance and costs can be achieved. This paper presents, in detail, the results of a comprehensive and large-scale laboratory testing program performed on outcrop salt samples that replicate drilling salt formations with undersaturated drilling fluids with flow rates of up to 1000 gpm. The laboratory-testing program includes tests performed in a CT-scanner to map hole enlargement in real-time, as well as variously sized borehole leaching tests with borehole diameters of up to 6". Flow rates are scaled to produce comparable levels of turbulence occurring in the field. An analytical hole enlargement prediction model is presented that incorporates the effects of ROP, pump rate, drill-fluid saturation, dissolving salt drill-cuttings and salt leaching from the borehole wall. This model accurately predicts, to within 10%, the measured hole enlargements produced in the scaled laboratory tests, for a wide range of flow rates and fluid saturations. Predictions of field performance are made, and the implications of the predicted hole geometry discussed. Provided high rates of penetration are realized, acceptable hole geometries will result, even when pumping seawater at flow rates of up to 1600 gpm. A field application is described, whereby the historical practice of drilling the last 200 ft of the 20" hole into salt using a saturated brine drilling fluid was discontinued in preference to continued use of seawater. The competing influences of increasing rate of penetration and avoiding the cost of a sacrificial mud system, offset by the increased cost of cement, resulted in cost savings of $250,000 per well. Implemented in a multi-well sub-salt development, cost savings of over $1.5 million will be realized in the drilling program. Further applications of this technology are now being sought.

This report summarizes recent reviews, observations, and analyses believed to be imperative to our understanding of the recent two million cubic feet salt fall event in Big Hill Cavern 103, one of the caverns of the Strategic Petroleum Reserve (SPR). The fall was the result of one or more stress driven mechanical instabilities, the origins of which are discussed in the report. The work has lead to important conclusions concerning the engineering and operations of the caverns at Big Hill. Specifically, Big Hill, being the youngest SPR site, was subjected to state-of-the-art solutioning methods to develop nominally well-formed, right-circular cylindrical caverns. Examination of the pressure history records indicate that operationally all Big Hill SPR caverns have been treated similarly. Significantly, new three-dimensional (3-D) imaging methods, applied to old (original) and more recent sonar survey data, have provided much more detailed views of cavern walls, roofs, and floors. This has made possible documentation of the presence of localized deviations from ''smooth'' cylindrical cavern walls. These deviations are now recognized as isolated, linear and/or planar features in the original sonar data (circa early 1990s), which persist to the present time. These elements represent either sites of preferential leaching, localized spalling, or a combination of the two. Understanding the precise origin of these phenomena remains a challenge, especially considering, in a historical sense, the domal salt at Big Hill was believed to be well-characterized. However, significant inhomogeneities in the domal salt that may imply abnormalities in leaching were not noted. Indeed, any inhomogeneities were judged inconsequential to the solution-engineering methods at the time, and, by the same token, to the approaches to modeling the rock mass geomechanical response. The rock mass was treated as isotropic and homogeneous, which in retrospect, appears to have been an over simplification. This analysis shows there are possible new opportunities regarding completing an appropriate site characterization for existing operating cavern fields in the SPR, as well as expansion of current sites or development of new sites. Such characterization should first be consistent with needs identified by this report. Secondly, the characterization needs to satisfy the input requirements of the 3-D solutioning calculational methods being developed, together with 3-D geomechanical analyses techniques which address deformation of a salt rock mass that contains inhomogeneities. It seems apparent that focusing on these important areas could preclude occurrence of unexpected events that would adversely impact the operations of SPR.

The Strategic Petroleum Reserve site at West Hackberry, Louisiana has historically experienced casing leaks. Numerous West Hackberry oil storage caverns have wells exhibiting communication between the interior 10 3/4 x 20-inch (oil) annulus and the ''outer cemented'' 20 x 26-inch annulus. Well 108 in Cavern 108 exhibits this behavior. It is thought that one, if not the primary, cause of this communication is casing thread leaks at the 20-inch casing joints combined with microannuli along the cement casing interfaces and other cracks/flaws in the cemented 20 x 26-inch annulus. An operation consisting of a series of nitrogen leak tests, similar to cavern integrity tests, was performed on Cavern 108 in an effort to determine the leak horizons and to see if these leak horizons coincided with those of casing joints. Certain leaky, threaded casing joints were identified between 400 and 1500 feet. A new leak detection procedure was developed as a result of this test, and this methodology for identifying and interpreting such casing joint leaks is presented in this report. Analysis of the test data showed that individual joint leaks could be successfully identified, but not without some degree of ambiguity. This ambiguity is attributed to changes in the fluid content of the leak path (nitrogen forcing out oil) and possibly to very plausible changes in characteristics of the flow path during the test. These changes dominated the test response and made the identification of individual leak horizons difficult. One consequence of concern from the testing was a progressive increase in the leak rate measured during testing due to nitrogen cleaning small amounts of oil out of the leak paths and very likely due to the changes of the leak path during the flow test. Therefore, careful consideration must be given before attempting similar tests. Although such leaks have caused no known environmental or economic problems to date, the leaks may be significant because of the potential for future problems. To mitigate future problems, some repair scenarios are discussed including injection of sealants.

