Some Problems of Superhigh-Pressure Gas Reservoirs In the Gulf Coast Area

As the search for oil and gas has progressed to deeper horizons, problems of abnormally high pressures have become increasingly difficult. Several of the world's oil- and gas-producing areas have extremely high pressures, and one of the most notable of these is the area about 50-miles wide along the coast of the Gulf of Mexico from the Rio Grande to the Mississippi delta. Hundreds of wells along the Louisiana coast have pressures greater than normal (gradient of .465 psi/ft of depth), and it is from this area that the data presented in this article were obtained. In earlier days completions in reservoirs with abnormally high pressures were unsuccessful for the most part, and the consensus then was that such reservoirs were small and, therefore, uneconomical. There was some basis for this belief, for until recently more money had been spent in discovering and developing high-pressure formations than the value of the oil and gas found. Operators tried deeper drilling, by which they hoped to drill past the high-pressure zones to other normal-pressure formations more favorable for the production of hydrocarbons. Once an abnormal-pressure zone is encountered, all deeper formations, as a rule, have abnormally high pressures; thus, the idea of finding normal-pressure reservoirs under high-pressure zones did not work out. At the same time, many big and valuable accumulations of hydrocarbons have been discovered in the high-pressure zones. After it was found that high pressures did not necessarily mean poor recoveries, it became apparent that drilling (though no less serious) was not the only problem. New problems of completion, production and reservoir performance, which did not occur in normal-pressure reservoirs, must be faced and solved. The way these new problems are handled will have much to do with the value of these high-pressure reserves. Drilling Excellent reports on the occurrence of abnormally high-pressure reservoirs have been made by Cannon and Craze, Cannon and Sullins, Watts, and Dickinson. The most credible theory of the cause of abnormally high pressures is that the formations cannot bear the overburden without compaction, and that they are effectively isolated from any other porous and permeable formation which contains normal hydrostatic pressure. Dickinson found that most of the abnormal-pressure reservoirs in south Louisiana are below the base of the main sand development because conditions favorable for isolation are seldom found within the main sand series. Therefore, a knowledge of the depth at which the change in facies from predominantly sand to predominantly shale occurs is an indication of the depth at which abnormally high pressures may be encountered. It follows then that, if the pressure to be encountered can be predicted fairly accurately, many drilling problems can be overcome. Drilling problems are magnified as the amount of uncased hole increases. Shallower formations often are unable to withstand the weight of the mud column needed to control the pressure of the deeper zones and will break down, causing a loss of returns. This quickly creates an imbalance between formation pressure and the pressure exerted by the mud column, and can result in a blowout. For years operators have wrestled with this problem of lost returns, but proper programming of drilling activities. such as the selection of casing setting depths, has brought success in drilling abnormally high-pressure wells. Some knowledge of how pressures are increasing with depth aids materially in deciding where to set protective casing. It does not necessarily follow, though, that the best depth was selected just because someone successfully drilled a well by setting pipe at that depth. Some very interesting facts relating to the increase of abnormal pressures with depth came to light during the development of the Lake Arthur field in Jefferson Davis Parish, La. The south half of the Lake Arthur field, which is the portion of abnormal pressures, has a productive area of some 7,000 acres (Fig. 1) and had original recoverable reserves in excess of 2 trillion cu ft of gas. In the area of abnormal pressures 40 wells have been completed, and there are 17 abnormal-pressure reservoirs in eight horizons, ranging in depth from 8,500 to below 15,000 ft. Surface pressures have been as high as 11,900 psi, and reservoir pressures have been computed to be in excess of 14,000 psi. JPT P. 935^