Directional/complex wells

Horizontal-Well Correlation in Geosteering Complex Reservoirs of Saudi Arabia

A new workflow technology has been launched that uses real-time logging-while-drilling (LWD) data and a new horizontal well-correlation technique to dynamically update a structural framework that can be used to geosteer an active drilling well.

A new workflow technology has been launched that uses real-time logging-while-drilling (LWD) data and a new horizontal well-correlation technique to dynamically update a structural framework that can be used to geosteer an active drilling well. The benefits of this new approach include faster well-log interpretation and the ability to dynamically and more-accurately update the subsurface model during drilling to efficiently steer the wellbore into the most productive parts of conventional and unconventional reservoirs.

Introduction

With increased competition in the energy market, oil companies face the constant challenge of improving accuracy while reducing cycle time. This goal has driven the advancement of technologies in horizontal drilling, real-time monitoring, and geological interpretation significantly during the last few years.

Previous well-correlation workflows used by Saudi Aramco were typically limited to applying the vertical-well approach even when drilling horizontal wells. This approach involved transforming the well logs from a measured-depth domain to a true-vertical-depth (TVD) domain and introducing some degree of uncertainty. However, accurate correlations using TVD can be obtained only in vertical wells or wells drilled through beds with no structural dip. TVD well correlations become meaningless when deviated wells penetrate dipping beds. This approach poses another problem in that the subsurface layers and the structural-framework model are not dynamically updated while drilling, which may negatively affect the accuracy of well planning, interpretation, and geosteering.

Additionally, the percentage of highly deviated and long horizontal wells in Saudi Aramco’s field portfolio has increased significantly in order to maximize reservoir contact for optimal development. Therefore, the shortcomings of the traditional well-correlation technique have become a major issue. Improvement of technology and tools for making accurate log correlations while geosteering to keep the well in the production zone on a real-time basis is not only critical but also essential.

The technology and methodology demonstrated in this paper enable real-time horizontal-well correlation, greatly enhancing the geologist’s ability to steer the well placement accurately and update the subsurface layers dynamically. This workflow has been applied successfully in several Saudi Aramco fields, including the largest onshore oil field in the world. By use of this technique, many long-reach horizontal wells achieving maximum reservoir contact have been drilled. Not only has accuracy been enhanced, but also the turn-around time has been reduced by an order of magnitude.

Technical Overview

Methodology. This advanced methodology works by first creating the structural-framework model with conformable geological surfaces, then predicting the log curve using offset wells. The predicted log is then correlated manually to the LWD log by interactive stretching and squeezing using true stratigraphic thickness (TST).

Fig. 1 describes a proposed workflow using TST-type logs from nearby wells, the reference framework surface, and the LWD curve to create a predicted log curve. Anchors can be set upon the predicted log curve to stretch and squeeze until a fit is attained with the LWD curve. Each anchor point creates an interwell point that can be used to create a structural-framework surface. This methodology will improve the time cycle for well-log interpretation workflows and subsurface mapping while drilling. Also, it will update the geological structural surfaces dynamically to allow geologists to look ahead and steer the well in the production and reservoir layers.

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Fig. 1—Horizontal well-correlation workflow and geosteering solution.

Mapping-Conformance Technique. This workflow starts with construction of a dynamic-framework model to facilitate the multisurface structural-mapping and dynamic-modeling processes. The dynamic-framework model combines traditional mapping, fault networking, conformance mapping, unconformity trimming, and interval modeling into one interpretation process. The conformance technique assumes that the rate of change in top-to-top thickness between two horizons is relatively low compared with structural variation.

The conformance tool improves the geological consistency of modeled surfaces in areas where data may be sparse or noisy. It is used to estimate a series of structural surfaces that are nearly parallel with each other, suggesting a conformable succession, in areas of little well control. The conformance tool works by first taking well-top pairs on wells where both tops and base picks exist, and then subtracting them to create isochore values. The resultant isochore map is then automatically added to the constraining surface to predict where the deeper surface would be in the absence of well control.

Well-Log Prediction. Predicted-log generation requires an intial structural framework created in the mapping-conformance technique, offset wells that penetrate the formations, selection of the log to be modeled, and a reference surface (which usually defines the top of the reservoir to be drilled). The predicted log for the drilling well is created by using the log from the offset well(s) and calculating the TST from the reference surface to depth along the offset well projected to the drilling-well path. The difference between predicted log and the LWD log signals a change in geology and indicates that the initial structural model is in need of updating.

