Optimization of Deep-Drilling Performance (November 2008)
Authors: Bybee Karen 
Published: February 2015

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 112731, "Optimization of Deep-Drilling Performance With Im provements in Drill-Bit and Drilling-Fluid De sign," by Alan D. Black, SPE, TerraTek; Ronald G. Bland, SPE, Baker Hughes; David A. Curry, SPE, and L.W. Ledgerwood III, SPE, Hughes Christensen; Homer A. Robertson, SPE, and Arnis Judzis, SPE, TerraTek; Umesh Prasad, SPE, Hughes Christensen; and Timothy Grant, US Department of Energy, originally prepared for the 2008 IADC/SPE Drilling Conference, Orlando, Florida, 4-6 March. The paper has not been peer reviewed.
Full-scale laboratory testing was conducted under a joint-industry and US Department of Energy program. Seven bits and 12 drilling fluids were tested in three different rocks at a variety of drilling conditions. Phase-1 results were reported in a previous paper. The goal of Phase-2 testing was to evaluate bit features and mud additives that might enhance rate of penetration (ROP) under high-pressure conditions. The test protocols developed in Phase 1 to simulate Arbuckle-play and Tuscaloosa-trend drilling at pressures greater than 10,000 psi were used to evaluate these features.

Introduction
An important factor in future gas-reserves recovery is the cost to drill a well. This cost is dominated by the ROP, which becomes increasingly important with increasing depth. The object of this study was to improve the economics of deep exploration and development.
In September 2002, the US Department of Energy's National Energy Technology Laboratory awarded funding to the Deep Trek program to develop technologies that make it feasible economically to produce deep oil and gas reserves and increase the ROPs 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. Phase-2 tests were an extension of Phase 1 and attempted to improve significantly the ROP and mechanical specific energy with modified drill bits and enhanced drilling fluids. There were 21 tests run during Phase 2. For consistency, the type of rock used in Phase 2 was the same as in Phase 1.
The drill bits selected for Phase-2 testing included a roller-cone bit to re-establish a baseline, three polycrystalline-diamond-compact (PDC) bits, and a diamond-impregnated bit. One of the PDC bits, a seven-blade bit, was identical to the one used in Phase 1; all others were modified. The drilling fluids used in Phase 2 included an 11-lbm/gal water-based mud for four tests, a 16-lbm/gal cesium formate brine for five tests, and 16-lbm/gal oil-based fluids for the others.

Methods and Materials

Test Facilities.
All drilling experiments were performed at a drilling-and-completions laboratory under simulated down-hole conditions in a wellbore simulator that applied overburden and confining stresses to the rock. The drillstring loading and rotation were provided by the full-scale laboratory drill rig. The drilling and rock-loading parameters used during Phase-2 testing are provided in Table 1 in the full-length paper.