Experimental Data Analysis of Lost-Circulation Problems During Drilling With Oil-Based Mud
- E.C. Onyia (Amoco Production Research)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- March 1994
- Document Type
- Journal Paper
- 25 - 31
- 1994. Society of Petroleum Engineers
- 1.14 Casing and Cementing, 1.6 Drilling Operations, 4.3.1 Hydrates, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 4.1.2 Separation and Treating, 5.8.2 Shale Gas, 4.3.4 Scale, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 1.11 Drilling Fluids and Materials, 3 Production and Well Operations, 2.5.2 Fracturing Materials (Fluids, Proppant)
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This paper presents interpretative analyses of experimental data from a joint industry project that investigated the phenomenon of lost circulation during drilling with high-density oil-based muds. These experimental results provide a better understanding of the mechanics of the oil-based-mud lost-circulation phenomenon and solutions to this problem.
In terms of cost, lost circulation during drilling with oil-based muds adversely affects the overall cost of a drilling operation. The type of lost-circulation problem addressed here occurs during drilling with high density (> 14-lbm/gal) oil-based mud in deep and sometimes overpressured drilling environments. The problem is exacerbated in the latter case because of the very narrow tolerance between drilling mud weight, formation pore pressure, and breakdown pressure.
Some fundamental issues this analysis seeks to address are as follows.
1. Is there any significant difference in the fracture-initiation pressure of oil- and water-based muds?
2. What role does fluid loss play in controlling lost circulation in oil-based muds?
3. What role does permeability and planar imperfections in the lithologies play in controlling lost circulation?
4. How effective are lost-circulation treatment techniques?
There are several possible causes of the lost-circulation problems described above. One is incorrect estimation of annular pressures in the hole sections where lost circulation occurred. Estimates of annular pressures may be wrong because of difficulties in estimating frictional losses and downhole pressures in wells. The complex rheological character of drilling fluids, especially oil-based muds, at different temperatures and pressures makes it difficult to determine correct annular pressures accurately. Another possible cause of lost circulation may be related to "fracturing" pressure, which was previously suspected to vary for water-, diesel-, and mineral-oil-based muds.Other factors that can lead to differences in the lost-circulation or fracturing pressures among water- and oil-based muds include the "sandout" or "mudout" phenomenon. For water-based mud, the mudout phenomenon, which is similar to a plugging process caused by various sealants used in drilling fluids, could be the controlling factor. A less likely but still possible cause of lost circulation is a weakening of rock strength by the surfactants mixed with diesel- or mineral-oil-based muds as emulsifiers. Rock strength can also be weakened by formation hydration and swelling because of exposure to various drilling fluids.
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