Seismicity Based Reservoir Characterization Case Study : Correlation of Microseismicity with Reflectivity at the KTB

An approach for the interpretation of microseismic data was proposed to provide in-situ estimates of the hydraulic diffusivity characterizing a geothermal o r hydrocarbon reservoir on the large spatial scale (on the order of 10m). This approach is called "Seismicity Based Reservoir Cha racterization" (SBRC, Shapiro et al. (1997-2002)). The SBRC is based o n the hypothesis that the spatial propagation of hydraulically induced seismicity is caused mainly by the pore pressure relaxation process. According to this hypothesis the SBRC uses a spatio -temporal analysis of fluid-injection induced microseismicity to reconstruct the tensor of hydra ulic diffusivity and to estimate the tensor of permeability in 3D. However, processes that can lead to tr iggering of microseismicity are not yet fully understood. A correlation of microseismic hypocente rs with structural images obtained from reflection seismics can help to better understand the physic s of microseismicity triggering and thus to test the main assumption of the SBRC. The SBRC approach was successfully applied to real data seve ral times. Recently, fluid injection induced microseismicity at the KTB site was analysed in term s of the SBRC method to reconstruct the tensor of permeability at the open hole section at 9.1 km d epth. Using new data sets acquired in 2000 we are able to observe indications of the depth-depende ncy of hydraulic diffusivity at the KTB for the first time. The analysis of fluid-induced microseismi city leads to an estimation of the hydraulic diffusivity at the KTB at different depths. A lower value of h ydraulic diffusivity was found in upper parts of the subsurface compared with the values at the openhol section. Correlations with structural images were obtained. For example, we observe that rock volu mes characterized by larger diffusivity also show larger reflectivity. INTRODUCTION The attention to microseismic monitoring during operation of geothermal or hydrocarbon reservoirs has grown considerably over the last several years. The observa tion of microseismicity occurring during borehole fluid injections or extractions has a large potential in characterizing rocks in terms of their hydraulic parameters at locations up to several kilometers from boreh oles (Talwani and Acree (1985); Adushkin et al. (2000); Fehler et al. (2001)). The most common applica tion has been hydraulic fracture imaging and growth characterization (e.g. Phillips et al., 1997; Ur bancic et al., 1999). Longer-term microseismic monitoring has been used to map oil-producing, natural frac tu es (e.g., Rutledge et al., 1998) and also shows promise in tracking flood fronts in the case of enhanced oil recovery (e.g., Maxwell et al.,1998). Beyond delineating conductive fracture geometry and infer ring fluid-flow paths, microseismic data could potentially be used to measure in-situ hydraulic propertie s of rocks at interwell scales, providing information that could further guide operations to optimize field pr oduction. Annual WIT report 2002 193 Recently, an approach for the interpretation of microseism ic data was proposed to provide in-situ estimates of the hydraulic diffusivity characterizing a geothe rmal or hydrocarbon reservoir on the large spatial scale (of the order of 10m). This approach is called “Seismicity Based Reservoir Char acterization“ (SBRC). It uses a spatio-temporal analysis of fluid-injecti on induced microseismicity to reconstruct the tensor of hydraulic diffusivity and to estimate the tensor of pe rm ability (see Shapiro et al., 1997, 1999, 2000, 2002 and the discussion of the method in Cornet 2000). The app ro ch assumes the following main hypothesis: Fluid injection in a borehole causes perturbations of the pore pressure in rocks. Such perturbations cause a change of the effective stress, which, if large enoug h, can trigger earthquakes along pre-existing zones of weakness. The SBRC approach considers that most of t he seismicity is triggered along critically stressed, pre-existing fractures. Furthermore, it assumes that the spatio-temporal evolutio n of the hydraulically-induced microseismicity is completely defined by the diffusion-like process of po re-pressure relaxation. The analysis of spatiotemporal features of the microseismicity then provides a po ssibility to invert for hydraulic diffusivity distributions in fluid-saturated rocks. The approach was successf ully applied to real data several times (Shapiro et al. (2000); Rothert et al. (2001); Shapiro et al. (2002); A udigane et al. (2002)) and numerically verified (Rothert and Shapiro (2002a,b)). In this paper, we present a case study from the German contine ntal deep drilling site (KTB) where microseismic events were induced by fluid injection within v arious depth ranges. This enables us to analyse the depth-dependency of hydraulic parameters at one locati on for the first time. The results obtained are correlated with structural images. Spatio-temporal featu res of the evolution of microseismic clouds as well as the differences in the estimations of depth-dependent hy draulic diffusivity are quite well supported by reflection seismic results. A SUMMARY OF THE CONCEPT OF SBRC In the low-frequency limit of the Biot equations of poroelas ticity (Biot 1962) the pore-pressure perturbation p can be approximately described by the following differenti al equation of diffusion: ∂p ∂t = ∂ ∂xi [ Dij ∂ ∂xj p ] . (1) Here,Dij are the components of the tensor of hydraulic diffusivity, xj (j = 1, 2, 3) are the components of the radius vector from the injection point to an observati on point andt is the time. Equation (1) corresponds to the second-type Biot wave (the slow P-wave) in the l imit of the frequency being extremely low in comparison with the global-flow critical frequency (Biot 1962). The tensor of hydraulic diffusivity is directly proportional to the tensor of permeability (see Sh apiro et al., 2002). Considering the power spectrum of a step function injection sig al, which can roughly approximate a real pore pressure perturbation, Shapiro et al. (1997) and Shapiro et al. (2000) introduced a heuristic concept of themicroseismic triggering front . This front is regarded as a spatial surface which separates the regions of the relaxed and unrelaxed pore pressure perturba tion. For example, in the case of a homogeneous and isotropic medium, Shapiro et al. (1997) obtained the fol lowing equation describing the spatial position of the triggering front in an effective isotropic homogeneo us poroelastic medium with the scalar hydraulic diffusivity D: