When drilling boreholes through the earth, it is the normal practice to circulate a drilling fluid down the drill string, through the drill bit and then back up to the surface through the annulus between the drill string and the borehole wall. Drilling fluids perform a variety of functions and their characteristics, such as density, viscosity and chemical composition are carefully selected depending upon the functions to be performed, to avoid negative effects on the geological strata being penetrated and to maintain borehole integrity.
Continuous phase oil based drilling fluids are well known in the drilling industry and have been used successfully particularly when drilling through water sensitive formations such as certain types of clay-bearing shales that swell when contacted by fresh water. Ideally therefore, the drilling fluid should be maintained as a pure oil but in practice, water contamination of the oil is almost inevitable. Water can enter the system either from the surface (rain etc.) or, more typically, by ingress of formation water present in some of the strata penetrated by the bore. Typically, the drilling fluid is continuously monitored at the surface for water content using two tests, namely electrical stability to detect the physical presence of water if drilling with a non-viscosified pure oil, and/or distillation of a carefully measured sample of the fluid in a retort or still to determine the actual amount of water present in the fluid. Using the electrical stability test, if the fluid sample remains non-conductive up to an applied potential of 2000 volts, the fluid is considered to be water free. This test cannot however be used with a viscosified oil as the viscosifiers can emulsify the water and mask its presence. If the electrical potential at which the fluid becomes conductive drops below 2000 volts, or if a stability test is not possible and the presence of water is to be detected, the sample is placed in the retort and the free water is essentially boiled off and its volume is then compared to the volume of the oil itself which is boiled off at a higher temperature to separate it from any entrained solids in the sample.
If free water is detected, the usual practice has then been to attempt to neutralize the water by emulsifying it to form an internal discontinuous phase of small droplets within the oil based fluid. This is done by the addition of emulsifiers, surfactants and oil wetting agents to create an invert oil emulsion, as is well known in the art.
The presence of emulsified water in the oil still has negative effects, not the least of which can be excessive increases in the drilling fluid's viscosity that robs considerable efficiency from drilling operations. The water itself, particularly if fresh, is in ionic imbalance with usually saline formation water and must therefore be salinated, usually with calcium chloride (CaCl.sub.2). The presence of chloride ions in the drilling fluid adds considerably to the cost of treating and disposing of the drill cuttings.
In some instances, free water is added to an oil based drilling fluid deliberately. This might be done to raise the fluid's viscosity or to decrease filtrate loss to formation. Whether water is added deliberately or not, the drilling fluid must still be emulsified and salinated. The deliberate addition of water reflected the heretofore near inevitability of having to create an invert emulsion in any event. The present invention eliminates or at least reduces this inevitability and the characteristics sought to be obtained by the deliberate addition of water can now be obtained in other practical ways. For example, viscosity increases can be obtained by adding organophyllic clays. Fluid loss control can be regulated by the addition of, for example, gilsonite, calcium carbonate or cellulose fiber. If the fluid must be weighted up to offset formation pressures, additives such as barite can be introduced.
There are therefore considerable operation and cost advantages to drilling with a pure oil system and being able to avoid having to convert the system to an invert emulsion. The applicant has found that this is possible, or at least the conversion to an invert emulsion can be delayed, by scavenging the water from the drilling fluid on a continuous basis while drilling.