Some Remarks Regarding Evaluation of the Potential Collapse of the I & W Brine Cavern

near Carlsbad, New Mexico

Christopher A. Rautman

Sandia National Laboratories

P.O. Box 5800, MS-0706

Albuquerque, New Mexico 87185

carautm@sandia.gov; (505) 844-2109

INTRODUCTION

The I & W brine cavern, located at the “South-Y” of U.S. Highways 285 and 62–180, south of the main part of the City of Carlsbad, was produced by nearly 25 years of uncontrolled solution mining. The salt interval, from which most production was obtained, is approximately 100 ft thick, beginning at a subsurface depth of about 450 ft. The salt unit is overlain by a roughly 8-ft thick anhydrite layer, which is in turn overlain by additional anhydrite, limestone, and terrigenous clastic units. The cavern was operated in both single well and two-well configurations. Most of the production is inferred to have been via single-well operation, with water injected through the casing annulus and brine produced through a hanging string within the casing.

This document attempts to provide some relevant comments regarding the collapse potential of the I & W brine cavern. Sandia National Laboratories is serving in an advisory role in the overall cavern investigation and mitigation effort. These remarks are considered preliminary and do not necessarily reflect a final official position of Sandia, ReSpec, Inc., the City of Carlsbad, or the New Mexico Oil Conservation Division.

REMARKS

In the following sections, we attempt to provide some semi-rigorous estimates of the size of the I & W brine cavern. Very little is information is available from the period of operations for this cavern, and thus the size and geometry of the present-day cavern are quite uncertain. However, records do appear to indicate the production of somewhat in excess of six-million barrels of essentially saturated brine, for use in oil field activities. The ratio of the volume brine to the vol-ume of salt dissolved is approximately 6:1. Applying the conversion factor of about 5.6 cubic feet per barrel of salt, we can estimate — to one significant figure— the volume of the cavern at about 6 x 106 ft3. Knowing the approximate volume of the cavern, we can make some educated esti-mates of the likely cavern geometry, even though recent (2010) attempts so obtain a sonar image of the cavern revealed only a small (~158 thousand barrels) open void.

Assuming that our estimates of the cavern radius — and the areal extent of the potentially affected real estate in the vicinity of the Carlsbad South Y — provide bounds on the lateral extent of the problem, we provide some comments on possible characterization techniques that might be used to better quantify the shape and horizontal size of the cavern.

Dec 13, 2010 Introduction 1

Attempt to Quantify the Diameter of the Cavern

We have attempted to quantify the areal extent of the I & W brine cavern using two similar, but slightly different, approaches. Both approaches assume that the vast majority of the leaching of salt will be immediately below the top of salt.

Method 1 is a relatively simplistic method, which assumes that the profile of the cavern wall is described simply by an exponential-decay function. This would be a fairly normal expected solu-tion profile for a single-well cavern leached by injecting fresh water through tubing near the bot-tom and removing brine through the well annulus near the top.

To make the exponential profile as realistic as possible, we extracted the 2-D profile that represents the greatest horizontal extent of the I & W brine cavern, from the sonar survey conducted in September 2010 by Socon Sonar Well Services. This profile is oriented approximately along azimuth 70°–250° (fig. 1, red outline). We neglected the roof por-tion of the 2-D radius-height profile, and fitted an exponential-decay func-tion (dashed blue lines) to the out-ward-and-upward expanding lower portion of the cavern profile. The resulting geometric approximation of the lower part of the cavern is also illustrated in figure1.

A complete three-dimensional cavern is then generated as a solid of revolution of this expo-nential function. We assume that the top of the leached volume corresponds to base of the overly-ing insoluble anhydrite layer at the top of the salt unit. This geometry is illustrated by profile view in figure 2. Depths are measured from the top of salt. The coefficients of the equation were manip-ulated, via trial and error, such that the volume of the resulting solid of revolution is approxi-mately 6 x 106 ft3. The maximum radius of the six-million barrel cavern represented by the cross section in figure 2 is approximately 613 ft.

