Interpretation of Microseismicity Resulting from Gel and Water Fracturing of Tight Gas Reservoirs

We provide a comparative analysis of the spatio-temporal dynamics of hydraulic fracturing-induced microseismicity resulting from gel and water treatments. We show that the growth of a hydraulic fracture and its corresponding microseismic event cloud can be described by a model which combines geometry- and diffusion-controlled processes. It allows estimation of important parameters of fracture and reservoir from microseismic data, and contributes to a better understanding of related physical processes. We further develop an approach based on this model and apply it to data from hydraulic fracturing experiments in the Cotton Valley tight gas reservoir. The treatments were performed with different parameters such as the type of treatment fluid, the injection flow rate, the total volume of fluid and of proppant. In case of a gel-based fracturing, the spatio-temporal evolution of induced microseismicity shows signatures of fracture volume growth, fracturing fluid loss, as well as diffusion of the injection pressure. In contrast, in a water-based fracturing the volume creation growth and the diffusion controlled growth are not clearly separated from each other in the space-time diagram of the induced event cloud. Still, using the approach presented here, the interpretation of induced seismicity for the gel and the water treatments resulted in similar estimates of geometrical characteristics of the fractures and hydraulic properties of the reservoir. The observed difference in the permeability of the particular hydraulic fractures is probably caused by the different volume of pumped proppant.     

A multiparameter radar case study of the microphysical and kinematic evolution of a lightning producing storm

Summary We examine the co-evolving microphysical, kinematic, and electrical characteristics of a multi-cell thunderstorm observed on 21 May 1993 along the Front Range of Colorado using data collected with the 11 cm, multiparameter, CSU-CHILL Doppler radar. The measured polarimetric variables provide information on the size, shape, orientation, and thermodynamic phase of hydrometeors. Recent modeling and observational advances in weather radar polarimetry now permit the inference of bulk-hydrometeor types and mixing ratios, and the measurement of precipitation rate in mixed-phase (i.e., hail and rain) environments. We have employed these and other radar techniques, such as dual-Doppler analyses, to investigate the correlation between the convective life cycle of a multi-cell storm and the evolution of lightning type and flash rate.The observations suggest a strong correlation between the radar-inferred graupel volume suspended in a vigorous updraft in upper-portions of the storm and the in-cloud (IC) lightning flash rate. Our analyses reveal that maxima in the hail rate are related to peaks in the cloud-to-ground (CG) lightning flash rate. Both correlation's are consistent with the non-inductive charging mechanism which relies on collisions between graupel/hail particles and ice crystals in the presence of supercooled water. Peaks in storm outflow are shown to either lag or to coincide with maxima in both the CG flash rate and hail rate. The amount and vertical location of ice in this storm was also related to the strength and polarity of the electric field through observations of a Field Excursion Associated With Precipitation (FEAWP) and a subsequent microburst. We demonstrate that the FEAWP was coincident with the descent of graupel and small hail below the charge reversal level as explained by the non-inductive charging mechanism, and that the further descent of graupel and small hail below the melting level aided in the generation of a microburst near the surface. Using observations of the FEAWP, we present some comparative speculation on the microphysics of the associated lower positive charge center and the applicability of various laboratory charging studies.With 14 Figures