Imaging Unstable Wellbores While Drilling
- S. Edwards (BP America) | B. Matsutsuyu (BP America) | S. Willson (BP America)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- December 2004
- Document Type
- Journal Paper
- 236 - 243
- 2004. Society of Petroleum Engineers
- 1.6 Drilling Operations, 1.2.3 Rock properties, 4.1.2 Separation and Treating, 4.1.5 Processing Equipment, 4.3.4 Scale, 5.1.2 Faults and Fracture Characterisation, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 1.12.2 Logging While Drilling, 1.6.1 Drilling Operation Management, 3 Production and Well Operations, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 1.11 Drilling Fluids and Materials, 3.3.2 Borehole Imaging and Wellbore Seismic, 1.14 Casing and Cementing, 2 Well Completion, 1.2.2 Geomechanics
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This paper presents a case study of borehole instability from four wellbores on the Gulf of Mexico (GOM) shelf, offshore Louisiana. Logging-while-drilling (LWD) borehole images are combined with observations of cavings and modeling of borehole shear failure to diagnose the mechanisms of instability and, thus, select the appropriate remedial action.
It is observed that instability caused by shear failure of intact rock (borehole breakout) can be suppressed by increasing the MW. However, where pre-existing planes of weakness (such as bedding planes and fractures) dominate the mechanism of instability, mud-weight increases do not necessarily lead to a more stable hole and can, in fact, further destabilize the wellbore.
Despite considerable effort from the drilling, subsurface, and geomechanics communities, many oil wells continue to suffer from wellbore-instability problems during drilling. Although instability is quite common, in the majority of cases a considerable amount of uncertainty exists concerning exactly where, when, and why the instability occurred.
Unfortunately, it is almost axiomatic that logs will not be run in an unstable wellbore. Direct measurements of the borehole shape and condition that can be obtained from caliper and image logs are, therefore, rarely acquired in the wellbores from which (from a geomechanics point of view) they would be most valuable. Modeling and cavings analysis alone can leave considerable uncertainty regarding the location and, to some extent, the mechanism of failure.
An exception to the axiom can be one in which LWD-image data are acquired. It is still unlikely that LWD-imaging tools would be run in a well in which significant instability was expected. However, LWD is often acquired in wells that turn out to be less stable than anticipated. In these cases, a rare glimpse of the unstable wellbore wall in the early stages of collapse may be captured. This is very useful information that would normally remain the secret of the well.
In this paper, a case study from the GOM shelf that includes wellbore instability, LWD imaging, cavings observations, and rock-failure modeling is presented. The authors of this paper were present to provide technical support to the drilling activities of this well soon after initial signs of instability were observed. We were fortunate enough to be able to acquire all the data presented below in a timely manner, such that analysis and recommended remedial action could impact drilling operations.
A detailed description and analysis of the data form the bulk of this paper; however, we first discuss mechanisms of wellbore instability within the context of pertinent literature on this subject.
Mechanisms of Wellbore Instability
We propose that mechanisms of mechanical wellbore instability can be grouped in two main classes:
- Instability caused by failure of intact rock (i.e., rock that is unbroken and isotropic in strength).
- Instability because of the failure of rock containing pre-existing planes of weakness (e.g., bedding planes, fractures, and/or cleavage).
Rock containing pre-existing weaknesses such as bedding planes or cleavage may be intact in the sense that it is unbroken. For the sake of this discussion, however, intact is defined as above.
The majority of quantitative wellbore-stability studies since the 1979 paper by Bradley1 have modeled the wellbore wall as intact rock subject to the stresses imposed from the far field and the wellbore fluid. This type of failure gives rise to symmetrical break- outs in the wellbore walls. Breakouts can be stabilized by increasing the MW, or they may stabilize after reaching a certain size under favorable combinations of stress and strength. Breakouts are quite often observed in image and multiarm-caliper log data and are clearly a common cause of wellbore instability.
Other mechanisms of instability in which pre-existing weaknesses are present do not necessarily stabilize with time or with increased MW. Instability because of such mechanisms is, therefore, rarely calipered or imaged, making the exact location and mechanism of instability uncertain.
Consideration of wellbore instability caused by pre-existing weaknesses in oil wells is, for the most part, relatively recent:2-6 evidence of these mechanisms came from observations, such as correlations of trouble time with wellbore trajectory, and the existence of pre-existing fracture planes, bedding planes, and/or cleavage in cavings.
A particularly insightful documentation of both field and laboratory evidence of this mode of failure from fissile shales in the North Sea is presented by Okland and Cook.4
From the data presented in the literature, as well as in this paper and in the author's experience, it seems that types of wellbore instability associated with pre-existing weaknesses can be grouped into two classes:
- Failure because of the existence of "impermeable" pre-existing weaknesses.
- Failure because of the existence of "preferentially permeable" planes of pre-existing weaknesses.
In the case in which the pre-existing weaknesses are not preferentially permeable, an increase in the MW tends to further support the wellbore wall. An example of this type might be where a single set of bedding planes intersected.
Where the mud and filtrate preferentially enter pre-existing planes of weakness, increasing the MW does not add support to the wellbore wall and may increase instability. Networks of pre-existing weakness (i.e., where two sets of weakness, such as bedding planes and fractures, intersect) are probably more likely to be permeable than a single plane of weakness (e.g., just bedding planes). In an extreme case, the body of rock may actually be composed of many discrete rock fragments with no cohesion between them, rather like a pile of rubble or the material seen in brittle/semibrittle fault zones. This type of rock mass could be referred to as a rubble zone - the rock having been effectively turned to rubble. These pre-existing weaknesses could be a combination of fractures, cracks, bedding planes, and cleavage planes. Naturally, fissile rock - such as thinly bedded shale - is likely to be particularly susceptible to becoming rubble where it is affected by faulting.
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