Stability analysis of a borehole wall during horizontal directional drilling

In this paper, numerical simulation strategies are proposed and numerical analyses are performed to investigate the stability of a borehole wall during horizontal directional drilling in loose sand with an emphasis on the role of the filter cake in borehole stability. Two computational scenarios, one in the absence of a filter cake and one with the presence of a filter cake in a borehole wall, are investigated by considering both deep and shallow borehole situations. In the case where no filter cake is formed, the soil–drilling fluid interaction analysis shows that the effective pressure on soil particles will quickly decrease to zero even at a low drilling fluid pressure because of the rapid drainage of the drilling fluids into the loose sands. This conforms to the classical liquefaction criterion, indicating that static (flow) liquefaction-based soil crumbling and sloughing will occur even at a very low drilling fluid pressure if an effective filter cake is not formed. Soil’s permeability effect on pore pressure and the transition to a steady flow are also studied. In the second scenario in which a filter cake is formed, the hydraulic fracture failures around the bores are investigated, which are caused by the expansion of the yielding zones. The yield zone sizes and critical drilling fluid pressures at the moment of hydraulic fracturing failure are calculated from the finite element analyses and the closed-form solution, which is based on classical plasticity theories. The critical fluid pressures from the finite element analyses and the closed-form solutions are very close, but there is a large discrepancy between the yield zone sizes.     

The return of drilling fluid in large diameter horizontal directional drilling boreholes

The return of drilling fluid (drilling mud) in large diameter horizontal directional drilling (HDD) boreholes is a very important issue related to mud pressure, cuttings transport and pull back force, but this problem has not been fully solved in previous research due to the complexity and uncertainty of downhole conditions. The calculation methods of mud pressure loss in the borehole annulus are briefly introduced and the equation using a Power Law model is adopted to study the drilling mud return. A typical reaming hierarchy of a large diameter HDD project has been used to study the return direction of drilling mud and volumes of drilling mud return to the exit and entry points. The research results disclosed that the drilling mud can return to both exit and entry points at the same time, with a large majority of the drilling fluid returning to the exit point if the reamer is closer to the exit point, and vice versa. A parametric study has been conducted to find out the effect of changing parameters in the calculation equation on the drilling fluid return. The drilling mud return for those boreholes with the exit point and entry point located at different elevations has also been discussed. Finally, different types of reamers and other factors such as borehole collapse and borehole shape were taken into the consideration of drilling mud return.     

致密灰岩储层压裂裂缝扩展形态试验研究

开发低孔、低渗的致密灰岩储层需要进行大规模的水力压裂。致密灰岩的成岩过程、矿物组成以及岩石力学性质与致密砂岩等储层差异很大,在不同应力状态以及施工参数条件下水力裂缝扩展形态有待研究。采用真三轴水力压裂试验系统对致密灰岩露头展开压裂物模试验,研究地应力差、压裂液黏度、变排量、酸处理等多种因素对水平井压裂裂缝扩展规律的影响。试验结果表明:当水平地应力差在2~8 MPa之间时,水力裂缝易于沟通天然裂缝形成复杂裂缝网络;压裂液黏度升高,会降低剪切滑移和滤失膨胀的可能性,从而降低裂缝的复杂程度;在走滑断层的应力状态下,即σ_Hσ_Vσ_h,容易形成水平缝,特别是当井眼方向沿着层理面时极易沿着层理起裂;变排量压裂可以激活更多的天然裂缝,有助于形成复杂的裂缝网络;酸液预处理裸眼井段能够显著降低破裂压力,随泡酸时间的增加,破裂压力下降幅度逐渐增大。研究成果为现场压裂施工提供参考。     

