Chnghua Ou ,Chnggang Liang ,Zhaoliang Li ,Li Luo ,Xiao Yang
a State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation,Southwest Petroleum University,Chengdu,610500,China
b Petroleum Engineering School,Southwest Petroleum University,Chengdu,610500,China
c Jiqing Operation Area of PetroChina Xinjiang Oilfield Branch,Fukang,831511,China
d Exploration and Development Research Institute of PetroChina Qinghai Oilfield Branch,Dunhuang,736200,China
e Southwest Branch of China Petroleum Group Logging Co.,Ltd.,Chongqing,400021,China
f Southwest Research Institute of Dongfang Geophysical Exploration Co.,Ltd.,Chengdu,610051,China
Keywords:3D visualization Micro-seismic monitoring Hydraulic fracture Jiao reservoir Reservoir modelling
ABSTRACT In this paper,a new 3D visualization technical method was developed for hydraulic fractures using micro-seismic monitoring.This technical method consists of four steps:i.interpret the geologic hydraulic fracture model based on seismic source location data from micro-seismic monitoring;ii.develop a hydraulic fracture indication model,relying on the 3D spatial freeze-frame of micro-seismic monitoring sources from hydraulic fracturing;iii.construct a hydraulic fracture density model using the intensity from the micro-seismic monitoring;and iv.implement a 3D visualization of the hydraulic fractures,relying on the spatial constraints of the density model,the hydraulic fracture indication model,and the properties of the hydraulic fractures.This proposed technical method was used to produce 3D visualizations of the hydraulic fractures in well X in the Jiao reservoir,China,and the 3D visualizations of the distribution,development,extent and cutting relationships of hydraulic fractures were successfully realized.The results show that this technical method can be used as a practical and reliable approach to characterize hydraulic fractures.
In recent years,exploration and production of tight oil and gas have become increasingly important [1-4].Hydraulic fracturing generates the fracture networks required for effective tight oil and gas production [5,6].One of the key techniques is to establish a discrete 3D network model to visualize the fracture network [7,8],evaluate hydraulic fracturing,simulate single-well production,and select an exploitation method [9-11].However,it remains a challenge to generate reliable 3D hydraulic fracture visualizations.
Hydraulic fracturing has been performed millions of times worldwide over the past 60+years [6,12,13].The effectiveness of hydraulic fracturing is usually measured by the production rate[14]or the stimulated reservoir volume(SRV)on reservoir performance[15].It is difficult to determine the exact distribution of the hydraulic fractures based only on the production rate or SRV.So the micro-seismic data are often used to quantify the effectiveness[16,17]of the hydraulic fracturing.
It has been 40 years since the initial application of micro-seismic monitoring in the petroleum field [16-18].During hydraulic fracturing,the formation pore pressure increases with the continuous injection of fracturing fluid,local stresses become strongly concentrated in the target strata,and the strain energy in the strata around the fracturing point accumulates rapidly.Once the strain energy reaches the fracture pressure of the rock,the rock will break and release elastic waves,inducing micro-seismicity [16,19].The micro-seismic monitoring technology for hydraulic fracturing was develcoped by monitoring and interpreting micro-seismic source location and energy from transient elastic waves [20,21].The spatial location and energy data from the micro-seismic monitoring during hydraulic fracturing are mainly used to characterize the hydraulic fracture.Data from micro-seismic monitoring have been widely and successfully applied to characterize the scale and scope of hydraulic fractures.However,the 3D visualization research on hydraulic fracturing is not very deep,there remains room for improvement in the field of 3D visualization of hydraulic fractures using micro-seismic monitoring [22,23].
Relying on micro-seismic monitoring data and Petrel platform,a new quantitative visualization technical method was developed to present a 3D visualization of the distribution,development,extent and cutting relationships of hydraulic fractures.Moreover,the hydraulic fractures of an actual well X in the Jiao reservoir were successfully visualized using this proposed technical method.
