Slimhole Exploration Requires Proper Technical Preparation

Abstract

Any application of new technology requires proper preparation to ensure a safe and efficient field program. Slim hole technology requires different preparation and procedures than conventional rotary drilling techniques. Application of the mining industry's coring techniques, small wellbore annuli, precise fluid flow measurement for kick detection, and specialized drilling fluids requires significant technical understanding and preparation. For Mobil in 1991, to prepare for a first application of Slim Hole Drilling Technology, the preparations included drilling a test well with Long year Company's Oil and Gas Division at Mobil's Dallas Research Lab and using the information to prepare for two slim hole oil and gas exploration wells to be drilled in Bolivia, South America. The test well gave Mobil drilling personnel practical hands on experience with the technology and provided a slim hole test laboratory to perform extensive borehole hydraulics tests and practice well control techniques. The information gained from the test well was used to plan two exploration wells in the Madre De Dios Block in Northern Bolivia. Industry has proven the economic advantages of utilizing this emerging new technology for Oil and Gas Exploration, especially in the remote frontier areas of the world. The planning, technical, and operational points of interest are highlighted in the paper. The advantages and disadvantages of the technology observed during the wells are illustrated. Methods available to allow the technology to adapt to Oil and Gas and API standards are discussed. Many technical aspects of the application of the technology are discussed which will give an insight to the minimum technical preparation a company needs to perform to be comfortable with a first application. Areas of technology are highlighted where additional development of the technology is needed. Introduction The drilling industry has applied many definitions to the term "Slim Hole Drilling." As applied to conventional drilling, slim hole typically indicates a well is designed to reach total depth (TD) at the next smaller hole diameter than would be normal. For the purposes of this paper, the definition will reflect the more common definition relating to mining slim hole technology. Most of all slim holes drilled to date have used the mining industry technology. The mining industry has been drilling slim hole for mineral and precious metal exploration for decades. A complete and separate infrastructure from the oil field has grown to support this industry. Therefore, while many same requirements exist between the mining and oil and gas industry, they have used different methods to accomplish the same tasks. This requires an operator to study the mining industry technology carefully so as to adequately transfer the technology to its exploration needs. Figure 1 illustrates the generic differences between the drilled hole diameters and sizes of casing typically run in conventional and slim hole wells. P. 235^

The viability of slim-hole drilling onshore Trinidad

“…A slim-hole well has been defined in a number of different ways (Shanks and Williams 1993;Finger and Jacobson 2000;Jahn et al 2008), all of which refer to the drilling of a well with a well-bore typically less than 7 in. in diameter.…”

“…in diameter. A comparison of hole size and casing programme for a slim-hole well versus a conventional well is shown in Fig. 1 (Shanks and Williams 1993).…”

“…The cost benefits to be derived from slim-hole drilling can be attributed to the following (Shanks and Williams 1993):…”

“…The annular pressure loss has been estimated by Bode et al (1991) to be as high as 90% of the total fluid circulation pressure loss. Because of this high annular pressure loss, Shanks and Williams (1993) concluded that the models for hydraulic design in conventional wells should be carefully considered for slimhole applications. To minimize annular pressure losses, Zhu and Carroll (1994) propose a shear-thinning viscosity profile-low viscosities while travelling through the bit and up a tight annulus at high velocities and higher viscosities for cutting transport and suspension nearer to surface at lower velocities.…”

Hydrodynamic modeling of gas influx migration in slim hole annuli

“…In conventional drilling, the annulus ratio is usually around 0.3, whereas in slim hole drilling, the annulus ratio can be as high as 0.9. Other reports consider drilling a well with a wellbore typically less than 7 inches in diameter as a slim hole well (Shanks II and Williams, 1993), regardless of the drill string dimensions.…”