L - WELL cementing is the process of mixing and displacing a cement slurry down the casing and up the annular space behind the pipe where it is allowed to set, thus bonding the pipe to the formation. No other operation in the drilling or completion process plays as important a role in the producing life of the well as does a success ful primary cementing job (Figure 16-1). The first verified use of portland cement in an oil well, for shutting off water that could not be held with a casing shoe, was in 1903. After placing the cement, the operator normally waited 28 days before drilling the cement and testing. Improvements in cements, understanding WOC times, and the use of admixes have reduced WOC time to a few hours under present-day practices. Cementing procedures may be classified into primary and secondary phases.
E cost of drill pipe failure has been estimated to be approximately $1 per foot of hole drilled. This is a significant fraction of drilling costs and is a point of extreme concern to those in the drilling business. Since some form of corrosion is the cause of the majority of drill string failures an understanding of the fundamentals of corrosion control is mandatory for minimum cost drilling. Most metals are found in nature as metallic oxides or salts. Refining to produce pure metal requires a large energy input. This energy is "stored" and is available to supply the necessary driving force to return the metal to its original state-an oxide or salt. This means that metals are unstable with respect to most environments and have a natural tendency to return to their original lower energy state, or "corrode." Corrosion is an electrochemical process. This means that electrical current flows during the corrosion process. In order for current to flow, there must be a driving force, or a voltage source, and a complete electrical circuit. The source of voltage in the corrosion process is the energy stored in the metal by the refining process.
This discussion includes advancements in inserts, seals and friction bearings; bit selections and limitations; and current operating practices and trends. Finally, the economics of a bit run will be considered along with a discussion of dull bit evaluations. By the late 1940 's the industry was venturing into deep drilling. In most areas this meant harder rocks such as limestone and chert, slow penetration rates, and reduced bit life. Conventional mill tooth bits were simply inadequate for the drilling environment encountered. Then in 1949 Hughes Tool Company introduced the first three cone bit using tungsten carbide inserts in the cutting structure and named it the "Chert As illustrated in Figure 14-1, the "Chert Bit" was characterized by short and closely spaced inserts. This cutting structure was durable and the "Che rt Bit" performed its task well. Typically, the five to ten foot runs were increased to fifty to one hundred foot runs; and the four to five hours became twenty-five to forty hours, and often as not failure was due to bearing failure and not structure failure. From this point the development and use of insert bits lagged. Conventional mud systems were characterized by high viscosities, high weights, and a high total solids content.
V I A TI O N in drilling operations is not a new problem. The diamond core drill was invented in 1865 and widely used as a cable tool drill in mining operations. The first evidence of concern about hole deviation was the invention by Nolten in Germany in 1874 of the use of hydrofluoric acid to etch and predict hole deviation. Later a South African miner named MacGeorge invented the clinostat to predict both deviation and direction. The clinostat consisting of a magnetic needle and a plumb immersed in gelatin was lowered into the hole and the gelatin was allowed to set. The instrument was then brought to the surface and deviation and direction were read directly. At a meeting of mining engineers in London in 1885 MacGeorge presented data illustrating deviations of 75 feet in 100 foot mine shafts. The Petroleum Industry did not become aware of the problem until the Seminole, Oklahoma, boom of the middle 1920's. Town lot spacing was the primary factor contributing to the experiences of the industry. There are actual recorded incidents of offset wells drilling into each other, drilli}1g wells drilling into producing wells, two rigs drilling the same hole, and wells in the geometric center of the structure coming in low or missing the field completely. It was common drilling practice at that time to use only large drill pipe with no drill collars and all available weight since weight indicators were not available. Engineers and the industry in general made a concentrated effort to solve the crooked hole problem. As a result, most of the practices commonly used today in an effort to correct and control deviation were conceived, experimented with and adopted in the 1920's-50 years ago.
