Study on borehole stability in fractured rocks in deep drilling conditions

Abstract

Wellbore or borehole stability is a serious and expensive problem in mining and petroleum industry. With the development of new exploration and production technologies, Australian miners are looking for mineral deposits in deep seated environments. Borehole instabilities can be encountered at any stage in the life of a well, including drilling, completion and production. Borehole instabilities are the main cause of difficulties encountered during drilling. This results in significant expenditure, excessive loss of time, sometimes it results in loss of borehole. One of the most integral part of rock formation is the presence of joints and fractures at a small scale. According to some researchers most of the rock formations have fractures at some scale. When boreholes are drilled in such formations, instability is a major concern. In order to accurately predict the behaviour in fractured media, the matrix and fracture deformations as well as fluid flow in fractures need to be fully coupled. A number of factors influence borehole instabilities in fractured rocks. This may include solid-fluid interaction (rock and chemically active mud), complex stress conditions, probable borehole deviation, heterogeneity in the formation and drilling operations. Vertical boreholes are usually stable where overburden is the maximum stress (σ1). However, drilling vertically does not guarantee a stable hole. Instability in a borehole is dominated by the in-situ stress system. When an undisturbed rock is penetrated by drill bit, the in-situ stresses are redistributed. As a result, in-situ stresses tend to concentrate around the excavation. This is presented by an increase in stress concentration in the vicinity of the borehole and induced stresses near intersection of discontinuities and fracture tips. These induced stresses can lead to rock failure of the borehole wall. This thesis represents three journal publications which represent simulation of an unsupported and mud supported vertical borehole in two dimensional and threedimensional analyses. Because the nature of rock media is considered as fractured with single permeability along discontinuities, Discrete Element Model (DEM) was considered to be the best tool for investigations. First of all, Numerical investigation on the behaviour of an unsupported vertical cylindrical borehole in heavily fractured rock mass is presented. DEM based code Universal Distinct Element Code (UDEC) is used as the simulation tool. With taking into account the in-situ stress conditions in Cooper basin, South Australia. A borehole of 0.15 m radius in the centre of the model was simulated comprising of two fracture sets. The vertical stress applied correlates with the 1.5 km depth of the Cooper basin. The effect of fracture orientation and in-situ horizontal stress ratio (σH /σh) on the stability of the rock mass around the borehole was investigated. It has been shown that the induced stresses due to excavation lead to the development of a yielded zone around the borehole. Borehole stability criteria relevant to the extent of yielded zone and maximum displacement around the borehole were introduced into stability analysis. Results show that when the in-situ stress ratio increases the rock blocks at borehole wall tend to move towards the centre of borehole, consequently yielded zone around the borehole increases. Similarly, the fracture orientation changes the angle of borehole fracture intersection which aids in displacement increase as well as the location of block detachment. Furthermore, the change in fracture orientation highly influences the formation of yielded zone. Secondly, a 3D discrete element model is presented which is developed to simulate a borehole drilled in fractured rock mass. A model with overbalanced drilling conditions is simulated in this study. In doing so, different depths of a borehole, MB-1 borehole, in Northern Perth basin was simulated. The developed model was validated against log measurements of Caliper log. Rock strength was found to be one of the governing factor in controlling the stability. Thereafter, hydro-mechanical models were generated and it was observed that high mud flow rates and high pore pressure increased the instability around borehole. Furthermore, a parametric study was performed to investigate the influence of viscosity and fluid flow on the stability. Shear displacement linearly increase with an increase in the flow rate while fluid pressure decreases due to the increase in fracture’s aperture with an increase in the flow rate. Similarly, increase in viscosity caused increase in fracture shearing and therefore instability around borehole. After most important rock mass and operational parameters were analyzed, their influence was determined. A detailed stress analysis of 3D model of Northern Perth basin was carried out. Apart from the regional stress constraints, stress distribution in a smallscale area has several influencers. Constraining these localized stress perturbations is a key element in analyzing borehole stability and related underground excavations. As a final part of this study stress perturbation near the well bore and fracture tips was analyzed. As part of the study a regional model with three major faults was generated which was further used to estimate boundary stresses on descriptive smaller model termed as ‘base model’. In addition to the magnitude of stresses at tips of discontinuity, it was observed that when stress tensor pass through a material of low stiffness in this case, a discontinuity, it tends to rotate parallel to the discontinuity. A borehole in such rock mass determined that yield zone is in agreement with high stresses along discontinuities. Base model was further subjected to strength anisotropy and stress anisotropy analysis. Effect of stress anisotropy on stress perturbation is found to be very significant whereas strength anisotropy which was studied by changing of friction angle and cohesion in one of the discontinuities slightly affected stress perturbation. In both cases, due to the effect of discontinuities the induced stress field is non-linear.