No abstract

Recorded drilling times may show significant variations from well to well even for the same total depth in the same field. Apart from the formation characteristics, engineers' technical ability plays an important role in determining drilling time. A drilling performance is usually compared with respect to the performances of previously drilled wells. This approach has two major drawbacks. One, in newly developed fields, there may not be sufficient number of drilled wells to make a healthy comparison. Two, past drilling practices may not represent good engineering practices. In this paper, a different approach is introduced in assessing drilling performance. The new approach suggests a comparison of drilling performance with respect to what is called ‘technical limit of drilling rate’, a maximum achievable drilling rate without risking drilling safety. Introduction The rate of drilling can be improved for a given field until it reaches its technical limit. Several variables affect drilling rate (1–4). Some of these variables originate from formations, and nothing can be done to change them practically. Formation properties such as; pore pressure, compaction, insitu stresses and mineral content are among the uncontrollable variables. On the other hand, several drilling variables can be selected carefully and drilling rate can be improved significantly. Mud weight (MW), weight on bit (WOB), rotary speed or rotation per minute (RPM), bit type and hydraulics are among the controllable drilling variables. It has long been observed that the drilling rate generally increases with increasing circulation rate (5), weight on bit (6), rotary speed (6) and bit tooth height. On the other hand, it decreases with increasing drilling fluid viscosity and density (7–11). Some of these variables may have a significant effect on drilling rate whereas others may have a marginal effect. Several authors have proposed mathematical relationships of drilling rate with major controllable and uncontrollable drilling variables for rolling cutter bits. (12–15) Among them, perhaps the most complete mathematical drilling model being used is Bourgoyne and Young's model (21,22).DR=f1f2f3f4f5f6f7f8 (1) The functions f1 through f3 represent the effect of uncontrollable drilling variables on drilling rate. For example, f1 represents formation strength on penetration rate. The functions f2 and f3 model the effect of compaction on penetration rate. For example, the function f2 accounts for the rock strength increase due to normal compaction with depth, and the function f3 models the effect of undercompaction experienced in abnormally pressured formations (14). The functions f4 through f8 represent the effect of controllable drilling variables on the drilling rate. The function f4 models the effect of overbalance on the drilling rate.f4=e2.303a4D(?f-?). 2

Inadequate weight on bit leads to drilling at lower rate of penetration which reflect itself with expensive drilling intervals. Weight on bit is provided by slacking of some of the weights of the tubulars on bit. From the mechanical point of view this means putting bottom section of drill string into compression. The upper limit of the compressional stress that can be imposed on a drill string is bounded by the minimum stress which can lead to the failure of the tubulars. One type of the failure of drill string is called 'drill string buckling'. As the size of tubular decreases, their ability to transmit weight on bit without buckling decreases. Curvature and inclination of drilled hole increase or decrease the amount of compressional load that can be imposed on tubular without buckling. This load is known as Critical Buckling Load. This paper presents a finite element and an experimental approaches to predict the critical buckling load for dropping and building sections of holes. Introduction Drilling at minimum cost necessitates optimum weight on bit. This weight is provided by slacking off some of the weights of drill string on bit. As the hook load decreases and slacked off length increases neutral point moves up in the drill string. At the same time, compressional stresses below the neutral point increase. There is a critical value of compressional stresses such that below which pipes are stable. In other word, pipes still have resistance against bending. However, if this value is exceeded then they will shown no resistance against bending. This phenomenon is known as buckling of tubulars and this critical value is known as Critical Buckling Load. Buckling is not desired in drillstring because of several reasons. One of the reasons is that it may lead to premature failure of the drillstring through fatigue and erosion. It may lead to termination of drilling due to drill string lockup especially in horizontal wells. Lockup occurs when the compression at any location within the drill string is equal to or is greater than the drillstring's resistance to buckling at that location. Therefore, it becomes impossible to provide the necessary weight on bit. Without the weight on the bit, drilling ceases and drilling of the reach is terminated. Therefore, it is essential to determine the maximum load that can be imposed on drillstring. Critical Buckling Load is not a unique number for a given diameter pipe. There are a number of borehole parameters affecting the magnitude of it. Such as, hole curvature, hole inclination, hole to pipe friction, etc. (Figure 1). Therefore, Critical Buckling Load for a given size pipe can not be determined independent without taking borehole data into consideration. Compressive load at which pipes buckle can be predicted based on Eigenvalue analyses or based on large Deflection Analysis of Finite Elements Methods. Eigenvalue analysis has the short coming of estimating the upper bound of critical buckling load (bifurcation load). Whereas, the more critical lower bound can be predicted with Large Deflection analysis of Finite Elements Methods (Figure 2). This study makes use of large deflection analysis of FEM to predict the critical buckling load in dropping holes. This paper also presents an experimental study to predict the critical buckling load in building holes. FEM modelling of the Drill Pipe and Bore Hole During a buckling simulation, the drill pipe is released and is allowed to slide down the wall of the borehole and settle into a stable position within the borehole. The pipe may or may not be buckled at the end of the simulation. If the pipe is buckled there will be a length of pipe in contact with the opposite side of the wall of the borehole (high side). The force created by the contact is called the contact force. It's direction is normal to the wall of the borehole. Whether the pipe buckles or only bends, two forces acting on the drill pipe will be created. P. 641

