When drilling wells through shales, the rock mechanics and physico-chemical properties of shales are critical to design the trajectory of wells, mud weight as well the mud chemical composition. In spite of their large occurrence, shales are also among the lesser known rocks. Laboratory campaign is essential in evaluation of potential borehole stability problems, been crucial to build and calibrate geomechanics models. This paper presents a laboratory investigation to characterize the mechanical and physical-chemical properties of a Norwegian North Sea Shale. The shale has been characterized and a wide range of petrographical, mechanical and physico-chemical properties were evaluated. An integrated characterization of the shale is presented. Procedures for sample handling, testing and interpretation of the laboratory campaign have been described and presented. The main purpose of this paper is to present a developed methodology and interpretation of the Norwegian North Sea results.
Wellbore instability problems issues while drilling through shales can lead to a high lost productive time and increase the drilling and completion costs, estimated conservatively to the industry around 500 million USD/year [1]. The petroleum industry is developing a considerable effort to the understanding of the mechanisms causing lost of productivity and wellbore instability problems. Some of these problems are directly related to the interaction between drilling fluids and shales, affected by many primary factors as: swelling behavior, rock strength, pore size, grain size distribution, porosity and permeability, rock thermal properties, pore water chemistry, native formation pore pressure, formation temperature, rock lithology and in-situ stress state. All of these factors are related to the formation properties and are considered as uncontrolled factors affecting wellbore stability. Broch [2] concluded that chemical effects can affects the rock strength of 30–100% due to deterioration of the cement. Risnes et al., [3] have shown that the chemical activity of the pore water have an impact on the strength of chalk. Shales constitute nearly 70% of the rocks present in a sedimentary basin, been responsible for approximately 90% of wellbore instability problems related to the drilling operations. The basic explanation for such a behavior is the rock mechanics properties, as it low stiffness, low rock strength, low permeability and its high reactivity potential when in contact with waterbased fluids. Some of these properties are directly affected by clay minerals constitution and tenor presented. Shales can swell and shrink when in contact with water, and generally tend to have low friction angles and can exhibit significant strength anisotropy due to weak bedding planes. To the better understanding of its interaction, it is become necessary a special characterization that must include additional tests to supply special effects that occur during the interaction through the mechanical, thermal, electric and hydraulic gradient between the drilling fluids and the shale. The electric nature of the mineral surface and its elevated moisture content, which result in exchangeable cations and ions adsorption or repulsion, can create an intensive 460 flux of ions and water when the chemical gradient between the shale and drilling fluids is intensive.
