In-well seismic methods include VSP, CT and other methods, which need to drill into or through one or more boreholes in the target layer. Compared with the ground earthquake, when the excited seismic wave reaches the geophone, at least one shallow low-speed layer absorbs less high-frequency energy, and the frequency of the seismic acquisition signal in the well is higher, so the resolution of the seismic in the well is higher. The coverage of borehole seismic data is only around or between boreholes. Borehole earthquake is generally used as an auxiliary means of conventional earthquake.
The acquisition equipment of different borehole seismic methods can interoperate, and the data acquisition methods are similar. Therefore, when using multiple borehole seismic methods at the same time, it is necessary to consider how to combine borehole seismic methods and use the characteristics of mutual borrowing to save resources and funds. In addition, the acquisition of borehole seismic data also has some influence on other methods. The combination of borehole seismic method and other methods should be considered in the design of actual exploration scheme. The following two typical borehole seismic methods (VSP, CT) focus on data acquisition, processing and interpretation, and expound the key points of their technical methods and related issues.
(1) vertical seismic profile
The observation of vertical seismic profile (VSP) requires at least one borehole penetrating or drilling near the target layer. Multilevel detection arrays are respectively arranged in the borehole, the seismic source position is designed in the ground or adjacent borehole, seismic wave is excited according to predetermined parameters, seismic wave field signals propagated through underground strata are collected, and the original data including upstream and downstream seismic wave field information are processed to obtain the seismic wave field imaging profile of the target exploration layer. On this basis, the geological stratigraphic structure and structure of the target interval of the reservoir point are inferred, especially the physical description and fault development of the reservoir-cap assemblage (Daley et al., 2005; Daley et al., 2007).
According to the exploration requirements, various observation systems such as zero offset VSP, non-zero offset VSP, Walkaway VSP, 3D VSP, cross-well VSP and their combinations can be selected for data acquisition.
VSP observation generally uses explosives, vibroseis, electric sparks or orbital vibration sources to excite longitudinal and transverse seismic waves. The geophone adopts multi-stage single-component or multi-component underwater geophone. The seismic source is placed on the ground or in the excitation hole, and the geophone combination is placed in the observation well. The combination of seismic source and geophone is connected with the ground receiving and excitation device through cables, and the combination of seismic source and geophone can move in their respective holes or on the ground.
Before formal observation, the initial model of the monitoring area is established according to the known geological data, and the numerical simulation is carried out by ray tracing method, and the acquisition parameters (imaging range, coverage times and other parameters) matching the exploration requirements are initially determined; Then, through field tests, the parameters such as observation system and combination mode, number of sources, source type, source position, source spacing, excitation energy, frequency range and stacking times, geophone series, geophone point spacing, sampling length, sampling interval, acquisition period, movement mode of source and geophone combination are determined, the sources of acquisition noise are analyzed, noise suppression measures are put forward, and the signal-to-noise ratio and signal reliability of wave trains picked up by each geophone are investigated. In the formal data acquisition, seismic wave signals are collected by using the parameters obtained from experiments, and the arrival time, amplitude and frequency of the first wave and the repeatability of the signal are investigated for each detection point to identify the wave field characteristics corresponding to the exploration target area.
VSP data processing generally includes conventional processing such as editing, amplitude recovery, wave field separation, first break picking, velocity analysis, attenuation coefficient calculation and imaging, and subsequent processing such as inversion and attribute analysis similar to conventional earthquakes. VSP data processing objects are divided into two categories: zero offset VSP data and non-zero offset VSP data. Zero offset VSP data processing content is as follows:
1) Decodes and displays the original records, and eliminates abnormal tracks;
2) spectrum analysis of first break wave and monitoring wavelet, and wavelet shaping;
3) Pick up the first break time (S wave should be synthesized by horizontal component vector first);
4) draw average velocity curve, interval velocity curve and time-depth conversion curve;
5) selecting appropriate deconvolution parameters to deconvolution the upward wave;
6) separation of upward wave and downward wave;
7) The corridor shall be removed, and the number of shallow and deep layers shall be the same as far as possible;
8) Corridor superposition. VSPLOG display of different filter files;
9) The results show that clear effective waves and weak interference background are needed. Header contents include: team number, construction well number and construction date. Seismic points, well source distance, observation interval, hole depth, wavelet detector depth, processing flow, processing unit and processing date.
Non-zero offset VSP data processing (Chen et al., 20 1 1) includes the following contents:
1) Decodes and displays the original records, and eliminates abnormal tracks;
2) Design the formation velocity model beside the well;
3) Three-component recording synthesis, that is, separation of uplink and downlink waves and separation of uplink P-SV waves;
4) selecting appropriate deconvolution parameters to deconvolution the upward wave;
5) Using the accurate calculation of the first arrival of P wave and P-SV wave, the records of P wave and SV wave are dynamically corrected respectively;
6) Migration stack (or CDP conversion stack) requires amplitude processing, and the time and amplitude characteristics of main horizons are consistent with the seismic profile near the well, and the scale selection is determined according to the user's requirements, at least one of which is consistent with the seismic profile near the well;
7) The result display requirements and header contents are the same as the zero offset VSP data processing.
