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The present techniques of air or gas drilling and their advantages and disadvantages are discussed. The relative merits of both air and mud drilling provide the basis for the advent of aerated fluids. For the purpose of this discussion, aerated fluids are divided into two phases: (1) physical composition and (2) circulation techniques. From the previous comparison, conclusions are drawn with respect to the future of aerated fluids.
The advent of aerated fluids was a result of an attempt to avoid the limitations and yet maintain the advantages of both conventional drilling muds and air or gas when used as a drilling medium. A brief philosophical review of the petroleum industry and a summation of air and gas drilling characteristics provide the foundation for this discussion of the development and the characteristics of aerated drilling fluids.
Many technological advancements into industry can often be attributed to either or both of two major demands: (1) a physical demand for new processes in order to accomplish heretofore technically unfeasible tasks, or (2) the economic demand for an improvement of present techniques that were formerly considered prohibitive from a sound investment viewpoint. The advent of aerated drilling fluids may be attributed to both demands.
Since drilling and completion annually of over 45,000 oil and gas wells provides a total operational outlay exceeding 30 per cent of the entire producing companies' expenditures, a small technological, and eventually in economic advancement, can be of paramount importance.
It appears that the most promising fields of improvement in drilling operations lie within three problems: (1) the proper completion procedures for productive formations which have been damaged by the present drilling techniques, (2) the reduction of lost circulation costs, and (3) the reduction of the over-all operational drilling costs.
Most new methods of oil well drilling achieve increased rates of penetration essentially by increasing the mechanical power input to the rock over the maximum practical power level obtainable by conventional rotary drilling. However, whether this would decrease the drilling cost per foot of hole depends upon other factors as well, such as the cost of operation per unit time, well depth and the distribution of rocks of various drillabilities at the well site.
Rotary drilling, introduced about 1900, is today the most widely used method of drilling oil wells in this country. The past 50 years have seen great strides in improved equipment and techniques for rotary drilling. However, the limitations of rotary drilling have long been recognized and there have been many attempts to develop new drilling methods to supplement or to supplant rotary drilling. The potentialities of some of these new drilling methods for decreasing the cost of drilling oil wells are discussed herein.
Drilling Rate and Mechanical Power
One significant result of a study of the fundamentals of rock drilling made at Battelle is that the fracture of brittle rock from the bottom of a hole by repetitive, indexed mechanical loading involves the expenditure of a certain amount of mechanical energy per unit volume of rock fractured out. Therefore, the rate of penetration of any mechanical drilling machine would be essentially directly proportional to the mechanical power developed in the rock per unit area of hole and inversely proportional to the drilling strength, which is the energy required to fracture off a unit volume of rock.
Limitations of Rotary Drilling
The mechanical power output to the rock for conventional rotary drilling is proportional to the torque required to turn the bit at the bottom of the hole and to the rate of rotation of the bit. The torque reaction to rotation associated with a given amount of static weight loading on the bit is less for the rocks of greater drilling strength. Since less mechanical power is developed in rocks of greater strength, the rate of penetration decreases more rapidly than in inverse proportion to the drilling strength. There is, consequently, a wide range in the drilling rates obtained by rotary drilling, perhaps about 50 to 1 from the weakest to the strongest rock of interest in oil well drilling.
Extensive drilling research, particularly in the last ten years, hasresulted in a better understanding of the effect of drilling variables andtheir interactions. The practical application of this knowledge has kepthole-making costs at the 1958 level in spite of inflation. Optimized drilling,which was first applied on a comprehensive basis in 1967, has significantlyreduced drilling costs, although it has yet to reach its full potential.
Optimized drilling is defined as the "mathematical treatment of the mostimportant controllable drilling variables to develop a comprehensiveminimum-cost drilling program". The variables involved in rotary drilling areclassified as alterable or unalterable and the variables selected formathematical optimization are herein described. A more detailed treatment willbe given to the most important variables optimized-mud, hydraulics, bits,weight and rotary speed. Rig selection and data acquisition are also discussed,as these factors play an important role in optimization. The paper includesdata which show that significant reductions in drilling costs have beenobtained when optimized drilling programs were effectively implemented. Aninteresting aspect of the data presented is that savings achieved in"second-round" optimums were significant and, in one case, more than thosesavings obtained in "first-round" optimums. This indicates that better data,more experience in applying optimized drilling principles and developingconfidence in the program are key factors in the successful use of this newdrilling approach.
THE DEVELOPMENT OF ROTARY DRILLING can be divided into four distinct periodsConception Period, 1900-1920; Development Period, 1920-1948; Scientific Period,1948-1968; and Automation Period, which began in 1968. The majoraccomplishments of the first three periods, and a prediction of what lies inthe future for the Automation Period, are shown in Table 1. In reviewing thesedevelopment periods, the question naturally arises as to the reason for theapproximate 30-year lapse between the end of the Conception Period and thestart of the Scientific Period. There are a number of reasons that can begiven, but undoubtedly the most significant one is that major oil fieldequipment firms, mud service companies and operators did not startappropriating the large amounts of money it takes to do high-quality drillingresearch until about 1948. When we look at the major accomplishments obtainedduring the Scientific Period, the most productive years are found to be from1958 to 1968. A measure of the impact of the drilling technology developedduring the latter part of the Scientific Period on hole-making costs, ascompared to total well costs, (can be seen from Figure 1). Total well costsincreased 14 per cent from 1958 through 1967, while hole-making costs remainedat the 1958 level; i.e., about $4.25/ft.111 Other costs, such as completion,logging and casing expenditures, increased 21 per cent. If the extensivedrilling research effort of the past 10 years had not been undertaken and hadnot been successfully reduced to practice in routine drilling operations, it isestimated that a typical 8,000-9,000foot hole would cost an additional$3.00/foot to drill today. This, would amount to a saving of about $500 millionfor 1967 alone, which is testimony to the fact that the investment in drillingresearch undertaken by many companies has paid off. Optimized drilling has beenone of the most significant accomplishments obtained during the ScientificPeriod, but it was not introduced on a comprehensive basis until 1967, andtherefore will not reach its full potential for several years. It is veryimportant to realize that optimized drilling would not be possible todaywithout the hard work of numerous researchers who have spent considerable timestudying the effects of drilling variables and how they relate to eachother.
This paper was prepared for presentation at the 1998 SPE Annual Technical Conference Conference and Exhibition held in New Orleans, USA, 27-30 September 1998.
Once again, we find ourselves in a time of extreme challenges on many fronts in the arena of well construction, with corresponding needs for technological advancements. Anyone who has been around the drilling-and-completion world during the past several years can attest to the unique environment in which we operate today. Ever-increasing drilling depths and formation temperatures and pressures are combined with depletion of mature basins and unprecedented geopolitical uncertainty. The good news is that human innovation and problem solving continue to accelerate commensurate with these challenges. In this feature, we specifically highlight the persistent need for high-performance drilling fluids.