Natures of Nahr Umr shale instability problems in Zakum field are studied in Geomechanical and chemical views. Literatures, past drilling data, wireline logging and well test data, and analysis of cuttings and cores are studied to define the cause of the problem and find the way to fix it.

By using the record of past drilling troubles, problems are likely in the highly deviated well, and relationship between drilling direction and trouble occurrence is not clear. It has obvious relationship with mud weight and chemicals, however the causality is not clear.

The Geomechanical model was set for the formation from the analyzed data, available literatures and measured data in the recently drilled well. Also, mechanical and chemical characteristics of the shale are examined with cuttings recovered from wells, and cores from past wells.

Mineralogical test are mechanical strength measurements are performed for the samples. Observation and testing of the samples unveil the fact that the shale has severe anisotropic feature of the mechanical strength and physical feature such as permeability.

By integrating the facts and findings, the authors determine the failure mode as bedding plane failure caused by its anisotropic feature and pressure penetration. Counter measures are proposed based on the numerical studies of poro-elastic and discontinuous modeling.


Nahr Umr formation constitutes the cap rock of Thamama formation, the main reservoir of Upper Zakum field. The shale formation spread widely in the southern part of Arabian Gulf, and hole instability problems occurred in the shale is one of the key interests for the oil industry in the Middle East. However, nature of the rock varies place-by-place, and depthby- depth, the troubles we encounter are unique for each field, and we do not expect any single solution to overcome them.

Recent development of Geomechanics and Rock Mechanics approaches for wellbore instability issue gives opportunity to find solutions of such problems. However, the shale instability is complicated phenomenon, and integration of the knowledge and expertise are required. (Fadaq and Tantawi, 2000). In the real situation, available data is limited, nature of the rock failure is far complicated than simplified mechanical models, and many elements are involved in the phenomena. Furthermore, the controllable factors for drilling engineers are limited. Thus, we should carefully watch the nature, and continuously improve the understanding of it.

ZADCO (Zakum Development Company), JODCO (Japan Oil Development Corporation) and JNOC (Japan National Oil Corporation) have jointly studied the nature and counter measures of the shale troubles. In this paper, we show available information about failure modes of the shale, and model the mechanical failures.

Scheme of the study

The scheme of the study is summarized in Fig. 1. Available data for us is past drilling trouble reports (morning reports and documents), rock samples (cores and cuttings), electrical logging, and knowledge and experiences of engineers.

Since a shale section is not an interest of production, rock samples and logging data is limited. Furthermore, critical hole conditions in the shale prevent us to gather new data. In the past two years, we have collected information available in Nahr Umr formation in Upper Zakum field with the cooperation of managements, geologists, and drilling engineers and crews, and looked over them. Chemical and mechanical information of the formation rock are collected.

Geomechanical condition is tentatively set up, and failure mechanism is anticipated. Both of them should be corrected and improved through the implementation of proposed counter measures, and monitoring.

This content is only available via PDF.
You can access this article if you purchase or spend a download.