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Abstract
Full-scale laboratory testing was conducted under a joint industry and Department of Energy program titled “Optimization of Deep Drilling Performance; Development and Benchmark Testing of Advanced Diamond Product Drill Bits and HP/HT Fluids to Significantly Improve Rates of Penetration.” In total, seven bits and twelve different drilling fluids were tested in three different rocks at a variety of drilling parameters. Phase 1 results have been reported in a previous paper (Arnis Judzeis et al., 2007). This paper presents the results from Phase 2 of the study. The goal of Phase 2 testing was to evaluate bit features and mud additives that might enhance ROP under high-pressure conditions. The test protocols developed in Phase 1 to simulate Arbuckle play and Tuscaloosa trend drilling at pressures in excess of 10,000 psi were employed to evaluate these features. Significant findings of Phase 2 include the following:

• Mud additives can substantially enhance ROPs in high-pressure conditions and may play a larger role than bit design features.
  
• A 16-ppg cesium formate brine increased ROPs 100% as compared to 16-ppg oil-based mud in Carthage marble and Mancos shale.
• The cesium formate improved ROPs by increasing both the efficiency and the aggressiveness of the bit.
  
• A 16-ppg oil-based mud weighted with fine particle size (D50 ≈ 1-3 microns) manganese tetroxide increased ROPs in Crab Orchard sandstone 100% as compared to a similar mud weighted with conventional barite. The manganese tetroxide improved ROPs by increasing the efficiency of the bit, but did not have a measurable effect on bit aggressiveness.
  
• Phase 2 tests continue to support the conclusion of Phase 1 that specific energy consumed while drilling is substantially higher than the confined compressive strength (CCS) of the rock.

Introduction
An important factor in future gas reserve recovery is the cost to drill a well. This cost is dominated by the rate of ROP that becomes increasingly important with increasing depth. The object of this study is to improve the economics of deep exploration and development.

In September 2002, the U.S. Department of Energy’s National Energy Technology Laboratory awarded funding to the Deep Trek program to assist in its goal “…to develop technologies that make it economically feasible to produce deep oil and gas reserves…” and “…focus on increasing the overall rates of penetration in deep drilling.”

The researcher’s proposal was to test drill bits and advanced fluids under high-pressure conditions. Phase 1 of the proposal was to establish a baseline of performance and provide data upon which to make design improvements. Phase 2 was to establish improvements in design.