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Stick-slip vibration has been a major problem in drilling operations in B basin, Chad, where rocks in basement formation are hard granite. Premature failure of drill bits and string has profoundly undermined the safe and economical drilling operations. Hence, it is urgent to reveal the mechanism of stick-slip vibration and find a way to eliminate or relieve stick-slip problem. Through the numerical and experimental study on the effects of the cutting parameters on cutting performance, root cause on depth of cut (DOC) or cutting depth is revealed as the mechanism of stick-slip vibration, optimized cutting depths for different rock layers are recommended as well. With it, a novel design of auto-response DOC controller is proposed and those structural parameters are modified and via numerical simulation research. After machining and testing, the structure of the auto-response DOC controller is preliminary determined and it has acquired the satisfactory motion performance and met the testing standards as it can slowly retract into the body by resisting the load and quickly extend outward the body. The auto-response DOC controller and the anti-stick-slip PDC bit have profound application potential in developing the ultra-deep and unconventional reservoirs, especially in this era shocked by low oil price and the pandemic.

A new type of hydraulic self-adaptive polycrystalline-diamond-compact (PDC) cutter (SAPC) is designed for controlling cutting depth of the PDC bit flexibly and reducing the harm of stick–slip vibration to drilling operation. The fluid–solid coupling movement of SAPC under different well-drilling conditions are simulated and analyzed. The results show that the up-going time of SAPC can be increased by reducing the diameter of the thin connecting pipe, increasing the diameter of the SAPC liquid-cavity, increasing the height of the cavity, increasing the liquid viscosity and increasing the initial spring force/decreasing the elasticity coefficient. Among them, reducing the diameter of thin connecting pipeline and increasing cavity diameter are the most sensitive. The up-going time can also be increased by the way of changing liquid viscosity without changing the structure of SAPC. Increasing the initial spring force and decreasing the elasticity coefficient can also increase the up-going time of the SAPC, however, the response window to the linkage force is reduced. The double-pipeline embedded SAPC can effectively reduce the volume and installation difficulty. The reasonable thin pipeline diameter is 0.2–0.5 mm, and the thick pipeline diameter is more than 2 mm, and the cavity diameter is more than 18 mm. Based on the results of the numerical simulation study, the up-going time of the SAPC is designed over 1.25 s, which meets the performance requirements of the hydraulic SAPC using in complex reservoir drilling, such as carbonate-sandstone reservoir.

abrasive supercritical CO2 (SC-CO2) jet perforation is the key procedure in the SC-CO2 fracturing, which will directly affect the exploitation of oil and gas. The properties of the SC-CO2 fluid, such as density, viscosity, diffusivity and so on, change greatly with the variation of temperature and pressure, which directly affect the particle-carrying ability and perforation performance. This paper investigates the influence of key parameters, such as ambient pressure, fluid temperature, jet standoff distance and jet pressure, on the perforation ability of abrasive SC-CO2 jet. The results indicate that the ambient pressure has no significant effect on perforation under the condition of a fixed jet differential pressure. When the confining pressure increases from 5 to 15 MPa, the hole depth and diameter decrease by 5.7% and 18.6% respectively. The hole depth increases slightly with the rising of jet temperature. In addition, with the jet temperature rising per 20℃ within the range from 40 to 100℃ and the standoff distances being 4 to 10 mm, the hole depths increase by 3.8% and 12.0% in average, respectively. Furthermore, the hole depths keep unchanged at first and then decrease rapidly with the increasing standoff distance. However, the hole diameters and effectively impinged areas increase with the standoff distance. The influences of SC-CO2 jet pressure on the perforation performance are similar to that of the conventional jet. Additonally, both effective hole depths and volumes increase linearly in general with the increasing of jet pressure. The hole depths averagely increase by 36.6% when the SC-CO2 jet pressure rising by 5 MPa. Furthermore, the research also shows that perforation performance of pre-mixed jet is better than the post-mixed jet, the volume ratio of the two kinds of perforating holes is 12.02 under these experimental conditions. All the above merits have provided a theoretical foundation and experimental proven for the field application of SC-CO2 jet perforation technology.

With the rapid development of global economy, an increasing amount of attention has been paid to the emission of greenhouse gases, especially CO2. In recent years, dominated by the governments around the world, several significant projects of CO2 sequestration have been conducted. However, due to the huge investment and poor economic effects, the sustainability of those projects is not satisfactory. Supercritical CO2 (SC-CO2) has prominent advantages in well drilling, fracturing, displacement, storage, plug and scale removal within tubing and casing, which could bring considerable economic benefits along with CO2 sequestration. In this paper, based on physicochemical properties of SC-CO2 fluid, a detailed analysis of technical advantages of SC-CO2 applied in oil and gas development is illustrated. Furthermore, the implementation processes of SC-CO2 are also proposed. For the first time, a recycling process is presented in which oil and gas are extracted and the CO2 generated could be restored underground, thus an integrated technology system is formed. Considering the recent interests in the development of enhancing hydrocarbon recoveries and CO2 sequestration, this approach provides a promising technique that can achieve these two goals simultaneously.

