Shale is often the most difficult of all formations to maintain a stable wellbore in when drillincr ::> for oil and gas. Time and money spent overcoming this problem during drilling, together with overall reduced profit margins. has led the oil industry to devote considerable time and effort to solve the problem of unstable boreholes in shales. It has long been established that the moisture adsorption (or desorption) of shale rocks can be controlled by the salinity of drilling fluid. When compacted shale (under constant compaction stress) adsorbs moisture, its total volume increases and swelling strains develop. Developed swelling strains then become an integral part of the effective radial stress acting on the shale formation contributing to borehole failure. A mathematical model has been developed for predicting the swelling behaviour of shale when placed in contact with water under moderate pressures and the effect of the swelling on borehole (in)stability. The model is based on thermodynamic theory which suggests that fluid movement into or out of a shale is driven by an imbalance in the partial molar free energy of the shale and the contacting fluid. Conversion of the free energy of each system (fluid and shale) into "total swelling pressure" made it possible to model transient pressures and strains generated in shale. The analytical solution of the radial diffusivity equation is reduced to a simpler form for the model. The model was validated using equipment and experimental techniques which allow continuous monitoring of shale swelling as function of time and distance from the wetting end. It was found that increasing the compaction stress acting on the shale reduced the rate of swelling, and increasing the hydraulic pressure of the fluid on the shale's wetted surface increased the rate of swelling. This behaviour was adequately described by the model which therefore represents a new method for predicting shale swelling as function of time and radial distance under different environments. Swelling strains are then used to predict related changes in shale mechanical properties (failure criteria) and well (in)stability. Several well-site index tests have been developed to study shale-drilling fluid interaction at wellsite. These index tests can provide input data for the mathematical model. Drilling fluids can be screened for their ability to control shale swelling, thus minimising the risk of well bore instability.

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Originally presented as author's Thesis (licentiate)--Chalmers tekniska ho̊gskola, 1997. / Includes bibliographical references.

Originally presented as author's Thesis (licentiate)--Chalmers tekniska ho̊gskola, 1997. / Includes bibliographical references.

Thesis (M.S.)--University of Wisconsin--Madison, 1980. / Typescript. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 138-151).

M.Sc. (Geology) / The purpose of this work was to attempt to understand the behaviour of the ground around a series of holes bored in gold bearing rock by a reef boring machine. Furthermore this ground behaviour is compared with existing theoretical concepts of rock behaviour. The experimental boring site and the sequence of events at that site are described. The state of the rock surrounding the excavations is observed to deteriorate with increase in mining. The fracturing observed is characterised by spalling on the sides of the bored holes. This spalling increases in severity with closeness to the bored out region. Calculations of the stress acting on the sides of the bored holes were made from elastic theory using two techniques. One of these was a quick graphical technique and the other was a more complex analytical one. The analytical technique was used as a check on the results of the graphical technique. Finally the extent of spalling on the sides of the holes is compared with the theoretical stresses. A close correlation is found, and spalling appears to start when the stress level reaches the strength of the rock. As the stress level increases spalling increases in severity until such time as the ground around the bored area is fractured by large through going fractures. At this stage holes bored in this ground no longer spall.

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