Deng Jingen

Brittle failure of shale under uniaxial compression

Shale gas reservoirs are characterized by tight matrix, well-developed micro-fissures, and laminations. The study about the failure of shale under compression is of great significance to safe drilling operation and the subsequent reservoir stimulation. The variation of rock mechanical properties with the angle between the axial stress and bedding plane normal (coring angle) is analyzed based on laboratory tests. A failure criterion is applied and verified to describe the strength of shale. Moreover, ultrasonic technology is used to study the damage characteristics of shale during the uniaxial compression process. The experimental results show that shale strength decreases initially and then increases with the increase of the coring angle. The Young’s modulus and Poisson’s ratio increase with the increase of coring angle. In a compression process, damage is essentially the development of new micro-cracks induced by the compression. Shale failure is the microscopical reflection of the process of the generation and expansion of axial micro-cracks, so it is the result of damage accumulation. The variation of the lateral p wave velocity can function as a monitor of the development process of shale damage. The damage factor will increase in the linear elastic stage and then enlarge rapidly after entering the stage of unstable micro-crack expansion.

Influence of H2S Content on CO2 Corrosion Behaviors of N80 Tubing Steel

Abstract The corrosion behaviors of N80 steel in pure CO2 and at different partial pressure ratios of CO2/H2S were tested by high-temperature and high-pressure autoclave. At 90°C, with the additional H2S to pure CO2, the surface corrosion condition improved greatly and the corrosion rates were lower than in pure CO2 condition. With the increase of partial pressure ratio, the corrosion rate reached a peak value at pCO2/pH2S = 100 and then declined. The corrosion products of the samples in different conditions were analyzed by scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and X-ray diffraction (XRD) methods. The inner film is finer and denser than the outer scale.

Wellbore Instability and Countermeasures in Offshore Bedding Shale Formations

Abstract The properties of bedding shales are different from that of normal formations. Strength anisotropy is their outstanding feature. Experimental study and theoretical analysis show that this kind of anisotropy of bedding shales is the main reason of wellbore instability. W-2nd section shales in WZ12-1N oilfield are in typical bedding formation, and there were some problems such as severe borehole collapse and pipe sticking in drilling. Bedding shale formation was assumed to be a transversely isotropic material, and a collapse pressure calculating method was established. The calculation result of directional drilling collapse pressure in W-2nd bedding shales shows that if hole deviation exceeds a certain critical value, collapse pressure will increase sharply. This result was used in mud density design and well trajectory optimization, which solved the wellbore instability problems of the W-2nd formation. This research method has reference value to solve problems of wellbore instability and improve drilling efficiency in other similar regions.

Borehole Stability Analysis in Deepwater Shallow Sediments

Deepwater shallow sediment is less-consolidated, with a rock mechanical behavior similar to saturated soil. It is prone to borehole shrinkage and downhole leakage. Assume the deepwater shallow sediments are homogeneous, isotropic, and ideally elastoplastic materials, and formation around the borehole is divided into elastic and plastic zone. The theories of small deformation and large deformation are, respectively, adopted in the elastic and plastic zone. In the plastic zone, Mohr–Coulomb strength criterion is selected. The stress and deformation distributions in these two zones, and the radius of plastic zone are derived. The collapse pressure calculation formula of deepwater shallow sediments under the control of different shrinkage rates is obtained. With the introduction of excess pore pressure theory in soil mechanics, the distribution rule of excess pore pressure in these two zones is obtained. Combined with hydraulic fracturing theory, the fracture mechanism of shallow sediments is analyzed and the theoretical formula of fracture pressure is given. The calculation results are quite close to the practically measured results. So the reliability of the theory is confirmed.

