Recent developments in drilling technology, such as increased sensory information, enhanced data processing and transmitting capacity and capability, and developments in computer controlled machinery, together with adaptation of already available process technology and know-how, are opening up new possibilities for drilling operations. Application of these combined technologies, together with advanced computer modeling, enables enhanced monitoring and increased optimization and control of drilling operations. This paper presents such an integrated system for monitoring and control of the drilling process, currently in the test phase. A key element in the methodology used here is that the models for fluid flow and drilling mechanics are continuously updated in real-time according to the measured data using Kalman filtering techniques. By comparing the calibrated models to real-time data, unwanted occurrences can be detected quickly, and mitigating actions may be taken, either through system control or through manual intervention. Using the calibrated models, safe limits for the drilling operation are computed and enforced, and procedures are optimized. The modules developed cover tripping and reaming, pump start up, friction tests, stick-slip prevention, bit load optimization and monitoring. The methodology may be applied to drilling operations where the drilling equipment is computer controlled. Surface and preferably downhole data must be available in real time. Rigorous testing with drilling data from offshore drilling operations has been performed, and several full-scale tests have been run on a test rig. The ability to maintain the drilling operation within critical limits has been demonstrated. The methodology may contribute to increased safety and reduced down time during drilling operations. Introduction A large part (25%, [1]) of the overall cost associated with drilling operations is a result of non-productive time due to unplanned well incidents. The main problems during drilling are related to events such as kick, stuck pipe, wellbore collapse, lost circulation and equipment failures, see [2]. Proper use of real time data has the potential to reduce the down time caused by these events significantly. Availability of real time drilling data is increasing, both from surface instruments and downhole gauges. Open standards for real time data access are being developed. Computer controlled drilling machinery like pumps, draw-work and top drive are available. High band-width communication between the rig sites and the office like fiber optic, high band width VHF and satellite are used. All these combined developments enable enhanced monitoring through data processing, and optimization and control of drilling operations through computer modelling and drilling machinery automation. For many years IRIS has developed advanced computer models for the oil industry. Among these are multiphase well flow models and torque and drag models. Testing and verification is done through studies with comparison to field data. Additional sub models have been incorporated to handle special effects and give the models special features. During the last few years the models have been developed in order to run real time and use available measurements of operational data such as flow rate, inlet temperature, surface torque and hook load. In order to run real time and to be a corner stone in a control system for the drilling operation, the models need to be fast and robust. Drilltronics - A Software System for Monitoring and Control IRIS (former RF-Rogaland Research) and National Oilwell Varco (NOV) have developed a new drilling, monitoring, control and prediction system called Drilltronics [3]. The main feature of the system is to combine existing hardware and software for monitoring and controlling the drilling process (system environment) with advanced mathematical models for the drilling process. In the implementation of the system we have used existing and upgraded system environment from NOV.

No abstract

The present work discusses some improvements that have been introduced in a dynamic model, which was developed for simulating the two-phase flow transient phenomena associated with underbalanced drilling operations. The model enhancements are basically obtained by implementing mechanistic closure relationships and more accurate numerical schemes. This process of improvement is validated through comparison to full-scale experimental data in transient scenarios, showing that the gains in terms of increasing the model accuracy are significant. Introduction Flow modelling has become more and more important in the whole planning process of an UBD operation. Steady-state models have been used for years for designing the operational window. The only drawback here is that steady-state models are not able to reproduce accurately the transient behaviour that occurs during e.g. unloading, connections, and other inevitable transient situations that occur while performing the operation. On the other hand, dynamic models have this capability. Proper modelling can ensure that the operation can be designed in an optimum manner, and predict the drawdown for various conditions. It is of direct importance to maintain the underbalanced conditions throughout the whole operation to avoid formation damage. Previous experiences indicate that even temporarily overbalanced conditions can reduce the formation productivity. In that sense, both steady-state and dynamic modelling can be of great importance and, in this respect; reliable models are necessary. The present work is concerned with improvements in transient modelling of underbalanced operations. The accuracy of the model, which is an approximation of the reality, depends heavily on using proper closure laws (mechanistic model) for flow pattern description, pressure losses and gas slippage. Another source of error is the basic numerical scheme that solves the fundamental flow equations. The process of improvement involves a new mechanistic approach that has been implemented in a transient model. The simulation results are compared with full-scale data in both steady-state and transient conditions, with the main focus on performing connections. The enhanced model not only matchs up very well with the experimental data but also shows a significant improvement compared to older models, particularly, with regards to describing gas dominated systems properly. The paper also focuses on how numerical schemes can be improved with regards to numerical diffusion. Schemes of high accuracy are required for giving a correct description of the maximum flowrates occurring at the separator (e.g. during the liquid unloading). This is of great importance for sizing properly the surface equipment, particularly the separator. Results are presented showing how a numerical scheme with reduced false diffusion differs from a conventional one that greatly underestimates the maximum flowrates. Constructing a Flow Model In general, multi-phase flow can be described by the fundamental two-fluid model1. It consists of separate conservation equations for each of the phases with respect to mass, momentum and energy. A simpler model can be obtained by adding the momentum conservations equations into a mixture momentum equation. This model is named drift flux. In addition, if the temperature modeling is not of large importance, it is also possible to neglect the energy equations and assume a fixed temperature gradient. Based on this assumption, a simplified version of the drift flux model is presented below.