Supercritical carbon dioxide (CO 2 ) jet-assisted radial drilling is regarded as a potential alternative drilling method because supercritical CO 2 jet has higher efficiency in rock-erosion efficiency than water jet. Although researchers have investigated how the rock-erosion performance by supercritical CO 2 jet impingement correlates with the influencing parameters, there is little on the rock failure mechanism and change of pore structure. Scanning electron microscopy (SEM) analysis has been an important means in mineral identification, material damage detection, and aiding in water jet research. Therefore, rock-erosion experiments and SEM observations and analyses were carried out on different rocks including sandstone, marble and so forth. Secondary electron (SE) images show obvious grain shapes on the complex impinged surface of sandstone and marble by supercritical CO 2 jet, and cleavages on the mineral grains, and cracks and holes between the grains in different sizes. It suggests that cementations in weaker rocks are firstly removed and then mineral grains broken, generating the intergranular and intragranular cracks and micro-fractures and fissures. Supercritical CO 2 jet cannot bring obvious changes to hard limestone and shale at the same condition because the rock substances are tightly packed and filled with large cementing forces and ultra-low permeability and porosity, but can produce a few micro-cracks vertical to bedding planes. The features show that supercritical CO 2 jet broke rock substances mainly in the brittle tensile failure mechanism and made the rock much easier to further break, accompanied with shear failure mechanism in particular locations of the perforation hole. In contrast, water jet impingement forms flat cutting sections in marble with continuous parallel breaking traces, indicating the shear failure mechanism. Low viscosity and high diffusivity of supercritical CO 2 are primarily attributed to the rock tensile failure mechanism, they facilitate supercritical CO 2 penetrates into deep into rock pores and produces static pressure on the inner wall and at the tips of the fractures, besides exerting jet impinging pressure. Moreover, the energizing effect caused by the phase change from supercritical to gas leads to the energizing effect and intensify rock breaking into more shattered pieces and, concurrently, improve general rock connectivity and porosity in the surface layers. As a consequence, supercritical CO 2 jet appears to be more efficient and suitable than water jet in slim-hole radial drilling and hydraulic fracturing, particularly in unconventional reservoirs with low permeability.

It is reported that supercritical carbon dioxide (SC-CO2) jet can efficiently erode rocks at a comparably low threshold pressure with high penetration rate. However, the influences of bottom-hole pressure and borehole irregularities on SC-CO2 jet cutting efficiency have not been studied. Therefore, in this research, comprehensive methods of numerical simulations and lab experiments were carried out to investigate the influences of the ambient pressure and the nozzle-to-target distance on the jet impinging pressure and perforation performance. Results show that, both the effective jet impinging pressure and the eroded depth of perforation hole notably decrease with the increase of the ambient pressure, when jet inlet pressure is constant. When the pressure difference between inlet pressure and ambient pressure is kept constant, the effective impinging pressure hardly changes with ambient pressure, but eroded depth increases at first and then decreases, bounded by the critical pressure of CO2. As the nozzle-to-target distance extends, both the depth and volume of the perforation hole decrease, and diameter of the perforation hole increases at first and then decreases. Under the research condition of certain pressure and temperature, a distance of 8 times the nozzle diameter is the critical distance that clarifies the different effect of nozzle-to-target distance on the hole diameter. Different features between using SC-CO2 jet and water jet, including the optimal distance for rock-erosion efficiency, are attributed to the difference in fluid properties. Further simulation results show that, unlike water, SC-CO2 fluid is compressible and it leads to specific variations of jet structure, namely the increase of length of SC-CO2 jet potential core, and flow type from non-submerged jet to submerged jet. As a consequence, under the simulated bottom-hole conditions, SC-CO2 jet is proven to be able to acquire high rock-erosion efficiency and the jet-assisted drilling rate at a larger application ranges. This research can promote the future application of SC-CO2 jets.

