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Radial drilling-fracturing, the integration of radial drilling and hydraulic fracturing, is an innovative approach to develop low permeability, thin target layer and naturally fractured reservoirs, etc. Understanding the fracture initiation process is required in the practical application to avoid high fracture initiation pressure (FIP) and complex fracture geometries near the wellbore. In this paper, we develop an analytical model to determine FIP, location of rock failure zones and initial fracture direction from the radial lateral borehole. This model is based on the stress superposition induced by cased main wellbore and radial borehole, and the maximum tensile stress criterion is adopted. Then, we perform a series of sensitivity analysis by examining different effects of in-situ stress regime, lateral orientation (the included angle between borehole axis and maximum horizontal in-situ stress), lateral length, and lateral diameter. Besides, the effect of pre-existing weakness plane, across which the tensile strength is much lower than that of the intact rock, is also investigated, when the plane is drilled through by the radial borehole. Results show that for intact rock with no weakness plane, the in-situ stress regime and lateral orientation are main factors influencing FIP, location of rock failure zones, and initial fracture direction. Under the in-situ stress regime of normal fault, fracture initiates vertically, and FIP enlarges as lateral orientation increasing; under the in-situ stress regime of strike slip, fracture initiates vertically with small lateral orientation, while fracture initiates horizontally with large lateral orientation; and under the in-situ stress regime of reverse fault, fracture initiates horizontally with any lateral orientation, and FIP decreases as the lateral orientation increasing. It is also found that when the fracture initiates vertically, the location of rock failure zones is at the base of radial borehole on its top and bottom sides; and when the fracture initiates horizontally, the location of rock failure zone is at the remote region of radial borehole on its right and left sides. The lateral length has a minor effect on fracture initiation, and the influence of lateral diameter is negligible. When the weakness plane is drilled through by the radial borehole, fracture initiation pressure from weakness plane is affected by its occurrence, i.e., azimuth and inclination. FIP is determined by taking the minimum between fracture initiation pressure from rock matrix and the weakness plane. The key findings of this work could provide critical insights into understanding radial drilling-fracturing initiation characteristics.

Based on bubble dynamics theory, a mathematic model describing the cavitation bubble size variation in the flow field of self-resonating cavitating jet was developed considering the pressure field and mass and heat exchange between cavitation bubble and ambient fluid. With this model, the influence factors on the cavitation intensity are investigated. The results show that the destructiveness of cavitating jet in breaking rocks depends on the bubble's first collapse, with decreasing intensity in the subsequent collapses. The self-resonating effect significantly enhances the cavitation intensity by promoting the collapse pressure and elongating its duration. Hydraulic parameters are proven to be the dominating factors influencing cavitation intensity: while collapse intensity monotonously increases with jet velocity, there exists an optimum ambient pressure where highest collapse intensity can be achieved. Conversely, the fluid properties show minor influences: cavitation intensity only slightly decreases with the increasing of fluid's density and barely changes with the variation of viscosity and surface tension. The results from this investigation help to uncover the mechanism of the enhanced erosion potential of self-resonating cavitating jet. The conclusions can be used to further improve the performance of self-resonating cavitating jet in field applications. Key words: self-resonating cavitating jet, cavitating bubble, collapse intensity, hydraulic parameters, fluid properties

Unconventional shale gas is rapidly increasing as an available source of natural gas in the worldwild. Horizontal well drilling and multistage fracturing are two principle techniques of shale gas resources production. Benefiting most from these two techniques, the United States, Canada, and China have become only three countries to produce shale gas in commercial quantities. The key technical issues emerged in the horizontal drilling and fracturing involve the rock fragmentation issues, wellbore trajectory optimization and control, wellbore stability, cementing and its quality control for long horizontal lateral, and hydraulic fracturing design. Although some progress associated with those technical issues have been achieved in China, there are still many geologic and engineering challenges: (1) the low rate of penetration and fast bit wearing for both PDC and Cone bits are still the primary issues, which makes hard achieve the “one-trip” drilling; (2) wellbore collapse and lost circulation are serious with a certain collapsing period due to natural fractures system; (3) complex multi-fractures propagation behaviors still need to be understood and the fundamentals of fracturing design should be established after undstanding multi-fractures propagation behaviors.

