This paper deals briefly with the history of methods of .drilling wells under high pressure, the reasons why special and heavy muds came into use and the subsequent development of apparatus for enabling the drilling of wells to be effected without the use of heavy muds. The conditions affecting the design and arrangement of apparatus of this kind are discussed. The main part of the paper describes the most recently developed apparatus which has been brought into use for the control of pressure when drilling. ‘Early rotary-drilled oil wells V5eré dulled without any form of formation control or formation pressure control. The fluid used was water. It was quickly found that when, instead of water, the mud formed during drilling was circulated, the walls of the hole stood up much better. Little effort was made to study muds until the use of rotary spread from Texas to California, because practically all wells in the Gulf Coast area made, when drilling, good enough mud to control both formation and the formation pressures found there. Blow-outs, however, did occur and master gate valves and two crude forms of control devices, similar in principle but different in construction from the "Cameron" and "Shaffer" hand-operated blow-out controllers, were brought into limited use. The formation drilled through in California did not generally produce good drilling mud, and formation pressures became higher as the depth of drilling increased. The study of mud’ was therefore taken up seriously. from the two angles of (1) obtaining clay muds most economically, and (2) of weighting the clay muds so that the pressure of the mud head should always be in excess of the formation pressure. Despite much scientific research and the development of muds having specific gravities of double that of the water used by the early rotary drillers, blow-outs still occurred and were brought under control after they had started by devices similar to those mentioned above or by inserting and compressing rubber packings around the drill pipe in ahead fixed to the casing. This method of attempted control of formation and formation pressures by mud, and the control of blowouts when they occurred, by temporarily fitted devices continued until the necessity for a more positive and continuous control prompted the developments of wellhead fittings designed to prevent blow-outs and to maintain continuous control over the drilling fluid. Among the first attempts at blow-out prevention-as opposed to blow-out control-were those made in Trinidad in 1920, and a number of high-pressure wells were successfully drilled-in there using the oil string with a bit on the bottom as the drilling string and a specially constructed head which could be speedily attached to the well head, and allowed of a few feet of drilling into the formation after the high-pressure oil-sand had been penetrat

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