Fracture geometry and breakdown pressure of radial borehole fracturing in multiple layers

Radial borehole fracturing that combines radial boreholes with hydraulic fracturing is anticipated to improve the output of tight oil and gas reservoirs.This paper aims to investigate fracture propagation and pressure characteristics of radial borehole fracturing in multiple layers.A series of laboratory experiments with artificial rock samples(395 mm×395 mm×395 mm)was conducted using a true triaxial fracturing device.Three crucial factors corresponding to the vertical distance of adjacent radial borehole layers(vertical distance),the azimuth and diameter of the radial borehole are examined.Experimental results show that radial borehole fracturing in multiple layers generates diverse fracture geometries.Four types of fractures are identified based on the connectivity between hydraulic fractures and radial boreholes.The vertical distance significantly influences fracture propagation perpendicular to the radial borehole axis.An increase in the vertical distance impedes fracture connection across multiple radial borehole layers and reduces the fracture propagation distance along the radial borehole axis.The azimuth also influences fracture propagation along the radial borehole axis.Increasing the azimuth reduces the guiding ability of radial boreholes,which makes the fracture quickly curve to the maximum horizontal stress direction.The breakdown pressure correlates with diverse fracture geometries observed.When the fractures connect multi-layer radial boreholes,increasing the vertical distance decreases the breakdown pressure.Decreasing the azimuth and increasing the diameter also decrease the breakdown pressure.The extrusion force exists between the adjacent fractures generated in radial boreholes in multiple rows,which plays a crucial role in enhancing the guiding ability of radial boreholes and results in higher breakdown pressure.The research provides valuable theoretical insights for the field application of radial borehole fracturing technology in tight oil and gas reservoirs.

Borehole stability in naturally fractured rocks with drilling mud intrusion and associated fracture strength weakening:A coupled DFN-DEM approach

Borehole instability in naturally fractured rocks poses significant challenges to drilling.Drilling mud invades the surrounding formations through natural fractures under the difference between the wellbore pressure(P w)and pore pressure(P p)during drilling,which may cause wellbore instability.However,the weakening of fracture strength due to mud intrusion is not considered in most existing borehole stability analyses,which may yield significant errors and misleading predictions.In addition,only limited factors were analyzed,and the fracture distribution was oversimplified.In this paper,the impacts of mud intrusion and associated fracture strength weakening on borehole stability in fractured rocks under both isotropic and anisotropic stress states are investigated using a coupled DEM(distinct element method)and DFN(discrete fracture network)method.It provides estimates of the effect of fracture strength weakening,wellbore pressure,in situ stresses,and sealing efficiency on borehole stability.The results show that mud intrusion and weakening of fracture strength can damage the borehole.This is demonstrated by the large displacement around the borehole,shear displacement on natural fractures,and the generation of fracture at shear limit.Mud intrusion reduces the shear strength of the fracture surface and leads to shear failure,which explains that the increase in mud weight may worsen borehole stability during overbalanced drilling in fractured formations.A higher in situ stress anisotropy exerts a significant influence on the mechanism of shear failure distribution around the wellbore.Moreover,the effect of sealing natural fractures on maintaining borehole stability is verified in this study,and the increase in sealing efficiency reduces the radial invasion distance of drilling mud.This study provides a directly quantitative prediction method of borehole instability in naturally fractured formations,which can consider the discrete fracture network,mud intrusion,and associated weakening of fracture strength.The information provided by the numerical approach(e.g.displacement around the borehole,shear displacement on fracture,and fracture at shear limit)is helpful for managing wellbore stability and designing wellbore-strengthening operations.

