本文選題：頁巖 + 水力壓裂　； 參考：《中國科學技術大學》2017年博士論文

【摘要】：頁巖氣是儲藏于頁巖基質中的一種非常規(guī)天然氣資源。隨著常規(guī)資源日益減少以及能源需求不斷增加,頁巖氣得到全世界越來越廣泛的關注。頁巖氣藏由于具有低孔隙度低滲透率特點,需要進行水力壓裂等增產(chǎn)措施改造儲層才能獲得可觀經(jīng)濟產(chǎn)量。水平井多段壓裂是一種能有效提高頁巖儲層產(chǎn)量的重要技術手段。多段壓裂一般分為順序壓裂和同時壓裂,施工設計要求人工裂縫能夠盡量生成橫向直裂縫以延伸到較遠地層區(qū)域,溝通更大面積儲層以提高導流率。由于裂縫之間存在應力陰影效應導致裂縫偏離其預設方向生成非平面裂縫,降低了壓裂效果。增大裂縫間距能降低應力干擾強度,但是過大的裂縫間距也會使整個儲層導流率降低。合理的裂縫間距是水平井多段壓裂設計的關鍵因素。頁巖作為一種典型的沉積巖,在垂直于層理方向和平行于層理方向呈現(xiàn)顯著的各向異性力學特征,可以簡化為正交各向異性材料處理。水力裂縫在各向異性巖石中的起裂和擴展規(guī)律與在各向同性巖石中顯著不同。考慮巖石各向異性對水力裂縫擴展形態(tài)的影響是壓裂施工設計的關鍵力學問題之一。頁巖地層中包含大量天然裂縫,水力裂縫在擴展過程中與地層中天然裂縫相交形成復雜縫網(wǎng),能顯著提高地層導流率。地層中天然裂縫可以分為兩類:摩擦型天然裂縫和粘結型天然裂縫。水力裂縫與這兩種類型天然裂縫相互作用機理不同。研究水力裂縫與不同類型天然裂縫相交行為對預測頁巖地層中復雜縫網(wǎng)形成過程具有重要意義。水力壓裂是涉及巖石骨架變形、裂縫起裂和擴展、裂縫中流體流動、壓裂液濾失和巖石基質中孔隙滲流的多場耦合復雜力學問題,受限于理論研究和實驗研究的局限性,數(shù)值模擬成為研究水力壓裂的一種有效手段。擴展有限元法(XFEM)通過在傳統(tǒng)有限元位移插值函數(shù)中引入增強函數(shù)來描述裂紋等不連續(xù)位移場,可以模擬裂紋沿任意路徑擴展而不需要網(wǎng)格重構,極大減少計算量。本文基于擴展有限元法,建立了二維非線性流固耦合水力壓裂數(shù)值模型,研究水平井多段壓裂、正交各向異性巖石中裂縫擴展、水力裂縫與天然裂縫相交等問題:(1)研究了水平井多段壓裂裂縫之間應力干擾問題,對比了順序壓裂和交替壓裂方法應力干擾強度和最優(yōu)裂縫間距。結果表明,地應力差對應力干擾區(qū)域大小有顯著影響;地應力差較小時,需要增大裂縫間距才能避免應力陰影效應的影響。裂縫附近的應力分布會隨著裂縫的擴展不斷變化,這是在預測裂縫擴展路徑時必須要考慮的重要因素。交替壓裂通過在兩條已壓裂完成的裂縫中間壓裂第三條裂縫,可以利用應力干擾相互抵消的效應縮短生成橫向直裂縫所需壓裂間距。(2)研究了正交各向異性巖石的材料角和楊氏模量比對水力裂縫擴展路徑的影響。結果表明,當水力裂縫初始方向和正交各向異性巖石材料主軸之間有夾角時,裂縫擴展過程中將偏離其預設方向而生成非平面裂縫。水力裂縫在第一個擴展步會發(fā)生明顯的轉向,初始偏轉角隨材料角呈現(xiàn)周期性變化。水力裂縫偏轉程度隨楊氏模量比增大而增加,水力裂縫傾向于沿著楊氏模量更小的材料主軸方向擴展。地應力差較小時,裂縫擴展方向主要由材料正交各向異性特征決定;地應力差較大時,裂縫擴展方向主要由地應力決定。(3)研究了水力裂縫與天然裂縫相交行為,對比了摩擦型天然裂縫和粘結型天然裂縫對水力裂縫擴展行為和復雜縫網(wǎng)形成過程的影響。結果表明,當水力裂縫與摩擦型天然裂縫相交時,水力裂縫能否穿透天然裂縫取決于裂縫相交角、地應力差、裂縫摩擦系數(shù)和巖石抗張強度。當水力裂縫與粘結型天然裂縫相交時,水力裂縫能否穿透天然裂縫取決于裂縫相交角、粘結裂縫斷裂韌度和巖石基質斷裂韌度。水力裂縫與粘結型天然裂縫相交常形成L型裂縫,而與摩擦型天然裂縫相交常形成T型裂縫:因此,對于含有相同初始幾何構型天然裂縫的地層,水力壓裂在摩擦裂縫地層形成的縫網(wǎng)結構比在粘結裂縫地層形成的縫網(wǎng)結構更復雜。(4)研發(fā)了二維擴展有限元水力壓裂程序Matlab-XFEM,可以模擬各向同性巖石和正交各向異性巖石中裂縫的起裂和擴展,多條裂縫同時擴展,水力裂縫與天然裂縫相互作用以及復雜縫網(wǎng)的形成過程。本文建立的數(shù)值模型、分析方法和計算結果,能為頁巖氣藏水力壓裂施工設計提供一定的技術指導,以期獲得最優(yōu)壓裂效果,提高地層導流率以實現(xiàn)整個儲藏產(chǎn)量的增加。
[Abstract]:Shale gas is a kind of unconventional natural gas resources stored in shale matrix. With the increasing reduction of conventional resources and increasing energy demand, shale gas has been paid more and more attention all over the world. Shale gas reservoirs need water pressure cracking and other measures to reconstruct reservoir because of low porosity and low permeability. Multistage fracturing in horizontal wells is an important technical means to effectively improve the production of shale reservoirs. Multi section fracturing is usually divided into sequential fracturing and simultaneous fracturing. The construction design requires artificial fractures to be able to generate transverse straight fractures as far as possible to extend to the farther formation area, and to communicate a larger area of reservoir to improve the conductivity. The existence of stress shadow effect between cracks causes the crack to deviate from its preset direction to generate non plane cracks and reduce the fracturing effect. Increasing the crack spacing can reduce the stress interference intensity, but the excessive gap spacing will also reduce the whole reservoir conductivity. The reasonable gap spacing is the key factor for the multi section fracturing design of the horizontal well. For a typical sedimentary rock, it can be simplified as orthotropic material in the direction of bedding and parallel to the bedding direction. It can be simplified as orthotropic material treatment. The crack initiation and expansion of hydraulic fractures in anisotropic rocks is significantly different from that in isotropic rocks. The influence of the slit expansion form is one of the key mechanical problems in the fracturing design. In the shale formation, a large number of natural cracks are included in the shale formation, and the hydraulic cracks intersected with the natural cracks in the formation to form complex seams in the process of expansion. The formation conductivity can be greatly improved. The natural cracks in the strata can be divided into two types: friction natural cracks and bonded days. The interaction mechanism between hydraulic fractures and these two types of natural fractures is different. It is of great significance to study the intersecting behavior of hydraulic fractures and different types of natural fractures. The hydraulic fracturing involves the deformation of the rock skeleton, the cracking