fax 01-972-952-9435.In order to realize the full potential of Drilltronics, surface and downhole drilling data must be available in real time. The drilling equipment will need to be computer controlled with an interface that supports automatic responses to the model's analysis.

The paper addresses the problem of predicting pressure peaks when starting pumps after a static period while drilling a well. It demonstrates that laboratory measurements with standard Fann viscometers and a gel breaking model can be used for predicting such peaks. Fluids with properties that are similar to those of real drilling fluids have been studied in detail in the laboratory. Gel breaking and thixotropic behaviour were first analysed in detail through rheometer measurements. The fluids showed a pronounced time dependent behaviour. Afterwards each fluid was placed in a flow loop where circulation was started after a static period. The time dependent signals were logged and analysed. An initial sharp pressure peak followed by a slow decrease in pressure was typical. A local model based on rheometer measurements that predicts gel breaking pressures versus time is presented. The authors are not aware of any other such model. The local model has been integrated in a transient drilling simulator which predicts the pressure peaks that follow when starting pumps after a static period. Gel strength is broken successively from the pump, down the drill string, and up the annulus. The model will help drilling engineers to determine whether extra care is required when starting pumps. Controlled field measurements have been made in two North Sea HPHT wells, one with water based and one with oil based drilling fluid. Gel breaking pressures were measured when starting pumps after static periods of different lengths. The effect of rotating the drill string prior to pumping was tested. The transient model reproduced measured data with reasonable values of gel model parameters. The observed effects are expected to be of importance when drilling any critical well, which may be deep water, HPHT, or extended reach wells. Introduction Most drilling fluids used in gas or oil wells are designed to build some degree of gel strength when not under shear flow, in order to prevent cuttings and mud weighting materials from precipitating out of the fluid during periods of stopped circulation. This is important in order to prevent slumping of cuttings and to maintain the pressure balance in the well. When circulation is resumed, sections of the well bore will experience a pulse of increased pressure during a short time until the gel strength has been broken. In addition, because of thixotropic effects in most gelled muds, the viscosity of the mud will stay enhanced for some time after the gel breaking. This can be a significant effect, especially in deep water, HPHT and extended reach drilling where gelling can have a big impact on pressure losses. Pressure effects due to gel braking have been investigated through a series of laboratory experiments. It has been shown that these effects partially can be predicted from measurements on the muds with laboratory equipment. Measurements with both a high precision CarriMed CSL 50 rheometer and a common Fann 35 instrument have been compared to gel breaking effects observed in an annulus equipped with pressure sensors along the outer tube. Such high-precision, loggable rheometers, like the CarriMed instrument, are seldom available on off-shore drilling installations, and scarce enough in on-shore laboratories. It is therefore of interest to determine to what extent the pressure pulse imposed on the well due to thixotropic effects can be predicted from measurements performed with a common Fann 35 viscometer.