The supercritical carbon dioxide (SC-CO2) jet can break rocks at higher penetration rates and lower threshold pressures than the water jet. The abrasive SC-CO2 jet, formed by adding solid particles into the SC-CO2 jet, is expected to achieve higher operation efficiency in eroding hard rocks and cutting metals. With the computational fluid dynamics numerical simulation method, the characteristics of the flow field of the abrasive SC-CO2 jet are analyzed, as well as the main influencing factors. Results show that the two-phase axial velocities of the abrasive SC-CO2 jet is much higher than those of the abrasive water jet, when the pressure difference across the jet nozzle is held constant at 20 MPa, the optimal standoff distance for the largest particle impact velocity is approximately 5 times of the jet nozzle diameter; the fluid temperature and the volume concentration of the abrasive particles have modest influences on the two-phase velocities, the ambient pressure has a negligible influence when the pressure difference is held constant. Therefore the abrasive SC-CO2 jet is expected to assure more effective erosion and cutting performance. This work can provide guidance for subsequent lab experiments and promote practical applications.

As a new jet technology developed in recent years, the supercritical carbon dioxide (SC-CO2) jet technology enjoys many advantages when applied in oil and gas explorations. In order to study the properties and parametric influences of the SC-CO2 jet, the flow fields of the SC-CO2 jet are simulated using the computational fluid dynamics method. The flow field of the SC-CO2 is compared with that of the water jet. The influences of several parameters on the flow field of the SC-CO2 jet are studied. It is indicated that like the water jet, the velocity and the pressure of the SC-CO2 jet could be converted to each other, and the SC-CO2 jet can generate a significant impact pressure on the wall, the SC-CO2 jet has a stronger impact pressure and a higher velocity than those of the water jet under the same conditions, the maximum velocity and the impact pressure of the SC-CO2 jet increase with the increase of the nozzle pressure drop, under the stimulation condition of this study, the influence of the SC-CO2 temperature on the impact pressure can be neglected in engineering applications, while the maximum velocity of the SC-CO2 jet increases with the increase of the fluid temperature. This paper theoretically explores the properties of the SC-CO2 jet flow field, and the results might provide a theoretical basis for the application of the SC-CO2 jet in oil and gas well drillings and fracturing stimulations.

In cased and perforation completion method widely applied in oil and gas wells, it is difficult to avoid the cementing slurry damage to the lower pore pressure formations. This paper presents a new type of down-hole device, consisting of a packer and valve on each side, to isolate the target interval. Several devices can be connected to the casing strings as necessary and are able to make the cementing slurry bypass such formations. The internal pressure resistance was tested and the pressure loss was calculated when the slurry, at a flow rate of 1.5 m 3 /min, flowing through it. The surface experiment results show that the device can bear 28 MPa without any leakage. Through calculation, the extra pressure loss when normal cementing slurry flows each such device is only about 2.2 MPa, which can satisfy the field requirements. In casing completion operations, multiple devices can be connected to the casing string at the corresponding intervals. Pressuring the liquid in the casing, the pins at the end of the string will be cut off, allowing cement slurry to circulate and bypass such target sections. Two successful field cases applied in coal bed methane wells proved its feasibility. This technology will be able to provide a new promising completion method to avoid cementing slurry damage to formations with lower pore pressure intervals in oil, gas or coal-bed methane wells.

In order to make the unstable three-phase power output of wind turbine can charge the lithium battery pack steadily through the control of topology circuit and controller, a hybrid energy storage system controlled by two degree-of-freedom (DoF) PID is proposed to realize stable charging of lithium battery pack in this paper. The circuit combines the control circuit, the detection circuit and the protection circuit to achieve normal charging of the lithium battery pack. The system model composed of each module is built and simulated by MATLAB/Simulink software. The simulation results show that when the unstable three-phase voltage of wind turbine varies in the range of 0.4V-400V, the hybrid energy storage system keeps the charge current of the lithium battery group within the range of the normal range of the 2.5A-3.5A. And the system can effectively charge the lithium battery pack with the unstable three-phase power output of wind turbine.