and expansion of the cracks, the fluid flow in the cracks, and the fracturing fluid filtration. The multi field coupled complex mechanics problem in the porous rock matrix is limited to the limitations of theoretical and experimental research. Numerical simulation is an effective method to study hydraulic fracturing. The extended finite element method (XFEM) is used to describe the discontinuous displacement fields, such as the crack, by introducing an enhancement function into the traditional finite element displacement interpolation function. In this paper, a two-dimensional nonlinear fluid solid coupling hydraulic fracturing numerical model is established based on the extended finite element method, which is based on the extended finite element method, and studies the multi section fracturing of horizontal wells, the crack propagation in the orthotropic rock, and the intersecting of hydraulic fractures and natural fractures. (1) study The stress interference between fractured fractures in multi section of horizontal wells is discussed, and the stress interference intensity and optimal gap between sequential and alternate fracturing methods are compared. The results show that the stress difference has a significant influence on the size of the force interfering region. The gap between the stress and the stress difference should be increased to avoid the effect of the stress shadow effect. The stress distribution in the vicinity will change with the expansion of the crack. This is an important factor that must be considered when predicting the crack propagation path. Through the fracturing of third fractures in the middle of the two fractured fracture, the fracture spacing of the fracture can be shortened by the effect of stress interference cancellation. (2) The effect of the material angle and the young's modulus of the orthotropic rock on the expansion path of the hydraulic fracture is studied. The results show that when the initial direction of the hydraulic fracture and the axis of the orthotropic rock material have a angle, the fracture propagation will deviate from the preset direction and become non plane cracks. The deflecting degree of the hydraulic crack increases with the ratio of Young's modulus, and the hydraulic crack tends to expand along the direction of the material spindle with smaller Young's modulus. The crack propagation direction is mainly determined by the orthogonal anisotropy of the material. When the stress difference is large, the direction of crack propagation is mainly determined by ground stress. (3) the intersecting behavior of hydraulic cracks and natural fractures is studied, and the effects of natural cracks and natural cracks on the expansion behavior of hydraulic fractures and the formation of complex seams are compared. Whether the force cracks can penetrate the natural cracks depends on the intersecting angle of the fracture, the difference of ground stress, the friction coefficient of the crack and the tensile strength of the rock. When the hydraulic crack intersects with the natural crack of the bond, it depends on the intersecting angle of the fracture, the fracture toughness of the bond crack and the fracture toughness of the rock matrix. Natural fracture intersecting often forms L type cracks, and intersects with friction natural fractures to form T type cracks. Therefore, the fractured net structure formed by hydraulic fracturing in the frictional fractured stratum is more complex for the stratum containing the same initial geometric configuration natural fissure. (4) the two dimensional extended finite element is developed. The hydraulic fracturing procedure Matlab-XFEM can simulate the cracking and expansion of cracks in isotropic rock and orthotropic rock, multiple cracks spread simultaneously, the interaction of hydraulic cracks and natural fractures and the formation of complex seams. The numerical model, analysis method and calculation results established in this paper can be used for hydraulic fracturing in shale gas reservoirs. The construction design provides some technical guidance for achieving the best fracturing effect and improving the formation diversion rate, so as to realize the increase of the whole storage production.

【學位授予單位】：中國科學技術大學
【學位級別】：博士
【學位授予年份】：2017
【分類號】：O346.1

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