Methodology Implemented in Assessing and Monitoring Hole Stability Concerns in Openhole Horizontal Wellbores in Carbonate Reservoirs

Abstract

Abstract Hole stability concerns are generally relevant to sandstone reservoirs, especially those characterized as being incompetent in terms of their rock mechanical properties. In general, carbonate rocks of the Middle East are competent with the vast majority of the wells completed openhole. Recent field development of a carbonate reservoir in the Middle East highlighted the weak nature of the reservoir rock in terms of its mechanical properties. A rock mechanics-testing program initiated on core samples revealed that the reservoir rock is relatively weak with the majority of the rocks tested having unconfined compressive strengths less than 2,000 psi. Hole stability concerns surfaced in two of the initial development vertical wells reflected by a stuck logging tool in one and a tight hole in the other. A comprehensive production-testing program was conducted to assess the extent of hole instability during shut-in and flowing periods, and to calibrate the rock mechanical properties. Four horizontal test wells were selected, and each tested by first running 4-arm caliper logs, followed by a 14-day flow period at maximum possible rates, and finally running production and caliper logs at the end of the test. Caliper logs from these tests established the baseline data for hole size changes and provided insights on the effect of production on wellbore stability. A monitoring program was initiated to monitor the hole stability before and after field start-up. Prior to field startup and during the shut-in phase, two caliper campaigns were conducted on the test wells. In addition, state-of-the-art sonic logs were implemented in several vertical and horizontal wells to evaluate the field stresses and provide insights on wellbore stability. Results of the shut-in monitoring campaign indicated that there was no immediate hole collapse. However, it was observed that hole size changed with time, with more significant changes occurring in wells characterized as having weak facies. Post-field start-up monitoring campaign was executed six months after field start-up. Meanwhile, experiments will be conducted in the lab to assess hole stability by predicting pore collapse through acoustic velocity measurements.