Recovering From Spectacular Failure - How In-Depth Root Cause Analysis Led to First Successful HP/HT Microseismic Monitoring in the Sultanate of Oman
Authors: Briner Andreas , Al-Yarubi Saeed , El-Taha Yasin Charles , Moiseenkov Alexey , Al Alqam Amin , El Gihani Mahmoud , Nadezdhin Sergey 
Published: December 2015

In 2013, Petroleum Development Oman (PDO) attempted hydraulic fracture monitoring (HFM) in its first deep, high- pressure/high-temperature (HP/HT) horizontal well. Despite appropriate planning and considerable effort spent on pre-job preparation at both monitoring and treatment wells and modeling that showed HFM to be feasible, several attempts at four fracturing stages failed to deliver useful data. PDO and the service company jointly conducted an in-depth root cause analysis that identified the initial shortcomings, and they performed a successful HFM job in late 2014.

The first deep, HP/HT horizontal well HFM job was part of a technology trial to assess the technical feasibility of microseismic monitoring of events caused by hydraulic fracturing in deep, tight gas reservoirs with static borehole temperatures of approximately 175°C. Although the technology had already delivered some good results for PDO and another operator in the Sultanate in more benign reservoirs, PDO still had no means to gauge the effectiveness of its hydraulic fracture programs for the deep and hot fields that are common in the northern part of its concession. Demonstrating stimulation effectiveness was considered one of the top technology challenges for the company. After the initial failure, therefore, it was not possible to trial the technology once more without further and significant de-risking of all contributing tools and practices.

The joint in-depth root cause analysis eventually led to a realization that significant improvements could be made in the deployment of the HFM technology in this kind of geological setting. The deployment of the geophone shuttles in the monitoring well needed to be deeper than in the first job, which was in direct conflict with the main cause of failure being temperature-induced damage to the shuttles. Also, the spacing of the shuttles needed to be optimized, and the overall effectiveness of the job needed to be increased dramatically to reduce exposure time at the hostile reservoir conditions.

A new type of purpose-maintained geophone was developed that can withstand the high temperatures, even at depths directly juxtaposed to the fracturing activities. It was also found that to maximize signal-to-noise ratio, the tool design and shuttle spacing needs to be revised and that the deployment of the tools, and critically also the orientation and calibration of them, can be shortened significantly when planned meticulously. These and other changes led to the recording of several hundred microseismic events in the most recent HFM deployment and enabled PDO to finally assess and adjust its hydraulic fracturing protocols.

This paper demonstrates the learning curve that the operating and service companies' jointly went through to finally achieve success in the probably world's deepest and hottest microseismic monitoring of hydraulically fractured reservoirs to date.