Abstract: When working on a horizontal well in the Austin Chalk area of south Texas, the wellbore is drilled vertically with drilling mud to maintain control of the well and casing is run to the bottom of the wellbore. Once the wellbore begins its curve and a horizontal drain hole is drilled, drilling mud cannot be used. Due to the presence of faults and/or fractures in the horizontal drain hole, oil, water, natural gas, and/or toxic materials are always present whether a drilling rig is drilling and has not reached total depth, or whether a workover service rig is performing service work. Due to the close proximity of the employees to the wellbore on a workover service rig and with the diesel engine on the workover service rig only a few feet above ground level--which in most cases is not approved for use in Class I Division I locations--an ignition source was provided. The employer did not install blowout preventers on the tubing hanger valve and/or the bradenhead of the casing of the wellbore. Employees #1 through #5 were left exposed to the sudden, unexpected release of flammable or toxic materials from the wellbore. Employees #1 and #2 died, and Employees #3, #4, and #5 were hospitalized.