The workflow of the proposed 3D visualization technical method of hydraulic fracture is shown in Fig.1,and the implementation of this technical method is described in detail below.
Detailed field operation and data processing of micro-seismic monitoring have been extensively studied [16-23].The process of micro-seismic monitoring after field operation and data collection can be described in four steps.
(1) A velocity model is determined by considering the acoustic logging curve and the detonator in the same well.First,the initial P-wave velocity is calculated from the acoustic logging curve.Then,different horizontal layer velocities along the wellbore are established by squaring the calculated P-wave velocity.At the same time,the two-way time is detected by the detonator,where a detected velocity value at a known depth is obtained.The detected velocity value is used to calibrate and optimise the horizontal layer velocities.Finally,the horizontal layer velocity model is built.Through this technical method,an anisotropic horizontal layer velocity model can be developed using one well(for example,well X discussed later).When there are multiple wells,an anisotropic horizontal layer velocity model is developed.
(2) Improve the signal-to-noise ratio (SNR) by suppressing the interference signals and enhancing the effective microseismic signals.Use the adjacent traces comparative method to identify and eliminate any pulse interference and abnormal local amplitudes induced by mechanical,transportation or artificial vibrations.Apply edge detection,crosscorrelation filters,and seismic event judgement to isolate and identify the effective micro-seismic signals generated by hydraulic fracturing [23,24].When SNR values are high,the micro-seismic data has high positioning accuracy,and when the SNR is low,positioning accuracy is low.On the basis of points (1) and (2),the micro-seismic source location can be determined directly.
(3) Locate the strong and weak micro-seismic events source using different methods [23-26].During the fracturing process,the stratum near the centre of the hydraulic fracturing experiences the greatest impact of force and energy,forming large fractures and strong micro-seismic events.Locations that are far from the centre of the hydraulic fracturing experience weak energies and a lower impact force.Here,the fracture opening degree is small,and the microseismic events are weak.Strong micro-seismic event sources can be accurately located by the superposition and source scanning,but weaker micro-seismicity needs to be processed differently.First,combine the strong micro-seismic event data with the detonator signal to optimise static correction.Next,determine the arrival time of the first wave by enhancing the weak signal.Finally,obtain precise event locations using the double difference relative location technique.Owing to varying SNRs for micro-seismic events,location accuracy also varies.Location error estimation can be implemented using SNR and perforation depth values.The located absolute error is equal to the difference between the perforation depth calculated from the micro-seismic data with a different SNR and the perforation measured depth.Generally,the vertical absolute error will be less than 40 m,and absolute error is ±10 m.
Fig.1.Technical workflow of the 3D visualization of hydraulic fractures using data from micro-seismic monitoring.
(4) Using the micro-seismic amplitude information,the energy intensity for micro-seismic events can be calculated based on the methods described by Maxwell [27]and Stanˇek et al.[28].
Hydraulic fracturing is a process of producing fractures by applying stress[5,6].Fracture orientation is controlled by the in situ stress field,and hydraulic fractures tend to develop along the natural faults and fractures [29,30].In other words,the main control factors for hydraulic fracture orientation are the pre-existing natural faults and fractures.After analysing the spatial position of these micro-seismic event locations,the tectonic and fault system orientations,the geo-stress characteristics of the monitored areas,and the hydraulic fracture geologic modes,including the different distribution,the development,extent and cutting relationship of different hydraulic fractures,can be understood.Accordingly,the micro-seismic monitoring location and energy data,as well as the interpretation of hydraulic fracture geologic modes,can be used to create a 3D visualization of the hydraulic fractures.
Since the extracted micro-seismic events were generated in the 3D space,it is necessary to locate the micro-seismic events for each series of modelled hydraulic fractures.Accordingly,the hydraulic fracture indication model F is established.
