Fuzzy Comprehensive Evaluation in Well Control Risk Assessment Based on AHP: A Case Study

To give a quantitative description of well control risk, a multi-layer fuzzy comprehensive evaluation based on AHP (analytic hierarchy process) is used. During the evaluation, risk factors and weight are given by Delphi method and AHP method. A multi-level and multi-factor evaluation system is built including four level-one factors of geologic uncertainty, well control equipments, techniques and crew quality, and fourteen level-two factors. Then a calculation is given with an oilfield in West China. The result shows geologic uncertainty is the primary factor leading to well control risks and the grade of well control risk is “higher risk”. The application result indicates that well control risk assessment by fuzzy comprehensive evaluation is feasible.Key words: Risk assessment; Fuzzy comprehensive evaluation; Analytic hierarchy process; Weight; Risk facto

Evaluation Method for Probability of Blowout after the Failure of Offshore Well Killing

249-259With the development of offshore oil industry, the influx and blowout are inevitable. Well control methods have been well researched, but how to recognize the failure of well control earlier and how to evaluate the probability of blowout for taking steps to avoid are imperfect. Based on the two-phase gas-liquid flow, the characteristic of well killing curve before and after killing are analyzed. Then the method for recognizing the failure of well killing is established by the probabilistic and covariance processing method. Then the blowout due to the failure of well killing is studied and the build-up pressure template is established. According to this, three evaluation methods for blowout probability are established, the shut-off pressure, the standing and casing pressure, formation parameters and underbalanced level varying methods. Final, four hardware systems and one evaluation system are recommended for decreasing or avoiding the risk during the failure of well killing

Evaluating essential features of proppant transport at engineering scales combining field measurements with machine learning algorithms

The behaviours of the particle settlement, stratified flow and inception of settled particles are essential features that determine the proppant transport in low-viscosity fracturing fluids. Although great efforts have been made to characterize these features, limited research work is performed at field scales. To test the laboratory outcomes, we propose a machine-learning-based workflow to evaluate the essential features using the measurements obtained from shale gas fracturing wells. Over 430,000 groups of fracturing data (1 s time interval) are collected and pre-processed to extract the particle settlement, stratified flow and inception features during fracturing operations. The GRU and SVM algorithms, trained by these features, are applied to predict fracturing pressure. Error analysis (the root mean squared error, RMSE) is carried out to compare the contributions of different features to the pressure prediction, based on which the features and the corresponding calculations are evaluated. Our result shows that the stratified-flow feature (fracture-level) possesses better interpretations for the proppant transport, in which the Bi-power model helps to produce the best predictions. The settlement and inception features (particle-level) perform better in cases where the pressure fluctuates significantly. The features characterize the state of proppant transport, based on which the development of subsurface fracture is also analyzed. Moreover, our analyses of the remaining errors in the pressure-ascending cases suggest that (1) an introduction of the alternate-injection process, and (2) the improved calculation of proppant transport in highly-filled fractures will be beneficial to both experimental observations and field applications

Promotion of CO2 fracturing for CCUS—the technical gap between theory and practice

CO2, used as an environmentally friendly fracturing fluid, has encountered a bottleneck in development in recent years. Despite great efforts in research work, limited progress has been made in field applications. In this study, an extensive literature review of research work and field cases was performed to summarize the technical issues and challenges of CO2 fracturing. The key issues of CO2 fracturing were analyzed to reveal the gap between fundamental research and field operations. The effects of CO2 properties on fracture creation and proppant transport were synthetically analyzed to extract new common research orientations, with the aim of improving the efficiency of CO2 injection. The hydraulic parameters of CO2 fracturing were compared with those of water-based fracturing fluids, which revealed a theory-practice gap. By studying the developing trends and successful experiences of conventional fluids, new strategies for CO2 fracturing were proposed. We identified that the major theory-practice gap in CO2 fracturing exists in pump rate and operation scale. Consequently, the friction reducer, effects of flow loss (due to leak-off) and distribution (within fracture networks), and shear viscosity of thickened CO2 are key factors in improving both fracture propagation and proppant transport. By increasing the scale of injected CO2, the CO2 fracturing technique can be enhanced, making it an essential option for carbon capture, utilization, and storage (CCUS) to reduce carbon emissions and mitigate climate change

