The constant al primarily represents the effect of formation strength on penetration rate. It is inversely proportional to the natural logarithm of the square of the drillability strength parameter discussed by Maurer. 2 It also includes the effect on penetration rate of drilling parameters that have not yet been mathematically modeled; for example, the effect of drilled solids.

An on-site computer system to control bit weight and rotary speed has been developed and successfully installed on a South Louisiana well. Early field tests indicate that the system could be invaluable in reducing drilling costs. Introduction Previous laboratory and field experimentation has Previous laboratory and field experimentation has demonstrated the effect of several variables on drilling rate. These results have been incorporated into optimization theories for the purpose of reducing drilling cost. Minimum-cost drilling theories rely on a combination of historical data and empirical prediction techniques for selecting optimum bit weight and rotary speed. Past solutions have required extensive use of computing equipment because of both the complexity of the mathematical formulations and the large number of parameters involved. Field attempts to apply these computed results have frequently failed because of the uncertainty of input data requirements. Humble has developed an on-site computer system to control bit weight and rotary speed and thereby implement the concepts of minimum-cost drilling. This system canperform short interval drilling rate tests for formation evaluation,use the results of these tests as input information to solve minimum-cost drilling formulas, andcontrol bit weight and rotary speed in accordance with these computed solutions. The purpose of this paper is to describe the computer control system and present results of initial field testing. Theoretical Considerations Minimum-cost drilling (MCD) requires a quantitative evaluation of the variables involved. Several forms of a basic mathematical model have been suggested. The relationships reported here form the basis for the solution programmed in this work. Four equations are used, expressing drilling rate, bit bearing wear, bit tooth wear, and cost. A solution for minimum-cost drilling assuming constant bit weight and rotary speed over the entire bit life has been programmed for use in computing MCD schedules. This solution is subject to certain limiting assumptions such as those discussed by Graham et al.:Drilling cost is the summation of bit cost, rotating cost, connection cost and hoisting cost.Diamond bits are excluded.Bit life is limited by either bearing failure or tooth wear, or by a combination of operational factors that make it cheaper to pull an incompletely consumed bit.Circulating hydraulics are adequate and do not limit drilling rate.Bit weight considerations exclude hole deviation.Drilling rate is a function of only bit weight, rotary speed, and degree of tooth dullness; that is, the effects of pressure, lithology, fluid property, hydraulics and drill string dynamics are ignored. JPT P. 483

Through the publication of 300 papers directly related to drilling, the journals of the Society of Petroleum Engineers have contributed significantly to the increasing effectiveness of rotary drilling over the past 25 years. Here is a brief summary of some of those reports dealing with aspects of planning and executing the rotary drilling operation. Introduction One of the impressive developments in the petroleum industry during the past 25 years has been the increase in depth of wells. In 1948 the depth record was 17,832 ft (set in 1947). Through 1947 only 14 wells had been drilled to depths below 15,000 ft. Now the depth record is 30,050 ft; and in 1972 alone, 484 wells exceeded 15,000 ft. Equipment has been made bigger and better. For example, the hoisting capacity of a deep-well rig now is 2 million lb, compared with 1/2-million lb in 1948. Bit life has been increased twentyfold through improved design and superior metallurgy. But progress has not been due solely to magnifying the strength and capacity of the equipment. In perhaps no other specialty of petroleum engineering has the impact of technology been so apparent as in rotary drilling. Improvements in equipment, materials, and techniques have combined to increase significantly the output per rig in the last 25 years. Records of the Joint Association Survey* show that the steady decline in active rigs over this period has been onset by a corresponding rise in output per rig per month (see Fig. 1), The rotary method has proved per month (see Fig. 1), The rotary method has proved to be so effective that 86 percent of the rigs are now drilling by this method to make 92 percent of the hole. Newly built rigs, representing an investment of more than $2 billion, are all being equipped for the rotary process. process. Offshore drilling has evolved from the novel to the commonplace. The innovations in equipment and practices engendered by this major development have practices engendered by this major development have markedly increased operating costs. Nevertheless, according to a recent survey, average cost per foot (expressed in 1948 dollars) increased only 20.4 percent from 1953 to 1971, whereas the Wholesale Price percent from 1953 to 1971, whereas the Wholesale Price Index for all industrial commodities increased 34 percent over the same period. (See Fig. 2.) percent over the same period. (See Fig. 2.) The purpose of this paper is not, however, to detail the progress in drilling since 1948, nor to compile a bibliography of all the significant publications of the past quarter century, but rather to sketch those past quarter century, but rather to sketch those developments that have found expression in the journals of the Society of Petroleum Engineers. This treatment is not to be construed as a casual dismissal of the numerous worthwhile contributions to drilling progress that have appeared in the publications of the API, the ASME, and other engineering societies, and in the industry magazines. Lack of space here precludes a thorough review of even the SPE-AIME publications. More than 300 papers relating directly to drilling (excluding logging, papers relating directly to drilling (excluding logging, cementing, and well completion practices) have appeared in the Society's journals. JPT P. 1347

Drilling tests with a Ilk-in. diameter roller bit w.we perjormed on Berea and Bandera sandstones and Leuders Iime.$tone lising water (.tnd two conventional drilling muds us circulating f7uids to evaluate the influence of dynamic fi[trationon penetration rate. The muds possessed widely difierirrg API fluid-loss properties. Mud filtrate was constrained to f70w benea(h the bit: no filtrare fiowed radially (hrough the borehole wall. Rock pore pressure at three different loca/ic)ns ahead ojtltebit, cl~mu[ative fillrate volume, borehole pressure and bit location were monitored during drilling. The rock samples, cored in three orientations with respect to bedding planer, possessed a ;zide range oj[iqu!d permeabilitie.s, and were drilled at horeholet[)-f[)rtllatiotl pre,f.~liredifiererltial,v[)f 0,250, 500 and 1,000 psig. Theeflec! on penetration rate of API fh~idloss, borehole pressure and rock pertneability was studied. Rock pet-meability damage and pore pressure gradients beneath the bit were evaluated. The importance of boreho!e-tojormation pressure differential was i[lus(rated for each dri[ling J7w!dand rock permeability combination. Penetration rares decreased with increased boreho[e pressure and reduced fluid 10SS. The observed penetration rote reduction d{(e to changing fluid lo.rs was attributed to decreased fiitratef/ow and improved mud cake pia.stering by the low fluid-k).r, rmud. The reduction in filtrate flow cordd not i)e related to spurt-loss phenomena. Rock permeability influences penetration rate through particle invasion ahead of the bit, which datnage leads to high pt-essure gradients. Penetration rate variance with satnple orientation was evident for water -drilled sample,s but iess obvious for mud-drilled rocks. Pernteabi[ity damage beneath the b!t ranged from 1 to 3 cm. Maximum damage occurred in the first OS cm. Pressure gradients varied with API finid loss and could he correlated with penetration rate, The pressure gradient was found (O inffuence penetration rate in rock,vof al!permeabilities tested.

United Airlines designed the Shuttle by United to compete in the short-haul air service market. Up to mid-1994, United had been steadily losing market share in the short-haul markets to new carriers with lower costs. It seized an opportunity created by an employee buyout through an employee stock ownership plan (ESOP) to reestablish itself as a competitor in those markets. Using a combination of market research, employee teams, and process analysis, United reduced its costs by 30 percent, increased plane utilization, and cut turnaround time in half from its existing air service. Its efforts have resulted in higher customer satisfaction, improved market share, reduced costs, and increased profitability.