In general,an oil or a gas reservoir is located in a stratigraphic system,divided into several fine layers [4,31,32].The top and bottom structure surface diagrams can be generated for each layer based on general requirements,forming a structure for the target stratigraphic system and defining the location of each microseismic event within 3D space [33,34].Each micro-seismic event can be associated with one layer in the stratigraphic system.All micro-seismic events are projected on the bottom surface of one layer in the stratigraphic system (see Figs.5 and 6),and the projected micro-seismic events on the edges of each layer surface are connected to form the outer envelope surface.The new 3D space is formed by combining all outer envelope surfaces,encompassing the locations of all micro-seismic hydraulic fracture events.Microseismic hydraulic fracture events for other hydraulic fracture groups are also processed using this method.
The 3D space covers all monitored micro-seismic events in the hydraulic fracture group.On the basis of the deterministic modelling approach,the grids in the 3D space are denoted as 1,and the grids outside the 3D space are denoted as 0.The hydraulic fracture indication model (F,dimensionless) of hydraulic fractures is thereby described as in equation (1).
Fig.2.The surface micro-seismicity survey line(a),and the 21-stage hydraulic fracturing section along the horizontal trajectory(b)of horizontal well X.The lower left corner is the plane projection of well X.Arabic numerals indicate the stage number of the hydraulic fracturing.
Fig.3.Micro-seismic events and source locations from hydraulic fracturing around horizontal well X.
Fig.4.The geologic mode of hydraulic fractures based on micro-seismic source location data for hydraulic fracturing around horizontal well X.
The energy intensity E (seismic amplitude squared sum per square second in one cubic metre,in units of cm2.s-2.m-3)can be calculated from the micro-seismicity monitoring data.The higher the energy intensity,the greater the density and spatial extent of hydraulic fracturing and vice versa.The micro-seismic energy intensity can be used as the input data,and the hydraulic fracture density model D(the product of the number and the average width of fractures per cubic metre of area,in units of num.mm.m-3)can be built using the sequential Gauss simulation method [33-36].However,the energy E of micro-seismic events cannot be considered as the same parameter as the fracture intensity D.First,the hydraulic fracture density model was established based on the energy intensity of the micro-seismicity.Second,the discrete hydraulic fracture model was built.After comparing the discrete model with the geologic interpretation of hydraulic fractures and eliminating the differences between the two models,the correction coefficient k (num.mm.cm-2.s2) for these two models is determined.Thus,a reliable hydraulic fracture density model can be achieved using equation (2).
The 3D visualization of hydraulic fractures was constructed using the hydraulic fracture density model,the auxiliary input of the azimuth and dip of the hydraulic fracture from geologic interpretation,and the spatial constraints of the hydraulic fracture indication model.This model can accurately characterize the hydraulic fracture features in 3D space,including shape,distribution,dip angle and azimuth.
The Jiao reservoir [32].lies in the Sichuan Basin,southwest China.It is currently one of the hot gas reservoirs in China[37,38].Well X,located at its core area with no faults,is a shale gas production well with a horizontal segment length of 1449 m,which was completed in September 2014.On the basis of a 21-stage hydraulic fracturing construction scheme(Table 1 and Fig.2b),surface micro-seismic monitoring was performed by the Sinopec Geophysical Research Institute.The surface micro-seismicity survey line is located at the centre of well X with a radial layout.There are 11 survey lines with a total length of 37,000 m and an intertrace distance of 20 m (Fig.2a).The detonator measured depth (to calibrate and optimise horizontal layer velocities)is 2712.82 m.The surface micro-seismic monitoring data were collected simultaneously with hydraulic fracturing over a period of 20 days.Using the isotropic horizontal layer velocity model and proposed methods,all 21 stages of hydraulic fracturing were completed,and a total of 695 micro-seismic events (including measurements for location and energy) are shown in Fig.3 and Table 1.
Table 1 Hydraulic fracturing data and micro-seismic monitoring events around horizontal well X.
