Drilling-Induced Fractures in Borehole Walls

Abstract

Summary Drilling-induced fractures in borehole walls are investigated by ring tests, flow tests, and microscopic studies. Each drilling method producescharacteristic fracture patterns in the hole wall. Weaker rock drilled withlarge drill bits shows more intense damage than stronger rock drilled withsmall bits. The fracture zones, usually less than 5% of the hole radius, havean insignificant effect on the rock permeability around borehole plugs. Introduction Knowledge of the hydrological and mechanical characteristics of rock nearboreholes is important for (1) determining the deformation and stability of aborehole; (2) calculating fluid flow into, out of, or parallel to holes; (3)evaluating the seating performance and axial strength of borehole plugs andperformance and axial strength of borehole plugs and of casing cement; and (4)determining any mechanical interactions at the borehole wall. Rock adjacent toa deep borehole usually is damaged by (1) the drill bit, (2) stress relief ofthe rock, (3) pressure and invasion of the drilling fluid, and (4) the scouringaction of rock fragments brought up from the bottom under high velocity andpressure. The complex influence of these factors makes analysis of the behaviorof the rock around boreholes difficult. Borehole instability can take severalforms. Hole size can he reduced when plastic or time-dependent flow squeezesrock plastic or time-dependent flow squeezes rock into the hole. Holeenlargement can he caused by caving of shales or by hard-rock spalling. Identification and analysis of potential instabilities require informationpotential instabilities require information about the conditions of the rockaround the borehole immediately after drilling. Borehole wall fractures inducedby drilling may be small. However, the rock around deep boreholes usually issubjected to a high stress gradient with the maximum tangential stress near theborehole wall. Deformation and failure of a borehole are therefore affected bythe presence of the damaged zone. The presence of a damaged zone also mayexplain why the tangential compressive stress required to induce boreholebreakouts is higher than the compressive strength of the rock obtained fromuniaxial and triaxial compression tests. For borehole sealing or cementing, microfractures induced by drilling could increase the annular zone permeabilityand thereby reduce the ability to permeability and thereby reduce the abilityto seal it. The microfractures could reduce the stiffness of the rock aroundthe borehole plug and may lead to a reduction of the mechanical interaction atthe plug/rock interface and a decrease of the axial strength of the plugs. Theobjective of the research reported here is to identity characteristics ofdrilling-induced fractures in the walls of different sized holes drilled bythree methods. This investigation concentrates on the influence of theinteraction between drill bit and rock, isolated from other factors likely tobe present in deep-hole drilling, such as stress present in deep-hole drilling, such as stress relief, high pressure and velocity of drilling fluids, and highthermal gradients. This approach allows a detailed investigation of suchparameters as bit type, bit size, energy input, and Petrographical andmechanical properties of the rock. properties of the rock. Methods of Investigation The experimental assessment determines the fractures in hole walls producedby diamond coring, percussion, and rotary drilling at low confining pressure. Hole diameters range from 38 to 102mm. Three main lines of investigation areused:an indirect damage investigation by identifying the change in tensilestrengths of the rock directly around the borehole walls by means of tensiontests,investigation of damage-zone permeability by flow testing, andavisual permeability by flow testing, anda visual inspection of damage bypetrographic microscopy and scanning electron microscopy (SEM). The rockstested are Pomona basalt (0.5 × 0.5 mm grain size) and Grande basaltic andesite(0.1 × 0.1 mm). Drilling in the field and in the laboratory is performed at lowconfining pressures. Table 1 summarizes drilling parameters. Fuenkajorn and Daemen described detailed drilling specifications and procedures. Making thesedrilling simplifications dramatically simplifies the experimental proceduresand allows the development of a much broader data base. Concerns about thesimplifications are valid and recognized, yet valuable information can begained from simplified tests. Ring Tension Tests The ring tension test measures the tensile strength of rock disks with acenter hole. Therefore, the strength and deformation results should besensitive to the condition of the borehole wall rock, which may have beenaltered by drilling. February 1992 JPT P. 210