This study was conducted as part of research concerned with borehole failures caused by shale materials during reservoir-related drilling activities. The examination of a particular shale of known mineralogical characteristics was completed to develop basic physical relationships between strength, p-wave velocity and moisture content The application of these relationships may lead to the prevention of drilling difficulties and the resultant increases in initial drilling costs associated with drilling through shales. Triaxial and uniaxial compression testing of the Pierre I shale indicates that as moisture levels in the shale increase, the strength decreases polynomially relating to burial depth, while p-wave velocity varies inversely and linearly with moisture content.


Cette etude a ete effectuee dans Ie cadre de recherches sur l'effondrement de trous de forage cause par des meteriaux argileux durant Ie forage de reservoir. L'examen d'une argile particulière, dont les characteristiques mineralogiques sont connues, a ete complète pour etablir les relations physiques de base entre Ie force, la velocite des ondes Pet la teneur en eau. L'application de ces relations pourrait conduire à la prevention des difficultes, reliees au forage dans des materiaux argileux, et des accroissements des coûts initiaux de forage qui en resultent. Des essais de compression triaxiaux et uniaxiaux des argiles Pierre I indiquent que lorsque la teneur en eau d'une argile augmente, la force decroît de facon polynomiale en fonction de la profondeur d'enfouissement, alors que la velocite des on des P varie de facon inverse et lineaire à la teneur en eau.


Diese Forschungsarbeit liefert einen Beitrag zum besseren Verstandnis von Bohrlochinstabilitaten, die durch das Antreffen von Schiefer beim Bohren nach fossilen Brennstoffen verursacht werden. Ein Schiefer bekannter Zusammensetzung wurde untersucht, urn grundlegende Zusammenhange zwischen Festigkeit und DruckweLlenausbreitungsgeschwindigkeit in Abhangigkeit vom Feuchtigkeitsgehalt mathematisch zu beschreiben, Die Beruecksichtigung dieser Zusammenhange kann Bohrschwierigkeiten verrneiden und so die Bohrkosten senken. Die mit dem 'Pierre I' Schiefer durchgefuehrten triaxialen und monoaxialen Drucktests zeigten, daß die Festigkeit als Funktion der Bodentiefe mit Zunehmendem Feuchtigkeitsgehalt abnimmL Die DruckweIJenausbreitungsgeschwindigkeit nimmt linear mit stelgendem Feuchtigkeitsgehalt abo


This study is composed of sections dealing with the particular shale that was tested, the method of control of moisture content, the testing procedure, the results of the moisture content control, the results of the testing in terms of strength and p-wave velocities, and the equations which are proposed that relate the two latter parameters to the measurable moisture content of the shale in question.


The procedure for performing tests required stringent controls on the homogeneity of the shale, and detailed monitoring during all stages of test preparation. The shale of interest during this test program was the Pierre I, with a clay to quartz ratio of 4.7. The Pierre I was chosen as an end member representative of the swelling, gumbo, or smectitic shales which are encountered in borehole drilling situations often resulting in stuck pipe. A similar study is currently being carried out on the Mancos Shale (a quartz-illite shale) which will aid in The Pierre 1 is an Upper Cretaceous marine shale from the Great Plains region of the northwestern United States. It is found relatively near the surface and outcrops in such states as Colorado (Chenevert, McKown et. a1.). Its dark grey colour and visible laminations give it a typical shale appearance and it is used by many researchers (A.H. Hale, F.K. Mody, M.E. Chenevert, C. Deem, R.P. Steiger). This formation spans a large physical region and is relatively uniform in moisture content, mineralogy, grain size, bedding thickness and CEC. The Pierre I shale is not buried too deeply and is fairly easy to recover in large amounts compared to offshore shales. Because of this, and its similarity of mineralogy to many other shales encountered during deep borehole drilling, it is an ideal shale to use as an end-member mineralogically and mechanically.


3.1 Equilibration with OBMs Moisture content alteration was carried out by submersing samples in an oil based mud (OBM) which was constantly circulated to maintain an equal distribution of fluid over the samples. The salinity of the water portion of this mud was used to maintain the Relative Humidity (RH) at a constant level within the sealed chamber. The difference in RH values between the shale samples and the surrounding fluid induced either hydration or dehydration of the samples, and thus, changes in the moisture content of the samples. Processes involved in this equilibration are assumed to be the driving force of concentration gradients, the resistance of pore channels to flow, and the driving force of a humidity gradient.

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