Low rate of penetration has been recognized as a major problem affecting the cost of operation in drilling ultra-deep wells (UDW) in the oil and gas industry. This study investigated hydraulic pulse jet (HPJ) technique for increasing rate of penetration in UDW drilling. Results of full-scale testing, numerical simulation, and field applications in the UDW conditions are presented in this paper. The full-scale testing indicates that the HPJ has characteristics of fluid-flow rate dependent amplitude and frequency of pressure fluctuation. This performance was confirmed by numerical simulation of fluid mechanics inside the tool. Field applications in the Tarim and Tahe oilfields, China, show that the rate of penetration (ROP) was increased by 22.2%–54.9% in drilling well sections ended at up to 6162 m. It has been concluded that the HPJ technique is suitable to drilling ultra-deep wells in China.

Hydro-jet fracturing is a unique hydraulic fracturing method, and has been recognized worldwide as an effective well-stimulation method for oil and gas production. However, because of controversy about the method's impact on water consumption and environmentally quality, hydraulic fracturing faces increasing restrictions. The demand for decreased water usage has promoted research on the use of non-aqueous fracturing fluids. In particular, the idea of using supercritical carbon dioxide (SC-CO2) fluid has attracted a lot of interest. Using SC-CO2 as a fracturing fluid is more effective and has significant advantages compared with water-based fluids. The unique properties of the SC-CO2 fluid facilitate fracture propagation, formation of complex fracture networks, and elimination of flow blockage in the rock matrix and pores. Therefore, the SC-CO2 jet fracturing technique may effectively enhance oil and gas recovery. Similar to hydro-jet fracturing, the pressurization effect in the perforation tunnel is a key precondition for successful jet fracturing with SC-CO2. This study investigated these effects using numerical simulations and experimental studies. Our results show that the pressurization effect during SC-CO2 jet fracturing improves with an increase in pressure difference, ambient pressure, nozzle diameter, and fluid temperature. However, pressurization becomes weaker with increasing casing hole diameter and annulus spacing. Comparing SC-CO2 and water jet fracturing pressurization effects suggests that using SC-CO2 jetting has advantages over water jet fracturing under in situ reservoir conditions. As the working depth increases, SC-CO2 jet fracturing achieves better pressurization than the water jet. The novel approach in our preliminary study begins to prove the feasibility of applying SC-CO2 jet fracturing to leverage unconventional resources.

Pulsating hydro-fracturing (PHF) technology, based on hydraulic fracturing and pulsating injection, is extremely applicable for coal seams because of the remarkably low mechanical strength of coal rock. Consequently, PHF is currently under development, and new applications are being explored in China. Current research studies on PHF mainly focus on coal rock damage via experimental methods; however, to the best of our knowledge, none of these studies consider stress disturbances, which significantly impact the stimulation effect. This paper presents a stress-disturbance numerical model in an infinite elastic formation that uses a high-order, staggered grid finite-difference scheme and boundary conditions that combine a perfectly matched layer with a quasi-static confining pressure load. We define the effective stress disturbance zone as an area where the principal tensile stress exceeds the tensile strength of the coal rock and investigate the influences of the technical parameters (frequency, amplitude, and mean stress) on the PHF stress disturbance effect. Several unique behaviors were observed. First, the effective disturbance zone caused by PHF is much larger than the zone caused by quasi-static fracturing. Second, although the disturbance zone created by PHF can be improved by increasing the source frequency, limitations in the stress parameters likely result in a threshold frequency. Third, in contrast with frequency, the disturbance zone created by PHF is positively correlated with the amplitude and mean stress, which have no threshold values. Finally, the impacts of the pulsating stress parameters are greater than those of the time parameter. As a result, the amplitude and mean stress should be maximized, while an excessively high frequency is unnecessary. These research results will provide guidance for optimizing the technical parameters for using pulsating hydro-fracturing technology in a coal seam.

Rock breaking with SC-CO2 has the advantages of low threshold pressure and high rate of penetration, so it has attracted full attention. In order to clarify the law of rock breaking with SC-CO2 jet, and enable it to serve SC-CO2 drilling better, the SC-CO2 jet system for rock breaking was used to carry out the experiments, which demonstrated that jet pressure, jet temperature, confining pressure, jet distance, rotary speed of core samples and jet time are the main factors to influence the rock-breaking performance and efficiency. The results showed the effects as follows. Jet pressure and confining pressure are the direct factors to affect rock-breaking performance. With the increase of jet pressure, the rock-breaking efficiency increases gradually, while as the confining pressure increases, it decreases with constant jet pressure or has the maximum around the critical jet pressure with constant pressure difference. With the increase of jet temperature, the efficiency increases at first and then decreases, and the relationship curve showed in parabolic function. Under the experimental conditions in this paper, the optimal jet distance is about 3–4 times of nozzle diameter. The rotary speed of core sample has no substantial influence on the erosion depth, but on the average width or diameter of eroded grooves. The average width and the size of the eroded cuttings are smaller when the rotary speed is higher. The erosion depth increases but the trend gradually decreases with the jet time being prolonged.

