Multiwell Borehole Image Interpretation Helps Resolve Structural Uncertainties in North Sea Field | Schlumberger
Case Study
North Sea, Norway, Europe, Offshore

Challenge: Overcome significant structural uncertainties in southern part of the field caused by reservoir complexity and limitations in depth-converted seismic data quality.

Solution:  Engaged Schlumberger to obtain EcoScope high-resolution borehole density images and to interpret 3D surfaces independent of seismic data using eXpandBG near-wellbore to reservoir-scale modeling and Petrel seismic-to-simulation software.

Results: Used precise dip interpretation to reveal a previously undetected unconformity and subseismic faults; achieved better resolution of structure in the gas-bearing zone.

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Multiwell Borehole Image Interpretation Helps Resolve Structural Uncertainties in North Sea Field

Expert interpretation of high-resolution density image data improves GDF SUEZ’s 3D model of the southern Gjøa field

Update the geological model

The Gjøa field in the Norwegian North Sea contains estimated reserves of 82 million bbl of oil and 40 billion m3 of gas. It has been part of the portfolio of GDF SUEZ, a French multinational energy company, since 2003. In recent years, Gjøa has been the largest industrial project in Norway. Developed in cooperation with Statoil, the field began production in November 2010, at which time GDF became operator. Gjøa is expected to produce oil and gas for at least 15 years.

Following an active drilling campaign in the southern part of the field, the company’s geoscientists sought to update the previous operator’s geological model in light of new well data. The initial model was based on depth-converted seismic surfaces and formation thickness or isochore maps. However, reservoir complexity and questionable seismic data quality, possibly caused by the gas effect on seismic velocities, contributed to relatively high uncertainties in the model.

In addition, the model had been built using software that was not integrated with Petrel software, which GDF now uses for reservoir modeling. This made it difficult to populate the model with detailed reservoir properties. The company turned to Schlumberger Data & Consulting Services (DCS) for a more integrated solution.

Generate 3D surfaces from density images

During the drilling of six highly deviated wells in the southern portion of the Gjøa field, Schlumberger used the EcoScope LWD and borehole imaging tool to obtain high-resolution geological information. GDF asked DCS consultants to provide an expert interpretation of these new data.

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EcoScope borehole density images and structural dips (left), and a 3D view of density images from two deviated wells (right). Large circles at right represent dips of key formation tops; small circles are individual bed boundaries. Colored flags along boreholes automatically indicate drilling polarity (red = updip, green = downdip).

Since GDF uses Petrel software, DCS proposed an integrated multiwell study using eXpandBG software. This Petrel plug-in enables density image log and dip interpretation, creates a near-wellbore 3D structural model, and extends it from borehole scale to reservoir or field scale. The result is a high-resolution model in depth, completely independent of the velocity model and depth-converted seismic.

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Validation of model based on density image interpretation (right) by comparison with depth-migrated borehole seismic (left). Colored lines represent the fault model.

DCS specialists processed, quality-controlled, and interpreted EcoScope density image data, performed structural correlation using  a true stratigraphic thickness display, and generated 3D surfaces. They validated those surfaces using two methods: correlation  with tops in a vertical exploration well, and comparison with a seismicVISION* depth-migrated borehole seismic survey. DCS geologists worked with the GDF SUEZ team to build the fault model.

Overcome limitations of existing interpretation

Expert analysis of high-resolution density images yielded a more precise and consistent dip interpretation on the scale of 10 × 10-m cells, which revealed previously undetected features including possible subseismic faults and a potential unconformity at the top of the reservoir. Borehole imaging also achieved more accurate resolution of the structure. As it turned out, the existing seismic interpretation had underestimated the depth of the reservoir, especially in the gas-bearing zone.

Use of Petrel technology for both near-wellbore and field-scale model-ing facilitated data transfer and collaboration between the GDF SUEZ team and Schlumberger. Also, the use of two different techniques to validate the 3D surfaces generated from borehole image data gave GDF SUEZ greater confidence in the structural model. As a result,  the company ordered two additional well interpretations in 2011.

GDF geoscientists can populate the improved Petrel model with lithofacies and petrophysical properties, enabling them to better anticipate formation depths in new wells, and use the model as  a local grid for wellbore stability analysis, among other uses.

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A 2D structural model (section) and 3D model (inset) based on density image log interpretation. The 2D model tied with vertical well tops shows how surface seismic underestimated depths within the high- resistivity (gas-bearing) zone, while matching the well in the low-resistivity (water-bearing) zone downdip.
“Our input from seismic depth-converted surfaces was limited. Therefore, our model had to rely mostly on isochore maps from well data. In this case, any improvement in data input had value for us. Schlumberger tools provided high-quality dip analysis and improved our confidence in the structural interpretation.”

Wouter Hazebelt, Senior Reservoir Geologist GDF SUEZ E&P Norway

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