In order to resolve the technical problem of cementing when drilling super high-pressure gas layers or saltwater layers of Guandu structure in Chishui region, Guizhou province, an ultra-heavy slurry was developed by optimizing weighting materials and designing particles, and applied to well Guanshen1 for liner cementing. According to the needs of preparing ultra-heavy slurry, the weighting additive MicroMAX in spherical particles was selected and combination of weight additives—reduced iron powder, iron powder, MicroMAX—was chosen for optimization. The particle size-distribution of the slurry was designed on the basis of the theory of tight packing, and the actual particle size-distribution is close to the ideal condition of tight packing. The ultra-heavy slurry with a density of 2.70-3.00 g/cm3 was designed, which is good in basic performance, flow ability and stability. In the simulated mixing test on the ground, a slurry with an average density of 2.71 g/cm3 was prepared by using conventional one-time cementing process. In the liner cementing of well Guanshen 1, the slurry with a density of 2.80 g/cm3 was used to successfully seal the super high-pressure saltwater layer: the average density of the slurry pumped into well was 2.78 g/cm3 and the maximum density was 2.82 g/cm3; amplitude log showed that the cementing quality was good; the well bore kept stable in follow-up drilling when the density of drilling fluid was reduced from 2.77 g/cm3 to 2.00 g/cm3. Key words: titania, photocatalysis, ceramic membrane, coupling reactor, methyl orange, degradation

The present trend of drilling deeper wells has increased the importance of estimating the wellbore temperature distribution and its variation with time. The need arises because it has a number of applications, such as: Cementing program design, drilling fluid rheology and design, drill tools planning, estimating mud properties and so on. The current wellbore temperature calculation method is through solving partial differential equation that obtaining the well temperature distribution. This method could obtain the well temperature distribution in different time and position, but the inherent characteristics could not be revealed. We plan to establish the well temperature state space model, and analyze the dynamic characteristics of well temperature according to this method and control theory. This work presents a study of the thermal behavior of drilling fluid during circulation and shut-in stages. A state space model is developed to calculate wellbore temperature distribution instead of solving partial differential equation. This model is based on wellbore heat transfer mechanism model and control theory. We analyze the dynamic characteristics of well temperature according to the state space model and control theory. Then we analyze how to adjust and control the bottom-hole temperature through operating surface parameters. The performance of the method is illustrated via the simulation of Holmes test well data. Wellbore temperature distribution profile and its variation with time are obtained according to the state space model. The calculation results shows that the maximum temperature is not appeared in the bottom-hole but appeared in some distance from the bottom-hole. The wellbore temperature will reach steady state in a few of circulation hours, the time to steady state is determined by fluid density, flow rate, specific heat. The bottom-hole temperature decreases as the flow rate or fluid density improving. Besides, we analyze the stability of wellbore heat transfer system according to Lyapunov theorem. The wellbore heat transfer system is steady system without large disturbance. This study can provide theoretical basis for automatic control of wellbore temperature in the future.

By analyzing the relationship between measured values of pressure loss in a high-pressure hose and calculated values of metal tubing pressure loss formula under the same condition, a calculation formula of pressure loss in a high-pressure hose is revised on the basis of the existing formula, and a model of calculating the circulating pressure loss in coiled tubing ultra-short radius radial drilling is established by theoretical derivation. The effects of tubing diameter, tubing length, pump delivery and fluid dynamic viscosity on each part of pressure loss in circulation system are studied by the pressure loss calculating model. The pressure loss in coiled and straight parts of 0.025 4 m (1 in) coiled tubing is about 8-10 times bigger than the pressure loss of 0.038 1 m (1.5 in) coiled tubing, and the pressure loss in high-pressure hose accounts for a large proportion in circulation system, about 86% in this study; and with drag reducer added in clear water, the pressure loss is about half of clear water pressure loss under the same condition. Key words: coiled tubing, radial drilling, high-pressure hose, circulation system, friction factor, pressure loss