Horizontal-Well-Correlation Workflow. This workflow allows geoscientists to correlate horizontal-well logs to logs from nearby offset wells during drilling. The update to framework layers is achieved through interactive correlation of the predicted log with the LWD log. Correlation of the predicted curve with the LWD curve involves editing of the predicted log by introducing anchor lines, a step that also establishes the corresponding interwell points on the modeled surface. Correlations between the predicted and LWD curves are performed by stretching or squeezing the predicted log until a match with the LWD curve is achieved. As the anchor lines are set and moved to obtain the best fit between curves, the corresponding interwell points also shift to maintain the TST between the modeled surface and the active drilling well. Once the changes are committed, the modeled surface ties to the new anchor points, and the changes are propagated to all subsequent layers per conformance rules defined in the model.

While points are established on the modeled surface, large discrepencies may be indicative of drilling through a fault that can be incorporated into the model. Additional options include the ability to select multiple offset wells that best correspond to the path of the drilling well and calculate multiple predicted curves.

Case Study

In the case study (see complete paper for more details), the authors demonstrated how wells with long horizontal sections of reservoir contact are correlated in a -real-time geosteering environment to enhance the accuracy and reduce the turn-around of updates to a structural model. The Geosteering Center at Saudi Aramco monitors tens of horizontal wells on average per day. Most of these wells are extended-reach horizontal wells in complex reservoir settings, which require careful predrill planning to place the wells in the reservoir accurately, because the changes in structural dip and stratigraphy are imminent. Reducing uncertainty in this process is a major challenge. Despite using all available data, such as seismic interpretation (which provides good structural control between the wells), maps, and cross sections based on offset wells, the process is still prone to errors because of limited seismic resolution, errors in velocity models, and the presence of structural dips and stratigraphic changes.

This horizontal-well-correlation workflow allows update of a dynamic-framework structural model, taking into account changes in the dip of the beds, and helps geosteerers update the well trajectory in a timely manner to maintain the well in good-quality reservoir rock.

There is no restriction on the number of grids in the initial structural model, the number of well-log curves that can be predicted, or the number of type wells that can be used to correlate along the trajectory of the drilling well. In the case study, the initial structural model consisted of more than 50 grids. For simplicity, only one type well with a gamma ray curve and six surfaces is used to explain the workflow. Fig. 2 represents a structural cross section showing the active drilling well and the offset well in 3D.

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Fig. 2—3D visualization showing active drilling and offset wells.

Conclusion

The use of real-time data and log prediction in horizontal-well correlation is of great value for geosteering horizontal wells and real-time Earth modeling while drilling. This new approach improves geosteering accuracy in complex and thin reservoirs by allowing for a dynamic subsurface model based on selected offset-well data, real-time logs, and available seismic data, thus improving the operational efficiency of horizontal-drilling programs.

The method allows for faster well-log interpretation, more-accurate subsurface models, and the ability to update the subsurface model dynamically during drilling to steer the wellbore into the most productive zones of the reservoir more efficiently. The turnaround time to drill a horizontal well is reduced by an order of magnitude by use of this technique. This method can simultaneously account for changes in the borehole in terms of the inclination, azimuth, structural dips, and seismic backdrop, and provides a forward view for anticipating faults, enabling more-timely informed decisions.

The use of TST in predicting the log curve from offset wells and correlation with the LWD curve means that the true location of the beds in 3D space and current location of the drill bit can be represented more correctly. This makes it easier to predict the path of the wellbore along its current trajectory and thus helps in minimizing uncertainties and maximizing well-path contact with the best reservoir sections. Unlike more-traditional vertical-correlation practices, this horizontal-correlation workflow can help eliminate erroneous assumptions while drilling horizontal wells. The application of this technology at several fields in Saudi Arabia has proved its validity in steering wells in thin and complex reservoirs.

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 164151, “Horizontal-Well Correlation Using Real-Time Data and Log Prediction in Geosteering Complex Reservoirs of Saudi Arabia,” by Abdul Mohsen A. Al-Maskeen, Roger R. Sung, and Sadaqat S. Ali, Saudi Aramco, prepared for the 2013 SPE Middle East Oil and Gas Show and Conference, Manama, Bahrain, 10–13 March. The paper has not been peer reviewed.