CO_(2)flooding in shale oil reservoir with radial borehole fracturing for CO_(2)storage and enhanced oil recovery

This study introduces a novel method integrating CO_(2)flooding with radial borehole fracturing for enhanced oil recovery and CO_(2)underground storage,a solution to the limited vertical stimulation reservoir volume in horizontal well fracturing.A numerical model is established to investigate the production rate,reservoir pressure field,and CO_(2)saturation distribution corresponding to changing time of CO_(2)flooding with radial borehole fracturing.A sensitivity analysis on the influence of CO_(2)injection location,layer spacing,pressure difference,borehole number,and hydraulic fractures on oil production and CO_(2)storage is conducted.The CO_(2)flooding process is divided into four stages.Reductions in layer spacing will significantly improve oil production rate and gas storage capacity.However,serious gas channeling can occur when the spacing is lower than 20 m.Increasing the pressure difference between the producer and injector,the borehole number,the hydraulic fracture height,and the fracture width can also increase the oil production rate and gas storage rate.Sensitivity analysis shows that layer spacing and fracture height greatly influence gas storage and oil production.Research outcomes are expected to provide a theoretical basis for the efficient development of shale oil reservoirs in the vertical direction.

Borehole stability in naturally fractured reservoirs luring production tests

Based on the plane of weakness theory, a model for predicting borehole stability in fractured reservoirs under different stress states was estiblisted and the equations for solving borehole stability were developed. The minimum downhole pressures required to maintain borehole stability under different natural fracture occurrences were calculated by using the data from a well in the Tazhong （central Tarim） area, Tarim Basin, west China. Several conclusions were drawn for naturally fractured reservoirs with a dip angle from less than 10° to greater than 30°. Application in three wells in the Tazhong area indicates that this model is practically useful.

Acoustic Borehole Images for Fracture Extraction and Analysis in Second Pre-pilot Drillhole of CCSD

Ultrasonic imaging logging provides continuous and oriented images of structures vs. depth. In the Chinese Continental Scientific Drilling (CCSD) Project, acoustic borehole images were recorded in the second pre-pilot drillhole which penetrates the metamorphic rocks. This paper focuses on fracture evaluation of the drillhole with these images. Both least square fit and a modified Hough transform are used for fracture extraction, and 269 fractures were mapped in the interval from 69.5 to 1 020 m. Most fractures dip steeply, with an average angle of 54°. Fracture dip directions are dominantly in the range of 220°-280° above the depth of 267 m, but 80°-120° in the lower zones. These observations may indicate the differences in structural movements or in-situ stress fields between the upper and lower zones in the drillhole.

THEORETICAL AND EXPERIMENTAL STUDIES ON FRACTURE PLANE CONTROL BLAST WITH NOTCHED BOREHOLES

１ＩＮＴＲＯＤＵＣＴＩＯＮＤｕｒｉｎｇｒｅｃｅｎｔｙｅａｒｓ，ｑｕｉｔｅａｆｅｗｉｎｖｅｓｔｉｇａｔｏｒｓｈａｖｅｗｏｒｋｅｄｏｎｆｒａｃｔｕｒｅｐｌａｎｅｃｏｎｔｒｏｌｂｌａｓｔｗｉｔｈｎｏｔｃｈｅｄｂｏｒｅｈｏｌｅｓ．Ｓｕｃｃｅｓｆｕｌａｐｐｌ...

Influence of fracture width on borehole radar response

Fracture is a common underground structure phenomenon,which can provide space and passage for the storage and migration of oil and gas.Borehole radar is a fast and high-resolution geophysical detection method,which has been widely used in engineering,exploration and other fields.This paper mainly uses theoretical analysis and numerical simulation to study the variation law of the characteristics of the received borehole radar signal with the variation of fracture width.The Ricker wavelet is used as the pulse signal of the borehole radar.The results show that the amplitude of the signal received by the borehole radar first increases,then decreases,and finally tends to be a stable value with the increase of fracture width.The results have guiding significance for the detection of underground fractures and the estimation of fracture width by borehole radar.