流固耦合作用下水平定向孔壁稳定性分析

孔壁失稳是目前水平定向钻进工程中经常遇到的一个实际问题,尤其是在地表的第四纪松散地层中钻进孔壁失稳更是屡见不鲜,严重影响工程施工。针对以往研究中没有考虑渗流场对孔壁稳定性的影响问题,本文在Biot固结理论的基础之上,建立了应力场和渗流场耦合数学模型,采用有限元方法对所建立的模型进行求解。通过对某一水平定向钻进孔壁稳定性进行数值模拟,分析了在流固耦合作用下地层孔隙压力、有效应力和位移的变化规律,并分析了在不同的冲洗液压力下,地层沉降情况。研究发现,在定向钻进过程中,流固耦合作用对地层特别是井壁附近围岩的应力和变形影响很大。研究结果为钻进过程中避免孔壁失稳,合理选择冲洗液及冲洗液压力提供理论依据。     

Analysis of formation fracturing for the Maxi-HDD Qin River crossing project in China

A new irrigation pipeline was installed using the Horizontal Directional Drilling (HDD) method in order to alleviate the water shortage situation in Jiaozuo City, Henan Province, China. This pipe installation project included two parallel HDD crossings, with a pipe diameter of 1016 mm and a driven length of 1750 m for both of them. Based on the preliminary detailed geological survey report of the riverbed, hydro-fracturing control under high slurry pressure would be a big challenge for this project. This paper summarizes the prediction methods for the maximum allowable mud pressure in the borehole based on different mechanisms of formation fracturing. Based on the geological data of this crossing project, the Delft equation and another prediction method based on tensile failure (termed here the Xia method) were used to calculate the maximum allowable annular pressure. The real pumping pressure of this project was recorded by the contractor. By comparing the theoretical predictions of allowable pressure with the actual pressures for the horizontal section of the borehole, it indicated that the Xia method is conservative and the limits of the Delft equation could not be verified in this project. As a solution dealing with potential hydro-fracturing, finite element modeling shown that increasing the ground surface load would rise the borehole allowable pressure at which fracturing would occur. This solution was used to deal with a blow-out in the entry section of one pilot hole on this project. Under the guidance of the theoretical and numerical simulation results, the Qin River crossing project was successfully completed.     

Drilling-induced tensile wall-fractures: implications for determination of in-situ stress orientation and magnitude

Detailed investigation of failure of the borehole wall in two scientific drilling projects, the German KTB (Kontinentales Tiefbohrprogramm der Bundesrepublik Deutschland) and the European geothermal research project at Soultz-sous-Forêts, France, has lead to new insight in the phenomena of tensile fractures induced in the wellbore wall during drilling. Comparison of the orientation of the fractures with the orientation of the horizontal principal stress known from breakout and hydraulic fracturing analysis demonstrates that these fractures are reliable indicators of the orientation of the maximum horizontal principal stress S_H. A model for the initiation of the fractures is presented which points out the important influences of (a) the tectonic stress state, (b) increased mud pressures during drilling operation and (c) thermal stresses induced by circulation of relatively cold drilling mud. Analysis of drilling-induced fractures in the GPK1 borehole at Soultz-sous-Forêts (where the magnitude of S_H is known from hydraulic fracturing experiments) demonstrates the validity of this model for the initiation of the fractures. Further, a new method is proposed to estimate the magnitude of S_H from the occurrence of drilling-induced fractures and knowledge of thermally induced stress and pumping pressure during drilling. The method is successfully applied to both KTB boreholes. An independent method to estimate the magnitude of S_H based on the analytical calculation of the stress intensity factor for drilling-induced fractures taking into consideration both, increased mud pressure and thermal stress, is also presented. Application of this method confirms the results derived with the analysis described above. Additionally, the evaluation of the orientation of the fractures with respect to the wellbore axis indicates that over major depth sections of the investigated wells the vertical stress is a principal stress.     

