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A new class of underbalanced drilling fluids being developed under U.S. Department of Energy sponsorship was recently successfully field tested. The fluid utilizes hollow glass spheres (HGS), also known as glass bubbles, to decrease the fluid density to below that of the base mud while maintaining incompressibility. A previous paper, SPE 30500, described the rheological properties and laboratory behavior of HGS fluids. An HGS fluid was formulated in the field and used to drill two wells in Kern County, California in the fall of 1996 for a major operating company. Concentrations of up to 20% by volume were used to decrease the fluid density to 0.8 lb/gal (ppg) less than normally used in the field. The techniques employed to mix and maintain the mud, the rheological properties measured in the field, and a discussion of future applicability of HGS fluid are addressed here. The field tests demonstrated that HGS drilling fluid can be easily and safely mixed under field operating conditions, is compatible with conventional drilling muds and rig equipment, and can be circulated through conventional mud motors, bits, and solids control equipment with little detrimental effect on either mud or equipment. Potential benefits of using these fluids include higher penetration rates, decreased formation damage, and lost circulation mitigation. When used in place of aerated fluid they can eliminate compressor usage and allow the use of mud pulse MWD tools. These benefits improve drilling economics. These and other recent advances in technology have spurred interest in underbalanced drilling to the highest level in 30 years. Industry-wide surveys indicate that more than 12% of wells drilled in the United States in 1997 will intentionally employ underbalanced techniques. Introduction The U.S. Department of Energy (DOE) recognizes the benefits of advanced technology to the oil and gas industry. Consequently, DOE manages a portfolio of drilling related research, development, and demonstration projects designed to reduce cost and increase process efficiency. This program is implemented by the DOE's Federal Energy Technology Center and is a market-driven balance of near-, mid-, and long-term efforts. These drilling related projects support the department's ultimate goal of developing the nation's large natural gas resource base and maintaining market-responsive supplies at competitive prices. Lightweight solid additives (LWSA) for drilling fluid density reduction were tested in the laboratory and in a test yard in drilling rig compatible equipment during 1994 and 1995. The Department of Energy (DOE) published a final report on this Phase I testing in the fourth quarter of 1995. The primary objective of the project since that time has been to test underbalanced drilling products in actual field operations. The LWSA tested consists of hollow glass spheres (i.e. glass bubbles) manufactured in the United States and commonly used as a filler material for other lightweight products. The spheres have an average specific gravity of 0.37 and average collapse strength of 3,000 psi. The spheres average 50 microns in diameter. The goal of the DOE project is to use the glass bubbles to generate drilling fluids having densities less than that of the base fluids. Much of the intangible cost of drilling wells is time sensitive, so techniques, which increase rate of penetration, are core to the DOE program. Underbalanced drilling products are investigated because of their potential for increasing drilling rate, as well as their potential to retain maximum well productivity by minimizing drilling induced formation damage. The LWSA fluids represent one such underbalanced drilling technology. A more comprehensive description of DOE drilling related research and development was provided in an earlier paper, SPE 30993. Mobil Oil Company provided the first opportunity to test LWSA in a field operation in September 1996. P. 699

Tuberculin skin testing is a notoriously poor marker of tuberculosis infection in patients who are serologically positive for the human immunodeficiency virus. A decision analysis was performed to determine if and when isoniazid should be prescribed to prevent tuberculosis in these patients. The decision was analyzed for an intravenous drug abuser who may have been anergic, while tuberculin test status, race, and gender were varied. The assumptions and parameter estimates selected for this investigation represent a highly conservative vantage point opposing the use of isoniazid as a preventive therapy. Nevertheless, results showed a benefit from the use of isoniazid as a preventive therapy for all groups, even without tuberculin testing, except tuberculin-negative black women. This benefit of isoniazid therapy increased patient life expectancy as much as 285 days. Further, the decisions continue to favor prescribing isoniazid even when the suspected incidence of tuberculosis infection falls as low as 3% to 8%. Tuberculin testing appears important primarily for black women, since those who have negative tuberculin skin tests may not be candidates for isoniazid therapy.