A suite of laboratory triaxial compression and triaxial steady-state creep tests provide quasi-static elastic constants and damage criteria for bedded rock salt and dolomite extracted from Cavern Well No.1 of the Tioga field in northern Pennsylvania. The elastic constants, quasi-static damage criteria, and creep parameters of host rocks provides information for evaluating a proposed cavern field for gas storage near Tioga, Pennsylvania. The Young's modulus of the dolomite was determined to be 6.4 ({+-}1.0) x 10{sup 6} psi, with a Poisson's ratio of 0.26 ({+-}0.04). The elastic Young's modulus was obtained from the slope of the unloading-reloading portion of the stress-strain plots as 7.8 ({+-}0.9) x 10{sup 6} psi. The damage criterion of the dolomite based on the peak load was determined to be J{sub 2}{sup 0.5} (psi) = 3113 + 0.34 I{sub 1} (psi) where I{sub 1} and J{sub 2} are first and second invariants respectively. Using the dilation limit as a threshold level for damage, the damage criterion was conservatively estimated as J{sub 2}{sup 0.5} (psi) = 2614 + 0.30 I{sub 1} (psi). The Young's modulus of the rock salt, which will host the storage cavern, was determined to be 2.4 ({+-}0.65) x 10{sup 6} psi, with a Poisson's ratio of 0.24 ({+-}0.07). The elastic Young's modulus was determined to be 5.0 ({+-}0.46) x 10{sup 6} psi. Unlike the dolomite specimens under triaxial compression, rock salt specimens did not show shear failure with peak axial load. Instead, most specimens showed distinct dilatancy as an indication of internal damage. Based on dilation limit, the damage criterion for the rock salt was estimated as J{sub 2}{sup 0.5} (psi) = 704 + 0.17 I{sub 1} (psi). In order to determine the time dependent deformation of the rock salt, we conducted five triaxial creep tests. The creep deformation of the Tioga rock salt was modeled based on the following three-parameter power law as {var_epsilon}{sub s} = 1.2 x 10{sup -17} {sigma}{sup 4.75} exp(-6161/T), where {var_epsilon}{sub s} is the steady state strain rate in s{sup -1}, {sigma} is the applied axial stress difference in psi, and T is the temperature in Kelvin.

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.

This report will explore the hypothesis that an underground cavity in gassy salt will eventually be gas filled as is observed on a small scale in some naturally occurring salt inclusions. First, a summary is presented on what is known about gas occurrences, flow mechanisms, and cavern behavior after abandonment. Then, background information is synthesized into theory on how gas can fill a cavern and simultaneously displace cavern fluids into the surrounding salt. Lastly, two-phase (gas and brine) flow visualization experiments are presented that demonstrate some of the associated flow mechanisms and support the theory and hypothesis that a cavity in salt can become gas filled after plugging and abandonment

The U. S. Department of Energy Strategic Petroleum Reserve currently has approximately 500 million barrels of crude oil stored in 62 caverns solution-mined in salt domes along the Gulf Coast of Louisiana and Texas. One of the challenges of operating these caverns is ensuring that none of the fluids in the caverns are leaking into the environment. The current approach is to test the mechanical integrity of all the wells entering each cavern approximately once every five years. An alternative approach to detecting cavern leaks is to monitor the cavern pressure, since leaking fluid would act to reduce cavern pressure. Leak detection by pressure monitoring is complicated by other factors that influence cavern pressure, the most important of which are thermal expansion and contraction of the fluids in the cavern as they come into thermal equilibrium with the host salt, and cavern volume reduction due to salt creep. Cavern pressure is also influenced by cavern enlargement resulting from salt dissolution following introduction of raw water or unsaturated brine into the cavern. However, this effect only lasts for a month or two following a fluid injection. In order to implement a cavern pressure monitoring program, a software program called CaveMan has been developed. It includes thermal, creep and salt dissolution models and is able to predict the cavern pressurization rate based on the operational history of the cavern. Many of the numerous thermal and mechanical parameters in the model have been optimized to produce the best match between the historical data and the model predictions. Future measurements of cavern pressure are compared to the model predictions, and significant differences in cavern pressure set program flags that notify cavern operators of a potential problem. Measured cavern pressures that are significantly less than those predicted by the model may indicate the existence of a leak.

The first sinkhole at the Weeks Island Strategic Petroleum Reserve (SPR) site was initially observed in May 1992. Concurrent with the increasing dissolution of salt over the mined oil storage area below, it has gradually enlarged and deepened. Beginning in 1994 and continuing to the present, the injection of saturated brine directly into the sinkhole throat some 76 m beneath the ground surface essentially arrested further dissolution, providing time to make adequate preparation for the safe and orderly transfer of crude oil to other storage facilities. This mitigation measure marked the first time that such a control procedure has been used in salt mining; previously all control has been achieved by either in-mine or from-surface grouting. A second and much smaller sinkhole was noticed in early 1995 on an opposite edge of the SPR mine, but with a very similar geological and mine mechanics setting. Both sinkholes occur where the edges of upper 152 m and lower 213 m mined storage levels are nearly vertically aligned. Such coincidence maximizes the tensional stress development, leading to fracturing in the salt. This cracking takes 20 or more years to develop. The cracks then become flow paths for brine incursion, which after time progress into the mined openings. Undersaturated ground water gradually enlarges the cracks in salt through dissolution, leading to eventual collapse of the overlying sand to form sinkholes. Other geologic conditions may also be secondary factors in controlling both mining extent and sinkhole location.