To reduce the current high completion cost for steel slotted screen pipes in coalbed methane wells and to develop low-intensity non-metal completion pipes to perform low-cost completion, this paper adopts electric survey to measure the collapsing strength and bending strength of J55 steel screen pipe and PVC non-metal screen pipe, and makes a comparative analysis of the effect of slot width and slot density on the strength of both kinds of screen pipes. The research results indicate that: the collapse of steel screen pipes mainly results from stiffness reduction or radial unstableness, showing the whole pipe deformation or the slot becoming wide or narrow at the internal or external wall along the short axis, accompanied with the tearing at the slot end-point; while the PVC screen damage mainly results from the material itself. For both steel and PVC screen pipes, the pipe's collapsing/bending strength declines with increase of the slot width and density, but the PVC screen pipes do not show such an obvious trend, making it possible to apply wider and denser slots on PVC pipes to increase the passing area. Key words: coalbed methane well, screen pipe completion, steel screen pipe, PVC screen pipe, slot width, slot density, strength, experiment

Acid fracturing in deep carbonate reservoirs is challenged by deep well stimulation with high temperature (>120 °C), high fracture pressure (>2.0 MPa/m), high flow friction, and strong reservoir heterogeneity. To meet these challenges, a new stimulation method, called the hydraulic jet acid fracturing technique, was developed. According to the mechanisms of hydraulic jet acid fracturing, the authors self-design the downhole injector and pipe strings used in multistage hydraulic jet acid fracturing and provide optimization standards for the nozzle number and diameter combination, abrasive perforating parameter, and pumping program. The technique realizes multistage acid fracturing by hydraulic separation and features simple downhole tools, high temperature resistance (160 °C), low cost and risk. In addition, hydraulic acid injection can extend effective acid corrosion distance nearby well and enhance the acidification effect. The optimal jet phasing is 60 degrees with spiral arrangement to lower formation fracture pressure. A relationship chart between optimal flow rate and wellhead pressure is established, which helps to increase flow rate as far as possible under wellhead assembly capacity and to determine nozzle diameter and number. Results from field tests show that this method can work at a maximum depth of 6 400.53 m, with a total acid volume of up to 618 m3. It is effective in creating acid fractures in ultra-deep horizontal wells. Key words: horizontal well, deep formation, water jet, multistage acid-fracturing, field test

Traditional permanent magnet synchronous motor model is established on the basis of excluding the eddy current and hysteresis losses, it is no longer suitable for analysis and control when the motor efficiency is considered. This paper establishes a new model considering iron loss under MATLAB environment, and based on which the maximum efficiency control are analyzed. When considering iron loss, the maximum efficiency control is affected by speed and torque; Because of nonlinear and coupling, the maximum efficiency control is a very complex calculation that can not be achieved online. In this paper, neural network is used to get the direct and quadrature axis current command based on speed and torque. Simulation results show that the proposed method effectively decreases the total loss of the motor.

Aiming to study the influence of formation water invasion on the wellbore temperature and pressure in SC-CO2 (supercritical CO2) drilling with coiled tubing, this paper builds up a wellbore flow model with formation water invasion in SC-CO2 drilling with coiled tubing, based on the comprehensive investigation of the influence of viscosity, density, thermal conductivity, isobaric heat capacity and Joule-Thompson coefficient of SC-CO2. The wellbore temperature and pressure distribution were calculated by the method of coupling among these parameters. The results show that the bigger the rate of invaded formation water and the bigger the density of mixed fluid, the bigger the convective heat transfer coefficient in the annulus. Because of the Joule-Thompson cooling effect caused by nozzle throttling, the annulus Mixed fluid density increased abruptly and the convective heat transfer coefficient decreased abruptly at the well depth of about 1 900 m (about 100 m to bottom hole). Meanwhile the wellbore fluid temperature increased with the invasion rate of formation water, and the same Joule-Thompson cooling effect caused the wellbore fluid temperature to decrease abruptly at the well depth of about 1 900 m. Moreover, the wellbore annulus pressure increased with the increasing of invaded formation water quantity. But the amplitude is not obvious. Key words: coiled tubing, supercritical CO2 drilling, formation water invasion, wellbore temperature, wellbore pressure

Transmission characteristics of differential phase shift keying (DPSK) pressure signals transmitted in a downhole mud channel are researched by theoretical and numerical methods. With the control logic analysis of a rotating valve in continuous-wave telemetry, a DPSK pressure signal mathematical model is built in accordance with principles of communications and mathematical analysis. In the condition of water-based drilling fluids, drill pipes of a straight well are divided into a number of sections in accordance with wellbore pressure distribution. By transfer function analysis of all the sections, a numerical expression of the signal along the entire length of the vertical drill pipes is obtained and the influence of transmission distance, internal diameter of drill pipes, carrier frequency, drilling fluids viscosity, and air voids on the signal amplitude are analyzed. Numerical calculation shows that the influence of drilling fluids viscosity and air voids on the DPSK signal are the most notab...

