Impact of Thief Zone Identification and Shut-off on Water Production in the Nimr Field
Authors: Medeiros Richard S. , Biswas Deepankar , Suryanarayana P. V. 
Published: December 2007

This paper describes ongoing efforts to improve the performance of Nimr wells by identifying and shutting off thief zones and summarizes results to date. The presence of a combination of unstable displacement, strong aquifer drive, complex geological settings and non-matrix permeability events has resulted in "scatter-shot" new well performance with progressively higher average initial water cuts. Although not a traditional fractured system, Underbalanced Drilling (UBD) data have shown that rogue fractures and bypass zones play a key role in the movement of water through Nimr reservoirs. The concept is to use UBD to identify "non-matrix" water bearing features (rogue fractures, bypass/thief zones), and shut them off using different shut-off techniques, including a new selective shut-off completion technology called EZIP. It is expected that this will lead to improved water cut behavior, and higher oil production rates and ultimate recoveries from new wells.

A sector model of the reservoir was used to evaluate the impact of the different fracture shut-off techniques. Using the unstructured grid capability of the simulator, rogue fractures intersecting the wellbore are introduced at arbitrary locations, and the impact of shutting them off either along fracture length or along wellbore face is investigated. The simulator model provided a means to quickly study several sensitivities such as dimensions, number and orientation of fractures, initial water saturation, reservoir pressure, etc. The results of the model showed that properly designed fracture shut-off methods could delay water cut by as much as 600 days. Equally importantly, the model showed that shutting the fractures along their length (by cement or gel squeezes, for instance) has almost no beneficial impact on the water cut behavior of the wells during production.

UBD was critical to the development and implementation of the shut-off technology. UBD was used to characterize not only the bypass features, but also their effectiveness and water production potential. Indeed, the authors argue that UBD is the only approach whereby static and dynamic characterization can be achieved while drilling. Based on interpretive analysis of data gathered during UBD, together with petrophysical data, non matrix features and their potential were identified, and used to decide upon the most effective shut-off strategy.

The concepts were tested in a number of wells where bypass features producing water were identified. The first well used abandonment plugs to shut off the whole lower section of the well, while the others have used EZIP technology to shut off the features. Results from the tests have been very encouraging, and confirm the key conclusions of the dynamic modeling. Based on the study and well results, the Nimr Asset team is embarking upon a long term campaign of using UBD to characterize the reservoir and optimally complete the wells to improve water cut behavior in wells.

The paper discusses the modeling, the reservoir characterization enabled by UBD, as well as the results from completed wells. The authors conclude that a combination of fit-for-purpose reservoir modeling, UBD enabled reservoir characterization and appropriate shut-off technologies can lead to significant value creation in fields like Nimr.