Research and Application of Reverse Circulation Drilling Technology

Abstract

Abstract The reverse circulation drilling technology is that drilling fluid is pumped down to the annulus, through drill bit and inside of the drilling tool, comes back to the ground. It has many advantages, such as reducing the formation leakage, protecting the oil and gas formation, and cutting sample very clear etc. It includes air lifting, air and pump sucking reverse circulation drilling. Air lifting reverse circulation drilling needs air compressor, air tank, air box, double wall drill tool, air mixer and reverse drilling bit etc. By applying the original drilling rig, these equipments are connected to drill string and carried out the drilling work, after finishing drilling work, these equipments can be removed totally, and have no influence to the normal drilling work. Through testing and on-site application, these equipments are coordinative, drilled 400m in the leaking formation and got a better effect. Introduction when reverse circulation drilling is applied, compressed air will be injected into the annulus between the inner pipe and outer pipe of the double wall drill pipe through a air swivel or other air injection sub(air box), the air flows down to the bottom of the drill pipe, spray into the inner hole by the air mixer and form millions of bubble, the bubbles move up rapidly along the inner hole and at the same time expand, the compressed air goes into the inner hole continually, and form gas-liquid mixture with low density at the top of the mixer, the density of the drilling fluid outside the drill pipe and below the mixer is low. Showed in fig. 1, h1 is the height of the mixing drilling fluid inside the inner hole of the drill pipe, the density of the mixing drilling fluid is ??1; h2 is the height of the unmixed drilling fluid, it's density is ??2, H is the height of drilling fluid of the annulus, it's density is ??, as ??gH>??1g h1+??2g h2, the drilling fluid of the annulus will go into inner hole of the drill pipe, under this condition, the reverse circulation will be happened, and the cuttings at the bottom of the hole will be carried to the surface and discharged to the sand pit continually by the reverse circulation. The deposited drilling mud will be re-injected into the hole, circulating like this continually forms the drilling process. At the same time, the compressed air goes into the drill pipe, expanding while it moves up, the expansion work produced by the expansion of the compressed air changes into water potential energy, which will drive the fluid to move. It is the two parts of the energy here above that make the drilling fluid circulate. The procedure of the drilling fluid (see fig. 2) is that mud tank---annulus---bit---inner hole of the conventional drill tool---air mixer(mix with the injected air)---inner hole of the double wall drill pipe---hose---discharging pipe line---mud tank. Advantage and purposeTo achieve the purpose of geology sampling.For the reverse circulation drilling technology, the drilling fluid up return to the ground directly from the inner hole of the drill tool, the ability to carry cuttings becomes stronger, and the sample will be clearer, even drilling in a leaking stratum, the geology purpose such as geology sampling can be achieved.To improve the drilling efficiency at the leaking stratum.While drill in reverse circulation mode, the drilling fluid at the bit creates a sucking action to the bottom of the hole, so the cuttings at the bottom will be carried away in time, and the compaction effect will be reduced. Drilling in a leaking stratum, reduce the repeated fragmentation for the cuttings, the penetration rate can be increased, and the drilling efficiency will be enhanced.The leaking of drilling fluid can be reduced or eliminated, and production zone protection will be achieved.The annulus pressure loss is low in reverse circulation drilling, so the pressure to the stratum is small, and for this reason, the leaking of drilling fluid can be reduced or eliminated, and production zone protection will be achieved, moreover, large quantity of drilling fluid material can be saved.Reduce the mud pump wastage, prolong its life.While drill in reverse circulation mode, the mud pump just pours mud into annulus(or use a charge pump), the burden of the mud pump will be reduced greatly, so it's life will be prolonged.Well control is flexible.