Tensile Fractures and in situ Stress Measurement Data Constraints on Cretaceous-Present Tectonic Stress Field Evolution of the Tanlu Fault Zone in Shandong Province,North China Craton

Tectonic stress fields are the key drivers of tectonic events and the evolution of regional structures.The tectonic stress field evolution of the Tanlu fault zone in Shandong Province,located in the east of the North China Craton(NCC),may have preserved records of the NCC’s tectonic history.Borehole television survey and hydraulic fracturing were conducted to analyze the paleo and present tectonic stress fields.Three groups of tensile fractures were identified via borehole television,their azimuths being NNW-SSE,NW-SE and NE-SW,representing multiple stages of tectonic events.Hydraulic fracturing data indicates that the study region is experiencing NEE-SWW-oriented compression and nearly-N-Soriented extension,in accordance with strike-slip and compression.Since the Cretaceous,the orientation of the extensional stress has evolved counterclockwise and sequentially from nearly-NW-SE-oriented to NE-SW-oriented and even nearly N-S-oriented,the stress state having transitioned from strike-slip-extension to strike-slip-compression,in association with the rotating and oblique subduction of the Pacific Plate beneath the NCC,with the participation of the Indian Plate.

Productivity enhancement in multilayered coalbed methane reservoirs by radial borehole fracturing

Coalbed methane(CBM)is an important unconventional natural gas.Exploitation of multilayered CBM reservoir is still facing the challenge of low production rate.Radial borehole fracturing,which integrates radial jet drilling and hydraulic fracturing,is expected to create complex fracture networks in multilayers and enhance CBM recovery.The main purpose of this paper is to investigate the mechanisms and efficacy of radial borehole fracturing in increasing CBM production in multiple layers.First,a two-phase flow and multi-scale 3 D fracture network including radial laterals,hydraulic fractures and face/butt cleats model is established,and embedded discrete fracture model(EDFM)is applied to handle the complex fracture networks.Then,effects of natural-fracture nonuniform distribution are investigated to show the advantages of targeted stimulation for radial borehole fracturing.Finally,two field CBM wells located in eastern Yunnan-western Guizhou,China were presented to illuminate the stimulation efficiency by radial borehole fracturing.The results indicated that compared with vertical well fracturing,radial borehole fracturing can achieve higher gas/water daily production rate and cumulative gas/water production,approximately 2 times higher.Targeted communications to cleats and sweet spots and flexibility in designing radial borehole parameters in different layers so as to increase fracture-network complexity and connectivity are the major reasons for production enhancement of radial borehole fracturing.Furthermore,the integration of geology-engineering is vital for the decision of radial borehole fracturing designing scheme.The key findings of this paper could provide useful insights towards understanding the capability of radial borehole fracturing in developing CBM and coal-measure gas in multiple-thin layers.

Effect analysis of borehole microseismic monitoring technology on shale gas fracturing in western Hubei

Hydraulic fracturing technology is an important means of shale gas development,and microseismic monitoring is the key technology of fracturing effect evaluation.In this study,hydraulic fracturing and microseismic monitoring were simultaneously conducted in the Eyangye 2HF well(hereinafter referred to as EYY2HF well).The target stratum of this well is the second member of the Doushantuo Formation of the Sinian System,which is the oldest stratum of horizontal shale gas wells in the world.A total of 4341 microseismic fracturing events were identified,and 23 fracturing stages of the well were defined.The fluctuation of the number of events showed a repeating“high-low”pattern,and the average energy of these events showed minimal differences.These findings indicate that the water pressure required for the reconstruction of the EYY2HF well is appropriate.The main body of the fracture network extended from northwest to southeast,consistent with the interpretation of regional geological and seismic data.The stimulated rock volumes showed a linear increase with the increase of the fracturing stage.Some technological measures,such as quick lift displacement,quick lift sand ratio,and pump stop for secondary sand addition,were adopted during fracturing to increase the complexity of the fracture network.Microseismic fracture monitoring of the well achieved expected eff ects and guided real-time fracturing operations and fracturing eff ect evaluation.

