Abstract

Granular salt can be used to construct high performance permanent seals in boreholes which penetrate rock salt formations. These seals are described as seal systems comprised of the host rock, the seal material. and the seal rock Interface. The performance of these seal systems is defined by the complex interactions between these seal system components through time. The interactions are largely driven by the creep of the host formation applying boundary stress on the seal forcing consolidation of the granular salt. The permeability of well constructed granular salt seal systems is expected to approach the host rock permeability (<10-21 m2 (10-9 darcy)) with time. The immediate permeability of these seals is dependent on the emplaced density. Laboratory test results such that careful emplacement techniques could result in immediate seal system permeability on the order of 10-16 m2 to 10-18 m2 (10-4 darcy to 10-6 darcy). The visco-plastic behavior of the host rock coupled with the granular salts ability to "heal" or consolidate make granular salt an ideal sealing material for boreholes whose permanent sealing is required.

INTRODUCTION

Effective sealing of penetrations (boreholes) in geologic materials is becoming increasingly important as it is recognized that groundwater resources need to be protected from potential contamination. Boreholes are drilled for a myriad of purposes including oil and gas production, water production, monitoring, site characterization, mineral exploration, and waste disposal. These boreholes penetrate all geologic environments and range in depth from very shallow to depths of up to several kilometers. In the undisturbed geologic system, potential contaminants are separated from freshwater aquifers by low permeability aquitards. Improper plugging, sealing, or abandonment of boreholes which penetrate aquitards can allow the borehole to act as a conduit between the contaminated and freshwater regions. The real or perceived failure of these seal systems to isolate contaminants from groundwater supplies can result in costly litigation and remediation, loss of public confidence and support, and destruction of groundwater resources. Borehole seal performance has been recognized as a potential environmental concern by numerous researchers (USEPA, 1987; National Research Council, 1985).

Typical well abandonment requirements and practice include sealing of any water-producing zones by the use of cements. Open-hole cementing is typically done to plug abandoned wells, to assist in directional or follow-on drilling, to minimize or restrict lost circulation, or for isolation of specific zones in the borehole to restrict water inflow. Secondary pressure grouting may also be used to reduce the permeability of the surrounding host rock. High density drilling muds are also used in abandonment of oil and gas wells. Typical tests to assure borehole seal quality include a complete review of well construction data as well as Mechanical Integrity Tests (MITs) such as pressure tests of the seal system as a check for cracks and leaks in the casing and seal/rock interface. These tests are conducted at the top of the plug and are intended to assure immediate performance. The nature of the seal system construction (placement across production and freshwater zones) and test techniques (pressure tests at the top of the seal) suggest that this standard practice may not identify seal problems until performance is compromised because the test techniques may only sample a small part of the seal.

Well founded concerns that such conventional cementitious seals will degrade with time or will otherwise be difficult or impossible to demonstrate permanent performance have led workers in radioactive waste disposal programs in the United States and elsewhere to develop new approaches to, and technology for the emplacement of truly permanent seals. or particular interest here is the approach being taken toward sealing of penetrations in bedded and domal salt formations. Native material (i.e., previously mined-out, granular salt) has been selected as the primary long-term component in these programs, in large part because of its natural compatibility with the host rocks. The experience acquired in evaluating seal materials and systems, necessitated by the requirement to ensure seal integrity and public safety over perhaps thousands of years has led the authors to develop new approaches to sealing boreholes through rock salt.

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