Djurre Hans Zijsling

Hyperbaric Rock Cutting

Under atmospheric conditions rock cutting is mostly brittle; either based on tensile cracks or based on shear cracks, although ductile cutting is possible. It is known from the drilling industry that under very high hydrostatic pressures often a sort of ductile cutting process occurs. Cutting under very high hydrostatic pressures is called hyperbaric cutting. Combining the cutting theory of water saturated sand of Miedema and the classic rock cutting theory of Merchant, the 2M theory, results in a new application of the theory for hyperbaric rock cutting. The new theory contains the pore pressure and the cohesive terms. To verify this theory the measurements of Zijsling (1987) are used, which go up to pressures of 1000 bar. A complication is that the chips cut will start to curl, but the contact length of the chip cut with the blade is unknown. Using the equilibrium of moment’s, results in a method for determining this contact length. In the specific case a contact length of about 4-5 times the layer thickness is found, resulting in an almost perfect match with the measurements. The paper shows the measurements and the theory. INTRODUCTION Rocks are collections of minerals where the grains are bonded chemically from very stiff clay, sandstone to very hard basalt. It is difficult to give one definition of rock or stone and also the composition of the material can differ strongly. Still it is interesting to see if the model used for sand and clay, which is based on the Coulomb model, can be used for rock as well. Typical parameters for rock are the compressive strength UCS and the tensile strength BTS and specifically the ratio between those two, which is a measure of the brittleness. Rock also has shear strength and because it consists of bonded grains it will have an internal friction angle and an external friction angle. It can be assumed that the permeability of the rock is very low, so initially the pore pressures do no play a role or cavitation will always occur under atmospheric conditions. But since the absolute hydrostatic pressure, which would result in a cavitation under pressure of the same magnitude can be neglected with respect to the compressive strength of the rock; the pore pressures are usually neglected. This results in a material where gravity, inertia, pore pressures and adhesion can be neglected. Merchant (1944), (1945A) and (1945B) derived a model for determining the cutting forces when machining steel. The model was based on plastic deformation and a continuous chip formation (ductile cutting). The model included internal and external friction and shear strength, but no adhesion, gravity, inertia and pore pressures. Later Miedema (1987) extended this model with adhesion, gravity, inertial forces and pore water pressures. Figure 1: The definitions of the cutting process. Figure 1 gives some definitions regarding the cutting process. The line A-B is considered to be the shear plane, while the line A-C is the contact area between the blade and the soil. The blade angle is named α and the shear angle β. The blade is moving from left to right with a cutting velocity vc. The thickness of the layer cut is hi and the vertical height of the blade hb. The horizontal force on the blade Fh is positive from right to left always opposite to the direction of the cutting velocity vc. The vertical force on the blade Fv is positive

Lubricants and Accelerated Test Methods for Expandable Tubular Application

Tubular expansion for well construction is a process accomplished by pulling (or pushing) a mandrel through a pipe downhole. This can be described as a tribological system that consists of the pipe and mandrel surface, a lubricant layer and a fluid. A successful expansion requires a lubricant that, not only ensures manageable expansion forces, but also provides a robust solution for downhole application. The desired lubricant should separate the mandrel from the pipe during a downhole plastic deformation and be thermally stable, chemically compatible with drilling fluids and withstand a long shelf life in harsh environments. This paper describes a process for optimal lubricant selection using accelerated testing with a dedicated setup that mimics demanding down hole conditions during expansion processes. Three commercially available lubricants, commonly used in tubular expansion or other similarly demanding applications, and a novel solid lubricant, developed specifically for tubular expansion applications, were tested. The test campaign focused on the influence of drilling fluids and temperature on the friction, wear and lubrication efficiency. The novel solid lubricant showed superior performance compared to conventional lubricants demonstrating lower friction factor with expansion forces between 13% and 49% lower than with conventional expandable tubular lubricants, outstanding resilience in coping with alien particles in the drilling fluid, ability to mitigate the onset of stick-slip phenomena and good corrosion protection for the expandable tubular. It was therefore selected and successfully used in field-trial applications.