The green trajectory was built without earth model knowledge. It is an achievable and acceptable trajectory with minimum dogleg passing through all targets. Bringing this trajectory back into its geological context shows that it is unacceptable — it misses most parts of the reservoir after landing due to fault throw not readily apparent without referencing the model. A revision based on the earth model provides the red trajectory, which increases reservoir exposure dramatically. (Figure courtesy of Baker Hughes INTEQ)

Today’s logging-while-drilling (LWD) measurements permit a greater level of real-time insight into the downhole environment. However, for these data to achieve their maximum value, one must be able to make accurate, real-time decisions based on the measurements. This is especially true in regard to directional drilling and reservoir navigation. Sharing these data between disciplines is a hurdle in this decision-making process

Sharing real-time LWD data using the familiar environment of a geological framework surrounding the well bore — a three-dimensional earth model — is one way to achieve common understanding. Using WITSML (Wellsite Information Transfer Standard Markup Language), the decision-making team, irrespective of their area of expertise or location, can view the well path in its surrounding interpreted environment with a common perspective — increasing understanding and facilitating better decisions.

In order to ensure that the 3-D earth model provides a reliable environment for while-drilling collaboration, it must pass through a series of stages including well planning and reservoir navigation modeling to facilitate good decision-making with the incorporation of while-drilling logs.

Well planning

In the well planning stage, the directional driller is typically provided with a starting location and a series of targets to intersect. From this, a well trajectory is designed, reviewed, and adjusted by a well planning team — an often lengthy process.

The use of the operator’s earth model could reduce the time required while increasing maximum reservoir exposure. Further calculations can be performed such as positional uncertainty, faults and beds proximity, and angle of intersection between geologic features and well to ensure that the well plan is robust and provides maximum upside for minimum risk. By simply allowing the directional driller and the oil company geologist to share their data (an earth model and a well trajectory), a decision upon the best possible trajectory can be reached more quickly. This has been demonstrated through the planning of numerous land and offshore wells.

Pre-well modeling

Once the trajectory has been agreed upon, it is then passed to the reservoir navigation engineer to build pre-well models and “what if” scenarios to be used along with real-time LWD data. Using the same data as the drilling engineer, the reservoir navigation engineer extracts the formation dips from the earth model along the planned trajectory to build a “3-D curtain section” (as it takes into account dip and dip azimuth of the formation beds). This is then populated using offset well data (LWD and wireline) including resistivity, gamma ray, density, and porosity to model the LWD responses expected while drilling the actual well bore. The team can then compare the anticipated responses to the actual, real-time while-drilling logs and navigate through the reservoir accordingly.

Navigation using an earth model

The reservoir navigation engineer monitors the LWD data in real-time for comparison to his modeled values and scenarios and adjusts the forward-modeled curtain section and well trajectory to find the best fit possible between the actual and predicted data. This demands a close working relationship with the directional driller. Of course, all of this must be done with the supervision and agreement of the operator. The conversations at this stage have to be fluent and fast to permit timely decisions based on a shared understanding of the downhole environment and what the real-time LWD measurements are telling them.

Reservoir navigation engineers can rebuild localized earth models in real time to enhance the interpretations that have been made for fault throw, bed thickness, formation dips, or geological deposition environment (i.e., changing bed conformability). Sending updated 3-D earth models to the directional driller and others allows the projected course of the wellbore to be modeled using fully integrated software and ensures wellbore positioning control that meets the reservoir objectives. In updating the model, the reservoir navigation engineer can interpret LWD curve separations, pick dips using image logs, and incorporate azimuthal propagation resistivity. And, when advising the directional driller and others, the reservoir engineer has all these data are available at one glance within the geological framework that can be shared in real time with a common view of the downhole environment.

The 3-D earth model represents the simplest, most accurate rendering to use in gaining consensus and common understanding. The key is updating this local model with real-time LWD measurements without slowing the drilling process. To date, building these models has taken approximately one minute with the model subsequently published using WITSML in real time.

Real-time LWD enhances navigation

On a recent well in the Far East, an operator enhanced reservoir navigation through sharing earth model visualization to precisely position the well bore. The operator’s desired well profile presented several challenges, including landing the well’s 121?4-in. section 9.8 ft (3 m) above the target and then placing and drilling the 81?2-in. lateral 3.3 ft (1 m) below the roof of the reservoir. Using a 3-D model in the well planning process, the directional driller worked with the operator to adjust the landing point to increase the likelihood of success. At the well site, real-time LWD data was used to update the forward model for sharing with the decision makers. As a result, the rig site team identified that the top of the reservoir was coming in a meter deeper than anticipated and quickly adjusted the well path to land the 121?4-in. section precisely 9.8 ft total vertical depth above the pay. Subsequently, the 81?2-in. section was landed just 60 cm below the reservoir roof. The LWD measurements confirmed that the well was in an extremely clean sand, and drilling continued to total depth while keeping 918.6 ft (280 m) of the 1,040-ft (317-m) long lateral within the desired target (less than 3.3 ft or 1 m below the reservoir roof) while maintaining a maximum dogleg of 2.7º/30 m.

Conclusion

Because the local earth models are built from logging data obtained while drilling, they are an extremely accurate tool for making time-critical reservoir navigation decisions.

For this reason the local, real-time models convey a better quality of information than the field model alone and represent a true advantage when it comes to placing a high-angle well in challenging reservoir conditions. These local models are built from the input of a reservoir navigation engineer without the need for additional interpretive workflows, which make them easy to update and share. Using a WITSML standard permits any personnel equipped with viewer technology to observe and collaborate in real time.