This is the introduction to shallow gas well control which will briefly describe the overview of shallow gas and some related information. We have few articles regarding this topics and we will separate into small parts for better understanding. Let’s get started.
Whenever offshore shallow gas accumulations are seen, they’re normally linked with down sand lenses enveloped by mudstones. Typically, lenses will be permeable, unconsolidated, and highly-porous when found in shallow depths. Although normally flat, thin, and normally pressured, many have previously encountered over-pressured lenses. When at this depth, one cause of over pressure is inclination of the lens; this can therefore increase both the lens height and pore pressure gradient (top of the lens).
Although rare, shallow gas can also be linked with vuggy limestone or buried reefs; these have the risk of being infinitely permeable and incredibly porous.
When drilling in the top-hole section, resulting kicks from shallow sands can be dangerous with short casing strings; there are many case histories to show this. Charged formations can also cause kicks from shallow sands and this itself can be a result of improper abandonments, previous underground blowouts, casing leaks, injection operations, and poor cement jobs.
The example of the shallow gas blow out is below.
Sedco 700 Shallow Gas Blow Out 6 June 2009
When it comes to shallow gas kicks, the most common cause is a loss of hydrostatic head and this can be a result of two common problems;
- Expanding drilled gas unloading the annulus
- Poor hole fill while tripping
- Losses through the overloading of the annulus with cuttings
In order to minimize the risk of inducing a shallow gas flow, we recommend some general precautions including restricting the rate of penetration, drilling a pilot hole, drilling riserless, and always monitoring the hole.
High flow rates of gas are often produced by shallow gas flows; high quantities of rocks from the formation are also possible. This is particularly true after long sections of sand have been exposed. When a shallow gas flow occurs, the representative responsible should contact a senior contract representative; all non-essential individuals should be evacuated from the rig. This eventuality should always be addressed, and there should be an implementation of the contractor’s emergency evacuation.
A shallow seismic anomaly, referred to as a bright spot, should never be drilled through since this suggests shallow gas. If bright spots are seen in upcoming drilling locations, the best solution would be to avoid a hazard by moving the rig. If possible, the new drilling location should be located on a shallow seismic shot point.
Just because no bright spots exist, this doesn’t mean you can be certain of a lack of shallow gas. In addition to this, it’s also important to note that subsequent directional wells can still hold shallow gas even if one well in a series fails to offer any (drilled from a surface location). Therefore, care should always be taken when drilling from the same surface location.
For the next topic, we will discuss about riserless drilling on a floater and what we should prepare for it.
Cormack, D. (2007). An introduction to well control calculations for drilling operations. 1st ed. Texas: Springer.
Crumpton, H. (2010). Well Control for Completions and Interventions. 1st ed. Texas: Gulf Publishing.
Grace, R. (2003). Blowout and well control handbook [recurso electrónico]. 1st ed. Paises Bajos: Gulf Professional Pub.
Grace, R. and Cudd, B. (1994). Advanced blowout & well control. 1st ed. Houston: Gulf Publishing Company.
Watson, D., Brittenham, T. and Moore, P. (2003). Advanced well control. 1st ed. Richardson, Tex.: Society of Petroleum Engineers.