Simulating corrosion condition of air foam drilling, corrosion inhibition performance and foaming properties of stable foam are evaluated. Corrosion product layer images and composites are investigated by scanning electron microscope, energy dispersion spectrometer and x-ray diffraction analysis. The results show that corrosion pitting with sodium chloride crystalline grain forms on the surface of steel corroded by formation water. Foam can retard corrosion, particles in foam uniformity, thinness and tightness. After adding foam and inhibitor no obvious corrosion phenomenon happens but existing tiny trace originating from washing and erosion. Foam corrosion inhibition mechanism is first put forward through laboratory data and theoretical analysis, which mostly consists in foam agent absorption on steel surface; viscosifier complexing characteristic; oxygen isolation; foam enwrapping sands, reducing erosion. Adding inhibitor and oxygen scavenger, foam corrosion inhibition ability is improved further. Indoor experiment and field application show foam drilling substituting for dry air drilling and adjusting pH value and adding inhibitor reduce sharply corrosion of drilling tools. Special formula in Western Gasfield of China X-14 well is 2.0%F +0.3%HPAM + 0.2%XC+ 0.5%CT2 + 0.05%NaSO.Practice proved that this method effectively restrained drilling tools corrosion and improved economic benefits.
This paper is the first in a series of studies designed to bring forward foam corrosion inhibition in air foam drilling. The goal is to better understand the mechanism of foam corrosion inhibition, to be able to select the proper inhibitor and extendair foam drilling techniques in suit region.
Definition of Air Drilling[1–2]. An air drilling operation can be defined as any drilling process that utilizes compressed air as the primary agent in an effort to cool the bit, to remove cuttings from the face of the bit, and to carry those cuttings to the surface. There are several drilling processes that fit this definition:
dry air drilling;
mist drilling; and
foam drilling(It can be detailedly classified stable foam drilling and rigid foam drilling).
During the dry air drilling or dusting process, dry air is injected into the drillstring and cutting carrying capacity for a particular application is determined by borehole annulus air velocity. This method is used when no formation liquids are present in the wellbore.
In mist drilling some water is injected along with the air into the drillstring, but air remains the continuous phase while water is the discontinuous phase. This practice can be used to handle relatively small water flows into the wellbore. Corrosion inhibitor and a surfactant solution are commonly injected along with the water in this method.
For foam drilling, enough water and foaming agent are injected along with the air to make air the discontinuous phase and water the continuous phase. Foam has a cuttings carrying capacity superior to that of any other system, including liquid drilling systems, yet it retains most of the advantages common to all air drilling procedures. In additional, it can be used to handle water influxes that are too large to be handled with mist drilling. Air drilling types and peremeter show in Table 1.