Feng Fuping

Study on Borehole Wall Real-time Stability of Coal Seam With Coal Cleat When Underbalanced Drilling

Coal seam as the gas productive reservoir and gas-bearing reservoir, it is different from the conventional sandstone reservoir. On the one hand, coal cleat is well-developed in coal seam reservoir. Its characteristics are low porosity, small permeability, large specific surface area, low mechanical strength, strong heterogeneity, low reservoir pressure and so forth. These characteristics determine that drilling has more influence on coal seam reservoir than on conventional sandstone reservoir. In the process of drilling, therefore, in order to reduce or avoid the pollution to coal seam, usually adopt underbalanced drilling way to keep negative differential pressure and to reduce the damage of fluid in borehole flowing into the reservoir. At the same time, when underbalanced drilling, formation fluid flows into the borehole, leading to the formation pressure near the borehole to change. On the other hand, due to coal seam with low mechanical strength, great brittleness, and well-developed coal cleat, in the process of drilling especially underbalanced drilling, borehole wall is prone to collapse. Coal cleat exists in the coal seam and affects its mechanical property. When studying coal seam borehole wall stability, coal cleat must be considered. Considering time effect, the paper established the borehole wall stability model of coal seam with coal cleat when underbalanced drilling, obtained the collapse pressure distribution, and analyzed influence factors of coal seam borehole wall stability, providing theoretical guidance to prevent borehole wall instability. Key words : Coal seam; Underbalanced drilling; Negative differential pressure; Borehole wall stability; Coal cleat

Study on Mechanism of Shear Swirling Flow Vibration CementingImproving Displacement Efficiency in Horizontal Well

When horizontal wells cementing, drilling fluid is easy to be held up at the narrow clearance due to casing eccentricity. At the same time, the density difference makes the lighter drilling fluid gather in the upper portion of borehole. Drilling fluid is hard to be displaced cleanly. Shear swirling flow vibration cementing technique is a new effective cementing technique, which can improve the cementing quality in several ways. On the one hand, shear swirling flow vibration cementing technique makes the annular fluid swirling flow. The circumferential velocity increases a circumferential shear stress for borehole wall, making it easy to carry drilling fluid near the wall and scour mud cake on the wall. Especially for the displacement at narrow clearance caused by the casing eccentricity in horizontal well, the effect will be more obvious. On the other hand, shear swirling flow vibration cementing technique makes the casing string periodically eccentric revolution. It changes the conventional pressure distribution and forms pressure difference on the annular cross section. The pressure at wide clearance is greater than that at narrow clearance, which is advantageous for the fluid flowing from wide clearance to narrow clearance and makes drilling fluid at the narrow clearance displaced effectively. In the paper, it analyzed the mechanism of shear swirling flow vibration cementing improving displacement efficiency from swirling flow and vibration two angles.

Calculating Method for the Axial Force of Washover String During Extracting Casing in Directional Well

In the process of extracting casing in directional well, the existence of inner casing and external centralizer makes the calculation of washover-string axial force more complex, which increases the difficulties for choosing a reasonable bit weight in construction with no doubt. In this paper, through taking three factors of casing contacting with washover tubing, centralizer and washover tubing contacting with well wall into consideration, an effective method for calculating the axial force conducting is established. Moreover, the axial force in different conditions is obtained through numerical solution. The calculation results are: (a) The casing has a less effect on the axial force conducting, and the axial force loss is mainly at centralizers and new contact points. (b) The axial force conducting can be improved effectively through setting a reasonable centralizer spacing. There is no doubt that the establishment of the method has an important significance for choosing the bit weight, judging whether the washover head cuts casing and designing washover string. Key words: Axial force; Washover string; Extracting casing; Frictional resistance

The Structure Parameters Optimization of Variable Direction Joint During Extracting Casing in High-Inclination Directional Well

During extracting casing in high-inclination directional well, the existence of variable direction joint (VDJ) can enhance the build-up ability of washover string. And it can increase the compatibility between washover head and casing to some extent. However, too large or too small bending angle of VDJ will result in that the washover head cuts the upper and bottom parts of casing. Therefore, it is necessary to design reasonable bending angles for casing damage wells with different borehole curvature. In this paper, through the geometrical and mechanical analysis on the washover string with VDJ, a build-up rate calculation model and an axial force calculation model are established respectively. Moreover, through establishing the finite element model of washover tubing-ball-casing combination, the contact stress distribution is obtained. Under the critical condition of casing breaking when the contact stress exceeds the yield stress, the reasonable bending angle of VDJ matching with different casing damage well is designed. We can conclude that the research results play an important significance for in assuring the safe extracting casing. Key words: Variable direction joint; Safe extracting; Parameters optimization; Casing damage well

