No abstract

A number of reservoirs around the globe are in deep environments, and often it is necessary to drill and cement through salt sections to reach them, such as in offshore Brazil. These reservoirs are mainly in deep water and are commonly referred to as pre-salt zones. It can be very challenging to drill and cement the salt section. The salt zones could be a few hundred meters thick, creep at high rates, and when in contact with drilling fluid and cement slurry, their chemical properties can change. The effect of salt movement or creep is addressed in this work.Salt formations pose a unique challenge to the structural integrity of the wellbore by applying a time varying load on the cement sheath and casing. This load is caused by the existence of deviatoric stress and temperature variations. Deviatoric stress is the difference between formation in-situ stress and the wellbore fluid's hydrostatic stress. When a salt formation creeps during a well operation, it does so to achieve a hydrostatic state such that the deviatoric stress is zero.The present work is a study of cement sheath integrity under downhole thermo-mechanical loading conditions using finite element analysis (FEA). The effect of salt creep on the stresses experienced by the cement sheath at various stages in the life of the well, including drilling, cementing, shut-in, completion, and production, are simulated. A material model for salt creep is validated against experimental data reported in the literature. Multiple cement systems are compared for their effectiveness in providing zonal isolation for the life of the well. The effect of structural and thermal loads due to salt creep on cement are quantified through stresses in cement.The method discussed in this paper should help the industry select cement formulations that can withstand stresses caused by salt movement during the life of the well. Method and case studies pertaining to offshore Brazil are presented and discussed in this paper.

Full-scale laboratory drilling tests investigated the drilling performance potential of a number of different drilling fluids in three analogue rocks for the pre-salt formations offshore Brazil: Bonne Terre dolomite, Carthage limestone, and Navajo Gold travertine. High-pressure drilling tests were performed in each of these rocks using a total of nine different drilling fluids, including a synthetic-based fluid used as reference for performance comparison. There was a consistent ranking of drilling fluids in terms of their penetration rate and mechanical specific energy in all three rocks. Several of the fluids tested showed penetration rates ∼50% higher and mechanical specific energies ∼35% lower than those seen with the baseline synthetic-based fluid. There was no clear correlation between good drilling performance and base fluid, rheology, or suspended solids. There was, however, evidence of a correlation between drilling performance and continuous phase viscosity; drilling performance apparently increased as the continuous phase viscosity decreased. This effect is consistent with a hypothesis that filtrate invasion into the damaged rock can relax so-called chip hold-down forces and thereby reduce their negative impact on the efficiency of the rock destruction process. The results presented here yielded what is believed to be the first evidence that increasing drilling fluid temperature can lead to an increase in penetration rate and a decrease in mechanical specific energy when all other conditions and operating parameters are held constant.

During drilling operations, the wellhead system and top hole casings shall be designed to support dynamic loads from the connected riser through the BOP stack/LMRP. As dynamic motions are associated to stress variations, fatigue becomes a major concern for designers. The accumulation of damage at the wellhead and close regions is dependent on several aspects, such as the riser components, the interactions soil-conductor and conductor-surface casing, and of course the environmental conditions. Consequently, fatigue analysis involves complex numerical models and requires the simulation of a huge number of loading cases. The present paper aims to estimate the fatigue damage at critical components of the top hole casings and at the wellhead. Two different approaches were investigated. In the first, a global model is analyzed in the time domain (TD), and the Rainflow cycle counting method is used to calculate fatigue damage. The global model includes the drilling riser, wellhead, casings, and interactions between components and with soil. In the second, the same model is analyzed in the frequency domain (FD), and the Dirlik method is used to calculate fatigue damage. Additionally, to allow a better evaluation of stresses at complex geometry regions, forces and moments obtained using the TD methodology were combined with load-to-stress transfer functions, defined by means of a local model and symbolic regression (SR) analysis. The local model includes a detailed 3D model of the pressure housings, and soil-to-casing interaction. The obtained results indicate that the pressure housings are not sensitive to fatigue, and also that the analyses performed are feasible, contributing to reduce computational costs in wellhead fatigue assessments.

Percussive air hammer tools have been used for many years to increase drilling ROP (rate of penentration) in air drilling applications. Similar developments for mud hammer tools have not been as successful. The incompressible nature of drilling mud makes the percussive action much slower to actuate using the same design methodology, rendering the tool ineffective. Other developments have suffered from reliability issues which have limited their drilling hours, therefore making them economically unfeasible. A novel percussive mechanically-actuated Hammer Motor, suitable for either mud or air drilling applications, has changed the landscape. This unique hammer assembly is assembled into a standard mud motor, without affecting the bit to bend distance. The percussive action of the tool is designed such that the bit remains in contact with the formation, while the hammering takes place against the top of the drive mandrel, driving the bit into the formation. The percussive impacts serve to greatly increase the effectiveness of the roller cone bit in crushing the rock, thus significantly increasing ROP. This paper illustrates a case study from Brazil, where the Operator has been using turbine motors or conventional motors to drill vertical wells through hard rock formations. The Hammer Motor displayed significantly higher ROP than the benchmark established by the other motors, while also reducing bit costs. These improvements in drilling performance improve the economics of drilling these hard rock formations, and are also applicable to other drilling applications.