2_RockFailureMechanisms.ppt.pptVIP

The elastic limit and ultimate strength of the formation are the most important formation properties affecting ROP. The shear strength predicted by Mohr failure criteria sometimes is used to characterize the strength of the formation. To determine the shear strength from a single compression test, an average angle of internal friction of 350 was assumed. The angle of internal friction varies from about 30 – 400 form most rocks The mineral composition of the rock also has some effect on ROP. Rocks containing hard, abrasive minerals can cause rapid dulling of the bit teeth. Rocks containing gummy clay minerals can cause the bit to ball up and drill in a very inefficient manner. Factors Affecting Penetration Rate Effecting of Formation Characteristics The permeability of the formation also has a significant effect on the ROP. In permeable rocks, the drilling fluid filtrate can move into the rock ahead of the bit and equalize the pressure differential acting on the chips formed beneath each tooth. This would tend to promote the more explosive elastic mode of crater formation. it also can be argued that the nature of the fluids contained in the pore spaces of the rock also affects this mechanism since more filtrate volume would be required to equalize the pressure in rock containing gas than in a rock containing liquid. Factors Affecting Penetration Rate Effecting of Formation Characteristics The properties of drilling fluid reported to affect the ROP include: density, rheological flow properties, filtration characteristics, solids content and size distribution, and chemical composition. Penetration rate tends to decrease with increasing fluid density, viscosity and solids content, and tends to increase with increasing filtration rate. The density, solid, and filtration characteristics of the mud control the pressure differential across the zone of crushed rock beneath the bit. The fluid viscosity controls the system frictional losses in the drillstring and thus the h