Nelson Inoue

Analyzing Geomechanical Effects While Drilling Sub Salt Wells Through Numerical Modeling

Santos Basin is one of the most promising basins of Brazil, recently it was discovered light crude oil of 30o API (American Petroleum Institute), this reservoir of high productivity is located below a salt layer of two thousand meters of thickness. Salt also known as an evaporite rock is found in many hydrocarbon basins around the world. Evaporites are sediments formed initially from minerals dissolved in water, the most common are: halite, gypsum, and anhydrite. These minerals are found in areas that passed by a geological time of high evaporation or precipitation. Evaporites in general have the structure of a dome, formed when a thick layer of salt found in the bottom begins to crossover the superior layers vertically, its process delays millions of years. The presence of saline structures takes into favorable conditions for creating a trap for hydrocarbons, increasing the success of probability in oil and gas exploration. In salt drilling the main problem is the closing of the well or known as squeeze or salt pinch, this phenomenon provokes the imprisonment of the drillstring also known as stuck pipe. Evaporite rock behavior is defined through a creep model. The creep model is the term used in the bibliography to define the tendency that a material has to move or to deform permanently to relieve stresses. Strains take place due to extended levels of stress; this stress is less than the rupture stress. In this matter it was chosen a numeric tool to simulate the behavior of salt drilling and the effects that this will have due to the overburden stress and drilling fluid pressure. This tool is Abaqus a program of finite elements in 3D (three dimensions). The objective of this work is to determine the effect of the fluency (creep) in salt during and after the drilling of the well, through numeric simulation. At first it is shown a validation of Abaqus finite element program comparing results from literature and then it is shown a simulation of a 3D vertical salt wellbore.

Answers to Some Questions About the Coupling Between Fluid Flow and Rock Deformation in Oil Reservoirs

In recent years operators have shown great interest in the coupling between the multiphase fluid flow and the rock deformation in oil reservoirs and surrounding rocks. Frequently, the geomechanical effects are approximated in a conventional reservoir simulation through only the rock compressibility. This means that the stresses in the reservoir and surrounding rocks may not be in equilibrium with the pore pressure, since the geomechanical behavior is not considered. The ideal solution for this coupled problem is to introduce the geomechanical effects through the stress analysis solution and to implement a scheme that assures that the governing laws of the flow simulation and stress analysis are obeyed simultaneously in each time step. It was developed one methodology to couple a conventional reservoir simulator (ECLIPSE) and a stress analysis program (Abaqus/CAE) that employs the pseudo-compressibility and the porosity as coupling parameters. It was implemented a C source code that manages all the work flow of the partial coupling in a fully automated manner. Two schemes of partial coupling were implemented: a) an iteratively two way coupled scheme and b) a one way coupled scheme. In this paper, it was examined the hydromechanical interaction results of these coupling schemes evaluating a problem of the soft reservoir and its surrounding stiff rock that was presented in others works. The results of average pore pressure in the reservoir, pore pressure field, subsidence and compaction were calculated. The results of the simulations using pseudo-compressibility as a coupling parameter in an iteratively two way coupled scheme are showed and compared with a fully coupled scheme.

Effects on Time-lapse Seismic of a Hard Rock Layer beneath a Compacting Reservoir

The compacting reservoir embedded in a homogeneous, isotropic and elastic medium, known as Geertsma’s model, is a model commonly used in feasibility studies for forecasting time-lapse changes due to hydrocarbon production. The scope of the thesis is to include a rigid basement to Geertsma’s formulation, and to study how breaking the model symmetry affects the estimated changes in reservoir monitoring. The objective of introducing the rigid basement is to capture a general increase in rock stiffness with depth. In this way, the model narrows the gap between the analytical modelling and the effective deformation of the rocks surrounding the reservoir . The ultimate goal is to get a better estimation of the time-lapse changes expected from reservoir compaction. An analytical solution for the displacement field caused by the compacting reservoir above a rigid basement is derived. The analytical model is introduced in a forward model for forecasting time-lapse changes in seismic monitoring and gravity monitoring. The results obtained in this way are compared with those obtained using Geertsma’s model for the same forward model. The most visible effects of the presence of the rigid basement are the increase of subsidence and the lowering of the top reservoir. Relevance should be also given to the increase of vertical stretching in the overburden and a corresponding stretching decrease in the underburden. In the time-lapse seismic modelling, these effects result in higher time-shifts in the overburden and lower in the underburden. In the time-lapse gravity modelling, the redistribution of the rocks around the compacting reservoir above the rigid basement causes a visible change in gravity, otherwise negligible in the homogeneous half-space. By extending Geertsma’s solution with a rigid basement below the compacting reservoir, the thesis provides a model that can reproduce the geomechanical behaviour of a subsurface where rock stiffness increases with depth. The model needs few parameters, and it can be implemented in a code that uses Geertsma’s solution. A feasibility study that includes the rigid basement model could bring interesting information to be taken into consideration in production management.

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.