Borehole Strengthening and Injector Plugging - The Common Geomechanics Thread
- Mehdi Loloi (Advantek International Corp.) | Karim S. Zaki (Advantek International Corp.) | Zongyu Zhai | Ahmed S. Abou-Sayed (Advantek International Corp.)
- Document ID
- Society of Petroleum Engineers
- North Africa Technical Conference and Exhibition, 14-17 February, Cairo, Egypt
- Publication Date
- Document Type
- Conference Paper
- 2010. Society of Petroleum Engineers
- 1.11 Drilling Fluids and Materials, 4.1.2 Separation and Treating, 5.4.1 Waterflooding, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 2.7.1 Completion Fluids, 1.6 Drilling Operations, 5.1 Reservoir Characterisation, 1.7 Pressure Management, 1.2.2 Geomechanics, 3 Production and Well Operations, 2 Well Completion, 1.2.3 Rock properties, 4.1.3 Dehydration, 6.5.2 Water use, produced water discharge and disposal, 2.4.3 Sand/Solids Control
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The high cost of offshore drilling and the safety aspects of penetrating depleted reservoirs bring borehole stability issues to the forefront of resource development. Drilling in deepwater fields, depleted reservoirs and/or low stress environments requires careful assessment of mud weights. Higher collapse pressures combined with lower fracture gradients limit or eliminate mud weight windows and lead to tight holes or lost circulation. Extended-reach wells require minimization of the number of casing shoes to reach the deeper targets. The need for long, open sections while drilling imposes restrictions on mud windows. Borehole strengthening has become the most effective method to address borehole stability in such circumstances.
In this paper, we provide geomechanics insights from the large knowledge base on waterflooding and injector performance to improve the practice of wellbore strengthening. The petroleum industry has morphed over its long history through fewer inventions but a long list of innovations derived from lessons learned from one sector to benefit another. Injector plugging, a nuisance to production/operation engineers, illustrates such an example. The vast amount of knowledge provides the fundamental reasoning and rationale for optimizing wellbore strengthening. Fractured injector geomechanics are extended to improve wellbore strengthening procedures. Strengthening is implemented through creation of plugged mini-fractures to raise loss circulation pressures. The mud contains particles to mitigate unstable fracture extension created by using higher mud weights. The particles deposit and plug initiated fractures and prevent further propagation. The propagation pressure is raised significantly and lost circulation stops. In the presented model, the stable fracture length and width are related to a given mud weight and rock characteristics. Accordingly, the plugging media (particle) size is optimized based on the fracture geometry and leakage scenarios. The model results are field-verified and demonstrate how the borehole stability problem could be optimized with proper design.
Altough the practice of including additives to prevent lost circulation events has been common for many years in wellbore drilling; the scientific approach to describe the process using physical and numerical models is quite recent. One of the earliest attempts belongs to the DEA-13 experimental works conducted during 1980's and partly published in the early 1990's by Morita et. al. and Fuh et. al. As described by Morita et. al.1, DEA-13 experiments demonstrated that before borehole breakdown occurs, a stable fracture develops; the fracture aperture is sealed by drilling fluid bridging over the fracture inlet; and, then the borehole breakdown occurs when the drilling fluid begins to enter into the fracture. Another phenomenon found in these tests was that a narrow fracture tip zone exists, which cannot allow drilling fluid invasion. In addition, the DEA-13 experiments illustrated that there exists a dehydrated mud zone behind this fracture-tip zone. This zone can also seal fracture pressure, although widening the fracture width can break the dehydrated mud plug. Consistent with DEA-13 experimental work, Morita et. al.1 developed a theory of fracture initiation and fracture propagation around a borehole whose stability is enhanced by drilling fluid interaction. Fuh et. al.2 later presented a new concept, theoretical formulations and field test results of the loss prevention material (LPM) developed for use in many possible applications such as high angle and horizontal well drilling, drilling through severely depleted formations, and drilling through highly tectonically active areas where in-situ stresses are high and directionally unequal. They described fracture tip screen-out mechanism of particulate material as the theoretical aspect of LPM. Aston et. al.3 described the approach taken by BP to produce a ‘designer mud' which effectively increase fracture resistance, while drilling in both sand and shale. They introduced the concept of a ‘stress cage,' which is a near wellbore region of high stress induced by propping open shallow fractures and plugging and sealing them with particles. Alberty and McLean4 described a physical model for the stress cage mechanism and used finite element analysis to demondtrate the effect of stress caging. The results were also used to determine concentrations of particles in the mud to build fracture resistance to a desired or targeted level. As all the previous developments addressed the strengthening concepts in sand permeable formations in general, Aston et. al.5 later presented a new treatment for wellbore strengthening in shale. In particular, they demonstrated that wellbore strengthening via stress cage techniques can be accomplished without the dependence upon fracture collapse due to leakoff, by maintaining a bridge at the mouth of the fracture and preventing additional fluid leakage into the fracture. Recently, the numerical solutions (boundary element method) have been used by Wang et. al.6,7 to perform near wellbore stress analysis for wellbore strengthening. In the latest publication, van Oort and Friedheim8 focused on the various theories and approaches to wellbore strengthening. They introduced the approach of gaining wellbore fortification through fracture propagation resistance and argue that an elevation in fracture propagation pressure may in fact be the actual mechanism underlying the borehole strengthening effect that has been attributed to ‘wellbore stress augmentation'.
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