Traditional stilt village Kampong Ayer on the Brunei River in Bandar Seri Begawan, the capital of Brunei Darussalam. Photography: Leonid Andronov via Adobe Stock.
Asia
Oil & Gas

Drilling around mud diapirs

Production geologist Peter Henneberg tells the story of how better seismic imaging, an improved subsurface understanding, and a healthy dose of perseverance led to the drilling of a successful well that tapped into a multi-million barrel resource that had been left stranded for a long time

Mud diapirs have long been enigmatic struc­tures in the subsurface. With older seismic data being of insufficient resolution to properly map them, companies often took a careful approach when it comes to drilling close. This is pri­marily related to the observation that the mud diapirs are often associated with overpressures.

The Tertiary basin in Brunei is a good example of this phenomenon.

Peter Henneberg, who worked in the country for seven years, was involved in drilling a well that took him a lot of convincing of manage­ment, because it was getting close to what was just before mapped as a mud diapir. For a long time though, it wasn’t even known that it was. But it was considered a no-go zone anyway, because a well had been drilled into it had experienced overpressures and was rather shaly.

“Early maps of the field based on 2D seismic data showed a zone that was surrounded by a series of faults, as if it was an isolated fault block,” says Peter. It shows how geologists tried to solve the pressure difference between the main field and in the well that drilled the diapir.

Only when 3D seismic volumes became available, the contours of the mud diapirs became more obvious. In time slices, circular zones can be seen, which are mostly transparent, but with clear higher impedance sliv­ers in it, interpreted as isolated over­pressured sands.

“It is these sands that are the problem,” says Peter. “They have kept the pressures from deeper down, and due to their isolated nature, they still record higher pressures than the sands in the reservoirs juxta­posed against the diapir at the same depth. However, we did observe that some pressure dissipation must have occurred though.”

The two maps on the left show the difference in structural interpretation of the section of the field where the mud diapir was found. The early generation interpretation based on 2D seismic lines shows the area of the mud diapir being surrounded by a set of faults, without necessarily having the right concept in mind what this area represented. All the team had available at the time was the observation of higher-than-normal pressures in the area, which had to be somehow isolated from the remaining part of the field where “normal” pressures dominated. The new well that Peter Henneberg suggested to drill, and which was ultimately drilled as planned, is indicated in red in the upper left map, coming closer to the mud diapir than any of the previously drilled wells had done and thereby tapping into a new resource base even though the same reservoir had already been drilled further to the east. Please note that not all wells drilled in the area are indicated on the maps; only three wells (black dots) have been included to help compare the two maps.

The easy bits first

The oilfield Peter worked on was large, and because there were plenty of are­as to go after for oil production first, the “difficult” high-pressure zone was avoided for a long time. Until the mo­ment that even this area appeared on the radar, invariably driven by the de­pletion of the easy parts of the field. But it was only the radar of the pro­duction geologist the area appeared at, not so much on the management’s radar because of the cloud of over­pressure issues hanging over the area. It took some convincing, or maybe to word it differently: Education.

As with so many things, once it is possible to explain why it happens, it becomes much more feasible to find ways to mitigate against the poten­tial issue. And the mud diapirs were a classic example of that.

Peter took the matter to heart and, having studied similar structures else­where, was clearly being able to show that the zones of overpressure tend to be confined to the diapir itself. This meant that the extent of overpressure could be mapped, which de-risked the areas immediately around it. That was the area Peter had in mind tar­geting, because he saw that a signif­icant undrained compartment must be present there. The existing produc­tion wells in the field were simply too far away to have properly done that.

Into the field

In order to make things easier to un­derstand, Peter even organized a field trip for management to show how a diapir looks like in outcrop and that the sands intercalated tend to be iso­lated and unconnected to the main reservoirs outside the diapir. This helped, and slowly the idea to drill a new well became more concrete. The continued depletion of the main field helped make the case as well.

Then came the question how to drill the well, what trajectory it should have?

It was decided to drill the well parallel to the sides of the diapir, in an attempt to tap into as many com­partments that may have arisen due to faults radiating away from the dia­pir. These faults are commonly found around these structures, and have a tendency to create compartmentali­zation of individual fault blocks.

After all, the well was drilled suc­cessfully, and a 2 million barrel re­source could be produced safely. No overpressures were observed, and all the reservoirs known from the wells at a dis­tance were found. This clearly demon­strated the value of the seismic data and the value of applying a valid model to explain overpressures observed at times when it all seemed so obscure.

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