In order to improve the rock breaking ability of jet nozzle in radial drilling technique, pulsed cavitating multihole nozzle was designed on the basis of multihole nozzle. And its modulation mechanism of pulsed cavitating jet was analyzed. Rock breaking test was carried out to compare the rock breaking results of pulsed cavitating multihole nozzle and multihole nozzle. The results indicated that the rock breaking result of pulsed cavitating multihole nozzle is better than that of multihole nozzle. The rock breaking volume is 1.23∼2.35 times. To guarantee the minimum hole diameter is bigger than nozzle diameter, jet impact pressure should not be lower than 30 MPa in the experiment. The experimental optimal standoff distance is 12 mm. The findings in this paper can increase the extension of horizontal holes and improve drilling rate in radial drilling technique.

The pipeline is the most economic and effective means to transport large quantities of oil and natural gas over long distances. However, in the long term, pipeline flaws may appear because of erosion, abrasion, unexpected damage and so on, which would weaken the pipeline safety performance and even result in leakage and pipe explosion accidents. For timely discovery of flaws and establishing optimal strategy of maintaining the pipeline, the in-served pipeline will be periodically online evaluated by using the smart PIG (pipeline inspection device) to detect flaws and locate them in the body of the pipeline. A method for flaw location of the pipeline is presented in this article, where a high resolution servo accelerometer CX-3 is used to measure the axial acceleration of the smart PIG in a pipeline to acquire the precise distance walked by the smart PIG by integral operation of the acceleration value to time variable and error correction. Basing on the linear relation between the distance and tim...

In order to improve the dissatisfied oil displacement results of conventional low molecular weight polymers in some thin and poor reservoirs of the Daqing Oilfield in China, two new polymers with low molecular weight are introduced. A series of laboratory experiments were performed to evaluate the performance index and oil displacement results of the novel polymers. The results show that the new polymers have the characteristics of higher viscosity, better salt and shearing resistance, and better solubility than conventional polymers, and the viscoelasticity and shearing resistance of the new polymers increases with a decrease in molecular weight. Bench-scale oil displacement experiments indicate that the oil displacement results of the new polymers are better than those of the conventional polymers with the same molecular weight and the performance will be better with a decrease in molecular weight. Novel polymers have a favorable matching relationship with low-permeability reservoirs that are f...

Deep penetrating perforation with a high-pressure water jet is an emerging advanced technique for enhancing oil well productivity because of its high cutting, breaking, and cleaning capabilities. Based on the analysis of productivity impairment caused by drilling fluid invaded zone and conventional charge perforating compacted zone, production-enhancing mechanisms of deep penetration perforating with a high-pressure water jet have been comprehensively investigated. The three major aspects are rock cutting with a high-pressure and high-velocity water jet, relieving the stress concentration of the near-wellbore region, and penetrating through the damaged zone. In addition, the feasibility of improving formation fracturing and acidizing treatment by using this technique is also discussed, along with future development and application.

The paper presents the investigations of a unique jet as self-resonating water jet including the modulating principles, characteristics of impact pressure and rock erosion, and field applications in petroleum engineering in China. The research showed that jet nozzles designed with organ pipe structure could generate significant self-resonating cavitating water jets. As compared with conventional cone-shaped nozzle with 120° taper angle, the amplitudes of pressure fluctuation and maximum impact pressure of self-resonating cavitating water jets increase by 24 and 37%, respectively, and rock erosion efficiency increases by 1 to 2 times. Self-resonating water jets have been successfully applied in petroleum drilling engineering, with enhanced average rates of penetration of tricone bits by 31.2% and improved bit footages by 29.1%. Treatment of near wellbore formation and self-excited oscillating water injection with self-resonating water jets have resulted in economic efficiency.