Slips and tongs produce permanent marks on pipe body and tool joints. Such marks develop high stress concentration that reduces strength of pipes. The remaining strength of pipes often falls below the pipe stresses which can lead to tubular failure. Slim pipes are most susceptables to failure due to die- marks. In many instances, slim pipes are handled using double elevator system to reduce pipe failure. In this paper, results of a recent study of various die-mark related failures of drill pipes under different loading conditions, with particular emphasis on fatigue damage are presented. Stress concentration due to die-marks is characterised by finite element analyses as a function of mark sizes to cover various gripping systems available in the market. Then a methodology is presented for the prediction of failure due to cumulative fatigue damage. Effect of stress concentration arising from die-marks is taken into account in the analysis. Results of this study suggest that the effect of stress concentration on the cumulative fatigue damage may be significant depending on particular gripping system in use. In most cases, the fatigue life evaluation based on conventional assumption of smooth pipe surface is found to be very unsafe. Thus, a new approach is proposed in this paper for prediction of true safe life of marked drillpipes against fatigue failure. Fatigue damage results are presented in graphical forms. Calculation of cumulative fatigue damage of drillpipes used in a number of drilling events is then presented systematically in tabular form to assist drilling engineers in the evaluation of actual remaining fatigue life of drillpipes for the target drilling event using a particular gripping system. P. 125

Premature screen-outs, and low in-place proppant concentrations occur frequently during hydraulic fracture treatments carried out in Central Australia. Two and three dimensional numerical modelling studies have been carried out, investigating factors affecting hydraulic fracture initiation, and near-wellbore fracture tortuosity, in highly stressed conditions. The 2D modelling suggests that drilling induced shear fractures, if oriented close to the maximum horizontal in-situ stress direction and inflated during treatment, may promote the initiation of multiple hydraulic fractures. Near-wellbore tortuosity and screen-outs are more likely in such situations. The results of this study also suggest that the elongated borehole geometry due to breakouts does not significantly alter the impact of preexisting fractures (either natural or induced) on hydraulic fracture initiation. 3D stress modelling indicated that fracture initiation may occur from perforations oriented even at large angles with respect to the maximum in-situ stress direction. Both numerical modelling and analytical analysis suggest that starter fractures initiate at the base, rather than the tip of perforations and that the initiation of horizontal starter fractures from perforations is independent of fluid pressure. For properly oriented perforations, horizontal starter fractures are unlikely to initiate because the strong reverse faulting regime required is rare at most reservoir depths. In strike-slip stress regimes, such as that experienced in Central Australia, however, the initiation of horizontal starter fractures is possible if perforations are misaligned with the minimum horizontal in-situ stress. This significantly increases the likelihood of near-wellbore tortuosity and the possibility of near-wellbore screen-outs. These studies highlight the benefits of aligning perforations in the maximum horizontal stress direction in eliminating reduced near-wellbore tortuosity. Introduction Hydraulic fracture treatments of tight formations in Central Australia often experience abnormally high treating pressures and fail to achieve adequate fracture conductivities due to low proppant concentration. The length of these fractures are commonly shorter than anticipated as a result of premature screen-out. A recent study has found that the hydrocarbon bearing formations in Central Australia have relatively high horizontal in-situ stresses and that the stress regime in the region is probably strike-slip faulting. In addition, FMS log images of selected wellbores in the region show widespread and consistent borehole breakouts in sandstones or coals, and tensile fractures in shales. Intuitively, these high horizontal stresses and associated induced shear fractures which cause borehole breakouts (see Fig. 1) may, in a manner similar to that of natural fractures, cause near-wellbore hydraulic fracture tortuosity However, the significant difference between these two types of fractures is that induced shear fractures occur in planes parallel with the minimum horizontal stress () (Fig. 1), whereas natural fractures are randomly oriented. It is therefore unclear what role such stress induced shear fractures play in the complication of hydraulic fracture initiation. The significant majority of high treating pressures and premature screen-outs in Central Australia, as experienced elsewhere, is ultimately near-wellbore fracture tortuosity which is manifested as multiple fractures or fracture reorientation. Regardless of form, the origin of near-wellbore tortuosity can be traced back to hydraulic fracture initiation, which is in turn controlled by the stress distribution around the wellbore during the breakdown stage. Therefore, a study of hydraulic fracture initiation in the presence of pre-existing fractures (either natural or induced shear fractures) under the influence of near-wellbore stresses, may help engineers establish a link between high in-situ stresses and an increased risk of premature screen-out. There exists much literature regarding experimental studies of hydraulic fracture initiation. P. 621^