The present invention relates generally to devices and methods for exploration of oil and gas. More particularly, the invention describes how to measure formation pressure and how to detect the presence of hydrogen sulfide gas in a rising gas kick so as appropriate prevention and safety measures can be promptly taken. The present invention can be advantageously used when drilling onshore and offshore oil wells. It is designed to allow prevention of blowouts and well explosions, which usually cause human losses, damage to environment and are hard and expensive to suppress.
During an exploratory drilling of an oil, gas or gas condensate wells, drilling fluid referred to in the industry as “mud”, is pumped into the drill pipe. The mud proceeds out through the drill bit and up the annular space between the drill pipe and the walls of the hole. It generally proceeds then further up the annular space between the drill pipe and the casing, after which it returns to the surface of the well. At the surface, the mud is typically examined for certain parameters, processed and returned to the circulation. The purpose of the circulating mud is to clean, cool and lubricate the bit, flush to the surface the cuttings from the bore hole and to protect the walls of the hole until casing is inserted. The density of the mud is carefully controlled at the surface so as to contain various pressures encountered in the hole.
As the well is drilled, gases saturated in highly pressurized fluids at the bottom may be released therefrom or from a porous rock and find their way into the circulating mud forming an annular gas bubble or a gaseous pack, also called a gas kick. This gas kick may ascend to the surface, result in a modification of the buoyancy of the drilling string and can cause extensive damage if it goes undetected. The gas or liquid contained in the gas kick reduces the hydrostatic head in the annulus. If the volume of the gas kick is not excessive and if it can be detected, gas kick removal procedures may be instituted so that drilling operations may proceed with minimal disruption.
Careful monitoring of formation pressure is highly desirable in order to control formation of gas kicks and to assure safe operation of the oil well.
In addition to monitoring for a possible formation and ascendance of a gas kick, containing mostly natural gas, there is an additional safety concern regarding formation of a gas kick containing hydrogen sulfide gas, H2S. This gas is highly toxic, heavier than air, flammable and can cause substantial damage and even death to the oil well service personnel—upon inhaling such gas is extremely irritating and harmful. Free hydrogen sulfide in the blood reduces its oxygen-carrying capacity, thereby depressing the nervous system. Hydrogen sulfide is oxidized quite rapidly to sulfates in the body, therefore no permanent after effects occur in cases of recovery from acute exposures unless oxygen deprivation of the nervous system is prolonged. Effects such as eye irritation, respiratory tract irritation, slow pulse rate, lassitude, digestive disturbances, and cold sweats may occur but these symptoms disappear in a relatively short time after removal from the exposure. At high concentrations of 500 ppm and above, hydrogen sulfide is fatal in as little as 30 minutes.
Surface monitors of hydrogen sulfide presence are not sufficient to assure safety in case of hydrogen sulfide exposure. Methods are needed to warn service personnel about the upcoming gaseous pack containing hydrogen sulfide which will give more time to assure personnel safety than that available with surface monitors.
Using acoustics for detection of the gas kick presence is known in the art. U.S. Pat. No. 4,273,212 for example discloses sending an acoustic pulse down the pipe and receive its reflection in the annular portion of the well head. Using high frequency positive acoustic pulses however does not allow full characterization of the gas kick as it only allows detection of its upper end and not allows detection of its lower end which is needed to detect its total volume.