Supercritical carbon dioxide (SC-CO2) jet is characterized by low rock breaking threshold pressure and high rock breaking rate. Swirling-round jet, taking the advantages of both swirling and round jet, enjoys high rock breaking efficiency. By integrating swirling-round jet and SC-CO2, it is prospective for swirling-round SC-CO2 jet to improve rock erosion efficiency further. Rock damage mechanisms of swirling-round SC-CO2 jet are investigated by means of rock-erosion experiments and scanning electron microscope (SEM) observation. The selected core samples include shale, dolomite as well as sandstone. For shale, no macro-deformation occurs and microscopic damage is discovered, forming tiny cracks. For dominate and sandstone, erosion holes form, and crystal particle cleavage as well as inter-particle rupture occur. According to the observation results, the rock damage mechanisms of swirling-round SC-CO2 jet are divided into two aspects. On the one hand, swirling-round SC-CO2 jet breaks rock by combining axial impinging, radical tension and circumferential shear. On the other hand, with characteristics of low viscosity, compressibility and strong diffusivity, numerous micro fractures are caused by SC-CO2, and existing fractures are expanded, which led to fractures joining together and massive rocks split off.

High pressure waterjet technology is a potentially advanced technology to improve the drilling efficiency of shale rock formation. The success of waterjet drilling requires more insight into the rock fragmentation behaviors and their mechanics. In the present study, a series of submerged waterjet impinging trials were carried out on the outcrop shale rock collected from the Longmaxi Formation, Sichuan Basin, China. Simultaneously, the sandstone was selected as the candidate of the control group. The macro fragmentation behaviors and their micro structures were acquired by the scanning electron microscope (SEM) and computed tomography (CT) techniques. The rock mechanics and mineral composition before and after experiments were measured by the standard ISRM method and X-ray diffraction, respectively. Particularly, the grain size distributions of cuttings were obtained by a laser particle size analyzer. Results show that the efficiency of shale waterjet drilling is ultralow, which the rate of penetration and the ultimate drilling depth cannot reach the half of those sandstone. SEM imaging shows that shale is ultra-tight in which the illite clay weaves into a network and the dolomite embeds into the network. It just likes a concrete material resulting in the high compression strength. Since the static pressure of waterjet is lower than the compression strength, the common compression-shear failure does not happen, but only the tensile fracture occurs and makes the rock spallation for the initial crushing. With the penetration depth increasing, the waterjet impinging force is not able to make the rock tensile or shear failure. Only the particle erosion becomes a dominant failure mechanism for shale rock where the mineral particles are removed or crushed from rock matrix as fine cuttings. This work provides the theoretical guide for improving the shale rock drilling efficiency by water jetting.

This paper reviews the developments of investigation and application of high pressure water jet technology in both deep drilling and oil production including jet-bit drilling, extended nozzles and their combination to enhance hydraulic effectiveness at bottomhole, combined jet and mechanical drilling, new efficient jets (self-resonating cavitating jet, self-excited oscillation jet, swirling jet) used in drilling, horizontal well drilling by water jet, near wellbore plug removal with water jet, and high pressure water jet penetrating. Laboratory investigation and field application have coherently shown that high pressure jet drilling can increase the penetration rate. The hydraulic power of rock bit with extended nozzles and with crossflow hydraulics can increase by 30∼40 percent, the bit footage by 40 to 50 percent and dynamic pressure by 90 to 110 percent compared with conventional bit. Combined jet and mechanical drilling can increase drilling rate by a factor of 3 in deep wells than convention...

this paper aims to improve efficiency in permanent magnet synchronous (PM) motor drives. The authors of this paper present a new algorithm of minimum input power search for vector-controlled interior permanent-magnet synchronous motor drives, through analysis and simulation show that the algorithm has faster convergence rate than existing methods.

In this paper, a nonlinear PI controller is developed to control speed of PMSM, it can adjustment of controller parameters automatically in order to achieve the best control effect in control process. Simulation results show that it was faster than the traditional controller response and have a smaller overshoot.