Well-Control Methods and Practices in Small-Diameter Wellbores

Abstract

Summary Slim-hole drilling and continuous coring for oil and gas exploration havebetween impeded by lack of documentation of well-control methods forsmall-annulus drilling. Research in annular pressure losses, kickidentification, wireline swab effects, and dynamic-kill well-controleffectiveness helped develop a slim-hole well-control methodology. Introduction Slim-hole drilling is increasingly used as an exploration tool. Thistechnique involves using compact, mobile rigs with drag bits and high rotaryspeeds to drill small-diameter, expendable wellbores. Walker and Millheimdescribed such a system, Stratigraphic High-speed Advanced Drilling System(SHADS), which uses mining exploration rigs for slim-hole oil and gasexploration. Mining rigs are equipped to core 6- to 3-in.-diameter (andsmaller) wells. The rigs have large-diameter (relative to hole size)flush-joint drillstrings that result in annular volumes 1/10 the size ofconventional wells. The concern about well-control capabilities insmall-annulus wells has hindered a greater use of slim-hole drilling. Thisconcern is justified for two reasons. First, the annular volume on a slim-holewell is so small that an influx must be detected more quickly than in aconventional well. Second, well-control practices for small-annulus wells havenot been documented in the literature. System pressure losses. The bestkick-detection methods and most effective kill procedures are unknown to manydrilling procedures are unknown to many drilling personnel. personnel. To solvethe well-control problems, a full-scale slim-hole well was drilled andinstrumented for well control research. Fig. 1 is a cross section of the SHADSwell-control well. The well is cased with 5-in. casing with an ID thatcorresponds to the most commonly drilled slim-hole size (for SHADS) of 4 3/8in. Eight 1/4-in. pressure transmission lines are attached at various depths tothe exterior of the casing. Special ported pup joints, similar to a side-pocketported pup joints, similar to a side-pocket mandrel, allow communication withthe casing ID. Two 1-in. lints are also attached to the exterior andcommunicate with the casing bore near the bottom of the casing. These 1-in. lines allow injection of nitrogen into the bottom of the well for kicksimulation. This well is called SHADS No. 7. Results of tests conducted tomeasure the annular pressure losses were used to develop correlations fordetermining system pressure losses based on hole size, depth, and pressurelosses based on hole size, depth, and fluid properties. Kick-detectiontechniques and kill methods were evaluated at the well. Drilling engineers andrig personnel were with a slim-hole rig over the well. A slim-hole well-controlphilosophy was developed and tested on a full-scale research well before fieldimplementation. This paper describes the techniques necessary to plan safe wellcontrol for a slim-hole well. Throughout the paper. typical 8,000-ftconventional and slim-hole wells are used to compare the two systems. Fig. 2illustrates the casing, hole, and drillstring sizes, along with otherspecifics. System Description. In this paper, the well-control system includes mudpumps, surface kick-detection system (pit volume totalizer, gas detector, pressure recorders, etc.), blowout preventers (BOP's) and choke manifold, drilling fluid, drillstring, and wellbore. The functionality of the system isidentical for conventional and slim-hole wells, although a slim-hole rig oftenuses smaller equipment. To apply the well-control practices of this paper, quantitative flowmeters practices of this paper, quantitative flowmeters arerequired on the mudlines into and out of the well. No additional equipmentshould be needed. The physical differences between slim-hole and conventionalwell control are analyzed first. They include annular volume effects, systempressure losses, and swab pressures. The next sections on kick pressures. Thenext sections on kick detection and the dynamic kill method apply the physicalresults to field operations. Finally, physical results to field operations. Finally, the approach to slim-hole well control is outlined. Physical Differences Physical Differences Annular Volume. A small annularvolume is the most apparent difference between slim-hole and conventionalwells. From a well-control standpoint, the height of an influx when a kick istaken is critical to the severity of a well-control situation. The greater theheight of the influx, the more serious the well-control problem. JPT P. 1380