Fracturing timing of Jurassic reservoirs in the Dibei-Tuziluoke gas field,Kuqa foreland basin:Evidence from petrography,fluid inclusions,and clumped isotopes

Determining the timing of fracturing is crucial for understanding reservoir evolution and hydrocarbon accumulation in foreland basins.Using fracturing data from cores,borehole images,and outcrops,combined with the clumped isotope(D47)and fluid inclusion analyses of carbonate minerals filled in pores and fractures,this study ascertained the fracturing timing of the Jurassic reservoirs in the Dibei-Tuziluoke Gas Field,Kuqa Foreland Basin.Data from outcrops and borehole images show two dominant fracture sets in the study area:W-E and NE-SW striking fractures.Some W-E striking fractures are carbonate-filled,while NE-SW striking fractures lack mineral fillings.Bitumen veins,not easy to be identified in borehole images,are prevalent in cores.The petrographic analysis reveals that these bitumen veins formed before the calcite cementation in pores and display high viscosity and low maturity.Homogenization temperatures(T_(h))from primary fluid inclusion assemblages in two representative calcite vein samples were notably lower than T_(△47) values from corresponding samples.This suggests the △_(47) signature underwent alteration due to partial reordering during burial.Thus,△_(47)-derived temperatures(apparent temperatures)may not faithfully represent the mineral precipitation temperatures.When plotting these apparent temperatures vs.the burial history,only the possible latest ages of fracturing emerged.These ages were further refined by considering petroleum charging,tectonic evolution,and stress orientation.Bitumen-filled fractures likely resulted from the Late Cretaceous uplift,marking the migration of low-maturity hydrocarbons in the study area.Carbonate-filled E-W striking fractures emerged during the late Miocene(~13-6.5 Ma)alongside fold development.NE-striking fractures that crosscut W-E ones possibly formed recently due to stress reorientation.

Influence of rock fractures on the amplitude of dipole-source refl ected shear wave

To identify refl ector fractures near borehole by using dipole-source refl ected-shearwave logging, we need to understand the relation between the amplitude of the refl ected shear wave and the source radiation, borehole conditions, and attenuation owing to the surrounding formations. To assess the effect of these factors on the amplitude of the refl ected waves, we first studied the radiation performance and radiation direction of the dipole source in fast, medium, and slow formations by using the asymptotic solution in the far fi eld of the borehole. Then, the relation between the fracture parameters, and the refl ected-shear-wave amplitude as well as the ratio of the refl ected-shear-wave amplitude to the direct-wave amplitude (relative amplitude, RA) was evaluated by the three-dimensional fi nite-difference (3D FDTD) method. Finally, the fracture detection capability of the dipole reflected-shear-wave logging tool in different formations was analyzed by using the RA. The results suggest that the radiation amplitude of the SH-wave in the slow formation is weaker than those in the fast and medium formations, and the amplitude of the refl ected shear wave is lower. However, the RA in the slow formation is close to or even greater than in the fast and medium formations, which means that dipole-source shear-wave logging has the same or even better fracture detection capability in the slow formation as in the fast and medium formations. In addition, when RA is small, there is a good correlation between the RA and the various fracture parameters in the different types of formation, which can be used in determining the lower limit of the fracture parameters identifi ed by refl ection logging.

Fracture evolution and pressure relief gas drainage from distant protected coal seams under an extremely thick key stratum

When an extremely thick rock bed exists above a protected coal seam in the bending zone given the condition of a mining protective seam, this extremely thick rock bed controls the movement of the entire overlying stratum. This extremely thick rock bed, called a ＂main key stratum＂, will not subside nor break for a long time, causing lower fractures and bed separations not to close and gas can migrate to the bed separation areas along the fractures. These bed separations become gas enrichment areas. By analyzing the rule of fracture evolution and gas migration under the main key stratum after the deep protective coal seam has been mined, we propose a new gas drainage method which uses bore holes, drilled through rock and coal seams at great depths for draining pressure relief gas. In this method, the bores are located at a high level suction roadway （we can also drill them in the drilling field located high in an air gateway）. Given the practice in the Haizi mine, the gas drainage rate can reach 73% in the middie coal group, with a gas drainage radius over 100 m.