VSP data interpretation objects include corridor stacking profile, non-zero offset imaging profile, cross-well seismic profile, well logging data and geological background data. The main interpretation means include stratigraphic correlation, structural interpretation, seismic wave field analysis, reservoir-cap property prediction and so on. The working ideas and main methods of VSP data interpretation are basically the same as those of conventional earthquakes, and its main contents generally include:
1)VSP profile, surface seismic profile and geological profile, determine the geological horizon of the main reflection layer, estimate the layer error, and determine the multiple component, generation horizon and propagation path;
2) Velocity analysis, which displays data such as average velocity, interval velocity and time-depth curve in the form of list or graph. Compare their relationship with the geological profile of this well, and comprehensively analyze their relationship with the lateral change of velocity in neighboring wells and the whole area;
3) During drilling, use VSP data to predict the depth of exploration target layer below the drill bit, and quickly put forward drilling correction opinions;
4) Analyze the seismic wave field, distinguish various types of waves, study the attenuation law of waves and its relationship with formation lithology, comprehensively interpret VSP data of P waves and S waves, and study seismic information related to lithology such as Poisson's ratio;
5) Study the changes of structural morphology and lithology around the well, make a careful comparison with the surface seismic profile, and put forward suggestions for modifying the interpretation of the original surface seismic profile;
6) Interpretation and research of other special geological tasks.
(2) Cross-well seismic tomography
Cross-well seismic tomography (CT) requires at least two boreholes penetrating or drilling near the target horizon. The seismic source and geophone are respectively arranged in the two boreholes, and the seismic wave field signal propagating through the target layer is excited according to the predetermined acquisition parameters. According to the extracted parameters such as the arrival time or amplitude of the first break wave, the spatial changes of wave field parameters such as velocity and attenuation coefficient of the target interval are inverted, so as to infer the geological stratigraphic structure and structure of the target interval where the reservoir is located, especially the reservoir-cap combination.
The source of cross-well seismic tomography is placed in the excitation hole, and the detector combination is placed in another borehole. The seismic source and detector array are connected with the ground receiving and excitation device through cables. The seismic source and detector array move up and down in their respective holes according to the exploration requirements. The focal distance, the number of detectors in the detector array and the distance between detection points are determined by the exploration target. Generally, the cross-well seismic tomography area should be slightly larger than the target area required for exploration. Time-lapse cross-well seismic monitoring generally uses electric spark or orbital source to excite longitudinal and transverse seismic waves. The geophone adopts multi-stage single-component or multi-component underwater geophone.
Before formal observation, parameters such as source spacing, excitation energy, frequency range, stacking times, detection point spacing, sampling length, sampling interval, acquisition period, combined motion mode of source and geophone are determined through experiments and exploration requirements, and the sources of acquisition noise are analyzed, noise suppression measures are put forward, and the signal-to-noise ratio and signal reliability of wave trains picked up by each geophone are investigated. In the formal data acquisition, the seismic wave signals are repeatedly collected by using the parameters obtained from the initial test, and the first wave time difference and signal repeatability of the repeatedly collected signals are investigated for each detection point to determine the error range of the first wave time difference of the signals.
Cross-well seismic tomography data processing generally includes preprocessing and parameter inversion. Pretreatment includes editing, filtering, ray angle constraint, borehole deviation correction and static correction. Parameter inversion includes picking up travel time or amplitude, establishing initial model, forward calculating travel time or amplitude, solving model correction by theoretical calculation and picking up travel time or amplitude difference, and smoothing model correction. According to the theoretical calculation and residual error, it is decided whether to terminate the iterative calculation of forward model modification.
The initial model of parameter inversion model generally adopts discrete parameter model; Forward calculation can use ray tracing method and wave equation solution method; The modified Jacobian equation is generally solved by LSQR (Zhao et al., 2011; Zhao Lianfeng, 2002).
The interpretation objects of cross-well seismic tomography data include velocity imaging profile, quality factor imaging profile, cross-well seismic profile, well logging data and geological background data. The main interpretation means include stratigraphic correlation, structural interpretation and reservoir-cap attribute prediction. The main contents of cross-well seismic tomography data interpretation generally include:
1) study the velocity and attenuation law of longitudinal and shear waves of seismic waves and their relationship with formation lithology, and the spatial change of formation lithology between wells;
2) Study the changes of cross-well stratigraphic structure and structural morphology.