Sergio Augusto Barreto da Fontoura

Lade and Modified Lade 3D Rock Strength Criteria

List of Symbols a, m, g1 Parameters of original Lade formulation c Cohesion C0 Uniaxial compressive strength I0 1 First invariant of the effective stress tensor I0 3 Third invariant of the effective stress tensor I00 1 Modified first invariant of effective stresses tensor in modified Lade I00 3 Modified third invariant of effective stresses tensor in modified Lade pa Atmospheric pressure Pp Pore fluid pressure Sa, g Parameters of modified Lade formulation T0 Uniaxial tensile strength a Biot’s parameter / Angle of internal friction r1 Major principal effective stress r2 Intermediate principal effective stress r3 Minor principal effective stress

Numerical modeling of borehole pressurization

Abstract This paper presents a method for obtaining the complete pressure versus time curve generated during pressurization of a wellbore considering the pumping rate as input. The rock formation is treated as a poroelasto-plastic material and the pressurization fluid is considered to have the same properties of the rock pore fluid. A fracture initiation criterion (maximum tensile stress) and a non linear fracture propagation model are used in order to handle the wellbore post failure behavior. The finite element method was used to formulate the solution for this coupled hydromechanical problem. It is shown that fracture energy, pumping rate and fluid viscosity affect both the maximum pressure on the borehole wall and the post failure pressure versus time curve. Result of a pressurization test carried out in a small diameter borehole is compared to the one predicted by the computer program and there is very good agreement. The paper investigates the fracture closure process upon shut in and shows the relationship between wellbore pressure and fracture closing pressure.

Rock Cutting Analysis Employing Finite and Discrete Element Methods

The petroleum industry has shown great interest in the study of drilling optimization on pre-salt formations given the low rates of penetration observed so far. Rate of penetration is the key to economically drill the pre-salt carbonate rock. This work presents the results of numerical modeling through finite element method and discrete element method for single cutter drilling in carbonate samples. The work is relevant to understand the mechanics of drill bit-rock interaction while drilling deep wells and the results were validated with experimental data raised under simulated downhole conditions. The numerical models were carried out under different geometrical configurations, varying the cutter chamfer size and back-rake angles. The forces generated on the cutter are translated into mechanical specific energy as this parameter is often used to measure drilling efficiency. Results indicate that the chamfer size does not change significantly the mechanical specific energy values, although the cutter aggressiveness is influenced by this geometrical characteristic. Results also show there is a significant increase in drilling resistance for larger values of back-rake angle.

Characterization of Carbonate Rocks through X‐Ray Microtomography

Image characterization of rocks is generally carried out by optical and scanning electron microscopy techniques. In this paper, the non-destructive technique of X-Ray micro tomography (μCT) was used in order to obtain microstructural parameters. The experimental program consisted in obtaining these properties from three different carbonate rocks: the Roman, Turkish and Itaborai Travertines. The rocks were selected since they possess similar properties to the Brazilian oil-bearing pre-salt rocks. The results have shown that µCT is very useful in a) determining the porosity and pore size distribution, as compared to traditional porosimetry methods, b) establishing the pore arrangement within the rock matrix and c) detecting pore connectivity. Additionally, the generation of 3-D models allowed a faithful representation of the samples and allowed the selection of regions for thin section preparation of their most representative features, which were then analyzed by Digital Optical Microscopy.