Using predrill stress to avoid distress

SEISMIC data can be processed to analyse geological stresses in the Earth’s crust and help avoid stress-related drilling problems, according to Brisbane-based geological software company Predrill Stresses International (PSI).
Using predrill stress to avoid distress Using predrill stress to avoid distress Using predrill stress to avoid distress Using predrill stress to avoid distress Using predrill stress to avoid distress

The company’s founder and chief executive, Australian geological consultant and structural expert John Davidson points out that the seismic data records subsurface deformation and says his “pep” software technology can interpret seismic data to predict the direction and magnitude of geological stress.

This not only reduces drilling problems but also enables petroleum explorers and producers to boost drilling performance and maximise oil recovery from new and existing fields, he claims.

“It’s a standalone technique that removes the need for expensive post-drill and conventional borehole geomechanics,” he said.

Three-dimensional seismic data is best but good quality 2D data is also acceptable.

Davidson suggests the stress-finding technique will aid a cross-section of petroleum industry tasks and professions.

“It is inviting problems if wells are drilled against the direction of maximum stress,” he said.

“This is particularly important when tackling a field development program from a fixed platform where the number of drilling slots is limited.

“Reservoir engineers can use the stress direction determination to predict which natural rock fractures will be open and therefore of use during production.

“Similarly, knowledge about the natural direction of stress in a reservoir is an aid when planning fraccing programs.

“And when looking at various prospects and leads, field geologists can use the sub-surface picture we generate to evaluate traps and determine which faults are likely to be sealed and which are leaking.”

Davidson says it has been known for some time that plate tectonics cause horizontal stress in the Earth’s crust, but small vertical stresses caused by the flexing up and down of the continents also complicate drilling operations.

“These vertical forces, sometimes referred to as ‘the Earth’s heartbeat’, change the curvature of the Earth and produce horizontal compression,” he said.

“Locally this causes circular wells to become elliptical.”

If the rock being drilled is hard, the distortion isn’t great and may not cause problems. But if the rock is soft, significant well distortion could become apparent within a day or even hours, resulting in a stuck bit or at worst, a complete hole collapse, forcing a costly abandonment.

The modern practice of drilling horizontal wells has made predicting and planning for this phenomenon even more critical, according to Davidson.

“We needed to know the direction of stress at the well location,” he said.

“Up until now the industry practice has been to estimate the direction and magnitude of the stress by extrapolating from other wells in the area.

“If there weren’t any nearby wells, we’d use distant earthquake data and even plate tectonic theory.”

“The trouble is, stress direction can change in less than 2km,” Davidson said.

“That means the extrapolation technique is not reliable. We needed a better way.”

It wasn’t until he was talking to a colleague in Norway about stress measurements taken in the Snorre field in the North Sea that Davidson realised the required stress information could be hidden in the seismic data that is routinely acquired by companies during the early stages of exploration in an area.

With the help of John Goodwood, formerly with Brisbane software solutions company Mincom, he put together the “pep” software package, which enables prediction of the degree and direction of geological stress.

The company has already completed more than 50 projects around the world with a high success rate and has caught the attention of French-headquartered global service company Schlumberger as well as Shell and other international oil majors.

PSI has undertaken several successful trials on a consultancy basis in the North Sea for Hurricane Exploration (west of Shetlands) as well as for Shell and Schlumberger (in the Viking Graben).

The company is also consulting with Adelaide explorer/producer Stuart Petroleum in South Australia’s Cooper Basin. PSI has already conducted several stress-from-seismic interpretations for Stuart that helped lead to the discovery of a number of small oil pools.

Davidson says he now wants to interest other Cooper Basin players such as Santos and Beach as well as other major companies in Australia and globally in the potential of his technology.