DRILLING FLUID LOSS EVENTS IN THE DENVER BASIN AQUIFERS

The Denver Basin aquifers are a widely developed ground water supply for many uses along the Front Range of Colorado. Wells are commonly constructed using reverse circulation mud rotary drilling methods. In the Denver Basin, prior to 2003, there were no incidents of major drilling fluid loss or lost circulation of properly conditioned drilling fluids resulting in the loss of the borehole. In 2003, three separate incidents of significant drilling fluid loss occurred within a sub-linear geographical area of approximately five miles in length in and near the City of Aurora. One of these incidents resulted in the loss of the borehole and the loss of the contractor’s down-hole drilling equipment.

In all four wells, the fluid loss occurred near the contact of the Cretaceous Fox Hills Sandstone and the overlying Cretaceous Laramie Formation. Fluid loss occurred in a highly permeable or fractured sandstone at depths of approximately 2,000 feet. Despite the fluid loss zone encountered in these wells, many other wells have been completed without significant fluid loss in the same geographical area.

Important lessons were learned as a result of these fluid loss incidents regarding the importance of drilling fluid volume, mud program control, reaction time in the case of a fluid loss incident, and awareness of the potential for fluid loss incidents to occur in the Denver Basin.