RING the 1960's, pressure control received more attention than any other phase of drilling practices. One rea son for this emphasis was an increasing awareness of the problems of deep drilling and the extremely high drilling costs associated with abnormally pressured formations. In addition, ecology became an important part of the American scene and several blowouts with the associated problems of pol lution gained international attention. Politicians, quick to seize an oppor tunity to condemn which seems to have become a stature building tech nique in some areas, made public statements and seemed to thrive on any problem that could be related to the industry. They also were working on methods to bring the blowout under control if it occurred. In addition methods were being developed to handle oil spills on open waters. No expense has been spared to develop methods of prevention and cure. One of the most important parts of any operation is the planning stage. With geological information and experience in the same or similar areas, wells are planned to reach total depth with no problems. Casing programs are designed to maximize safety, mud programs are selected carefully and control equipment is designed to handle potential problems. Specific knowledge of the new area is not available; however, geophysical methods of exploration can now be used to estimate formation pressures and give an insight into formation characteristics. Continued development of these tools will be a substantial aid in future exploratory drilling.
Very little attention was directed towards fluid-circulation programs prior to the introduction of jet nozzles in bits in 1948. Conclusions on the value of jet bits were not made until 1949 and very limited industry acceptance was noted for several more years. Even today many operators do not recognize the importance of bottomhole cleaning. They may use jet bits, but the circulation program is so poorly designed that bottom-hole cleaning is not much better than that achieved with conventional bits. We need to go back to the analogy of the child cleaning mud off the driveway with a garden hose. First he turns on the water full pressure, directs the water discharge on the mud, and, if the hose is equipped with a nozzle, he reduces the nozzle size until he blasts away the mud. No engineering is involved; by simple observation he has noted that this is the fastest way to remove the mud. For some reason, maybe because the nozzle is hidden, the industry first resisted turning the nozzle on the mud and then it resisted the nozzle adjustment necessary to remove the mud in the shortest time. Maybe we have not been in as big a hurry as the child, or perhaps intuition is better than scientific conclusions. In any event, 25 years have elapsed since jet bits were proven to be superior to regular bits for bottom-hole cleaning. Nu merous articles have been written on hydraulics and all conclusions have shown the importance of adequate bottom -hole cleaning.
Swab pressures are associated with fluid flow, caused by pulling equipment out of a liquid filled bore-hole. Procedures used for estimating the magnitude of these pressures are similar to estimating the pressure losses in conventional fluid circulation. To reduce problems of calculation, the swab pressure is estimated by calculating the surge pressure and assuming that this is equal to the swab pressure for the same rate of pipe movement. More than 25 per cent of the blowouts result from pressure reductions in the bore-hole due directly to swabbing when pulling pipe. This may result in expensive mud treating costs and cause other hole problems. The adverse effects of surge and swab pressures were recognized very early in rotary drilling.
N I T I A L LY, the primary purpose of the drilling fluid was to remove formation solids continuously. No time was spent on a scientific evaluation of the carrying capacity of the fluid and very little effort was made to control the fluid properties. Even with the advancement of science in mud treating, operators ignored the lifting capacity of muds. Based on many months of controlled research on the lifting capacity of fluids, Williams and Bruce concluded the following: 1. Turbulent flow in the well annulus was the most desirable flow pattern for removing formation cuttings.
E determination of pressure losses in the circulating system has been an objective of technology for almost as many years as rotary drilling has been in existence. The first dedicated efforts to determine the pressure losses came when drilling hydraulics were introduced in 1948. The development of jet bits and hydraulics programs was responsible during the early 1950's for the introduction of graphs, charts and sliderules, by bit companies, for the determination of pressure losses in turbulent flow. The rotating viscometer was introduced at about the same time to measure mud properties in laminar flow. In general, the initial calculation of laminar flow pressure losses was performed assuming a Bingham Plastic flow model. The Bingham model has now been replaced generally by the Power Law flow model, which has been shown to be more accurate. In general, the methods introduced by the bit companies to calculate pressure losses in turbulent flow have been sufficiently accurate in field applications for hydraulics programs. However, efforts are continuing in the drive to improve pressure loss calculations in laminar flow. Operators, at least in geologically young formations, can measure directly the fracture gradients of open hole formations close to the casing seats.