Using Microseismic Monitoring and Crosswell Tomography to Map Hydraulic Fracture Treatments in Tight Gas Formations

Puckett, Mark; Le Calvez, Joel
Schlumberger, Clamart, France.

The goal of most hydraulic fracture monitoring campaigns is to understand the relation between hydraulically-induced fracture systems and estimated ultimate recovery (EUR). Integrating observations from various disciplines such as geology, geophysics, geomechanics, fluid physics, stimulation and engineering is critical.

One key step in properly mapping hypocentral locations of hydraulically-induced microseismic events is to understand the spatio-temporal evolution of the formation velocities in the zone of interest. We present the results of a dual borehole microseismic monitoring exercise in a tight gas sand formation undertaken as part of a drilling and completion program, and interpret its results in relation to pre- and post-treatment crosswell tomographic surveys performed between nearby vertical wells.

In this study, three vertical wells along an approximately N40 bearing are evenly spaced with the treatment well located in the middle. In a first phase, one monitoring array is deployed in each of the two wells at each end to perform a crosswell tomography survey yielding an initial velocity profile. Such profile matches the sonic log within acceptable limits. In a second phase, hydraulic stimulations take place at various depths in the treatment well. Similar microseismic results are observed from both arrays; confirming that great confidence may be given to the mapped geometries of the N130-oriented hydraulically-induced fracture systems. In a third and final phase, a post-treatment crosswell tomography survey is performed using the same acquisition geometry that was used in the first crosswell survey. Both crosswell surveys yield high-resolution P- and S- imaging which indicate some change in P-wave velocity up to 3% which could relate to pressure change and/or changes in effective stress. Observed changes in S-wave velocity may be related to variations in total effective porosity, and changes in S-wave attenuation are likely associated with the presence of the hydraulically-induced fracture network.

Three-dimensional rendering of the tomographic profiles and associated velocity changes with the mapped hypocentral locations provide additional understanding of the uncertainties associated to microseismic event location mapping. An assessment of uncertainty estimation is carried out for all measurements involved in the survey.

AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012