Tripping Friction Model for Multi-Stage Fracturing and Completion String in Horizontal Well

The structure of multi-stage fracturing completion string in horizontal well is complicated. The downhole tools such as packers and sliding sleeves whose dimensions are very close to the size of the borehole, and the completion string has strong stiffness as well. Thus, it leads to larger frictional restriction when running string. Based on the above reasons, it is essential to calculate the tripping capacity before the strings running into the well in case of sticking off. However, calculation errors of conventional string tripping models are relatively larger. This paper took the structure of multi-stage fracturing completion string into consideration, divided completion string by contact points between string and borehole to establish the stress and bending model of the string between two contact points, and established the tripping friction and hookload model for multi-stage fracturing completion string. An applied example of multi-stage fracturing horizontal well in Hong 90-1 block of Jilin Oil Field shows that the created model in the paper is more accurate. The accuracy of hookload while the string running in form curved section to bottom is 95.80%. The established model is more accurate and reliable. It can be used to estimate the tripping ability of the multi-stage fracturing completion string. Key words : Multistage fracturing; Tripping; Tripping friction; Mechanical model

The Influence Factors Analysis of Abrasiveness Based on the Grey Theory

Aimed at the phenomenon of many factors affecting rock abrasiveness, classification is not clear, and the influences of each factor on abrasiveness have different increase or decrease, different weights, and the influence mechanism are also different, in addition there may be a mutual influence between various factors, this paper takes into account the mutual influence between various factors and the weight of influence on abrasiveness, found out the main influence of abrasiveness by correlation analysis and grey correlation degree, so that guide the field better work by the main influence. Key words: Rock abrasiveness; Main influence; Correlation analysis; Grey correlation degree

The Effects of Density Difference on Displacement Interface in Eccentric Annulus During Horizontal Well Cementing

It is known that the interface of slurry displacement during cementing reflects the extent of two-phase fluid in- termixing. A longer interface means that more displacing fluid becomes contaminated. When the interface becomes too long, it can occur that the trailing edge of the interface fails to reach a designed cementing segment at the end of opera- tion, which reduces displacement effectiveness and adversely affects cementing quality and well integrity. In this paper, a 3D numerical model was developed and employed to analyze the effects of slurry density difference on the displacement interface in the eccentric annulus of horizontal wells. The simulation results showed that smaller density difference can create shorter interface and lead to better displacement efficiency when casing is centrally placed in a horizontal borehole. When the casing is eccentric, slurry tends to advance in the annular upper side under smaller density difference; other- wise, the opposite result will be obtained under greater density difference. Therefore, there exists an optimal density dif- ference under which a minimum interface length can be achieved for a given casing eccentricity. In addition, the optimal density difference increases as the shear thinning index, consistency index and yield point of a cement slurry increase. In order to minimize the intermixing extent of two-phase fluids and slurry fingering effects in cementing operations, it is necessary to take into consideration the comprehensive influence of casing eccentricity, slurry density, and rheological parameters to design slurry parameters properly, which will achieve higher displacement efficiency and better cementing quality.

Abhesion Mechanics Model of Horizontal Well Cement-Formation Interface

Good sealing between layers of the isolation of low permeability oil field was the effect premise to realize the horizontal section fracturing, the fluid channeling passageway was easy formed between the interface. Because the difference between formation and cement mantle, by the action of fracturing fluid it can produce deformation difference between them, this paper draw on the experience of the steel and concrete composite structure plane destroy study, for the first time, the cement-formation interface damage was described as two interface happen shear “abhesion” damage, considering the cement-formation interface cementing strengths,crustal stress and casing and the cement mantle and other factors, the cementing cement-formation interface abhesion damage mechanics mode was established, finally obtained cement-formation interface abhesion pressure and abhesion length calculation formula under the condition of different fracture fluid, the result of the research was to evaluate the horizontal well cement-formation interface pressure can provide theoretical basis.