Fig.5.3D spatial freeze-frame of micro-seismic monitoring sources from hydraulic fracturing around horizontal well X.(a)2D section and(b)3D spatial distribution of eight layers(nine surfaces)from bottom to top along the trajectory of well X.The envelope polygons at the bottom surfaces of layers 1(c)and 7(d)are shown.Other layers are omitted due to limited space.Units are measured in meters.
Relying on the fault-related fold theory [30,39]and the two large faults that limit the Jiao reservoir,the study area has been subjected to compressional tectonic stress in both NW-SE and NE-SW directions [39-41].On the basis of the analysis of microseismic event hypocenter spatial relationships,the directions of hydraulic fractures in well X are divided into NW and NE directions,where different micro-seismic event source points are connected with different lines according to these two directions.This results in a complete geological interpretation of the hydraulic fracture distribution pattern for well X.As shown in Fig.4,a NW-and NEoriented intersected network is formed during the fracturing of well X.However,owing to the implementation of different stages of fracturing along the horizontal well trajectory,changes in working conditions inevitably lead to disparities.Therefore,the geological mode of the fracture network differs with each fracturing stage,and characterising the distribution,size,density and cross-sectional relationship of hydraulic fractures in well X is extremely complex.
The WL formation in the Jiao reservoir can be subdivided into eight layers from top to bottom and nine layer interfaces based on sedimentary facies and lithofacies characteristics[37,38].The upper and lower interfaces of each layer are considered boundaries.Using these regional divisions,the monitored micro-seismic events in well X were classified into nine spatial regions(Fig.5a and b).The horizontal trajectory of well X mainly passes through the bottom part of the WL formation.The fracturing affected shale layers 1-8 and extended to other formations below the WL formation.In this study,only the fracturing in the WL formation is considered and discussed.Layer 8 at the top of the WL shale was not affected by the fracturing of the horizontal section of well X (Fig.5a and b),and micro-seismic hydraulic fracturing events were mainly distributed in layers 1-7.
Fig.6.The 3D hydraulic fracture indication model of horizontal well X.(a) Section visualization along the trajectory of well X,(b) 3D visualization of hydraulic fractures,(c) 3D visualization of stratigraphic hydraulic fractures in layer 1,and(d)3D visualization of stratigraphic hydraulic fractures in layer 7.Other layers are omitted due to limited space.The eight layer numbers (nine surface numbers) are the same as those in Fig.5 (b).Units of axes are meters.
The outer envelope of the micro-seismicity region was formed by projecting the micro-seismic events of well X to the bottom surfaces of layers 1-7 (Fig.5c and d).The outer envelope volume gradually decreases with the increment of horizontal well distance and decreases dramatically in the spatial area of layer 7(Fig.5a and d),again reinforcing that almost no hydraulic fracturing microseismic events exist in the spatial area of layer 8.
By connecting each outer envelope in well X,the hydraulic fracture indication model was formed on Petrel platform.The grid dimensions in the X,Y,and Z directions are 5,5 and 12 m,respectively,and the grid numbers in the X,Y and Z directions are 381,321 and 9,respectively.As shown in Fig.6,the 3D model of fractures and their distributions,and the modelling results of the strata and fracture distributions show that the fracture extent in well X decreases gradually from bottom to top and decreases sharply at the seventh layer to zero in the eighth layer.
By applying the proposed method,the correction coefficient was determined by repeated trial calculations on the fracture pattern characteristics of well X to be k=1/60.The hydraulic fracture intensity of well X was obtained through micro-seismic event energy calculations.The hydraulic fracture indication model was employed as a trend constraint,and using the sequential Gaussian method on Petrel platform,the hydraulic fracture density models with the same grid size and cells as the hydraulic fracture indication model were established for both the NW-and NE-oriented hydraulic fractures of well X (Figs.7 and 8).In comparing the hydraulic fracture intensity in the NW and NE directions,it was found that the hydraulic fracture intensity values in NW trending fractures are greater than those in the NE trending fractures;that is to say,the hydraulic fracture intensity in NW fractures is stronger than that in NE fractures.
