An unconventional unfolding of events

Saudi Field Gets ‘Advanced’ Approach

The EXPLORER editor suggests a story about an advanced geophysical technique for identifying fractures, already in use by Saudi Aramco.

“Fractures are a big deal in developing shale gas and tight sands. It sounds like an important story for unconventionals,” I say.

He likes the fact this new approach is being used for unconventional development in Saudi Arabia.

I send an e-mail to AAPG member Khalid Shokair, geophysical explorationist for Saudi Aramco’s Concept Generation Team in Dhahran.

“This is a robust technique that can be used to model and evaluate a wide variety of structures in different tectonic settings – extensional, compressional, transpressional and transtensional,” Khalid writes back.

“Within these geological settings, tight reservoirs can be restored and the amount of deformation can be calculated to assess unconventional plays.

“Our 3-D restoration techniques help to evaluate structure and to assess strain and fracture, which often play key roles in defining the viability of these deposits.”

Picture this: Saudi Aramco was conducting an exploration program, drilling to a Permian-Carboniferous natural gas target south of the Ghawar Field.

But, unexpectedly, the well started to produce oil from the higher Tuwaiq-Mountain Formation.

That formation was supposed to have poor, tight reservoir qualities in the area.

Why was there so much oil?

“Several exploration wells in the south Ghawar area were drilled for deep gas targets,” Khalid explains. “However, numerous oil shows were coming from a shallower, tight Jurassic carbonate section known to be deposited in a deep carbonate environment.

“From our understanding of the fold geometry and the seismic expression of the master fault at depth, we have interpreted the structure in question as a fault-propagation fold,” he continues.

“The fold limb above the upper fault tip has been deformed by intense shearing over the past tens of millions of years. This zone is highly strained, and is characterized by numerous small scale faults and fracture sets.”

Khalid is getting ready to leave for AAPG’s International Conference and Exhibition in Rio de Janeiro, where he’ll present the paper “3-D Structural Restoration Approach for Fracture Prediction – A Case Study from the Kingdom of Saudi Arabia.”

His co-authors are AAPG members John H. Shaw, professor and head of the Structural Geology and Earth Resources Group at Harvard University, and Andreas Plesch, research associate with that group.

“In this case study, core and image logs of an exploratory well indicated two sets of fractures. While many of the fractures are drilling induced, several natural fractures sets were identified in the area of intense strain predicted from the structural restoration technique,” he says.

“This tight rock is about 120 feet thick and has low shale content within high organic-rich formation.

“The combination of highly fractured reservoir with the source rock defines this nonconventional oil accumulation, and establishes an attractive play or sweet spot for exploration in other large structures within the Kingdom.”

Communication is a little challenging.

>I’m in Tulsa; Khalid is at his office in Dhahran from 7 a.m. to 4 p.m.

That’s 10 p.m. to 7 a.m Tulsa time.

When I talk to Khalid by phone for the first time, it’s 12:30 a.m. on a Sunday in Tulsa.

“For the last several decades, Aramco structural geologists have attempted to understand and describe the evolution of the major folds in Saudi Arabia that provide traps for our large oil and gas accumulations,” he says.

“Detailed studies in the field, interpretations of 2-D seismic data, and gravity and magnetic modeling have provided a basic understanding of the fold geometries and have successfully supported many hydrocarbon exploration programs.

High tech in Saudi Arabia, from start to finish: A view of the general setting (grids, wells, faults), followed by the generating of 3-D volumetric meshes layers (2), followed by calculation of 3-D displacement and 3-D strain fields (3), followed by final validation of structural deformation histories and analysis of reservoir-scale strain fields.
High tech in Saudi Arabia, from start to finish: A view of the general setting (grids, wells, faults), followed by the generating of 3-D volumetric meshes layers (2), followed by calculation of 3-D displacement and 3-D strain fields (3), followed by final validation of structural deformation histories and analysis of reservoir-scale strain fields.

“Significant advances in 3-D seismic imaging, combined with advanced technologies in structural analysis, now provide more detailed information about the structure of these folds as well as smaller, reservoir-scale deformations.

“Imaging these kinds of faults and fractures is possible using wide-azimuth seismic 3-D technology. This type of advanced 3-D seismic data is not often available in the early stage of exploration due to the high cost and long time period required to acquire, process and analyze these data.

“Thus, 3-D geomechanical structural restoration has proven useful as a complimentary technology that enables explorationists to model and predict the most strained regions in the fold under investigation.

“Integration with well data and seismic attributes helps to constrain further the fracture patterns associated with these highly strained fold zones.

Khalid tells me about the department where he works.

