Analysis of Geomechanical Properties in Well Design and Well Optimization
Main aspect in wellbore instability is the selection of an appropriate rock failure criterion. This criterion involves only the maximum and minimum principal stresses 1  and 3  and therefore assumes that the intermediate stress 2  has no influence on the rock strength. When Mohr-Coulomb criterion was developed it justified by experimental evidence from conventional triaxial test ( 1  > 3 2    ). Based on triaxal failure machine the Mohr-Coulomb criterion has been extensively used to represent rock failure under the polyaxial stress state ( 1  > 3 2    ). In contrast to the prediction of redistribution Mohr-Coulomb much evidence has been accumulating to suggest that 2  thus indeed have a strengthening effect. This research shown that Mohr-Coulomb failure criterion only represent, that triaxial stress state ( 1  = 1 2 3     or ) which only occasionally encountered in-situ.The linear failure criterion has been justified by experimental evidence from triaxial tests as well as polyaxial test. It is natural extension of the classical Mohr-Coulomb criterion into three dimensions. As the Mohr-Coulomb criterion only represents rock failure under triaxial stress state, it is expected to be conservative in predicting well bore instability. The 3D analytical model employed by MogiCoulomb has been achieved by using linear elasticity theory to calculate the stresses. A model was developed used in analyzing shale intrusion, at depth 6100ft the well bore pressure of the formation is found to be 0.9180psi/ft and decreases to 0.420psi/ft at depth 7300ft. The membrane efficiency of the formation increases from 0.698 to 0.719 at the depth considered above.
The influence of chemical potential on wellbore stability
An experimental investigation was begun to understand fundamental borehole stability mechanisms. This laboratory study was directed toward understanding why and how drilling-fluid chemistry affects borehole stability. The ability of drilling-fluid chemistry to alter shale mechanical behavior and the borehole stress state as a result of shale water content alteration was investigated. Experimental results demonstrate the influence of various drilling-fluid parameters (e. g., salt concentration, salt type, and diesel/water ratio) in oil-based drilling fluids on the water content, compressive strength, and mechanical properties of five shales. The influences of temperature and drilling-fluid exposure time on shale strength alteration also are addressed. The results are explained on the basis of chemical potential differences between oil-based drilling fluid and shale. The change in the shale water content caused by these differences is identified as the predominant factor leading to alteration of shale mechanical behavior and hence borehole stability.
Nonsteady flow through porous media in the presence of a threshold gradient
SummaryIn this paper the threshold gradient effect in the nonsteady flow through a porous media is investigated and its role on the pressure and flow rate distributions in a flow system is evaluated. The system considered in this investigation is a linear oil reservoir. Two situations of practical interest, i.e. reservoir depleted at a constant pressure and at a constant flow rate, are considered. The solutions in a closed form for this flow system are obtained. A comparison between the pressure distributions obtained analytically, numerically and by the integral method is also presented. In order to illustrate the effect that threshold gradient would have on the flow behavior, several numerical examples are given. The results derived from this investigation show that the flow behavior in the presence of a threshold gradient depends strongly on two dimensionless parameters. These parameters are directly related to the threshold gradient, the physical properties of the porous medium and the flow parameters at the outface flow.ZusammenfassungIn dieser Arbeit wird der Einfluß des Schwellgradienten in der instationären Strömung durch ein poröses Medium behandelt und seine Auswirkung auf Druck und Strömungsgeschwindigkeitsverteilung in einem Strömungssystem berechnet. Das in dieser Abhandlung betrachtete System ist ein linearer Ölbehälter. Zwei Fälle von praktischem Interesse, das sind Entleerung des Behälters bei konstantem Druck und bei konstanter Ausflußgeschwindigkeit, werden behandelt. Es werden geschlossene Lösungen für dieses Strömungssystem erhalten. Ein Vergleich zwischen den analytisch, numerisch und aus Integralgleichungen erhaltenen Druckverteilungen wird gebracht. Um den Einfluß zu zeigen, welchen der Schwellgradient auf das Strömungsverhalten haben kann, werden einige numerische Beispiele angeführt. Die aus dieser Untersuchung abgeleiteten Ergebnisse zeigen, daß das Strömungsverhalten unter einem Schwellgradienten stark von zwei dimensionslosen Parametern abhängt. Diese Parameter werden direkt auf den Schwellgradient, die physikalischen Eigenschaften des porösen Mediums und die Strömungsparameter der außenseitigen Strömung bezogen.