Shallow gas presents a major worldwide problem to the drilling industry. The records show that approximately one third of all blowouts have been caused by shallow gas. This proportion is equally applicable over the recent past. During the period June 1988 to June 1989 shallow gas was responsible for four of the twelve blowouts that occurred. Fifteen people were killed in three of the four shallow gas blowouts. Drilling rigs were lost in all four incidents.
Planning a well to reduce the chance of a shallow gas blowout depends to a major extent on the provision of an accurate prognosis of the geology and gas risk in shallow sections of the well.
The purpose of this paper is to illustrate to those involved in preparing the shallow prognosis the nature of the shallow gas problem in drilling operations and the variety of precautionary drilling practices which can be adopted depending upon the estimated risk.
Shallow gas kicks cause a unique problem in drilling operations because they cannot be controlled using conventional techniques. Conventional secondary well control techniques for handling a flow of formation fluid into the wellbore require the well to be closed in at the wellhead to stop the flow. The influx of fluid is then circulated out and the well displaced to a drilling mud of sufficient density to prevent further flow from the offending formation. The action of closing in at the wellhead results in additional pressure being applied to the entire wellbore. In shallow formations with low fracture pressures the action of closing in the well on an influx of gas is highly likely to cause the open hole formations to fracture. This may subsequently result in an underground blowout to a shallow formation or in the formation broaching to the surface (Figure 1). The result of formations broaching and causing a suface crater is catastrophic for bottom supported drilling rigs such as Jack Ups and Production Platforms. Since the well cannot be closed in the gas must be allowed to flow in a safe manner until the producing zone is isolated or depleted. (Fig. 1 and 2 are available in full paper) Further problems can arise when the gas flow is routed through pipework on the drilling installation.
Gas flowing from the producing formation carries with it a high volume of abrasive sand particles and boulders often found in shallow formations. This combination of material will have a tendency to erode or plug any pipework through which it flows. The rate of erosion or plugging is dependent on the design of the pipework. The result of erosion of the flow diversion system could be the release of gas to areas of the drilling installation where there is a potential for explosive ignition of the gas. The effect of a blocked or partially blocked flow path is the same as closing in the well and can have the same castastrophic results.