Emergency Pump-Rate Regulation to Mitigate Water-Hammer Effect—An Integrated Data-Driven Strategy and Case Studies

Pump-rate regulation is frequently used during hydraulic fracturing operations in order to maintain the pressure within a safe range. An emergency pump-rate reduction or pump shutdown is usually applied under the condition of sand screen-out when advancing hydraulic fractures are blocked by injected proppant and develop wellhead overpressure. The drastic regulation of the pump rate induces water-hammer effects—hydraulic shocks—on the wellbore due to the impulsive pressure. This wellbore shock damages the well integrity and then increases the risk of material leakage into water resources or the atmosphere, depending on the magnitude of the impulsive pressure. Therefore, appropriate emergency pump-rate regulation can both secure the fracturing operation and enhance well-completion integrity for environmental requirements—a rare mutual benefit to both sides of the argument. Previous studies have revealed the tube vibration, severe stress concentration, and sand production induced by water-hammer effects in high-pressure wells during oil/gas production. However, the water-hammer effect, the induced impulsive pressures, and the mitigation measures are rarely reported for hydraulic fracturing injections. In this study, we present a data-driven workflow integrating real-time monitoring and regulation strategies, which is applied in four field cases under the emergency operation condition (screen-out or near screen-out). A stepwise pump-rate regulation strategy was deployed in the first three cases. The corresponding maximum impulsive pressure fell in the range of 3.7~7.4 MPa. Furthermore, a sand screen-out case, using a more radical regulation strategy, induced an impulsive pressure 2 or 3 times higher (~14.7 MPa) than the other three cases. Compared with the traditional method of sharp pump-rate regulation in fields, stepwise pump-rate regulation is recommended to constrain the water-hammer effect based on the evolution of impulsive pressures, which can be an essential operational strategy to secure hydraulic fracturing and well integrity, especially for fracturing geologically unstable formations (for instance, formations near faults)

A new high-precision timely monitoring and metering system for early kick and loss

Kick and loss are two complicated incidents that affect the construction safety in oil and gas well drilling. The commonly-used kick/loss monitoring methods are disadvantageous with monitoring lag and low metering precision, which may cause well collapse, pipe sticking and well blowouts due to untimely detection and improper treatment. In this paper, a new type of kick/loss monitoring and metering system was designed based upon a comparative analysis of several kick/loss monitoring methods commonly used on rig sites. This new system has the functions of early monitoring and alarm, kick/loss velocity metering, total loss metering and automatic filling, and its feasibility was verified through laboratory experiments. And the following research results were obtained. First, the monitoring tank of this new system is divided into two chambers, i.e., a main chamber and a secondary chamber. The return of drilling fluid partially flows back to the shale shaker through the main chamber, and the rest overflows into the secondary chamber. Second, the internal cross section area of the secondary chamber is small, which increases the response sensitivity to liquid level change, so kick/loss can be detected in time. Third, the water head of the outlet pipeline of the main chamber remains constant and the outlet flow is stable, so kick/loss velocity and total kick/loss can be calculated quantitatively based on the change of liquid level in the secondary chamber. And the monitoring error of kick/loss velocity is less than 8%. Fourth, in the process of tripping out, the drilling fluid in the monitoring tank flows into the wellbore under the action of self weight to keep the full liquid level of the wellbore all the time. As a result, lagged filling and partial filling are eliminated. In conclusion, this new kick/loss monitoring system is economical and practical, and giving a full play to the advantages of ground survey and alarm timely and accurately. Keywords: Early kick, Early loss, Monitoring and metering system, Main and secondary chambers, Monitoring tank, Leakage velocity, Total loss, Automatic fillin