Cretaceous-Cenozoic regional stress field evolution from borehole imaging in the southern Jinzhou area, western Liaoning, North China Craton

The Mesozoic Yanshanian Movement affected the tectonic evolution of the North China Craton(NCC).It is proposed that Mesozoic cratonic destruction peaked~125 Ma,possibly influenced by subduction of the western Pacific Plate beneath the Euro-Asian Plate in the Early Cretaceous.The southern Jinzhou area in the eastern block of the NCC preserves clues about the tectonic events and related geological resources.Studies of the regional stress field evolution from the Cretaceous to the Cenozoic can enhance our understanding of the tectonics and dynamics of the NCC.Borehole image logging technology was used to identify and collect attitudes of tensile fractures from 11 boreholes;these were subdivided into four groups according to dip direction,i.e.,NNW-SSE,NWW-SEE,W-E and NE-SW.The development of these fractures was controlled primarily by the regional tectonic stress field;temperature,lithology,and depth contributed to some extent.In 136-125 Ma in the Early Cretaceous,the area was characterized by extension that was oriented NNW-SSE and NWW-SEE;from 125-101 Ma the extension was oriented W-E;after 101 Ma it was NE-SW.This counterclockwise trend has persisted to the present,probably related to oblique subduction of the Pacific Plate,and is characterized by ongoing extension that is nearly N-S-oriented and NEE-SWW-oriented compression.

2.5D boundary element simulation of wave propagation in a vertical fluid-filled borehole with irregular shape

In this paper, a boundary element formulation in the wave-number space domain for solving the wave equation for a borehole with arbitrary shape in acoustic logging problems is presented. The problem is treated as a two-dimensional medium with the discrete wave- number method in the vertical direction. The method is validated by comparing the results obtained by this method with those obtained by the finite-difference method. The method is used to study the effect on wave propagation in a vertical borehole of a vertical fracture. For a monopole source, the dispersion curves for Stoneley waves yield three branches. For dipole and quadrupole sources, different orientations of the source yield different results. When the dipole source is orthogonal to the fracture, the dispersion curve is similar to that of the open hole, while the curves are quite different when the source is parallel to the fracture. These characteristics enable us to determine the orientation of the vertical fracture.

Fracture Plane Control in Rock Blasting

Blasting operation dissipates much of the blasting energy in crushing the rock at the borehole and the resulting cracks are randomly oriented. There is very little control of the fracture plane. In order to control the fracture plane, many methods have been applied. This paper discusses a new blasting method in which a high degree of fracture control can be achieved while minimizing the ground shock.

THEORY ANALYSIS AND EXPERIMENT STUDY ON THE FRACTURE PROPAGATION UNDER BLASTING LOAD

This paper theoretically analyzed and discussed the fractures’ initiation, propagation and arrest of grooved borehole blasting under the function of explosion, and provided the optimistic values of blasting parameters. The stress intensity factor and velocity of crack propagation were measured through dynamic caustics measuring system.

倾斜碎软煤层群煤层气协调开发关键技术-以艾维尔沟矿区为例

为了明确与倾斜碎软煤层开采条件相适配的煤与煤层气协调开发关键技术,基于倾斜碎软煤层群瓦斯难抽采与煤层群非对称采动特征,形成了与之匹配的非对称卸压时空协同“三孔四区五量”煤与煤层气协调开发模式,明确了非对称采动下煤与煤层气协调开发策略,阐明了井上下联合煤与煤层气协调开发时空协同机制。针对倾斜碎软突出首采煤层消突困难的问题,优化了倾斜碎软煤层下向长钻孔施工工艺,研制了钻头电磁波无线随钻测量系统,实现了更高精度的钻孔轨迹防偏。针对非对称采动区地面井防护缺乏针对性的难题,阐明了非对称采动覆岩卸压破坏时空演化特征,获得了非对称采动下覆岩采动裂隙演化特征,确定了倾斜煤层地面井井位安全位置,研发了能够兼顾地面井稳定性和抽采效率双因素的采动区地面三开套管结构。应用结果表明:时空协同的“三孔四区五量”模式能够实现煤层气高效抽采,保障倾斜煤层群的抽掘采接替平衡;倾斜煤层下向钻孔递进式抽采方式拓展了递进式抽采的应用范围,破解了倾斜煤层煤层气抽采过程中的时间不协调问题;优化后的采动地面井维稳结构能够适应非对称采动作用,实现了采动区地面井煤层气高效抽采。上述成果在新疆焦煤集团艾维尔沟矿区得到推广应用,初步形成符合艾维尔沟矿区主要高瓦斯矿区倾斜煤层特点的煤层气抽采关键技术和典型模式。

