Multicomponent wide-azimuth seismic data for defining fractured reservoirs

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1 Multicomponent wide-azimuth seismic data for defining fractured reservoirs Evaluating and exploiting azimuthal anisotropy Data Processing Figure 1 A typical surface outcrop showing aligned fractures Figure 2 Drilling deviated or horizontal wells perpendicular (a) or parallel (b) to fractures Tight rock reservoirs such as limestone often produce more from secondary fracture porosity than from primary porosity. Consequently, mapping fracture density and direction is important not only to identify sweet spots, but also to optimize production. Because of in-situ stress within fields, most fractures are aligned and often close to vertical, rendering a reservoir azimuthally anisotropic (Figure 2). However, azimuthal anisotropy is captured in recorded seismic data, which exhibits similar azimuthal variations. WesternGeco has many technologies in seismic acquisition and processing that can assist the explorationist in detecting, quantifying, and compensating for azimuthal anisotropy.

2 Amplitude Figure 3 Narrow-azimuth P-amplitude maps provide no azimuthal information. The amplitudes are the same at 0 (left) and 180 (right). Data courtesy of Eni Agip divsion Amplitude Figure 4 Wide-azimuth P-amplitude maps showing azimuthal variations due to seismic anisotropy Data courtesy of Eni Agip division In narrow-azimuth marine seismic surveys such as those acquired by towed streamers, there is little azimuthal information. Because of reciprocity, the amplitude is the same every 180 (Figure 3). However, data acquired using ocean-bottom cables for a marine multicomponent (4C) survey produce a wideazimuth data set that can be split into a number of azimuth-limited cubes. This is because the shooting vessel often traverses perpendicularly to the recording cables, (almost all land 3D surveys are shot in this way). As illustrated in Figure 4, clear azimuthal variations can even be observed on a set of P-amplitude maps for different azimuth groups. In azimuthally anisotropic rocks, the initial shear wave splits into two (Figure 5). A faster-speed wave polarizes along the fractures, and a slower-speed wave moves perpendicularly to the fractures. This information is used extensively when analyzing seismic data for azimuthal anisotropy and can provide information about fracture density (fracture porosity) and orientation (directions of preferred permeability). Figure 5 Azimuthal variation in seismic velocity and shear-wave splitting

3 a. Supergather analysis - radial (left) and transverse (right) horizontal components c. 2C x 2C convertedwave Alford rotation analysis b. Common-offset ring analysis Direction Magnitude Figure 6 Quantifying azimuthal anisotropy WesternGeco has developed a number of methods (supergather analysis, common-offset ring analysis, and 2C x 2C converted-wave Alford rotation analysis) to analyze azimuthal anisotropy. The methods are illustrated in Figure 6. These options allow the analysis to be performed with the required level of lateral resolution to capture all the details of the subsurface anisotropy.

4 Supergather analysis displays the radial and transverse horizontal components of the data. In Figure 6a, clear bands of low amplitude are observed on the transverse component corresponding to the fast and slow directions. Common-offset ring analysis also utilizes the variation in the transverse component with azimuth and is applied prestack. Figure 6b shows the analysis at five different locations on two different layers. Traces are gathered together in common-offset rings, then a stack of all data for all azimuths and limited offsets with different trial directions for the fast shear direction is generated for each offset and trial direction. The stacked values are displayed as a function of azimuth and offset. Radial lines with high stack values highlight the fast and slow directions. The 2C x 2C converted-wave Alford rotation is generally used poststack. The left image in Figure 6c shows the data before rotation to pure fast and slow directions, while the right image shows the data after rotation. There are four panels in each of the two displays; the middle two are mixed fast and slow directions that we aim to minimize. Interpretation of these analyses delivers a detailed lateral image of the anisotropy magnitude and direction variations. Starting from the shallowest layer and working downwards by layer stripping, the anisotropy levels are determined independently. Figure 7 Fracture characterization at depth derived from marine 4C data Data courtesy of Eni Agip division Example results Example final results of a full layer-based azimuthal anisotropy analysis are shown in Figure 7. The anisotropy for a deep horizon is analyzed by direction in the left display and by magnitude in the right display. This process identifies rapid changes in anisotropy.

5 In Figure 8, the fracture information data derived from a 3-component land seismic survey is mapped. The display shows color-coded anisotropy magnitude, with the direction shown by the lines. The overlaid interpreted fault pattern clearly indicates anisotropy consistent with the fault blocks. Figure 8 Top reservoir fracture mapping from 3-component US land data

6 Well - 3 Well - 7d Well - 3 Well - 7d Well - 4 Well - 4 Figure 9 High azimuthal anisotropy in separate compartments Data courtesy of Eni Agip division Figure 9 shows the type of benefit that fracture mapping with wide-azimuth 4C seismic data can provide. The green areas represent high levels of anisotropy. The interpretation indicated a high-fracture-density zone to the west of Well-4. With the faulting overlaid in the right image, we can see that this zone is in a different compartment from the producing wells, as indicated by the white circles. It is, therefore, probably not in communication with the producing wells and offers potential for future drilling. New development at WesternGeco - Analysis method to detect tilted fractures Kink in SSE direction indicates fractures dipping in that direction Symmetrical shape indicates vertical fractures Kink in NNW direction indicates fractures dipping in that direction WG0473P London: Houston: Dubai: A Schlumberger/Baker Hughes company