Shallow Gas & the Near Surface
There are only 6 shallow hazards.
by Ralph W. Baird
BPI with its affiliate joint venture partners provide high resolution geophysical services including the data acquisition, data processing, interpretation, report production and data base management of land and marine engineering geophysical, geologic, engineering soils, hydrographic and environmental field survey information. These services are used by engineering departments, consulting engineering and governmental authorities to identify and plan for potential problems of the earth soils and sediments. Projects are listed following this article.
Here is a link to the original study prepared by the author for the benefit of the Offshore Operators Committee: LINK.
THERE ARE ONLY SIX SHALLOW HAZARDS
There are several instances every year where the six shallow hazards: shallow gas, near surface faults, sediment strength, water bottom anomalies, glacier or river channels, and man-made objects, have delayed man's quest for oil. These "geologic" features have cost the oil industry hundreds of millions of dollars due to sediment failures, lost circulation, and shallow blowouts. Many of these problems could have been avoided by judicious use of digital high resolution geophysical data.
Engineers can change their drill plan to compensate for potential geologic hazards. Losses can be avoided if near surface information from a wellsite survey is examined and incorporated into the well plan.
Shallow Hazard No. 1: Shallow Gas (and Shallow Water Flows)
If shallow gas of a large enough quantity is encountered unexpectedly during drilling operations, a blowout might occur. The driller has interest in shallow gas from mudline to 3,000 feet and below. Shallow trapped gas areas can be avoided by changing the wellsite location, or if required, and only if the frac gradient is sufficient, can be penetrated by cementing a string of casing firmly above the gas zone, increasing mud weight to penetrate through the gas zone, and continue the drilling operation. Gas that is trapped in the shallow sediments usually originates from deeper gas reservoirs but can also come from biogenic activity in the shallow sediments. Shallow gas can only be confidently interpreted from high resolution seismic data that has been digitally processed and displayed in true amplitude. See attached Figures 1. and 2.
Our experience with shallow water flows (SWF) is similar to shallow gas. In the case of water flow, the problem to drilling is it can be water under very low over-pressure, usually in an area of rapid sediment deposition, but there are exceptions. Our recommendations regarding SWF is to not let even a small flow develop. Another aspect, yet not fully understood is the long term effect of a casing/asset set through a potential shallow water flow zone.
Shallow Hazard No. 2: Near Surface Faults
Near surface faults can create surface anomalies hazardous to jack-up and drilling rigs including anchors and guy-wire bases. The fault plane itself can pass gas from a deeper gas zone and if not controlled, a blowout will occur. The ocean bottom is unstable around fault traces. Casing should not be terminated in or near a fault zone because shear strength (frac gradient) of the sediments in the fault zone is much less than "non-faulted" sediments. In deep water and to improve the resolution of faults, time or depth migration of the digitally processed high resolution seismic data is recommended.
Shallow Hazard No. 3: Sediment Strength
Both slightly hard sediments and slightly soft sediments can create problems to drilling operations. Jack-up rigs require ample leg penetration for stabilization for high shear capacity for their legs. If leg penetration is too deep, the well may not be drilled because the limit of penetration is the leg length. The water depth, the leg penetration, and the required air gap (for insurance and safety purposes) must add up to less than the leg length available. If this is greater, then the well cannot be drilled and another drill rig or drillship should be chosen. Prediction of jack-up leg penetration is based on the first good seismic reflector deeper than 20 feet below mudline. This reflecting horizon can be interpreted from seismic data. Correlation to known engineering data from local soil borings or leg penetration depth provide a more accurate estimate.
Anchor systems, including primary and piggy-back anchors, require at least 20 - 25 feet (thickness) of mud for adequate shear strength. If a hard silt, sand, limestone, coral reef, or salt is encountered shallower than 25 feet within the anchor's path, the anchor will slide along the layer and not "dig in". This situation calls for additional piggy-back anchors to be set in order for the combined shear strength of all anchors to provide adequate tension carrying ability by the total anchoring system. High resolution geophysical can be used to determine anchoring conditions.
Note: In addition, jack-up legs cannot penetrate hard materials such as coral reefs or hard calcareous cemented sandstone or limestone. Sometimes these layers appear as thin high velocity zones and are confused with shallow gas "Bright Spots" due to their anomalous seismic acoustic velocity.
Shallow Hazard No. 4: Old Rivers and Glaciers
Old river channels can be filled with clay, porous material, mud, gravels, and/or boulders. Any channel is potentially hazardous and should be planned for in the wellsite location and drilling plan. Lost circulation in channels has cost the industry greatly in the North Sea and the Gulf of Alaska, especially due to glacial boulder channels causing high bit torque and lost circulation at shallow depth. Channels are interpreted from high resolution data.
Shallow Hazard No. 5: Water Bottom Anomalies
Steep bottom slopes create sediments bottom stability problems for both jack-ups and drillships. Water bottom features such as mud lumps, trenches, faults scarps, pockholes, ridges and depressions can be interpreted from high resolution seismic data. The vertical exaggeration in high resolution records makes the identification of water bottom hazards and accurate description of the seafloor possible.
Shallow Hazard No. 6: Man-made Objects
Man-made objects exist wherever man travels. These include pipelines, debris, wrecks and cultural items that can be detected with multi-sensor geophysical surveys. Each must be avoided. Car bodies, garbage and schools of fish are commonly also seen on high resolution records and have been misinterpreted by even the most experienced analyst.
Shallow, near surface geological engineering information and features are not visible on conventionally stacked exploration seismic data. Shallow hazards are interpreted from high resolution multi-fold, multi-sensor geophysical data properly digitally processed. The risk and costs to offshore drilling has been reduced by the judicious use of wellsite surveys with processing. The costs of the survey are small when compared to the costs of even a small delay to drilling in the hostile marine environment. Application of high resolution geophysical methods are everywhere, from environmental disposal problems to land-based construction and offshore drilling.
MINI 3-D SURVEYS
3-D methods are equally applicable, is not more so, for high resolution geophysical surveying then for conventional, lower resolution surveys. BPI offers both 3-D acquisition and data processing of high resolution reflection seismic data. In addition, offshore platforms may be undershot for sediment analysis in-situ by BPI high resolution, high frequency telemetry hydrophones and receivers. The attached CDP center plot is a map of such a survey recorded and processed by BPI.
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