Fig.7.The 3D NW-oriented hydraulic fracture density model around horizontal well X.(a) Section visualization along the trajectory of well X,(b) 3D visualization of hydraulic fractures,(c) 3D visualization of stratigraphic hydraulic fractures in layer 1,and (d) 3D visualization of stratigraphic hydraulic fractures in layer 7.The eight layer numbers (nine surface numbers) are the same as those in Fig.5 (b).Other layers are omitted due to limited space.Units of color legend (development density):num.mm.m-3.
The 3D visualization of hydraulic fractures in well X in the Jiao reservoir was carried out successfully by using the module of creating discrete fracture network in Petrel platform.The main input is the hydraulic fracture density models and the hydraulic fracture indication models for well X and the auxiliary inputs are the fracture azimuth and dip angle (Fig.9).
Fig.9(a) and b were coloured according to the dip angle and azimuth angle,which show the distribution of hydraulic fractures,the intersecting relationships of hydraulic fracture groups,the inclination of each fracture in well X in 3D,and the changes in dip angle and azimuth angle.Fig.9(c) and d show the frequency distribution of the dip angle and azimuth angle.Results indicate that the dip angle changes from 75°to 90°,and the azimuth angle varies between 60°and 240°.
Fig.8.The 3D NE-oriented hydraulic fracture density model around horizontal well X.(a) Section visualization along the trajectory of well X,(b) 3D visualization of hydraulic fractures,(c) 3D visualization of stratigraphic hydraulic fractures in layer 1,and (d) 3D visualization of stratigraphic hydraulic fractures in layer 7.Other layers are omitted due to limited space.The eight layer numbers (nine surface numbers) are the same as those in Fig.5(b).Units of axes scales:meters;units of color legend:num.mm.m-3.
The 3D visualization of hydraulic fracturing helps to quantitatively characterize the geometric size,the occurrence characteristics,the cutting relationships,the distribution location and scale of the artificial hydraulic fractures in well X.The 3D visualization results provide a typical example for the artificial hydraulic fracturing evaluation of similar shale gas horizontal wells in the Jiao reservoir.In fact,the bedding fractures and the structure surfaces also have some effect on the production performance in the Jiao reservoir.However,as other shale gas fields,the contribution of artificial hydraulic fractures is critical and tremendous to shale gas production in the Jiao reservoir,and the evaluation of the contributions is necessary to use the 3D visualization method proposed in this paper.After assigning porosity and permeability to the fracture model,the numerical simulation method can be used to simulate the flow change in the fractures and obtain the production for a single well.In a word,the technical method formed in this paper provides the model basis and method support for hydraulic fracturing evaluation and production simulation of shale gas horizontal well.
Fig.9.3D discrete network model of hydraulic fractures around horizontal well X.Colors show the(a)dip angle and(b)azimuthal angle.Frequency distribution histogram of the(c)dip angle(°),and (d) azimuthal angle(°).
(1) A technique to establish the 3D discrete network model of hydraulic fracturing based on micro-seismic monitoring was proposed.Procedures include extracting the spatial position and intensity of the micro-seismic monitoring data and then developing the hydraulic fracture indication model,the hydraulic fracture density model,and the hydraulic fracture discrete model.This technique allows for 3D visualization of the position distribution,development,extent and cutting relationships of each group of hydraulic fractures.
(2) Using the proposed technique,3D discrete hydraulic fracture models for the WL formation in well X in the Jiao reservoir were successfully developed.These models visualized the 3D spatial distribution and characteristics of hydraulic fractures in the study area and verified the applicability and the reliability of the technique.
Acknowledgements
We are grateful for the support of the National Major Basic Research Development Program of China (2014CB239205),the Sichuan Science and Technology Program (Grant No.2021YFQ0049)and the sub-project of the National Science and Technology Major Project (Grant No.2017ZX05035003).
扩展阅读文章
推荐阅读文章
老骥秘书网 https://www.round-online.com
Copyright © 2002-2018 . 老骥秘书网 版权所有