However, it is known that the I & W brine cavern — during the majority of its history — was not operated by injecting fresh water low and withdrawing brine near the top. Instead, fresh water was generally injected through the annulus of the well, and brine withdrawn using a hanging string positioned deeper in the cavern. Because of this operational geometry, the negative-expo-nential form of the cavern walls may not be fully appropriate.

Because of the density differences between fresh water and even partially saturated brine, the fresh water will float on top of the brine, immediately below the overlying anhydrite. Because the anhydrite layer is not soluble, no leaching will take place until the fresh water reaches the farthest extent of the cavern. Leaching will be most aggressive in the upper portion of this horizontal edge to the cavern, and as the fluid becomes more saturated in halite, the brine will be displaced towards the bottom of the leached interval and towards the center of the cavern.

Figure 1. Exponential-decay function fitted to the lower portion of the sonar profile of the I & W brine cavern. No vertical exaggeration.

2 Remarks Dec 13, 2010

If we assume that virtually all leaching takes place at the cavern wall, and that the brine becomes fully saturated at this location, such that the bottom of the cavern is subjected to only minimal leaching, then it would appear that the overall form of the cavern void might be a thin, flat-lying cylinder. It is then a relatively simple matter to calculate the radius of a cylinder of height, h, for which the volume is 6 x 106 ft3. Because sonar surveying of the I & W cavern does indicate a void, which lower portion exhibits a more-or-less negative-exponential profile (fig. 1), we have superimposed our cylindrical cavern volume on top of this sonar profile. The resulting cavern geometry is represented in figure 3. Note that the volume of the cone-shaped lower part of the cavern is virtually negligible (about 1,400 ft3 total) compared with the volume of the overly-ing cylindrical form. Nonetheless, this small volume has been considered in constructing a series of potential caverns with cylindrical radius, r.

The second estimation technique does not yield a unique estimate for the radius of the cavern. We therefore have selected a small number of essentially arbitrary cavern heights, h, and com-

Figure 2. Cavern profile used by Method 1 for estimating the volume of the I & W brine cavern. The volume is generated by revolving the exponential-decay function about the depth axis.

Figure 3. Conceptual representation of the I & W brine cavern used by Method 2 for estimating the maximum horizontal cavern extent. Not to scale; radius is varied.

hr

Dec 13, 2010 Remarks 3

puted the cylindrical radius required to generate the nominal six-million cubic foot cavern. The results of this what-if exercise are presented in table 1.

These results are portrayed graphically on the attached large-format map of the I & W brine cavern site. As indicated by the large-format map, the maximum areal extent of the I & W brine cavern may approach, or possibly exceed one mile. However, the larger the estimated horizontal extent, the thinner the associated cavern. The maximum postulated cavern extent, shown in table 1 at a radius of nearly 4,000 ft (8,000 ft diameter), corresponds to a cavern height of a mere inch and a half.

This cavern radius-to-height trade off may possibly be used to examine the lateral extent of leaching that would be of material importance, were the cavern to collapse. Clearly, the effect of total collapse of a void 1.5 inches high, at a depth of roughly 500 ft, is probably negligible, in terms of surface influence. However, the collapse of a 10-ft high void extending over an area slightly more that 800 ft in diameter, might well propagate a collapse crater 500 ft vertically to the surface. If sufficient material properties information can be obtained, or assumed, for the overly-ing sedimentary units, it should be possible to estimate the minimum height of a void necessary to propagate collapse to the ground surface.

Possible Characterization Approaches

Efforts to understand the precise geometric configuration of the I & W brine cavern, along with the implications of this configuration for potential collapse to the surface, are significantly impeded by an absence of meaningful data from the period of operations. Since I & W ceased brining operations at this facility, efforts have been made to obtain a sonar image of the cavern, by reentering one of the cavern wells. This survey failed to reveal a cavern of the size necessary to accommodate even crude estimates of the total salt production.

Given the paucity of historical subsurface data, and the limitations of the attempts made to-date to map the cavern extents, we here consider some surface geophysical methods as character-ization alternatives. The intent is to identify the horizontal extent of a “lens” (broadly defined) of highly conductive brine solution, positioned within a much less conductive “matrix” of intact salt, generally non-porous sediments, and fresh-water-filled alluvium.

