Effective well planning and accurate wellbore placement are vital tools in reducing survey, seismic, structural or geological uncertainties. There has been an increased development of well planning tools to reduce uncertainties and a growing relationship between the drilling and geosciences disciplines — between the geological construction of a well and the constraints imposed by the physics of drilling.

Today, well targets are digitized directly in 3-D using all available data such as seismic,

Figure 1. Co-visualization of the well trajectory with bottom hole assembly and grid properties. The visible objects of the BHA are the bit and the MWD and LWD tools. The log measurement points are represented by grey circles on the tools. On a vertical fence following the well trajectory the grid property as well as the real-time log curves can be visualized. (All graphics courtesy of Roxar)
horizons, faults and property models. The well path is automatically generated from the surface location to the target respecting predefined constraints. The data are then incorporated into a shared earth model, a single model of the reservoir that incorporates observed and interpreted data and is integrated across the reservoir lifecycle.

A survey program can be specified to evaluate the positional uncertainty of the well path relative to horizons or faults, for example. A driller’s target will then be calculated to see if the geological target can be reached using the planned survey program.

Integrated well planning in 3-D
Constraint-based well planning has also come to the fore. Traditional drilling planning software relied on the user to choose the shape of the well. Today’s well planning software has moved the process closer to the geological model. Yet there is still more data that can be collected while drilling and an industry need for integrated, real-time tools that make drilling operations more efficient.

Today’s modeling applications already allow for real-time access to data on the desktop through Wellsite Information Transfer Standard Markup Language (WITSML) and the updating of the model through workflow management tools. However, when it comes to drilling, there are inevitable delays between the analysis and integration of real-time drilling data and the updating of geological models.

Real-time, integrated software
Geosteering is becoming a key well planning tool. In the Haradh III project in Saudi Arabia, Saudi Aramco describes geosteering as a key enabling technology in the accurate placement of multilateral wells to achieve the desired production rates of 10,000 b/d.

Funded by the Norwegian Research Council and in partnership with operators Norsk Hydro
Figure 2. Warnings are activated when the monitored object crosses the vicinity of the targeted object (Cross-It), when the monitored object leaves the vicinity of the targeted object (Stay Within), and when the monitored object does not stay above (Stay Above) or does not stay below (Stay Below) the targeted object. Different rules apply to different objects.
and India’s national oil and gas company ONGC, Roxar’s goal was to develop a geosteering product able to identify and predict combined geological and drilling events and update the reservoir model during drilling. The geosteering tool would focus on the integration of the real-time drilling data monitoring systems within the company’s reservoir modeling software, IRAP RMS.

Operator benefits would be a better characterization of reservoir entry, an optimization of the wellbore positioning within the reservoir (thereby increasing well production) and the ability to update the structural model while drilling, to automatically reposition the targets and redesign the planned trajectory ahead of the bit.

Geosteering diagnosis
The geosteering diagnosis process is designed to automatically quantify the position of the well under monitoring within the geological model through the automatic and real-time updating and extrapolation of well paths from current positions and targets.

The monitoring consists of gathering different types of real-time information such as survey
Figure 3. The real-time log modeling and reservoir updating component of the pilot focuses on the geological and structural aspects where the real-time log data is co-visualized with the geological model information to validate the model.
data, logging-while-drilling (LWD) data, drilling information and bottomhole assembly (BHA) information for the current drilling phase. All this data can be co-visualized into multiple views in combination with elements of the geological model (Figure 1).

The company’s geosteering diagnosis methodology has:
• An understanding of the position of each of the LWD tool’s sensors behind the bit within the geological model;
• The ability to differentiate between multiple types of objects within the geological model;
• Different methods for calculating distances between the bit or a measurement sensor behind the bit and a defined object belonging to the 3-D geological model;
• Rules to monitor distances from the bit or any measurement sensor behind the bit relative to defined objects within the 3-D geological model; and
• The ability to apply the rules on the already drilled section and on projected sections.
An alarm methodology was also defined based on the distances between objects and on the differences between property values to help the geologist’s decision-making.
The result is that distances between the objects of the bottom hole assembly and geological objects can be calculated and the measurement-while-drilling (MWD)/LWD information can be monitored against the geological model to predict future trends of the data based on the project-ahead trajectory.

Putting the diagnosis to the test
Geosteering diagnosis has been applied (for demonstration purposes only) to the North Sea’s Troll field to monitor drilling against the predefined geological model. The objective was to demonstrate how geosteering diagnosis can help in validating the interpretation while monitoring real-time information. As geosteering diagnosis is integrated into a geological model, the geologist is able to perform updates on the model while drilling and can visualize the status of the diagnosis rules along the drilled and project-ahead sections.

Two lateral wells drilled within the field were used as examples with the real-time
Figure 4. A 3-D view of the well currently under drilling with the log data.
environment simulated by receiving the MWD/LWD data at regular time intervals. The rules are automatically applied anytime new information is received, allowing analysis at the LWD and bit location and prediction ahead of the bit.

The high concentration of wells on the Troll field leads to challenging directional planning. This includes the crossing of existing wells, while staying within the productive reservoir zone. As part of the analysis, the rule methods defined for monitoring trajectories are “Cross It” or “Stay within.” The “Cross It” rule is used to help avoiding drilling too close to surrounding trajectories, whereas the “Stay Within” can be used to keep the well close to its plan trajectory.

Figure 3 shows geosteering diagnosis at 12,647 ft (3,856 m). The first rule is monitoring the oil/water contact (OWC) vertical distance. The second rule is monitoring the shortest distance from the bit to the drilled well A. The third rule is monitoring the resistivity.

The step-by-step process
The geological model is built before drilling, using all available data sources, such as log data from the already drilled wells and seismic data.

During the drilling session, the WITSML link is on and the well can be monitored. Real-time drilled data and MWD and LWD data are loaded automatically into the reservoir modeling software, IRAP RMS.

The user can co-visualize the data with the geological model and model logs can be calculated along the real-time trajectory and compared with the real-time models. Any mismatches between the real-time and model log triggers a need for a model update.
The structural model is updated by changing the horizon and fault position along the well. Workflows can be composed on a gridding part where horizons are recalculated, and the grid updated. Parameters of the grid can be updated as well using petrophysical and facies modeling capabilities within IRAP RMS. In this way, the “modeling while drilling” workflow allows the model to be updated in real time as new data is incorporated.

Figure 4 shows a 3-D view of the well currently under drilling with the log data. The planned trajectory is visible ahead of the drill bit reach and the expected target (semi-transparent grey cube). The log data can also be visible in correlation view (1-D view), in intersection view (vertical 2-D view), and in 2-D view (map).

Reducing uncertainty
A robust, real-time, integrated geosteering software tool will have a major impact on reducing uncertainties in well planning and drilling. A high-resolution geological model around the well bore allows forward modeling of real time log data, enabling wellsite geologists to see what log responses to expect. There will also be links to the target and trajectory planning process, allowing “look ahead” trajectories and targets to be updated in real time.

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