超深勘察钻孔ASR法地应力测试技术应用

宝灵山隧道是某铁路先开段的重难点控制性工程,其中宝灵山DZ-06勘察钻孔深2118.00 m,是该段深度最大的控制性勘察钻孔。在钻孔的2072.53、2084.27 m深度选取合适岩心,采用ASR法(非弹性应变恢复法)开展了地应力测试,并与该钻孔采用水压致裂法获取的地应力结果进行对比,同时验证了ASR法对于估测超深钻孔的三维地应力大小和方向的实用价值。结果表明:在钻孔2072.53~2084.27 m深度范围,ASR法2个测点获取的最大主应力、中间主应力、最小主应力平均值分别为59.70、54.03、30.74 MPa,最大主应力近水平,方向近SN向,说明该区域以水平构造应力为主;ASR法的最大主应力测试结果均值与水压致裂法测试结果回归值一致性很好,最小主应力测试结果均值与水压致裂法测试结果回归值的一致性处于合理范围,两种方法的水平主应力方向测试结果基本一致。

瓦斯抽采SCMC-Bent基封孔水凝胶漏失特性及黏度演化机制

为实现瓦斯抽采钻孔漏气裂隙的有效动态封堵,防止负压驱动下浆液在裂隙中漏失,以羧甲基纤维素钠(SCMC)与钠基膨润土(Bent)为主料,通过引入系列添加剂制备了SCMC-Bent基封孔水凝胶,并测试了该水凝胶黏度的演化过程。然后通过开展负压作用下的浆液运移实验,研究了SCMC-Bent基水凝胶在煤颗粒间裂隙网中的运移及漏失特性。最后测试了SCMC-Bent基封孔水凝胶pH演化过程,采用傅里叶变换红外光谱(FTIR)、扫描电镜(SEM)及X射线衍射(XRD)表征了其微观结构,进而揭示SCMC-Bent基水凝胶黏度演化的微观机制。研究表明:①在恒温条件下,SCMC-Bent基水凝胶的黏度随时间呈现“先上升、后降低”趋势;水料比越大,SCMC-Bent基水凝胶的黏度越低、越早发生后期黏度下降。②水料比越高的水凝胶在裂隙网络中运移越快,但负压作用下却越早发生漏失;相较其他水料比,水料比10∶1的水凝胶在负压抽气第24天仅运移41 mm,更容易长时间密封漏气裂隙。③SCMC-Bent基水凝胶内部的蒙脱石层结构随时间先增多后减少,存在中间产物(SCMC-CXP)插入蒙脱石层间的现象,SCMC-CXP插入量随时间先增多后减少并随水料比增大而减少;蒙脱石层与层间SCMC-CXP形成了类似“三明治”的夹层结构,在水凝胶体系中逐渐形成了由蒙脱石层、层间SCMC-CXP、层外SCMC-CXP组成的三维网络。④三维网络结构的形成是造成SCMC-Bent基水凝胶黏度前期升高的原因;但随时间持续,中蒙脱石层结构减少且SCMCCXP大分子链断裂,导致了三维网络结构消散进而致使水凝胶黏度降低;另一方面,高水料比削弱了SCMC-CXP在蒙脱石层间的插入量及大分子间的相互作用、增强了三维网络结构的过水能力,进而导致水凝胶黏度下降。