Table 1: Estimated maximum radii of I & W brine cavern under various assumptions for Method 2 (flat-cylindrical approximation). Total cavern volume is approximately six million cubic feet.

Height of Cylinder, ft Radius of Cylinder, ft10 4375 618

2.5 8741 1382

0.5 19540.25 27630.125 3908

4 Remarks Dec 13, 2010

Time-Domain Electromagnetic Soundings

Time-dome electromagnetics offers the potential to map conductive layers embedded in less-conductive media. The technique uses vertical sounding profiles, in which a transmitter loop of wire is laid out on the ground, through which a current is run and then abruptly shut off. The decay of the current induces a magnetic field within the earth, which in turn induces a secondary current in a much smaller receiver loop positioned at the center of the transmitting loop. By ana-lyzing the received current and voltage behavior as a function of time since the instant of termina-tion of the initiating current, it is possible to back-out an apparent resistivity profile of the subsurface materials as a function of depth.

The depth of investigation is determined largely by the size of the transmitter loop. For a con-ductive layer positioned at roughly 500 ft below the ground surface, a transmitter loop roughly 100-m on a side would be required. Although a transmitter loop of this size may pose logistical issues with respect to site infrastructure, the fact that the survey is conducted as independent, indi-vidual vertical soundings may work to alleviate some of these difficulties. Although multiple ver-tical TEM soundings are often conducted in close proximity to one another, to form a nominally 2-D profile of apparent resistivity, there is no requirement that the soundings be conducted adja-cent to one another. It is possible to obtain vertical apparent resistivity profiles at essentially arbi-trary positions located at varying distances from the known center (brine well) of the cavern.

Electrical Resistivity

Conventional direct-current resistivity geophysical methods make use of identifying voltage variations, measured between two relatively closely spaced electrodes inserted into the ground at various separation distances, in response to a current induced in the ground by creating a voltage across two other electrodes that are much more widely spaced. As the distance separating the measurement electrodes increases, the observed voltage is affected by the current flowing through progressively greater depths. The result is a profile of apparent formation resistivities as a func-tion of depth. The depth of investigation of the electrical resistivity method is determined, in part, by the separation distance between the two current electrodes.

Because it is necessary to measure the voltages directly in-line with the current electrodes, conventional DC electrical resistivity surveying is a line technique. The I & W brine cavern is cen-tered in between two branching major highways and the main irrigation canal of the Carlsbad Irri-gation District. Although more detailed planning of possible resistivity surveying would be required before eliminating this geophysical method, it seems likely that the various site infra-structure constraints might make maintaining the “line” nature of the survey configuration of the current electrodes and the progressively and symmetrically increasing separations of the voltage electrodes somewhat problematic.

Gravity

Given that the target of a subsurface investigation is a brine-filled void (with density ~1.1–1.2 g/cm3, sandwiched within lithified layered sediments (density ~2.5–2.8), it seems likely that a sensitive gravity survey might be capable of identifying the presence/absence of an underground cavity, based upon the density contrast. The height of the actual void will strongly influence the

Dec 13, 2010 Remarks 5

resolution of a gravity survey. However, it should be possible to forward model the likely of cav-erns of various heights on the observed gravity a priori.

Gravity surveying is a “vertical” method, in that one measures the gravity at a given spatial position. Accordingly, the spatial constraints imposed by site infrastructure are relatively relaxed. Gravity may be measured virtually anywhere it is possible to set up the equipment. That said, the extant infrastructure surrounding the I & W cavern site does pose some significant difficulties for the gravity method. Gravimeters measure very slight deflections of a large mass with respect to a reference. Significant ground vibration — such as will be present during much of the day in the vicinity of two major highways — could easily swamp any variations in gravity produced by the presence of a cavern void. It is currently unknown if a gravity survey could be conducted during the small hours of late night and early morning, when traffic in the vicinity of the site would be minimal.