钙芒硝盐岩水力压裂裂纹端面特征试验研究

为研究钙芒硝水力压裂作用下裂纹扩展-溶解机理,对钙芒硝矿层的流体化开采提供参考,同时填补水力压裂中只有物理作用而无化学作用的空白,丰富水力压裂理论,作者采用室内试验的方法,分别进行了钙芒硝试件在轴压/围压为5/4 MPa、7/4 MPa、9/4 MPa三种应力条件下的水力压裂实验,并采用3D形貌扫描仪对破坏断面的裂纹形态进行扫描,结合裂纹粗糙度表征公式对试验数据进行计算分析。研究发现:钙芒硝水力压裂是应力和溶解的共同作用,压裂过程大致可以分为水压升高阶段、裂纹扩展阶段、水压下降阶段3个阶段;3个阶段的划分与前人研究成果相同,但裂纹扩展阶段的压力波动剧烈,与其他岩石(例如,煤,页岩)水力压裂试验结果相差较大,其为钙芒硝溶解现象影响所致;不同应力下,钙芒硝水压致裂后裂纹形态不同;当垂向应力差异系数(k)≤0.75时,钙芒硝盐岩产生横向裂纹,k≥1.25时,钙芒硝盐岩将产生纵向裂纹;同一应力状态下钙芒硝清水致裂后裂纹端面粗糙度大于饱和盐水致裂后的粗糙度,不同应力状态下,水压致裂后粗糙度随轴压的增大而增大。研究成果对钙芒硝矿层的原位溶浸流体化开采具有重要的工程价值。     

应用水压致裂法测量三维地应力的几个问题

以新疆某抽水蓄能电站地下厂房区地应力实测资料为基础,探讨了当前应用水压致裂法测量三维地应力存在的几个问题,提出了孔壁出现倾斜水压破裂时计算三维地应力的简便方法,并对水压致裂三维地应力测量方法的应用前景进行了讨论。     

Prediction of rotational torque and design of reaming program using horizontal directional drilling in rock strata

Horizontal directional drilling (HDD) has been widely used to efficiently and cost-effectively install pipelines in a variety of conditions. In particular, during application of HDD in the recent West-East Natural Gas Transmission Project in China, rocky conditions were encountered. In such difficult situations, it is necessary to predict the torque required for performing the reaming operation, as well as to select the appropriate type and size of reamers. However, there is presently not an appropriate and convenient method to accomplish these tasks, especially for large pipelines, such as 1219 mm (48 in.) diameter, which typically require a borehole of approximately 1600 mm (64 in.). The process of expanding the initial borehole to the ultimate required diameter in rocky strata is often a complicated procedure involving several passes of intermediate size reamers. Although available information for drilling techniques does provide some means of predicting the torque, it is not sufficient for meeting the present needs, including providing guidelines for determining the corresponding reaming program. Therefore, the results of recent research, which addresses rock crushing energy, have been incorporated to develop an improved prediction method. The results reflect rock characteristics collected at different job sites, which may be applied to new projects containing similar rock strata. The improved methodology is a valuable aide for engineers in their selection of an appropriate back-reaming program.     

基于钻液压力的非开挖近孔壁位移响应数值分析

水平孔壁的位移特征直接影响非开挖水平定向钻进敷设管道的质量,而钻孔液的压力是影响孔壁位移的重要因素之一。采用快速拉格朗日的计算方法,利用Mohr-Coulomb准则对美国洛杉矶地下燃气管道敷设中的近水平孔壁位移响应特征进行了数值计算分析。获得了在不同钻孔液压力(0~2 MPa)条件下近水平孔壁x、y和z三个方向的位移矢量、应力分布数据。分析结果表明,随着钻液压力的不断增加,近孔壁各点的x和z位移呈线性增加趋势,z方向变化更为明显,y方向位移基本保持不变。同时,位移变化区域集中在距离孔壁1.6~1.8 m范围之内。因此,合理控制钻液压力的范围对于减少孔壁位移扰动具有重要意义。     

CO2–水两相条件下砂岩致裂特征与有效应力模型的试验研究

 CO2–水两相条件下的岩石力学行为是CO2地质封存中的关键科学问题之一。利用自主研发的厚壁圆筒式两相致裂仪进行CO2–水两相条件下砂岩压裂试验,并研究CO2–水两相有效应力模型。通过向试样加载内压、围压和轴压形成与井壁围岩类似的最小主应力为负的真三轴应力状态,采用半透隔板法进行CO2驱水试验以精确控制试样孔隙中CO2–水两相流体的组分压力和饱和度,最后通过内压致裂试样获得致裂压力。还进行了不同有效围压下含单相水孔流试样的致裂试验,得到其致裂压与有效围压的拟合函数。为研究CO2–水两相有效应力模型,选用4种可应用于岩石的非饱和介质有效应力公式与拟合函数联立推导得到破裂压力预测模型,与试验结果对比以间接验证这4种有效应力公式对于表征CO2–水两相流体条件下有效应力的适用性。结果表明,考虑Biot系数的Bishop非饱和有效应力模型能够较好地描述CO2–水两相流体下所测试砂岩的张性开裂行为,可直接作为CO2–水两相有效应力模型使用。     

