Gonghui Liu

The characteristics of recycling gas drilling technology

Recycling gas drilling is a new drilling technology. This paper can be divided into three parts, with the purpose of introducing and analyzing the characteristics of this new technology. First, the major equipment characteristic of this technology was introduced. Secondly, compared with conventional gas drilling, Angel’s model was used to analyze the wellbore flow characteristics. Due to the closed loop and the effect of back pressure caused by the equipment, the gas ow rate decreases dramatically during drilling. Apart from this, it is also found that the kinetic energy at the casing shoe is always smaller than that at the top of the collar. The proposing of the drilling limit concept points out the basic difference between the two gas drilling technologies. Lastly, according to the results of the theoretical analysis, gas supplement operations for the wellbore must be conducted. Thus, two gas supplement schemes are presented in this paper, to provide some guidance for field operations.

Numerical Model for Evaluating Well Kick and Lost Circulation During Managed Pressure Drilling

Based on the rapidly varying casing pressure method and multiphase flow theory, we have constructed a transient mathematical model for managed pressure drilling (MPD) that combines kick with mud loss in a single fracture. The finite difference method was used to iteratively solve this model. The model calculated values agree well with the experimental results, which confirms the reliability of the model. Casing pressure, casing shoe pressure, pit gain, outlet flow, and choke opening during the entire well control process are analyzed. Experimental results show that equality of inlet flow and outlet flow may not imply influx stoppage when a kick is coupled with mud loss. In addition, the loss rate at the casing shoe initially increases linearly over time and then decreases exponentially, ultimately becoming constant. After completion of well control, drilling can be safely continued until loss is negligible. This study may provide a theoretical basis for better control of wells with complicated kick and circulation loss in managed pressure drilling (MPD).

Modeling the pressure characteristics of parallel chokes used in managed pressure drilling and related experiments

Based on the local resistance computation model for a choke valve and using the flow characteristics of choke valves, we studied the relationships between the back pressure of a parallel choke assembly and the opening extent of choke valves and developed a model to characterize the pressure regime of the manifold assembly. A comparison of pressure characteristic curves shows that a parallel choke manifold assembly has obvious advantages over the conventional serial type including high linearity of pressure-regulating characteristics curves, the elimination of the overshoot interval, wider effective regulating interval and the higher system security. Laboratory hydraulic experiments have validated the capability of a back pressure control model for the parallel choke assembly to accurately control pressure. This study is of great theoretical and practical significance to further improve the performance of chokes used in managed pressure well drilling.

The influence on casing stress for shale gas fracturing wells considering thermo-pressure coupling effect

The casing deformation problems are presented over 36 wells (among 112 horizontal wells by 2016) during fracturing processes in Weiyuan-Changning shale gas play in China.1 Subsequent tools could be blocked in the wellbore.2–4 The segments with serious deformation had to be abandoned before completing fracturing operations.5 The casing deformation issues make a great influence on production of shale gas wells.

Influence of Cement Defection on Casing Stress Under Cyclic Loading During Multi-stage Hydro-Fracturing

The casing integrity problem presents severe challenges for shale gas wells regarding the multi-stage fracturing operation. Multiple factors during fracturing process may have individual or combined influence on it. However, most of the researchers only studied one fracturing stage. The influence of cyclic fracturing loading on casing of shale gas well has been rarely studied. This paper presents a finite element model approach to simulate the multi-stage fracturing operations. Based on the theory of elasto-plasticity, the nonlinear isotropic/kinematic hardening material was used to simulate the response of the casing under cyclic loading. The transient temperature-pressure coupling model of casing-cement sheath-formation (CCF) was established. Multiple analysis steps were used to simulate the multi-stage fracturing processes. The cement channel angle, pump rate, and fracturing fluid temperature were the variables taken into account. An attempt was made to reveal how the factors affect the casing stress. Sensitivity analyses showed that the cement channel had the greatest influence on casing stress. The casing had the highest Von Mises stress and equivalent plastic strain (PEEQ) at the channel angle between 60° and 90°. The stress could be easy to exceed the casing yield stress, causing the casing failure. The more the fracturing stage was, the larger the Von Mises stress and PEEQ of casing were. Larger pump rate tended to dramatically reduce the downhole temperature. The lower the temperature of the fracturing fluid was, the greater the reduction of the downhole temperature was. When the cement channel angle was smaller than 60°, the casing had the higher Von Mises stress and PEEQ for the larger pump rate and lower fracturing fluid temperature. The results indicated that good cement sheath was the basic requirement to ensure the safety of casing. The pump rate and fracturing fluid temperature should be in a reasonable range. During multi-stage fracturing construction, the pressure should be not too high to reduce the casing stress, avoiding the risk of casing deformation.

Design and Field Testing of a Nitrogen Circulation Drilling System

In the recent decades, the merit of enhancing drilling rate by wellbore gas scavenging has been described comprehensively in many papers. This method began to be used widely and successfully in field conditions. In comparison with air and natural gas, due to its inertness and inflammability, gaseous nitrogen has become the most preferred fluid for drilling. The biggest shortcoming of using nitrogen gas as drilling fluid is high cost of its production. If the nitrogen gas circulated out of the wellbore could be recycled by some ground-based equipment and injected into the wellbore again, nitrogen gas drilling may become more viable. In this work, we developed a new nitrogen gas circulation drilling technology and carried out a systematic study of the proposed project, namely, the technological process, development of separation equipment and control system, etc. To verify the feasibility of this technological system, a comprehensive on-site experiment was designed and implemented for testing the prototype system.