It was started specifically to generate new exploration plays in Saudi Arabia, using new ideas.

“The newly formed Aramco Department of Exploration Resource Assessment has put in place a team to exploit these new technologies and apply them to identify and develop new concepts for future hydrocarbon resource exploration,” he says.

“One of these unconventional concepts is tight reservoir exploration. Our group used these new technologies to help characterize the unconventional resource in collaboration with the Structural Geology and Earth Resources Group at Harvard, headed by professor Shaw.

“Most traditional methods of structural restoration rely on geometric or kinematic constraints to restore structures through time. While these methods have proven very useful in helping to validate structural interpretations and define paleo-trap geometries, they generally do not work well to predict patterns of reservoir-scale deformation.

“In contrast, we employed a new, 3-D, geomechanical restoration approach based on finite element techniques that restores the deformed area while taking into account rock properties and stress.

“This advanced technique was developed by the Nancy School of Geology (in France) in collaboration with the Harvard structural group.”

I ask Khaild to send some figures to help illustrate the 3-D reconstruction technique. He also e-mails me his photograph for the story.

“This restoration method requires surfaces representing geomechanical stratigraphic layers and faults extracted from 3-D seismic data. These horizons are triangulated and used to generate tetrahedral meshes.

“The interactions between these surfaces and faults have to be set in order to represent how the blocks can move during the restoration steps. The restorations also incorporate information about each rock layer’s physical properties, such as lame’s constant, shear modulus and density.

“This information can be direct measurement from cores or logs. Internal deformation within regions of the model is controlled by these rock measurements.

“During a sequential restoration of each layer, restoration vectors at each node in the entire volume are calculated. Strain and dilatation – volume change – can also be calculated and visualized at each restoration phase.

“The total restoration values then can be posted over the reservoir layer under investigation.

“Zones of high strain generally represent highly deformed areas within the reservoir. Based on calibration with well, seismic and production data, these strain patterns can be used to understand the heterogeneity, the fracture density and direction, of the reservoir.”

Khalid is in Rio now.

He gives me a phone call from there.

“What we found out in this example is that we can expect to predict the same things in other places,” he tells me.

“We don’t need to shoot any extra seismic – we have everything. It’s just a matter of continuing to apply this technique in Saudi Aramco.

“To see the strain variation we need some significant structuring or variation in the rock. If the fracturing is very gentle, we won’t be able to see it using the 3-D reconstruction.

“That’s the only limitation I see so far.”

When he talks about the reconstruction technique, Khalid sounds genuinely excited.

By now, Aramco has worked out a standard approach for using it.

“The workflow starts when exploration geologists use regional information about tectonics and stratigraphy to define play fairways,” he explains.

“Then surface grids are constructed from seismic data, preferably 3-D, for prospective structures using commercial structural-modeling software.

“Defining the initial geometries of the structures accurately is extremely important, and this can be aided by understanding of the kinematic of folding.

“Modeling and restoration of multiple cross-sections (1-D restoration) using kinematic balancing techniques (2.5-D restoration) can help to understand the best approaches and boundary conditions for the 3-D restoration.

“The next step is performing 3-D restoration based on the best restoration algorithms. A series of restoration scenarios, capturing the degree of uncertainty of such features as the dip of the master fault, can be preformed and the restoration result (dilatation) can be compared from all scenarios.

“The final step in this workflow involves the integration of strain results with well information, cores, fracture density from formation imaging tools, production test results and seismic attributes such as curvature and coherency to characterize the reservoir properties.”

I’m getting ready to leave for a conference in London.

So I send Khalid one last e-mail to confirm a few details for the story.

“Many of these unconventional fractured reservoirs in the Kingdom were accidentally drilled and discovered, but not initially developed,” he notes.

“Now that we have tools that we can use to structurally characterize the fracture sets in these reservoirs, we plan to incorporate this technology in our exploration and development efforts.

“Fractured, tight and unconventional petroleum reservoirs, including fractured shale-gas reservoirs, represent great resources. Emerging technologies such as this one will make more of these accumulations economic.”

Later, he expands on the last point.

“We have not applied this technique yet to other unconventional plays. Now we are nearly finished from evaluating 3-D restoration softwares within our Aramco exploration environment.

“After that we will start applying this technique in some known similar exploration cases, as well as in tight sand unconventional plays.”

The EXPLORER editor also had gone to Rio.

Now he’s back in Tulsa, working on the next issue of the EXPLORER.

“How’s the Aramco story going?” he asks.

“To be honest,” I tell him, “I’m not sure how to write it.”

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Emphasis: World Development