Mechanisms of Shallow Waterflows and Drilling Practices for Intervention
- M.W. Alberty (BP Exploration) | M.E. Hafle (BP Exploration) | J.C. Mingle (BP Exploration) | T.M. Byrd (BP Exploration)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- June 1999
- Document Type
- Journal Paper
- 123 - 129
- 1999. Society of Petroleum Engineers
- 5.6.1 Open hole/cased hole log analysis, 1.11 Drilling Fluids and Materials, 5.3.4 Integration of geomechanics in models, 1.14.3 Cement Formulation (Chemistry, Properties), 1.14 Casing and Cementing, 1.10 Drilling Equipment, 1.12.1 Measurement While Drilling, 1.1.6 Hole Openers & Under-reamers, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 1.7 Pressure Management, 1.2.3 Rock properties, 1.6.10 Running and Setting Casing, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 4.3.1 Hydrates, 3 Production and Well Operations, 5.2 Reservoir Fluid Dynamics, 1.7.5 Well Control, 1.6 Drilling Operations, 2.4.3 Sand/Solids Control, 4.2.4 Risers, 1.2.1 Wellbore integrity
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Geopressured water sands near the mudline in deep water, greater than 1,000 ft, have been shown to be hazards when these sands are permitted to flow outside structural and 20-in. conductor pipe. Special drilling practices are required to contain the pressure during drilling and casing operations.
Four mechanisms have been identified as causes of shallow waterflows: (1) induced fractures, (2) induced storage, (3) geopressured sands and (4) transmission of geopressure through cement channels. Geoscience techniques have been developed to aid in the detection of the shallow waterflow mechanisms, prior to drilling. These techniques include seismic stratigraphic interpretation of shallow hazard airgun data and specially processed three-dimensional surveys, and special pore pressure and fracture gradient prediction methods.
Drilling and cementing practices have been developed to minimize the risk of inducing flow behind structural and conductor casings. Each of these flow mechanisms require adapted drilling and cementing practices to prevent potentially damaging flow.
This paper presents best practices developed by this operator along with our contractors to detect, drill, case, and cement shallow waterflows in the deepwater Gulf of Mexico. These practices should be transferable to other similar sites around the world.
A shallow waterflow can be defined as water flowing on the outside of structural casing to the ocean floor. This flowing water can erode the structural support of the well which may lead to buckling of the casing and subsequent casing failure. This flow path will also compromise the wellbore integrity which can result in loss of well control.
A study has been conducted on 74 industry wells in areas of the deepwater Gulf of Mexico (GoM) where geopressured sands within 2,000 ft of the mudline were known to exist. Well results associated with shallow waterflow incidents were classified into one of four categories: (1) the well had a shallow waterflow which rendered the well unable to achieve its designed objectives, (2) the well had a shallow waterflow and was able to reach planned total depth, but because of the flow, only limited formation evaluation was possible, (3) the well had a shallow waterflow but successfully completed all of it's planned objectives, and (4) the well had no known shallow waterflows. See Table 1 .
Using this criteria, 12% were never able to reach the final objectives due to the occurrence of shallow waterflows. One percent (1%) of the wells were able to reach total depth, but evaluation was limited due to the influence of the shallow waterflows. Fifty-three percent (53%) had known shallow waterflows but were able to complete all depth and evaluation objectives. Thirty-four percent (34%) of the wells did not encounter any shallow waterflow problems.
In one of the more severe cases the formation fluid escaping through the shallow waterflow resulted in significant compaction of the shallow sand. The surface expression of this compaction and subsequent subsidence produced a fissure extending up to 300 ft from the wellhead with as much as 7 ft of displacement between the two opposing sides of the trench. This surface damage could have resulted in significant financial losses if allowed to occur at a subsea template development. See Fig. 1.
Shallow waterflows represent a significant financial risk to deepwater developments if appropriate methods are not used to reduce drilling risks.
The same study identified four different mechanisms which cause the shallow waterflows: (1) induced fractures, (2) induced storage, (3) geopressured sands in conductor intervals, and (4) transmission of geopressure through cement channels. In all wells studied, the shallow waterflows could be classified into one of these four mechanisms.
In the induced fracture mechanism, the pressure generated at the casing shoe exceeds the formation strength resulting in the generation of a fracture which provides a flow path for wellbore fluids back to the surface. These fractures occur in the conductor (20 in.) or surface (16 or 13-3/8 in.) hole sections—Fig. 2. The pressure may be the result of wellbore friction, well packoff, suspended cuttings, or increased base mud weight. It should be noted that these fractures can occur in the conductor hole section which is drilled riserless (returns to seabed). No sands need be present to produce this failure.
The nature of the surface expression of fluid escape through an induced fracture will vary depending upon the soil mechanics in the general area of the wellhead. The fracture may provide a bypass channel to the isolation cement near the shoe, in which case the flow will occur between the conductor (typically 20 in.) and structural casings (typically 30 or 36 in.). The fracture may provide an upward path to unconsolidated mud near the surface, in which case the flow may produce small surface volcanoes which release the escaping fluid and transported formation grains; these collect, building a small crater-like mound. Cases have also been observed in subsea template locations where the fractures connect to adjacent wells producing flow on the outside of nearby casings.
Induced storage describes a condition that is produced when the pressures generated in the mud column charge shallow permeable and porous sands or silts that were previously normally pressured. This is a common phenomena that takes place in many deepwater sediments lying above the first sealing formation. In very shallow sediments, shales may even have sufficient porosity and permeability to become charged.
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