Wellbore stability analysis and its application in the Fergana basin, central Asia

Wellbore instability is one of the major problems hampering the drilling speed in the Fergana basin. Comprehensive analysis of the geological and engineering data in this area indicates that the Fergana basin is characterized by high in situ stress and plenty of natural fractures, especially in the formations which are rich in bedding structure and have several high-pressure systems. Complex accidents such as wellbore collapse, sticking, well kick and lost circulation happen frequently. Tests and theoretical analysis reveals that the wellbore instability in the Fergana basin was influenced by multiple interactive mechanisms dominated by the instability of the bedding shale. Selecting a proper drilling fluid density and improving the sealing characteristic of the applied drilling fluid is the key to preventing wellbore instability in the Fergana basin. The mechanical mechanism of wellbore instability in the Fergana basin was analysed and a method to determine the proper drilling fluid density was proposed. The research results were successfully used to guide the drilling work of the Jida-4 well; compared with the Jida-3 well, the drilling cycle of the Jida-4 well was reduced by 32%.

Borehole Stability in Shale Formation for Extended Reach Wells

In recent years, Extended reach well (ERW) drilling technology is widely used in the offshore oil & gas fields in order to reduce the number of the drilling platforms. As it has notable characteristics such as the high deviation angle, large horizontal displacement and long open borehole interval the borehole stability increases the drilling risk and cost dramatically. To research the ERW borehole stability, including mechanical model, shale hydration test and the effect of circulating pressure loss in this paper, rock mechanics theory and hydraulics principle were comprehensively applied . The results show that, the safer drilling azimuth of the ERW in normal fault lies in the minimum horizontal principle stress direction; hydration radius increases with the passage of time, and the hydration collapsed rock has important influence on cutting beds and circulating pressure loss in annulus; the upper limit value of safety mud density decreases with the well depth increases, and when it deceases to the equivalent mud density of collapse pressure, the limit depth of the ERW is obtained. The research results provide relevant guidance on the ERW drilling, which can be used to determine the upper and lower safety mud weight limits, the best well trajectory selections and the well structure design.

Effects of Long-term Development on Wellbore Stability: A Case Study of Bohai Bay Basin

This paper describes the method used in the depleted reservoir for analyzing horizontal in-situ stresses in order to define a stable mud weight window to maximize the efficiency of drilling process. The method combines wellbore sta- bility modeling, in-situ stress prediction, and pore pressure depletion during production process. In the presence of any hydraulically isolated fault blocks or other permeability barriers, the pore pressure depletion will cause horizontal stress changes in both magnitude and orientation. Furthermore, the changes of horizontal stress affect the wellbore stability of inclined wells. The results indicate that the reservoir depletion has notable effect on the safe mud weight window, espe- cially the fracture pressure. The fracture pressure may be overestimated in previous model, and the most stable well azi- muth is not static but varies over the lifetime of the oilfield. The research conclusions can provide significant reference for the mud weight design of directional well in depleted reservoir.

A Discussion of the Compound Sand Control Technique of High Pressure Packing and Moderate Acidification

Acidification can both eliminate formation damage and improve flow passage, which can be good for improving single well productivity. Based on this, the authors bring up the thought of moderate acidification technique for sandstone reservoir, and combine it with high pressure packing technique, and one step further, presents a compound sand control technique of high pressure packing and moderate acidification. At the same time, the authors preliminarily discuss the main points of this technique and analyze its mechanism of sand control and yield increase as well as its adaptability. The main purpose of this technique is to clean out the clay and mud in the immediate vicinity of wellbore, decrease the fine content or the particle size in that area, enlarge the seepage channel, and fill in optimized gravel, so as to make it conducive for transpiring the fine sand in the formation and keeping the gravel layer unblocked, as a result making the sand control life prolonged and single-well productivity raised. The keys of this compound technique are moderate acidification and gravel size optimization.

Analysis of Gravel-Sizing Optimization Method for High-Pressure Gravel-Packing Sand Control

Abstract This article takes the Saucier method as the foundation and aim at fracturing packing, under ideal gravel conditions, through the analysis of the pore of gravel bed in the condition of fracturing or high-pressure packing and the arch bridge theory to revise the Saucier method and put forward the design space of gravel-to-sand median diameter ratio (GSR) is 5.464–6.831, thereby providing the foundation for optimizing and gravel size decision with high-pressure packing control sanding.