Gas hydrate formation during deep-water offshore drilling is a well recognized operational hazard. At water depths in excess of 1000 ft[30S m), the sea-bed ambient conditions of pressure and temperature become conducive for hydrate formation. This can impose serious well control difficulties during the containment of a gas kick.Salts and glycerol are the main hydrate inhibitors commonly used in water-based muds. To determine the required salt and/or glycerol concentration for a particular drilling fluid, costly and time consuming experiments are required. This paper presents a simple correlation to predict the hydrate point suppression using mixtures of salts and glycerol. The correlation is based on rigorous thermodynamic principles and represents a substantial improvement over Hammerschmid's correlation. Example calculation is proVided on hqw to utilize the proposed correlation..
Discussion of this paper is invited. Three copies of any discussion should be sent to the Society of Petroleum Engineers office. Such discussions may be presented at the above meeting and, with the paper, may be considered for publication in one of the two SPE magazines. Abstract A new downhole tool has been developed to indicate the presence of gas in the mud immediately above the drill bit. The new tool has many potential applications beyond the early anticipation of a kick. For instance, under favorable conditions, shallow gas bearing sands can be located without interrupting drilling operations. Consequently, drilling schedules and mud programs can be optimized for safe and programs can be optimized for safe and efficient rig operation. No electronics is used in the downhole gas detector and no modification of the drill string is necessary. operating the tool requires only a minor variation in routine drilling operations. Introduction Whenever formation gases flow into the well bore during drilling operations, a potentially hazardous condition exists. As the gas rises up the well with the drilling mud, it expands, reduces the apparent density of the well fluids and, consequently, reduces the pressure which the mud can exert pressure which the mud can exert against drilled formations. Most often, the reduction of hydrostatic pressure is small and the flow of gas into the well is quickly stopped by the buildup of a mud cake on the bore walls. However, if the initial amount of gas which enters the well as the bit cuts into the formations is high or if the mud density is abnormally low, the reduction in the well fluid hydrostatic pressure caused by the expansion-of the pressure caused by the expansion-of the gas permits more gas to enter the mud system. The additional gas causes a further reduction in the hydrostatic pressure and a corresponding additional pressure and a corresponding additional flow of gas. Unless the well is closed off, a blowout situation quickly develops. It is obviously very important to measure as accurately and as early as possible how much gas is being added to possible how much gas is being added to the mud stream near the bit in the process of drilling. If the amount is process of drilling. If the amount is small or if the flow is only temporary, the situation will correct itself and there is no reason to stop drilling operations. However, if the flow of gas is steady, it is inevitable that a dangerous condition is developing and immediate measures must be taken. Even when the gas kick is kept under control, the time required to correct the situation and the costs incurred to do so increase rapidly with the amount of gas which is permitted to enter the well bore. The laws of physics which apply to the expansion of formation gases in the well bore do not permit a simple and early detection of the kick.
Drilling highly reactive gumbo shale sediments offshore in the Gulf of Mexico has been and currently still is problematic, particularly when drilling deviated weI/bores. Borehole instability can result from erosion and dispersion of formation clays. Bit balling may induce high torque and drag, swabbing, and stuck pipe. The multitude of drilling fluid systems that have been used vaty greatly, from simple 2 or 3-component to vety complex systems containing a wide range of shale-control additives with differing chemistries.It is not uncommon that a particular fluid performs well in one area or on one well, but is not successful on the next well or in a different area.A water-base drilling fluid system has been developed containing proprietaty shale control additives coupled with a fluid selectively formulated to drill reactive formations.The additives analyzed and SUbsequently field tested possess dual functionality in both controlling shale swelling and limiting the tendency for the gumbo to stick or "ball up" bottom hole assemblies. A unique feature of these additives is that they do not alter the rheological properties of the fluid.The design and application of the fluid has been an evolutionary process. Systematic laboratoty testing of shale samples from the area to be drilled were analyzed References and illustrations at end of paper. and fluid formulations tested for inhibitive qualities. Once a formulation was selected, the system was field tested and further refinements were made to adjust the system chemistty for optimum performance. Over 25 wel/s have been drilled with the fluid showing exemplaty performance. Borehole stability has been achieved and bit balling has been minimized or eliminated. Refinements to the original shale control additives utilized have created "second generation" materials to meet environmental concerns and provide more flexibility, particularly if used in other mud systems.This paper describes the shale control additives, the fluid systems they can be used in, performance testing, and field results from several wells.
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