Putting the Reservoir to Test: An Innovative Multi-Zone Drill Stem Test Method Revealed Critical Reservoir Characteristics, Horizontal and Vertical Connectivity for Field Development Decisions and Reserve Calculations
Authors: Al-Fawwaz Abdullah A. , Al-Obaid Rashid H. , Sarac Sukru , Kamas Ibrahim 
Published: September 2021

Information from dual-zone drill-stem tests (DSTs) and vertical interference tests, especially between oil and water zones, is very valuable for reservoir characterization under dynamic conditions. Despite the critical information that can be gathered for field development decisions, it is also an uncommon test type at a DST scale because of the advanced downhole test string design and operational complexity.

This paper describes the objective-driven test design for a multi-layered carbonate reservoir and how the test sequence was modified in real-time to increase the value of information. An unconventional multi-zone well test with flexible on-demand control functionality was executed to resolve vital reservoir uncertainties for field development, including vertical interference between zones, individual zone characteristics and to confirm reservoir boundaries.

Reservoir simulations were performed to decide the test sequence to create interference between different reservoir layers, while considering other test objectives, simultaneously. A DST string with wirelessly-activated downhole tools and flexible control options allowed to modify the test program on the fly, based on real-time data analysis and performance of each zone. One of the zones was also tested and characterized without flowing fluids to surface by making use of real-time downhole data.

This complex, unorthodox well test operation was chosen to resolve multiple reservoir uncertainties in a single DST run, which would have normally required multiple different DSTs. Operations included flowing different zones individually, including acidizing operations, and the sequence was monitored and modified on the fly to maximize the value of data. Vertical interference between oil and water zones were also monitored to determine test conditions. Reservoir simulation studies helped choose the optimum test sequence to create the required interference by avoiding any disturbance in the interference data from other pressure transients during the test. During the test, downhole and surface testing data was transmitted to town in real-time and analyzed by the petro-technical team to guide the operational procedures. Downhole equipment was controlled wirelessly by acoustic commands to change the test design, depending on the data analysis.

This thick carbonate formation was tested pre- and post-acid, to evaluate stimulation efficiency. Also, by testing the well at different flowrate and pressure conditions, information regarding future production well behavior was obtained. A low-pressure water zone was also tested without bringing fluids to surface, using a type of closed chamber test which was made possible by analyzing real-time downhole data from different depths.

Multiple crucial objectives for field development decisions were planned and successfully obtained using an engineered DST string design. The confirmed reliability of complex DST string and wireless telemetry in difficult environment enabled well test efficiency gains to solve multiple challenging dynamic reservoir evaluation problems, simultaneously. These novel solutions bring new types of data and information to the exploration and appraisal teams to answer connectivity questions in a cost and time effective manner.