定向水力压裂控制煤矿坚硬难垮顶板试验

 针对煤矿坚硬难垮顶板控制的研究现状及存在的问题,进行定向水力压裂控制煤矿坚硬难垮顶板井下试验。通过在压裂孔两侧布置监测孔和在压裂过程中实时监测泵压变化,深入分析煤矿坚硬难垮顶板水力压裂特点。试验结果表明：(1) KZ54型切槽钻头能够在坚硬岩层中预制横向切槽,可有效降低裂缝破裂所需压力;(2) 采用跨式膨胀型封隔器可对岩层坚硬段分段逐次压裂,压裂过程中可在顶板中产生多条裂缝,从而有效弱化顶板;(3) 随着压裂处与孔口距离的增大,裂缝破裂和扩展所需的压力也相应增大,裂缝的扩展半径最大可达20 m;(4) 在压裂过程中,由于岩层均匀性、渗透性、地应力场、岩层结构面等影响因素的变化,压力–时间曲线呈现出多种形态;(5) 岩石抗拉强度与地应力值较为接近时,岩石强度对水力压裂有较大影响。     

Borehole stability analysis considering spatial variability and poroelastoplasticity

Typically, stability analysis of oil-producing wells is carried out considering that both the hydraulic and mechanical parameters of the rock mass are deterministic. When analyzing borehole stability, two failure mechanisms are generally considered, namely, failure due to either tensile or compressive (shear) stresses. These mechanisms are produced, respectively, by either too high or too low drilling fluid pressure. Mechanical and hydraulic properties of rock masses, and in particular sedimentary rock masses, may show a considerable degree of spatial variability. This paper focuses on the evaluation of the limits for the internal pressure associated with a target probability of failure, taking into account both the spatial variability of hydraulic and mechanical properties, and the simple variability of the initial pore pressure and in-situ stresses. The analysis is performed with a finite element program that incorporates coupled fluid-mechanical effects and elastoplastic behavior of the rock. In this way, the proposed borehole stability analysis can be set into a reliability-based framework described in the paper. Examples are shown, using stochastic data from the literature, and conclusions are drawn regarding the effect of spatial variability on the borehole stability.     

万福煤矿深部水压致裂地应力测量

为了克服煤田上覆冲击层厚度高达700m所造成的困难，保证超千米深部地应力测量的成功，对传统水压致裂法测量方法和设备进行改进，增加测量系统的耐压能力和测试系统的精度。在完成的7个钻孔、37个测点的现场实测中，6个钻孔的深度接近和超过1100m，最大测点深度为1105m，创造了我国水压致裂法测量地应力钻孔深度的新记录。通过实测，确定了矿区的地应力状态及其分布规律。研究结果表明，37个测点的最大水平主应力与垂直主应力的比值为1．441～3．224，平均为1．97，反映了矿区地应力场以水平应力为主导的特点。测量结果可为矿山建设和开采设计提供科学的依据。     

注液方式对深层页岩裂缝形态影响实验研究

开发深层页岩需要通过对储层进行水力压裂,而注液方式会对压裂效果产生很大的影响。采用真三轴水力压裂实验系统,研究不同的注液方式对深层页岩的裂缝形态的影响规律。实验结果表明:高地应力差下水力裂缝会沿着垂直于最小地应力方向起裂并扩展成横切缝,遇层理或天然裂缝会沟通并形成分支缝;采用先小排量后提升至大排量的注液方式可以开启更多的裂缝;先注入高粘度的压裂液制造主缝,然后替换为低粘度压裂液开启分支缝,形成主缝+分支缝的裂缝形态,有利于提升储层SRV。本实验研究结果可以为深层页岩压裂设计提供参考。     