Reflection Seismography

A shallow reflection seismic survey has been conducted over the immediate vicinity of the main brine well servicing the I & W brine cavern. However, the lateral extent of the several 2-D lines that were surveyed is sufficiently small, with respect to the potential diameter of the cavern — if the most of the radial estimates from the flat-cylindrical model is correct — that the reflec-tion survey almost certainly did not capture the actual margins of the cavern. The seismic survey did produce apparently high-quality reflections. Thus, a more laterally extensive repeat survey might prove quite successful in imaging the margins of the subsurface void.

SUMMARY OF REMARKS

This report presents estimates that provide semi-quantitative bounds on the likely lateral extent of the I & W brine cavern, south of Carlsbad, New Mexico, which are based on a small number of reasonable assumptions and the very minimal amount of objective data available. Most importantly, the radius of the cavern — corresponding to void heights that are likely to be mean-ingful — would appear to be on the order of, perhaps, a thousand feet. A “meaningful” cavern height, in this context, might potentially be on the order of one to three feet. A cavern taller than this — say 5 to 10 ft high — would almost certainly induce noticeable surface subsidence over a radial area on the order of 500 ft. Although these estimates of “meaningful” are subjective and arbitrary, it should be possible to conduct more detailed mechanical analyses regarding the height of a given cylindrical subsurface void that is required to produce significant surface subsidence 450–500 ft above, should it collapse.

The logistical constrains imposed by the intensive development of the I & W brine cavern site significantly impact the types of geophysical investigations that might be conducted to more closely identify the edges of the cavern, and thus the radius of the area potentially affected by any collapse. A series of two-dimensional reflection seismic profiles were obtained over the central portion of the site, including one line that crossed a major highway to the north. However, the remaining 2-D seismic lines were confined within the region bounded by the two highways and the Carlsbad Irrigation District main canal. Whether or not a set of seismic lines could be obtained extending over a much larger horizontal distance is not known.

6 Summary of Remarks Dec 13, 2010

Potentially, time-domain electromagnetic profiling and microgravity surveying offer the best hope of minimizing the infrastructure constraints. Both of these geophysical methods are “verti-cal” techniques, not requiring survey activity along an extended linear profile. Gravimetric mea-surements are essentially a “point” technique, whereas time-domain electromagnetics requires the lay-out of transmitter loop of non-trivial extent.

Very subjective assessment suggests that both gravity and time-domain electromagnetics should have the discriminating capability to identify a brine-filled void of “meaningful” height. However, prior to committing to either type of survey in the field, forward modeling of the expected magnitude of the geophysical response should be conducted to improve this assessment.

Dec 13, 2010 Summary of Remarks 7

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UNITED STATES DEPARTMENT OF ENERGYSANDIA NATIONAL LABORATORIES

I & W BRINE CAVERNCARLSBAD, NEW MEXICO

Projection and 2,000-ft grid ticks: New Mexicocoordinate system, in feet (Lambert Conformal Conic)North American Datum of 1927National Agricultural Imagery Program imagery copyright 2010 Digital Globe, used under license;nominal resolution 1-m

Potential Maximum Radius of the I & W Brine Cavern,Carlsbad, New Mexico

Under Varying Assumptions

13 December 2010

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Explanation© Cavern Well

Estimation Method 1Exponential Profile

Estimation Method 210 ft high cylinder5 ft high2.5 ft1 ft0.5 ft0.25 ft0.125 ft

Known Cavern ExtentSonar Survey, Sept. 2010

0 500 1,000 1,500250Feet

0 100 200 300 400 50050Meters

This map shows the maximum potential radius of the I & W brine cavern. ASSUMING the following: Both Cases -- Cavern Volume: 6 million cubic ft.

Method 1 -- Cavern is conceptualized as a solid of revolution generated by an exponential leaching profile (below), corresponding to the cavern volume, and positioned at the top of salt.

Method 2 -- Cavern is conceptualized as a thin laterally extensive cylinder of the indicated height, correspon- ding to the indicated cavern volume, and positioned directly below the top of salt (below).

No vertical exaggeration

No horizontal scale