A comprehensive study of Laffan Shale Formation in Sirri oil fields, offshore Iran: Implications for borehole stability

• The process and a workflow for data-acquisition and interpretation in a shale formation were proposed.
• Detailed laboratory investigation complemented with geological and petrophysical studies provided significant information on type and quantity of clay minerals, cation exchange capacity, drilling fluid and pore pressure.
• Laffan Formation is not a swelling but a hard brittle shale formation and after drilling, because of fissility, the rock will split along the planes of weakness and spalls to the well.
• This study suggests that for borehole stability purposes the perpendicular well to the fissility plane is the best well orientation and the NW–SE direction is the best well direction.

This study aims to characterize the Laffan Shale Formation in Sirri oil fields from a borehole stability point of view. This paper describes the process and workflow for data-acquisition and interpretation in a shale formation characterization program and not only demonstrates the benefits of acquiring specific data, but also highlights the uses of the data to aid the exploration decision process. The next purpose of this paper is to provide a research process that can be applied in similar geological settings. In the study process, we collected a complete set of information and samples from the fields and presented a detailed case study, including laboratorial studies of formation samples and interpretation of the information. Available samples and information sources from Laffan Formation include drillhole core samples, daily drilling and geology reports and petrophysical logs of drilled wells and structural geological data of the area. Type and quantity of minerals were determined by direct and indirect methods. Bulk XRD analyses performed on core samples showed presence of traces of clay minerals. For determination of the exact clay mineral type, clay minerals were extracted and treated by heat and ethylene glycol saturation. Treated samples were subjected to XRD analyses. Interpretation of the gamma ray and natural gamma spectrometry logs allowed the determination of the type and content of clay minerals. In a next step, in order to study the distribution of minerals types and fabric of the shale samples, SEM photomicrographs were acquired. Cation exchange capacity of the samples was determined by methylene blue method and hot roll dispersion test in a water based drilling fluid was carried out to experimentally confirm the results. The caliper logs show washouts in the Laffan Formation in most wells. Pore pressure of the Laffan Formation was determined by using data obtained from both acoustic and resistivity logs. From the identification results it can be concluded that Laffan is not a swelling but a hard and brittle shale formation. The instability problem is not due to the dispersion or high pore pressure and cannot be completely solved by drilling fluid design and mechanical causes must be addressed. A noticeable point in this study is the similar results of direct and indirect methods of mineralogy determination. The practical conclusions of the study can be used in the field by engineers.