脆性岩石热–水–力耦合断裂的断口分析

 利用自行设计的热–水–力(THM)耦合断裂试验和扫描电镜试验,研究脆性岩石THM耦合断裂的宏微观特征;通过有限元法和新型应力强度因子比断裂准则,判断THM耦合断裂模式,揭示出THM耦合断裂机制。结果表明：红砂岩试件具有3种THM耦合宏观断裂轨迹和对应的3种THM耦合微观断裂特征,即低温、低水压、高围压条件下的断裂轨迹为横向断裂,微观上表现为穿晶的剪切断裂;高温、高水压、低围压条件下的断裂轨迹为纵向断裂,微观上表现为沿晶的拉伸断裂;中等温度、中等水压、中等围压条件的断裂轨迹为双向断裂,微观上表现为含沿晶和穿晶的拉剪复合型断裂。基于有限元法和新型最大应力强度因子比断裂准则判断出的红砂岩试件THM耦合断裂模式与基于宏微观断口分析得到的THM耦合断裂机制完全一致。     

单钻孔中水压致裂法三维地应力测量的新进展

由于测量深度深和其他突出优点,水压致裂法地应力测量已被国内外广泛应用,但以往只能测量钻孔横截面上的二维地应力状态。长江科学院在国内率先提出在3个不同方向钻孔中和单钻孔中进行三维地应力测量的原理和方法,扩大其应用范围,并已在工程中得到广泛应用。近期,长江科学院又提出新的单钻孔水压致裂法三维地应力测量原理,只需选择2个或2个以上原生裂隙段进行重张试验的测量,或者在完整岩体段常规压裂试验的同时,选择1个或1个以上原生裂隙段进行重张试验的测量,就可确定岩体的三维地应力状态,并且可以应用到深钻孔的测量中,在工程上有广泛的应用前景。对该测量方法资料整理的计算公式严格进行推导,在实例中,三维地应力测量的测量成果,不仅得到水压致裂法钻孔横截面上实测的二维应力状态的检验,还得到同一钻孔对应深度的套芯应力解除法实测的三维地应力状态的印证。     

Strength criterion for rocks under compressive-tensile stresses and its application

Estimating in-situ stress with hydraulic borehole fracturing involves tensile strength of rock. Several strength criteria with three parameters result in tensile strengths with great differences, although they may describe the relation between strength of rock and confining pressure with low misfits. The exponential criterion provides acceptable magnitudes of tensile strengths for granites and over-estimates that for other rocks, but the criterion with tension cut-off is applicable to all rocks. The breakdown pressure will be lower than the shut-in pressure during hydraulic borehole fracturing, when the maximum horizontal principal stress is 2 times larger than the minor one; and it is not the peak value in the first cycle, but the point where the slope of pressure-time curve begins to decline.     

Acoustic emission monitoring of hydraulic fracturing in laboratory and field

In the hydraulic fracturing tests, water was injected into four kinds of granitic rock specimens with different grain sizes. The fault plane solutions of acoustic emission (AE) indicated that the shear fracturing is dominant in the specimens with larger grains whereas the tensile fracturing is dominant in those of smaller grains. In addition, the experiments were conducted in granitic rock specimens of the same kind, under three different conditions: injection of water, injection of viscous oil and pressurization via a urethane sleeve. The shear fracturing was dominant due to water injection and the pressurization whereas the tensile fracturing was dominant due to viscous oil injection. Further in a field, AE was monitored by a borehole sonde in the hydraulic fracturing to measure rock stress. The AE count rates increased with closing and reopening hydraulically induced cracks. The result suggested that AE monitoring could improve the reliability and the accuracy of hydraulic fracturing stress measurements.     

在单钻孔中水压致裂法的三维地应力测量

 提出了利用钻孔中原生裂隙的重张试验, 在单钻孔中进行水压致裂法的三维地应力测量的原理和方法。对利用原生裂隙的重张试验及其与完整岩体段常规压裂试验相结合的两种基本情况, 推导了计算公式, 可操作性很强。