A user-defined function and a numerical simulation method were used to analyze the pressure drop of foam flow in the coiled tubing spiral section, and the calculated pressure drop was compared with the simulated pressure drop. For foam flow in the straight section, as the influences of acceleration pressure drop and actual velocity profile were considered, the simulated pressure drop is higher than the calculated pressure drop. For foam flow in the spiral section, due to the compressibility, the foam physical properties change with pressure. Therefore, at different outlet pressures, the velocity profile and density profile are different at the same spot. Centrifugal force makes the velocity profile unsymmetrical. Outlet pressure, gas injection rate, and curvature have significant influence on pressure drop. During the hydraulics design for coiled tubing drilling with foam, the foam injection pressure should be as high as possible to decrease the pressure drop of foam flow in the spiral section.

The Aktobe project is at the east margin of the Pre-Caspian Basin, where horizontal well is mainly used to develop the low porosity and permeability layers. To improve production per well, the horizontal interval increased from +/-400m to +1000m. Due to complicated geological structure, variable boundary and thickness of target layers, the RSS was employed to improve the rate in layer. But the average ROP was just 2.36m/hr, and average footage was just 335m per bit due to high strength of the formation. To solve this problem, the wearing characteristic of PDC bits was analyzed. The BHA and table rotary speed were optimized on the basis of drilling pipe vibrating calculation. Thus, the drilling technology for hard and long horizontal interval, including high performance PDC bit, RSS, PDM, BHA with 1/2 stabilizer, has been worked out. The technology was used in 5 wells, of which 4 wells at 60 rpm, had stick-slip index of less than 80%, the drill bits out of the wells had few cutters broken by vibrating; 1 well had stick-slip index of higher than 80% at 70-80rpm, and the drill bits out of the well had many cutters broken by vibrating. Compared with Well H814, the horizontal section drilling time shortened from 22.5d to 9.6d, by 57.7%, ROP increased by 261.4%, and the footage increased by 195.5%. The optimized horizontal drilling technology for hard formation could provide reference for the drilling of similar hard formation, especially for the other oilfields in the Pre-Caspian Basin.

With the commercial extraction of hydrocarbons in shale and tight reservoirs, efficient methods are needed to accelerate developing process. Supercritical CO

With the commercial extraction of hydrocarbons in shale and tight reservoirs, efficient methods are needed to accelerate developing process. Supercritical CO2 (SC-CO2) jet has been considered as a potential way due to its unique fluid properties. In this article, a new setup is designed for laboratory experiment to research the SC-CO2 jet’s characteristics in different jet temperatures, pressures, standoff distances, ambient pressures, etc. The setup is composed of five modules, including SC-CO2 generation system, pure SC-CO2 jet system, abrasive SC-CO2 jet system, CO2 recovery system, and data acquisition system. Now, a series of rock perforating (or case cutting) experiments have been successfully conducted using the setup about pure and abrasive SC-CO2 jet, and the results have proven the great perforating efficiency of SC-CO2 jet and the applications of this setup.

Sand production and blockage are common during the drilling and production of horizontal oil and gas wells as a result of formation breakdown. The use of high-pressure rotating jets and annular helical flow is an effective way to enhance horizontal wellbore cleanout. In this paper, we propose the idea of using supercritical CO2 (SC-CO2) as washing fluid in water-sensitive formation. SC-CO2 is manifested to be effective in preventing formation damage and enhancing production rate as drilling fluid, which justifies tis potential in wellbore cleanout. In order to investigate the effectiveness of SC-CO2 helical flow cleanout, we perform the numerical study on the annular flow field, which significantly affects sand cleanout efficiency, of SC-CO2 jets in horizontal wellbore. Based on the field data, the geometry model and mathematical models were built. Then a numerical simulation of the annular helical flow field by SC-CO2 jets was accomplished. The influences of several key parameters were investigated, and SC-CO2 jets were compared to conventional water jets. The results show that flow rate, ambient temperature, jet temperature, and nozzle assemblies play the most important roles on wellbore flow field. Once the difference between ambient temperatures and jet temperatures is kept constant, the wellbore velocity distributions will not change. With increasing lateral nozzle size or decreasing rear/forward nozzle size, suspending ability of SC-CO2 flow improves obviously. A back-propagation artificial neural network (BP-ANN) was successfully employed to match the operation parameters and SC-CO2 flow velocities. A comprehensive model was achieved to optimize the operation parameters according to two strategies: cost-saving strategy and local optimal strategy. This paper can help to understand the distinct characteristics of SC-CO2 flow. And it is the first time that the BP-ANN is introduced to analyze the flow field during wellbore cleanout in horizontal wells.